GEOL-GEOM

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Prof. Stephen A. NelsonEENS 111

Tulane UniversityPhysical Geology

Introduction to Course

This page last updated on 27-Aug-2003

Geology, What is it?

Geology is the study of the Earth. It includes not only the surface process which have shapedthe earths surface! "ut the study of the ocean floors! and the interior of the Earth. It is not only

the study of the Earth as we see it today! "ut the history of the Earth as it has evolved to its

present condition.

• Important point: The Earth has evolved #changed$ throughout its history! and will

continue to evolve.

• The Earth is a"out %.& "illion years old! hu'an "eings have "een around for only the

past 2 'illion years.o Thus! 'an(ind has "een witness to only 0.0%3) of Earth history.

o The first 'ulti-celled organis's appeared a"out 700 'illion years ago. Thus!

organis's have only "een witness to a"out *+) of Earths history.

Thus! for us to have an understanding of the earth upon which we live! we 'ust loo( at

processes and structures that occur today! and interpret what 'ust have happened in the past.

,ne of the 'aor difficulties we have is with the ti'e scale. Try to i'agine * 'illion years--Thats +0!000 ti'es longer than 'ost of you have lived. It see's li(e a long ti'e doesnt it

/et! to geologists! * 'illion years is a relatively short period of ti'e. ore discussion a"out

ti'e in later lectures. 1ut one thing we have to re'e'"er when studying the earth is that

things that see' li(e they ta(e a long ti'e to us! 'ay ta(e only a short ti'e to earth.

Examples:

• A river deposits a"out *'' of sedi'ent #'ud$ each year. ow thic( is the 'ud after

*00 years -- *0 c' hardly noticea"le over your lifeti'e.

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• hat if the river (eeps depositing that sa'e * ''4yr for *0 'illion years Answer

*0!000 'eters #&.2 'iles$. Things can change drastically5

Why Study the Earth?

• ere part of it. 6ust to 6ust. u'ans have the capa"ility to 'a(e rapid changes. All

construction fro' houses to roads to da's are effected "y the Earth! and thus reuire

so'e geologic (nowledge. All life depends on the Earth for food and nourish'ent. The

Earth is there everyday of our lives.

• Energy and ineral resources that we depend on for our lifestyle co'e fro' the Earth.

At present no other source is availa"le.

• Geologic a8ards -- Earthua(es! volcanic eruptions! hurricanes! landslides! could

affect us at any ti'e. A "etter understanding of the Earth is necessary to prepare for

these eventualities.• 9uriosity-- e have a "etter understanding of things happening around us. :cience in

general. Ill try to relate geology #and science in general$ to our everyday lives.

Earth Materials and Processes

The 'aterials that 'a(e up the Earth are 'ainly roc(s #including soil! sand! silt! dust$ . ;oc(s

in turn are co'posed of 'inerals. inerals are co'posed of ato's!

<rocesses range fro' those that occur rapidly to those that occur slowly

• E=a'ples of slow processes

o >or'ation of roc(s

o 9he'ical "rea(down of roc( to for' soil #weathering $

o 9he'ical ce'entation of sand grains together to for' roc( #diagenesis$

o ;ecrystalli8ation to roc( to for' a different roc( #metamorphism$

o 9onstruction of 'ountain ranges #tectonism$

o Erosion of 'ountain ranges

• E=a'ples of faster processes

o 1each erosion during a stor'.o 9onstruction of a volcanic cone

o ?andslides #avalanches$

o 6ust :tor's

o 'udflows

<rocesses such as these are constantly acting upon and within the Earth to change it. any of

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these processes are cyclical in nature.

• Hydrologic Cycle (see figure 1.17b & 1.18 in your text)

o ;ain co'es fro' clouds - falls on surface! pic(s up sand! silt and clay! carries

particles to river and into ocean. ater then evaporates to "eco'e clouds!which 'ove over continents to rain again.

Rock Cycle (see figure 1.19 and 1.20 in your text)

o

ost surface roc(s started out as igneous roc(s- roc(s produced "y crystalli8ation fro' a

liuid. hen igneous roc(s are e=posed at the surface they are su"ect to weathering #!emial and me!anial proesses t!at redue ro"s to partiles$. Erosion 'oves particles

into rivers and oceans where they are deposited to "eco'e sedimentary roc(s. :edi'entary

roc(s can "e "uried or pushed to deeper levels in the Earth! where changes in pressure andte'perature cause the' to "eco'e metamorphic roc(s. At high te'peratures 'eta'orphic

roc(s 'ay 'elt to "eco'e magmas. ag'as rise to the surface! crystalli8e to "eco'e igneous

roc(s and the processes starts over.

External Processes

Erosion- roc(s are "ro(en down #weathered$ into s'all frag'ents which are then carried "y

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wind! water! ice and gravity. E=ternal "ecause erosion operates at the Earths surface. The

energy source for this process is solar and gravitational.

Internal Processes

<rocesses that produce 'ag'as! volcanoes! earthua(es and "uild 'ountain ranges. Energyco'es fro' the interior of the Earth! ost fro' radioactive decay - nuclear energy.

Principle of Uniformitarianism

<rocesses that are operating during the present are the sa'e processes that have operated in

the past. i.e. the present is the (ey to the past. If we loo( at processes that occur today! we caninfer that the sa'e processes operated in the past.

<ro"le's@

• ;ates -- rates of processes 'ay change over ti'e for e=a'ple a river 'ight deposit *

'' of sedi'ent 4yr if we loo( at it today. "ut! a stor' could produce higher runoff and

carry 'ore sedi'ent to'orrow. Another e=a'ple@ the internal heat of the Earth 'ay

have "een greater in the past than in the present -- rates of processes that depend on thea'ount of heat availa"le 'ay have changed through ti'e.

• ,"servations -- we 'ay not have o"served in hu'an history all possi"le processes.

E=a'ples@ t. :t. elens! :i8e of earthua(es.

<erhaps a "etter way of stating the Principle of Uniformitarianism is that the laws of nature

have not changed through ti'e. Thus! if we understand the physical and che'ical laws ofnature! these should govern all processes that have ta(en place in the past! are ta(ing place in

the present! and will ta(e place in the future.

Energy

All processes that act on or within the Earth reuire energy. Energy can e=ist in 'any differentfor's@

• Gravitational Energy -- Energy released when an o"ect falls fro' higher elevations tolower elevations.

• eat Energy -- Energy e=hi"ited "y 'oving ato's! the 'ore heat energy an o"ect has!

the higher its te'perature.

• 9he'ical Energy -- Energy released "y "rea(ing or for'ing che'ical "onds.

• ;adiant Energy -- Energy carried "y electro'agnetic waves #light$. ost of the :uns

energy reaches the Earth in this for'.

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• Ato'ic Energy -- Energy stored or released in "inding ato's together. ost of the

energy generated within the Earth co'es fro' this source.

Heat ransfer

eat oves through 'aterial "y the following 'odes@

• 9onduction - ato's vi"rate against each other and these vi"rations 'ove fro' high

te'perature areas #rapid vi"rations$ to low te'perature areas #slower vi"rations$.-

eat fro' Earths interior 'oves through the solid crust "y this 'ode of heat transfer.

• 9onvection - eat 'oves with the 'aterial! thus the 'aterial 'ust "e a"le to 'ove.

The 'antle of the Earth appears to transfer heat "y this 'ethod! and heat is transferred

in the at'osphere "y this 'ode.

• ;adiation - eat 'oves with electro'agnetic radiation #light$ eat fro' the :un orfro' a fire is transferred "y this 'ode

Geothermal Gradient

Te'perature and pressure increase with depth in the Earth. ear the surface of the Earth the

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rate of increase in te'perature #called the Geothermal Gradient $ ranges fro' *+ to 3+o9 per

(ilo'eter. Te'perature at the center of the Earth is a"out %+00o9

he Earth !! What is it?

The Earth has a radius of a"out &37* ('! although it is a"out 22 (' larger at euator than at

poles.

Internal Structure of the Earth@

6ensity! #'ass4volu'e$! Te'perature! and <ressure increase with depth in the Earth.

• "ompositional #ayering

o Crust - varia"le thic(ness and co'position

9ontinental *0 - +0 (' thic(

,ceanic B - *0 (' thic(

o antle - 3%BB (' thic(! 'ade up of a roc( called peridotite

o Core - 2BB3 (' radius! 'ade up of Iron #>e$ and s'all a'ount of ic(el #i$

• #ayers of $iffering Physical Properties

o !ithosphere - a"out *00 (' thic( #deeper "eneath continents$

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o "sthenosphere - a"out 2+0 (' thic( to depth of 3+0 (' - solid roc(! "ut soft and

flows easily.

o esosphere - a"out 2+00 (' thic(! solid roc(! "ut still capa"le of flowing.

o #uter Core - 22+0 (' thic(! >e and i! liuid

o Inner core - *230 (' radius! >e and i! solidAll of the a"ove is (nown fro' the way seis'ic #earthua(e waves$ pass through the Earth aswe will discuss later in the course.

Surface %eatures of the Earth

,ceans cover 7* ) of Earths surface -- average depth 3.7 ('. ?and covers re'aining surface

with average of 0.B (' a"ove sea level

,cean 1asins

o Continental $helf% $lope% and rise

o "&yssal Plains

o #ceanic ridges

o #ceanic 'renches

Plate ectonics

'ectonics C 'ove'ent and defor'ation of the crust! incorporates older theory of continental

drift.

Plates@ are lithospheric plates - a"out *00 (' thic(! which 'ove around on top of the

asthenosphere.

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Plate &oundaries

• 6ivergent 1oundaries occur at

#ceanic Ridges! where new #ceanic

lithosphere is for'ed and 'oves

away fro' the ridge in oppositedirections

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• 9ontinental rifting 'ay create a new

divergent 'argin and evolve into an oceanic

ridge! such as is occurring in East Africaand "etween the African <late and the

Ara"ian <late.

• 9onvergent 1oundaries occur where

oceanic lithosphere is pushed "ac( into

the 'antle! 'ar(ed "y oceanic trenches

and su&duction (ones. Two types are

possi"le -

o hen two plates of oceanic

lithosphere converge oceanic

lithosphere is su"ducted "eneath

oceanic lithosphere.

o hen ocean lithosphere runs

into a plate with continental

lithosphere! the oceaniclithosphere is su"ducted "eneath

the continental lithosphere.

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o 9ontinental 9ollisions@ 'ay

occur at a convergent "oundary

when plates of continentallithosphere collide to oin two

plates together! such as has

occurred recently where theIndian <late has collided with theEurasian <late to for' the

i'alaya ountains.

• Transfor' 1oundaries occur where two plates slide past one another hori8ontally. The

:an Andreas >ault! in 9alifornia is a transform fault)

<late tectonics e=plains why earthua(es occur where they do! why volcanoes occur where theydo! how 'ountain ranges for'! as well as 'any other aspects of the Earth. It is such an

i'portant theory in understanding how the Earth wor(s that we cover it "riefly here! "ut will

return for a "etter understanding of later in the course.

Prof. Stephen A. NelsonEENS111

Tulane University

Physical Geology

Minerals

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The Earth is co'posed of roc(s. ;oc(s are aggregates of 'inerals. inerals are co'posed ofato's. In order to understand roc(s! we 'ust first have an understanding of 'inerals. In order

to understand 'inerals we 'ust have so'e "asic understanding of ato's - what they are andhow they interact with one another to for' 'inerals. ell start with the definition of aineral.

$efinition of a Mineral'

• aturally for'ed it for's in nature on its own #so'e say without the aid of hu'ansD

• :olid # it cannot "e a liuid or a gas$

• ith a definite che'ical co'position #every ti'e we see the sa'e 'ineral it has the

sa'e che'ical co'position that can "e e=pressed "y a che'ical for'ula$.

• and a characteristic crystalline structure #ato's are arranged within the 'ineral in aspecific ordered 'anner$.

E=a'ples

• Glass - can "e naturally for'ed #volcanic glass called o"sidian$! is a solid! its che'ical

co'position! however! is not always the sa'e! and it does not have a crystalline

structure. Thus! glass is not a 'ineral.

• Ice - is naturally for'ed! is solid! does have a definite che'ical co'position that can "e

e=pressed "y the for'ula 2,! and does have a definite crystalline structure when solid.

Thus! ice is a 'ineral! "ut liuid water is not #since it is not solid$.• alite #salt$ - is naturally for'ed! is solid! does have a definite che'ical co'position

that can "e e=pressed "y the for'ula a9l! and does have a definite crystalline

structure. Thus halite is a 'ineral.

(toms

Ato's 'a(e up the che'ical ele'ents. Each che'ical ele'ent has nearly identical ato's. Anato' is co'posed of three different particles@

• Protons -- positively charged! reside in the center of the ato' called the nucleus

• Electrons -- negatively charged! or"it in a cloud around nucleus

• *eutrons -- no charge! reside in the nucleus.

Each ele'ent has the sa'e nu'"er of protons and the sa'e nu'"er of electrons.

• u'"er of protons C u'"er of electrons.

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• u'"er of protons C atomic num&er .

• u'"er of protons u'"er of neutrons C atomic weight .

Isotopes are ato's of the sa'e ele'ent with differing nu'"ers of neutrons. i.e. the nu'"er ofneutrons 'ay vary within ato's of the sa'e ele'ent. :o'e isotopes are unsta"le which results

in radioactivity.

• E=a'ple@

o F #potassiu'$ has * protons. Every ato' of F has * protons. Ato'ic nu'"er

of F C *. :o'e ato's of F have 20 neutrons! others have 2*! and others have22. Thus ato'ic weight of F can "e 3! %0! or %*. %0F is radioactive and decays

to %0Ar and %09a.

Structure of (toms

Electrons or"it around the nucleus in

different shells! la"eled fro' the

inner'ost shell as F! ?! ! ! etc. Eachshell can have a certain nu'"er of

electrons. The F-shell can have 2

Electrons! the ?-shell! B! the -shell *B! -shell 32.

Helectrons C 22 ! where C* for the F

shell! C2 for the ? shell! C3 for the

shell! etc.

A :ta"le electronic configuration for an ato' is one B electrons in outer shell #e=cept in the F

shell! which is co'pletely filled with only 2 electrons$. Thus! ato's often loose electrons or

gain electrons to o"tain sta"le configuration. o"le gases have co'pletely filled outer shells! so

they are sta"le. E=a'ples e! e! Ar! Fr! e! ;n. ,thers li(e a! F loose an electron. Thiscauses the charge "alance to "eco'e uneual. In fact to "eco'e #positive$ charged ato's

called ions. <ositively charged ato's C cations. Ele'ents li(e >! 9l! , gain electrons to "eco'e# - $ charged. # -$ charged ions are called anions.

The drive to attain a sta"le electronic configuration in the outer'ost shell along with the fact

that this so'eti'es produces oppositely charged ions! results in the "inding of ato's together.

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hen ato's "eco'e attached to one another! we say that they are "onded together.

ypes of )onding@

• Ionic &onding - caused "y the force of attraction "etween ions of opposite charge.

E=a'ple a* and 9l-*. 1ond to for' a9l #halite or salt$.

Co+alent &onding - Electrons are shared "etween two or 'ore ato's so that each

ato' has a sta"le electronic configuration

#co'pletely filled outer'ost shell$ part of

the ti'e.

E=a'ple@ has one electron! needs to 2 to "e sta"le. , has & electrons in its outer shell!

needs 2 to "e sta"le. :o! 2 ato's "ond to

* , to for' 2,! with all ato's sharing

electrons! and each ato' having a sta"leelectronic configuration part of the ti'e.

• etallic &onding -- :i'ilar to covalent "onding! e=cept inner'ost electrons are also

shared. In 'aterials that "ond this way! electrons 'ove freely fro' ato' to ato' andare constantly "eing shared. aterials "onded with 'etallic "onds are e=cellent

conductors of electricity "ecause the electrons can 'ove freely through the 'aterial.

• ,an der aals &onding -- a wea( type of "ond that does not share or transfer electrons.

Jsually results in a 8one along which the 'aterial "rea(s easily #clea+age$. A good

e=a'ple is graphite #see figure 3.+ in your te=t$.

:everal different "ond types can "e present in a 'ineral! and these deter'ine the physical

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properties of the 'ineral.

"rystal Structure

<ac(ing of ato's in a crystal structure reuires an orderly and repeated ato'ic arrange'ent.

:uch an orderly arrange'ent needs to fill space efficiently and (eep a charge "alance. :ince the

si8e of ato's depends largely on the nu'"er of electrons! ato's of different ele'ents have

different si8es.

E=a'ple of a9l @

>or each a ato' there is one 9l ato'. Each a is surrounded "y 9l and each 9l is surrounded

"y a. The charge on each 9l is -* and the charge on each a is * to give a charged "alanced

crystal.

The structure of 'inerals is often seen in the shape of crystals. The law of constancy of

interfacial angles --- Angles "etween the sa'e faces on crystals of the sa'e su"stance are

eual. This is a reflection of ordered crystal structure #:ee figure 3. in the te=t"oo($

9rystal structure depends on the conditions under which the 'ineral for's. Polymorphs are'inerals with the sa'e che'ical co'position "ut different crystal structures. The conditions are

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such things as te'perature #T$ and pressure #<$! "ecause these effect ionic radii.

At high T ato's vi"rate 'ore! and thus distances "etween the' get larger. 9rystal structure

changes to acco''odate the larger ato's. At even higher T su"stances changes to liuid and

eventually to gas. ?iuids and gases do not have an ordered crystal structure and are not'inerals.

Increase in < pushes ato's closer together. This 'a(es for a 'ore densely pac(ed crystal

structure.

E=a'ples@

• The co'pound Al2:i,+ has three different polymorphs that depend on the te'perature

and pressure at which the 'ineral for's. At high < the sta"le for' of Al2:i,+ is (yanite!at low < the sta"le fro' is andalusite! and at high T it is silli'anite.

• 9ar"on #9$ has two different poly'orphs. At low T and < pure car"on is the 'ineral

graphite! #pencil lead$! a very soft 'ineral. At higher T and < the sta"le for' isdia'ond! the hardest natural su"stance (nown. In the diagra'! the geother'al gradient

# how te'perature varies with depth or pressure in the Earth$ is superi'posed on the

sta"ility fields of 9ar"on. Thus we (now that when we find dia'ond it ca'e fro'so'eplace in the Earth where the te'perature is greater than *+00o9 and the pressure is

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higher than +0!000 at'ospheres #euivalent to a depth of a"out *70 ('$.

• 9a9,3 - ?ow <ressure for' is 9alcite! igh <ressure for' is Aragonite

Ionic Su)stitution *Solid Solution+

Ionic su"stitution - #also called solid solution$! occurs "ecause so'e ele'ents #ions$ have the

sa'e si8e and charge! and can thus su"stitute for one another in a crystal structure.

E=a'ples@

• ,livines >e2:i,% and g2:i,%. >e2 and g2 are a"out the sa'e si8e! thus they can

su"stitute for one another in the crystal structure and olivine thus can have a range of

co'positions e=pressed as the for'ula #g!>e$2:i,%.

• Al(ali >eldspars@ FAl:i3,B #orthoclase$ and aAl:i3,B! #al"ite$ F * can su"stitute for

a*

• <lagioclase >eldspars@ aAl:i3,B #al"ite$ and 9aAl2:i2,B #anorthite$ a:i+ can

su"stitutes for 9aAl+ #a co'ple= solid solution$.

"omposition of Minerals

The variety of 'inerals we see depend on the che'ical ele'ents availa"le to for' the'. In the

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Earths crust the 'ost a"undant ele'ents are as follows@

*. ,! ,=ygen %+.2) "y weight

2. :i! :ilicon 27.2)

3. Al! Alu'inu' B.0)%. >e! Iron +.B)

+. 9a! 9alciu' +.*)

&. g! agnesiu' 2.B)7. a! :odiu' 2.3)

B. F! <otassiu' *.7)

. Ti !Titaniu' 0.)*0. ! ydrogen 0.*%)

**. n! anganese 0.*)

*2. <! <hosphorous 0.*)

ote that 9ar"on #one of the 'ost a"undant ele'ents in life$ is not a'ong the top *2.

1ecause of the li'ited nu'"er of ele'ents present in the Earths crust there are only a"out 3000

'inerals (nown. ,nly 20 to 30 of these 'inerals are co''on. The 'ost co''on 'inerals arethose "ased on :i and ,@ the $ilicates) :ilicates are "ased on :i,% tetrahedron. % ,=ygens

covalently "onded to one silicon ato'

Properties of Minerals

<hysical properties of 'inerals allow us to distinguish "etween 'inerals and thus identify

the'! as you will learn in la". A'ong the co''on properties used are@

• Ha&it - shape

• 9olor

• $treak #color of fine powder of the 'ineral$

• !uster -- 'etallic! vitreous! pearly! resinous #reflection of light$

• Clea+age #planes along which the 'ineral "rea(s easily$

• .ensity #'ass4volu'e$

• Hardness@ "ased on ohs hardness scale as follows@

*. Talc

2. gypsu' #fingernail$

3. calcite #penny$%. fluorite

+. apatite #(nife "lade$

&. orthoclase #glass$7. uart8

B. topa8

. corundu'

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*0. 6ia'ond

%ormation of Minerals

inerals are for'ed in nature "y a variety of processes. A'ong the' are@

• 9rystalli8ation fro' 'elt #igneous roc(s$

• <recipitation fro' water #che'ical sedi'entary roc(s! hydrother'al ore deposits$

• 1iological activity #"ioche'ical sedi'entary roc(s$

• 9hange to 'ore sta"le state - #the processes of weathering! 'eta'orphis'! and

diagenesis$.

• <recipitation fro' vapor. #not co''on! "ut so'eti'es does occur around volcanic

vents$

:ince each process leads to different 'inerals and different 'ineral poly'orphs! we can identifythe process "y which 'inerals for' in nature. Each process has specific te'perature and

pressure conditions that can "e deter'ined fro' la"oratory e=peri'ents. E=a'ple@ graphite anddia'ond! as shown previously.

oc-s ! Mixtures of Minerals

i=tures or aggregates of 'inerals are called roc(s. There are three "asic (inds of roc(s! each

type is deter'ined "y the process "y which the roc( for's.

• Igneous ;oc(s - for' "y solidification and crystalli8ation fro' liuid roc(! called

'ag'a.

• :edi'entary ;oc(s - for' "y sedi'entation of 'ineral and other roc( frag'ents fro'water! wind! or ice and can also for' "y che'ical precipitation fro' water.

• eta'orphic ;oc(s - for' as a result of increasing the pressure and4or te'perature on a

previously e=isting roc( to for' a new roc(.

Each of these roc( for'ing processes results in distinctive 'ineral asse'"lages and te=tures in

the resulting roc(. Thus! the different 'ineral asse'"lages and te=tures give us clues to howthe roc( for'ed. An understanding of the roc( for'ing processes and the resulting 'ineral

asse'"lage and te=ture will "e the 'ain goal of the ne=t part of this course.



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Magmas, Igneous Rocks, Volcanoes, and Plutons

This page last updated on 2&-Aug-2003

.inds of Igneous oc-

Igneous ;oc(s are for'ed "y crystalli8ation fro' a liuid! or 'ag'a. They include two types

• ,olcanic or e/trusi+e igneous roc(s for' when the 'ag'a cools and crystalli8es on the

surface of the Earth

• Intrusi+e or plutonic igneous roc(s wherein the 'ag'a crystalli8es at depth in the

Earth.

agma is a 'i=ture of liuid roc(! crystals! and gas. 9haracteri8ed "y a wide range of che'ical

co'positions! with high te'perature! and properties of a liuid.

ag'as are less dense than surrounding roc(s! and will therefore 'ove upward. If 'ag'a

'a(es it to the surface it will erupt and later crystalli8e to for' an e/trusi+e or +olcanic rock . If

it crystalli8es "efore it reaches the surface it will for' an igneous roc( at depth called a

plutonic or intrusi+e igneous rock . 1ecause cooling of the 'ag'a ta(es place at a differentrate! the crystals that for' and their interrelationship #te=ture$ e=hi"it different properties.

• >ast cooling on the surface results in 'any s'all crystals or uenching

to a glass. Gives rise to aphanitic te/ture #crystals cannot "edistinguished with the na(ed eye$! or o&sidian #volcanic glass$.

• :low cooling at depth in the earth results in fewer 'uch larger crystals!

gives rise to phaneritic te/ture.

• Porphyritic te/ture develops when slow cooling is followed

"y rapid cooling. Phenocrysts C larger crystals! matri/ or

groundmass C s'aller crystals.

ypes of Magma

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9he'ical co'position of 'ag'a is controlled "y the a"undance of ele'ents in the Earth. :i!

Al! >e! 9a! g! F! a! ! and , 'a(e up .). :ince o=ygen is so a"undant! che'icalanalyses are usually given in ter's of o=ides. :i,2 is the 'ost a"undant o=ide.

*. 0asaltic or ga&&roic -- :i,2 %+-++ wt)! high in >e! g! 9a! low in F! a

2. "ndesitic or .ioritic -- :i,2 ++-&+ wt)! inter'ediate. in >e! g! 9a! a! F

3. Rhyolitic or Granitic -- :i,2 &+-7+)! low in >e! g! 9a! high in F! a.

Gases ! At depth in the Earth nearly all 'ag'as contain gas. Gas gives 'ag'as their e=plosive

character! "ecause the gas e=pands as pressure is reduced.

• ostly 2, with so'e 9,2

• inor a'ounts of :ulfur! 9l ! and >

• ;hyolitic or granitic 'ag'as usually have higher gas contents than "asaltic or ga""roic

'ag'as.

emperature of Magmas

• 1asaltic or Ga""roic - *000-*200o9

• Andesitic or 6ioritic - B00-*000o9

• ;hyolitic or Granitic - &+0-B00o9.

/iscosity of Magmas !

,iscosity is the resistance to flow #opposite of fluidity$. 6epends on co'position!

te'perature! K gas content.

• igher :i,2 content 'ag'as have higher viscosity than lower :i, 2 content 'ag'as

• ?ower Te'perature 'ag'as have higher viscosity than higher te'perature 'ag'as.

Summary a)le

ag'aType

:olidified

Lolcanic;oc(

:olidified

<lutonic;oc(

9he'ical9o'position

Te'perature Liscosity Gas 9ontent

1asaltic 1asalt Ga""ro%+-++ :i,2 )!high in >e! g!

9a! low in F! a

*000 - *200 o9 ?ow ?ow

Andesitic Andesite 6iorite

++-&+ :i,2 )!

inter'ediate in

>e! g! 9a! a!

F

B00 - *000 o9 Inter'ediate Inter'ediate

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;hyolitic ;hyolite Granite

&+-7+ :i,2 )!

low in >e! g!

9a! high in F! a

&+0 - B00 o9 igh igh

Eruption of Magma

hen 'ag'as reach the surface of the Earth they erupt fro' a vent. They 'ay erupt

e=plosively or non-e=plosively.

• on-e=plosive eruptions are favored "y low gas content and low viscosity 'ag'as

#"asaltic to andesitic 'ag'as$.

o Jsually "egin with fire fountains due to release of dissolved gases

o <roduce lava flows on surface

o <roduce <illow lavas if erupted "eneath water

• E=plosive eruptions are favored "y high gas content and high viscosity #andesitic to

rhyolitic 'ag'as$.

o E=pansion of gas "u""les is resisted "y high viscosity of 'ag'a - results in

"uilding of pressure

o igh pressure in gas "u""les causes the "u""les to "urst when reaching the low

pressure at the Earths surface.

o 1ursting of "u""les frag'ents the 'ag'a into pyroclasts and tephra 1ash2)o 9loud of gas and tephra rises a"ove volcano to produce an eruption column that

can rise up to %+ (' into the at'osphere.

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Tephra that falls fro' the eruption

colu'n produces a tephra fall deposit)

If eruption colu'n collapses a

pyroclastic flow 'ay occur! whereingas and tephra rush down the flan(s of

the volcano at high speed. This is the

'ost dangerous type of volcaniceruption. The deposits that are

produced are called ignim&rites.

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?ateral "lasts and de"ris avalanches occur when gas is

released suddenly "y a large landslide or de"risavalanche ta(ing out part of the volcano

Plutons

Igneous roc(s cooled at depth. a'e co'es fro' Gree( god of the underworld - <luto.

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• $i-es are s'all #M20 ' wide$

shallow intrusions that show a

discordant relationship to theroc(s in which they intrude.

6iscordant 'eans that they cut

across pree=isting structures.They 'ay occur as isolated

"odies or 'ay occur as swar's of

di(es e'anating fro' a large

intrusive "ody at depth.

• Sills are also s'all #M+0 ' thic($

shallow intrusions that show a

concordant relationship with the

roc(s that they intrude. :illsusually are fed "y di(es! "ut these

'ay not "e e=posed in the field.

• #accoliths are so'ewhat large

intrusions that result in uplift andfolding of the pree=isting roc(sa"ove the intrusion. They are also

concordant types of intrusions.

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• &atholiths are very large

intrusive "odies! usually so large

that there "otto's are rarelye=posed. :o'eti'es they are

co'posed of several s'aller

intrusions.

• Stoc-s are s'aller "odies that

are li(ely fed fro' deeper level

"atholiths. :toc(s 'ay have

"een feeders for volcaniceruptions! "ut "ecause large

a'ounts of erosion are reuired

to e=pose a stoc( or "atholith!the associated volcanic roc(s are

rarely e=posed.

Methods of intrusion

• elting - crystalli8ation

• :toping - =enoliths

• Inection

hy do we see intrusive igneous roc(s at

the surface of the Earth Answer - They

are e=posed "y erosion which has

re'oved all of the 'aterial a"ove theintrusion.

0rigin of Magma

In order for 'ag'as to for'! so'e part of the Earth 'ust get hot enough to 'elt the roc(s present. Jnder nor'al conditions! the geothermal gradient is not high enough to 'elt roc(s!

and thus with the e=ception of the outer core! 'ost of the Earth is solid. Thus! 'ag'as for'

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only under special circu'stances. To understand this we 'ust first loo( at how roc(s and

'ineral 'elt.

As pressure increases in the Earth! the 'elting te'perature changes as well. >or pure 'inerals!

there are two general cases.

• >or a pure dry #no 2, or 9,2

present$ 'ineral! the 'eltingte'perate increases with increasing

pressure.

• >or a 'ineral with 2, or 9,2

present! the 'elting te'perature

first decreases with increasing

pressure

:ince roc(s 'i=tures of 'inerals! they "ehave so'ewhat differently. Jnli(e 'inerals! roc(s do

not 'elt at a single te'perature! "ut instead 'elt over a range of te'peratures. Thus! it is possi"le to have partial 'elts fro' which the liuid portion 'ight "e e=tracted to for' 'ag'a.

The two general cases are@

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• elting of dry roc(s is si'ilar to

'elting of dry 'inerals! 'elting

te'peratures increase withincreasing pressure! e=cept there is a

range of te'perature over which

there e=ists a partial 'elt. The

degree of partial 'elting can rangefro' 0 to *00)

• elting of roc(s containing water or

car"on dio=ide is si'ilar to 'eltingof wet 'inerals! 'elting

te'peratures initially decrease with

increasing pressure! e=cept there is arange of te'perature over which

there e=ists a partial 'elt.

9lues to how 'ag'as originate in the Earth and the special circu'stances necessary for'ag'as to for' can "e found "y loo(ing at the distri"ution of volcanoes on the Earths surface.,"viously! if a volcano occurs on the surface! it 'ust "e telling us that the special circu'stances

reuired for 'ag'a to for' 'ust e=ist "eneath the surface in this locality. To a large e=tent the

location of volcanoes is related to plate tectonics.

$i1erging Plate &oundaries

6iverging plate "oundaries are 'ostly "eneath the oceans and occur at oceanic ridges. ere!

"asaltic 'ag'a is erupted at the oceanic ridge and is intruded "eneath the ridge where it for's

new oceanic crust.

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,nly rarely does the oceanic ridge "uild itself a"ove the oceans surface. ,ne e=a'ple of wherethis occurs is the island of Iceland in the northern Atlantic ,cean. Eruptions of 'ag'a inIceland are 'ostly "asaltic.

"on1erging Plate &oundaries

here lithospheric plates converge! oceanic lithosphere su"ducts "eneath either another plate

co'posed of oceanic lithosphere or another plate co'posed of continental lithosphere.

• If an oceanic lithospheric plate su"ducts

"eneath another oceanic lithospheric

plate! we find island arcs on the surface

a"ove the su"duction 8one. These arevolcanoes "uilt of 'ostly andesitic lavas

pyroclastic 'aterial! although so'e

"asalts and rhyolites also occur.

• If an oceanic plate su"ducts "eneath a

plate co'posed of continental

lithosphere! we find continental margin

arcs. Again! the volcanoes found here

are co'posed 'ostly of andesitic lavasand pyroclastics. It is li(ely that so'e

'ag'as cool "eneath the volcanic arc to

for' dioritic and granitic plutons.

Hot Spots

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Areas where rising plu'es of hot 'antle reach the surface! usually at locations far re'oved

fro' plate "oundaries are called hot spots.

1ecause plates 'overelative to the underlying

'antle! hot spots "eneathoceanic lithosphere produce a chain of

volcanoes. A volcano is

active while it is over the

vicinity of the hot spot! "ut eventually plate

'otion results in the

volcano 'oving awayfro' the plu'e and the

volcano "eco'es e=tinct

and "egins to erode.

1ecause the <acific <late is one of the

faster 'oving plates! this type of

volcanis' produces linear chains of

islands and sea'ounts! such as theawaiian - E'peror chain! the ?ine

Islands! the arshall-Ellice Islands!

and the Austral sea'ount chain.

In the oceans! the volcanoes that occur

in relation to hot spots erupt 'ostly "asaltic 'ag'a.

here hot spots occur "eneathcontinental lithosphere! large volu'es

of rhyolite are produced #:ee figure

%.*& in your te=t$.

0rigin of &asaltic Magma

uch evidence suggests that 1asaltic 'ag'as result fro' dry partial 'elting of 'antle.

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• 1asalts 'a(e up 'ost of oceanic crust and only 'antle underlies crust.

• 1asalts contain 'inerals li(e olivine! pyro=ene and plagioclase! none of which contain

water.

• 1asalts erupt non-e=plosively! indicating a low gas content and therefore low watercontent.

The antle is 'ade of garnet peridotite #a roc( 'ade up of olivine! pyro=ene! and garnet$ --evidence co'es fro' pieces "rought up "y erupting volcanoes. In the la"oratory we can

deter'ine the 'elting "ehavior of garnet peridotite.

Jnder nor'al conditions the te'perature inthe Earth! shown "y the geother'al

gradient! is lower than the "eginning of

'elting of the 'antle. Thus in order for the'antle to 'elt there has to "e a 'echanis'

to raise the geother'al gradient. ,nce such

'echanis' is convection! wherein hot

'antle 'aterial rises to lower pressure ordepth! carrying its heat with it. If the raised

geother'al gradient "eco'es higher than

the initial 'elting te'perature at any pressure! then a partial 'elt will for'.

?iuid fro' this partial 'elt can "e

separated fro' the re'aining crystals

"ecause! in general! liuids have a lowerdensity than solids. 1asaltic or ga""roic

'ag'as appear to originate in this way.

0rigin of Granitic Magma

ost Granitic or ;hyolitic 'ag'a appears to result fro' wet 'elting of continental crust. The

evidence for this is@

• ost granites and rhyolites are found in areas of continental crust.

• hen granitic 'ag'a erupts fro' volcanoes it does so very e=plosively! indicating highgas content.

• :olidified granite or rhyolite contains uart8! feldspar! horn"lende! "iotite! and

'uscovite. The latter 'inerals contain water! indicating high water content.

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:till! the te'perature in continental crust is usually not high enough to cause 'elting! and thus

another heat source is necessary. In 'ost cases it appears that this heat source is "asaltic'ag'a. The "asaltic 'ag'a is generated in the 'antle! then rises into the continental crust.

1ut! "ecause "asaltic 'ag'a has a high density it 'ay stop in the crust and crystalli8e! releasing

heat into the surrounding crust. This raises the geother'al gradient and 'ay cause wet partial'elting of the crust to produce rhyolitic 'ag'as.

0rigin of (ndesitic Magma

Average co'position of continental crust is andesitic! "ut if andesite 'ag'a is produced "y

'elting of continental crust then it reuires co'plete 'elting of crust. Te'peratures in crustunli(ely to get high enough. Andesitic 'ag'as erupt in areas a"ove su"duction 8ones - suggests

relation "etween production of andesite and su"duction. ,ne theory involves wet partial 'elting

of su"ducted oceanic crust. 1ut! newer theories suggest wet partial 'elting of 'antle.

Magmatic $ifferentiation

hen 'ag'a solidifies to for' a roc( it does so over a range of te'perature. Each 'ineral

"egins to crystalli8e at a different te'perature! and if these 'inerals are so'ehow re'oved

fro' the liuid! the liuid co'position will change. 6epending on how 'any 'inerals are lost

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in this fashion! a wide range of co'positions can "e 'ade. The processes is called 'ag'atic

differentiation "y crystal fractionation.

9rystals can "e re'oved "y a variety of processes. If the crystals are 'ore dense than the liuid!

they 'ay sin(. If they are less dense than the liuid they will float. If liuid is suee8ed out "y pressure! then crystals will "e left "ehind. ;e'oval of crystals can thus change the co'position

of the liuid portion of the 'ag'a. ?et 'e illustrate this using a very si'ple case.

I'agine a liuid containing + 'olecules of g, and + 'olecules of :i, 2. Initially theco'position of this 'ag'a is e=pressed as +0) :i,2 and +0) g,. i.e.

ow lets i'agine I re'ove * g, 'olecule "y putting it into a crystal and re'oving thecrystal fro' the 'ag'a. ow what are the percentages of each 'olecule in the liuid

If we continue the process one 'ore ti'e "y re'oving one 'ore g, 'olecule

Thus! co'position of liuid can "e changed.

&o2en3s eaction Series

1owen found "y e=peri'ent that the order in which 'inerals crystalli8e fro' a "asaltic 'ag'adepends on te'perature. As a "asaltic 'ag'a is cooled ,livine and 9a-rich plagioclase

crystalli8e first. Jpon further cooling! ,livine reacts with the liuid to produce pyro=ene and

9a-rich plagioclase react with the liuid to produce less 9a-rich plagioclase. 1ut! if the olivine

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and 9a-rich plagioclase are re'oved fro' the liuid "y crystal fractionation! then the re'aining

liuid will "e 'ore :i,2 rich. If the process continues! an original "asaltic 'ag'a can changeto first an andesite 'ag'a then a rhyolite 'ag'a with falling te'perature

/olcanoes and /olcanic oc-s

1asalts! Andesites! 6acites! and ;hyolites are all types of volcanic roc( distinguished on the "asis of their 'ineral asse'"lage. 6epending on conditions present during eruption and

cooling! any of these roc( types 'ay for' one of the following types of volcanic roc(s.

• #&sidian - dar( colored volcanic glass showing concoidal fracture. Jsually rhyolitic or

dacitic.

• Pumice - light colored and light weight roc( consisting of 'ostly holes #+esicles$ that

were once occupied "y gas! Jsually rhyolitic! dacitic or andesitic.

• ,esicular roc( - roc( filled with holes #li(e :wiss cheese$ or vesicles that were once

occupied "y gas. Jsually "asaltic and andesitic.

• A'ygdaloidal "asalt. If vesicles in a vesicular "asalt are later filled "y precipitation of

calcite or uart8! the fillings are ter'ed a'ygdules and the "asalt is ter'ed ana'ygdaloidal "asalt.

Pyroclasts and ephra

• Pyroclasts C hot! "ro(en frag'ents. ;esult fro' e=plosively ripping apart of 'ag'a.

?oose asse'"lages of pyroclasts called tephra. 6epending on si8e! tephra can "eclassified as "o'"s. lapilli! or ash.

• ;oc( for'ed "y accu'ulation and ce'entation of tephra called a pyroclastic rock or

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tuff. elding! co'paction and deposition of other grains cause tephra #loose 'aterial$ to

"e converted in pyroclastic roc(.

/olcanoes

• $hield +olcano - volcanoes

that erupt low viscosity

'ag'a #usually "asaltic$ that

flows long distances fro' thevent.

• Pyroclastic cone or cinder cone - a

volcano "uilt 'ainly of tephra falldeposits located i''ediatelyaround the vent.

• $trato+olcano

#co'posite

volcano$ - avolcano "uilt of

inter"edded lavaflows and

pyroclastic'aterial.

• Crater - a depression caused "y e=plosive eection of 'ag'a or gas.

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• Caldera - a depression caused "y

collapse of a volcano into the

cavity once occupied "y 'ag'a.

• !a+a .ome - a steep sided

volcanic structure resultingfro' the eruption of high

viscosity! low gas content

'ag'a

• 'hermal $prings and Geysers - hot water resulting fro' heating "y 'ag'a at depth in

the Earth. :prings flow! geysers erupt.

• 3issure Eruptions - An eruption that occurs along a narrow crac( or fissure in the

Earths surface.

• Pillow !a+a - ?avas for'ed "y eruption "eneath the surface of the ocean or a la(e.

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Sedimentary Rocks

This page last updated on 03-:ep-2003

;ivers! oceans! winds! and rain runoff all have the a"ility to carry the particles washed off of

eroding roc(s. :uch 'aterial! called detritus! consists of frag'ents of roc(s and 'inerals. henthe energy of the transporting current is not strong enough to carry these particles! the particles

drop out in the process of sedimentation. This type of sedi'entary deposition is referred to as

clastic sedimentation. Another type of sedi'entary deposition occurs when 'aterial isdissolved in water! and che'ically precipitates fro' the water. This type of sedi'entation is

referred to as chemical sedimentation. A third process can occur! wherein living organis's

e=tract ions dissolved in water to 'a(e such things as shells and "ones. This type of

sedi'entation is called &iogenic sedimentation) Thus! there are three 'aor types ofsedi'entary roc(s@ Clastic $edimentary Rocks! Chemical $edimentary Rocks! and 0iogenic

$edimentary Rocks.

"lastic Sediments

"lassification ! 9lastic sedi'entary particles are classified in ter's of si8e

a'e of <article

:i8e ;ange ?oose:edi'ent

9onsolidated ;oc(

1oulder N2+& '' Gravel9onglo'erate or 1reccia #depends on

rounding$9o""le &% - 2+& '' Gravel

<e""le 2 - &% '' Gravel

:and *4*& - 2'' :and :andstone

:ilt *42+& - *4*& '' :ilt :iltstone

9lay M*42+& '' 9lay 9laystone! 'udstone! and shale

The for'ation of a clastic sedi'entary roc( involves three processes@

*. 'ransportation - :edi'ent can "e transported "y sliding down slopes! "eing pic(ed up

"y the wind! or "y "eing carried "y running water in strea's! rivers! or ocean currents.

The distance the sedi'ent is transported and the energy of the transporting 'ediu' allleave clues in the final sedi'ent that tell us so'ething a"out the 'ode of transportation.

2. .eposition - :edi'ent is deposited when the energy of the transporting 'ediu'

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"eco'es too low to continue the transport process. In other words! if the velocity of the

transporting 'ediu' "eco'es too low to transport sedi'ent! the sedi'ent will fall outand "eco'e deposited. The final sedi'ent thus reflects the energy of the transporting

'ediu'.

3. .iagenesis - 6iagenesis is the process that turns sedi'ent into roc(. The first stage of

the process is co'paction. 9o'paction occurs as the weight of the overlying 'aterialincreases. 9o'paction forces the grains closer together! reducing pore space and

eli'inating so'e of the contained water. :o'e of this water 'ay carry 'ineral

co'ponents in solution! and these constituents 'ay later precipitate as new 'inerals inthe pore spaces. This causes ce'entation! which will then start to "ind the individual

particles together. >urther co'paction and "urial 'ay cause recrystalli8ation of the

'inerals to 'a(e the roc( even harder.

,ther conditions present during diagenesis! such as the presence of a"sence of free

o=ygen 'ay cause other alterations to the original sedi'ent. In an environ'ent where

there is e=cess o=ygen ##/idi(ing En+ironment$ organic re'ains will "e converted to

car"on dio=ide and water. Iron will change fro' >e

2

to >e

3

! and will change the colorof the sedi'ent to a deep red #rust$ color. In an environ'ent where there is a depletion

of o=ygen # Reducing En+ironment $! organic 'aterial 'ay "e transfor'ed to solidcar"on in the for' of coal! or 'ay "e converted to hydrocar"ons! the source of

petroleu'.

extures of "lastic Sedimentary oc-s

hen sedi'ent is transported and deposited! it leaves clues to the 'ode of transport and

deposition. >or e=a'ple! if the 'ode of transport is "y sliding down a slope! the deposits that

result are generally chaotic in nature! and show a wide variety of particle si8es. Grain si8e andthe interrelationship "etween grains gives the resulting sedi'ent te=ture. Thus! we can use thete=ture of the resulting deposits to give us clues to the 'ode of transport and deposition.

$orting ! The degree of unifor'ity of grain si8e.

<articles "eco'e sorted on the "asis of density! "ecause of the energy of the transporting

'ediu'. igh energy currents can carry larger

frag'ents. As the energy decreases! heavier particles are deposited and lighter frag'ents

continue to "e transported. This results in sortingdue to density.

If the particles have the sa'e density! then the heavier particles will also "e larger! so the sorting

will ta(e place on the "asis of si8e. e can classify this si8e sorting on a relative "asis - well

sorted to poorly sorted. :orting gives clues to the energy conditions of the transporting 'ediu'

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fro' which the sedi'ent was deposited.

E=a'ples

o 1each deposits and wind "lown deposits generally show good sorting "ecause

the energy of the transporting 'ediu' is usually constant.

o :trea' deposits are usually poorly sorted "ecause the energy #velocity$ in a

strea' varies with position in the strea'.

Rounding - 6uring the transportation process!grains 'ay "e reduced in si8e due to a"rasion.

;ando' a"rasion results in the eventual

rounding off of the sharp corners and edges ofgrains. Thus! rounding of grains gives us clues

to the a'ount of ti'e a sedi'ent has "een in the

transportation cycle. ;ounding is classified on

relative ter's as well.

"hemical Sediments and Sedimentary oc-s

"herts ! che'ically precipitated :i,2

E+aporites - for'ed "y evaporation of sea water or la(e water. <roduces halite #salt$ and

gypsu' deposits "y che'ical precipitation as concentration of solids increases due to water loss "y evaporation.

&iogenic Sediments and Sedimentary oc-s

#imestone ! calcite #9a9,3$ is precipitated "y organis's usually to for' a shell or other

s(eletal structure. Accu'ulation of these s(eletal re'ains results in a li'estone.

$iatomite - :iliceous oo8e consisting of the re'ains of radiolarian or diato's can for' a light

colored soft roc( called diato'ite.

"oal - accu'ulation of dead plant 'atter in large a"undance in a reducing environ'ent #lac( of

o=ygen$.

0il Shale - actually a clastic sedi'entary roc( that contains a high a"undance of organic

'aterial that is converted to petroleu' during diagenesis.

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%eatures of Sedimentary oc-s hat Gi1e "lues to the En1ironment of $eposition

Stratification and &edding

• Rhythmic !ayering -

Alternating parallel

layers having different properties. :o'eti'es

caused "y seasonalchanges in deposition

#,ar+es$. i.e. la(e

deposits whereincoarse sedi'ent is

deposited in su''er

'onths and fine

sedi'ent is depositedin the winter when the

surface of the la(e isfro8en.

• Cross 0edding - :ets of "eds that are inclined relative to

one another. The "eds are inclined in the direction that

the wind or water was 'oving at the ti'e of deposition.

1oundaries "etween sets of cross "eds usually representan erosional surface. Lery co''on in "each deposits!

sand dunes! and river deposited sedi'ent.

• Graded 0edding - As current velocity decreases! first the larger or 'ore

dense particles are deposited followed "y s'aller particles. This results in

"edding showing a decrease in grain si8e fro' the "otto' of the "ed to thetop of the "ed.

• 4on!sorted Sediment ! :edi'ent showing a 'i=ture of grain si8es results fro' such

things as roc(falls! de"ris flows! 'udflows! and deposition fro' 'elting ice.

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Surface %eatures

• Ripple arks -

9haracteristic of shallowwater deposition. 9aused

"y waves or winds.

• udcracks - result fro' the drying out of wet sedi'ent at

the surface of the Earth. The crac(s for' due to

shrin(age of the sedi'ent as it dries.

• Raindrop arks - pits #or tiny craters$ created "y falling rain. If present! this suggests

that the sedi'ent was e=posed to the surface of the Earth.

• 3ossils - ;e'ains of once living organis's. <ro"a"ly the 'ost i'portant indicator of

the environ'ent of deposition.

o 6ifferent species usually inha"it specific environ'ents.

o 1ecause life has evolved - fossils give clues to relative age of the sedi'ent.

o 9an also "e i'portant indicators of past cli'ates.

"olor

• Iron o=ides and sulfides along with "uried organic 'atter give roc(s a dar( color.

Indicates deposition in a reducing environ'ent.

• 6eposition in o=idi8ing environ'ent produces red colored iron o=ides.

Sedimentary %acies

A sedimentary facies is a group of characteristics which reflect a sedi'entary environ'ent

different fro' those elsewhere in the sa'e deposit. Thus! facies 'ay change vertically through aseuence as a result of changing environ'ents through ti'e. Also! facies 'ay change laterally

through a deposit as a result of changing environ'ents with distance at the sa'e ti'e.

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"ommon Sedimentary En1ironments

• on-'arine environ'ents

o :trea' sedi'ents

o

?a(e sedi'entso Glacial #ice deposited$ sedi'ents

o Eolian #wind deposited$ sedi'ents

• 9ontinental :helf sedi'ents

o Estuarine sedi'ents

o 6eltaic sedi'ents

o 1each sedi'ents

o 9ar"onate shelf sedi'ents

o arine evaporite sedi'ents

• 9ontinental slope and rise sedi'ents

o Tur"idites

o 6eep :ea >anso :edi'ent drifts

• 6eep :ea :edi'ents

o 6eep -:ea oo8es

o ?and-derived sedi'ents


Tulane University

Physical Geology

Metamorphism and Metamorphic Rocks

This page last updated on 0-:ep-2003

$efinition of Metamorphism

The word O etamorphismO co'es fro' the Gree(@ eta C change! orph C for'! so

'eta'orphis' 'eans to change for'. In geology this refers to the changes in 'ineralasse'"lage and te=ture that result fro' su"ecting a roc( to pressures and te'peratures different

fro' those under which the roc( originally for'ed.

• ote that .iagenesis is also a change in for' that occurs in sedi'entary roc(s. In

geology! however! we restrict diagenetic processes to those which occur at te'peratures

"elow 200o9 and pressures "elow a"out 300 <a #<a stands for ega <ascals$! this is

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euivalent to a"out 3!000 at'ospheres of pressure.

• eta'orphis'! therefore occurs at te'peratures and pressures higher than 200o9 and

300 <a. ;oc(s can "e su"ected to these higher te'peratures and pressures as they

"eco'e "uried deeper in the Earth. :uch "urial usually ta(es place as a result oftectonic processes such as continental collisions or su"duction.

• The upper li'it of 'eta'orphis' occurs at the pressure and te'perature of wet partial

'elting of the roc( in uestion. ,nce 'elting "egins! the process changes to an igneous

process rather than a 'eta'orphic process.

Grade of Metamorphism

As the te'perature and4or pressure increases

on a "ody of roc( we say that the roc(undergoes prograde metamorphism or thatthe grade of 'eta'orphis' increases.

etamorphic grade is a general ter' for

descri"ing the relative te'perature and pressure conditions under which 'eta'orphic

roc(s for'.

• ?ow-grade 'eta'orphis' ta(es place at te'peratures "etween a"out 200 to 320o9! and

relatively low pressure. ?ow grade 'eta'orphic roc(s are characteri8ed "y ana"undance of hydrous minerals #'inerals that contain water! 2,! in their crystal

structure$.

o E=a'ples of hydrous 'inerals that occur in low grade 'eta'orphic roc(s@

9lay inerals

:erpentine

9hlorite

• igh-grade 'eta'orphis' ta(es place at te'peratures greater than 320o9 and relatively

high pressure. As grade of 'eta'orphis' increases! hydrous 'inerals "eco'e less

hydrous! "y losing 2, and non-hydrous 'inerals "eco'e 'ore co''on.

o E=a'ples of less hydrous 'inerals and non-hydrous 'inerals that characteri8e

high grade 'eta'orphic roc(s@ uscovite - hydrous 'ineral that eventually disappears at the

highest grade of 'eta'orphis'

1iotite - a hydrous 'ineral that is sta"le to very high grades of

'eta'orphis'.

<yro=ene - a non hydrous 'ineral.

Garnet - a non hydrous 'ineral.

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etrograde Metamorphism

As te'perature and pressure fall due to erosion of overlying roc( or due to tectonic uplift! one

'ight e=pect 'eta'orphis' to a follow a reverse path and eventually return the roc(s to theiroriginal un'eta'orphosed state. :uch a process is referred to as retrograde metamorphism. If

retrograde 'eta'orphis' were co''on! we would not co''only see 'eta'orphic roc(s at

the surface of the Earth. :ince we do see 'eta'orphic roc(s e=posed at the Earths surfaceretrograde 'eta'orphis' does not appear to "e co''on. The reasons for this include@

• che'ical reactions ta(e place 'ore slowly as te'perature is decreased

• during prograde 'eta'orphis'! fluids such as 2, and 9,2 are driven off! and these

fluids are necessary to for' the hydrous 'inerals that are sta"le at the Earths surface.

• che'ical reactions ta(e place 'ore rapidly in the presence of fluids! "ut if the fluids are

driven off during prograde 'eta'orphis'! they will not "e availa"le to speed upreactions during retrograde 'eta'orphis'.

%actors that "ontrol Metamorphism

eta'orphis' occurs "ecause so'e 'inerals are sta"le only under certain conditions of

pressure and te'perature. hen pressure and te'perature change! che'ical reactions occur tocause the 'inerals in the roc( to change to an asse'"lage that is sta"le at the new pressure and

te'perature conditions. 1ut! the process is co'plicated "y such things as how the pressure is

applied! the ti'e over which the roc( is su"ected to the higher pressure and te'perature! and

whether or not there is a fluid phase present during 'eta'orphis'.

• Te'perature

o Te'perature increases with depth in the Earth along the Geother'al Gradient.

Thus higher te'perature can occur "y "urial of roc(.

o Te'perature can also increase due to igneous intrusion.

• <ressure increases with depth of "urial! thus! "oth pressure and te'perature will vary

with depth in the Earth. <ressure is defined as a force acting eually fro' all directions.

It is a type of stress! called hydrostatic stress! or uniform stress) If the stress is not

eual fro' all directions! then the stress is called a differential stress)

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o If differential stress is present during 'eta'orphis'! it can have a profound

effect on the te=ture of the roc(.

rounded grains can "eco'e flattened in the

direction of 'a=i'u' stress.

• 'inerals that crystalli8e or

grow in the differential

stress field can have a

preferred orientation. Thisis especially true of the

sheet silicate 'inerals #the

'icas@ "iotite and

'uscovite! chlorite! talc!and serpentine$.

These sheet silicates will grow with their sheets orientated perpendicular

to the direction of 'a=i'u' stress. <referred orientation of sheetsilicates causes roc(s to "e easily "ro(en along appro=i'ately parallel

sheets. :uch a structure is called a foliation.

• >luid <hase - Any e=isting open space "etween 'ineral grains in a roc(s can potentially

contain a fluid. This fluid is 'ostly 2,! "ut contains dissolved 'ineral 'atter. Thefluid phase is i'portant "ecause che'ical reactions that involve one solid 'ineral

changing into another solid 'ineral can "e greatly speeded up "y having dissolved ions

transported "y the fluid. ithin increasing pressure of 'eta'orphis'! the pore spacesin which the fluid resides is reduced! and thus the fluid is driven off. Thus! no fluid will

"e present when pressure and te'perature decrease and! as discussed earlier! retrograde

'eta'orphis' will "e inhi"ited.

• Ti'e - The che'ical reactions involved in 'eta'orphis'! along with recrystalli8ation!

and growth of new 'inerals are e=tre'ely slow processes. ?a"oratory e=peri'ents

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suggest that the longer the ti'e availa"le for 'eta'orphis'! the larger are the si8es of

the 'ineral grains produced. Thus! coarse grained 'eta'orphic roc(s involve longti'es of 'eta'orphis'. E=peri'ents suggest that the ti'e involved is 'illions of

years.

esponses of oc- to Increasing Metamorphic Grade

E=a'ple - 'eta'orphis' of a shale

#'ade up initially of clay 'inerals anduart8$

• $late - :lates for' at low 'eta'orphic grade "y the growth of fine grained chlorite and

clay 'inerals. The preferred orientation of these sheet silicates causes the roc( to easily

"rea( along the planes parallel to the sheet silicates! causing a slatey clea+age) ote that

in the case shown here! the 'a=i'u' stress is applied at an angle to the original "edding planes! so that the slatey cleavage has developed at an angle to the original "edding.

• $chist - The si8e of the 'ineral grains tends to enlarge with increasing grade of

'eta'orphis'. Eventually the roc( develops a near planar foliation caused "y the preferred orientation of sheet silicates #'ainly "iotite and 'uscovite$. Puart8 and

>eldspar grains! however show no preferred orientation. The irregular planar foliation at

this stage is called schistosity.

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• Gneiss As 'eta'orphic grade increases! the sheet silicates "eco'e unsta"le and dar(

colored 'inerals li(e horn"lende and pyro=ene start to grow. These dar( colored

'inerals tend to "eco'e segregated in distinct "ands through the roc(! giving the roc( a

gneissic &anding . 1ecause the dar( colored 'inerals tend to for' elongated crystals!rather than sheet- li(e crystals! they still have a preferred orientation with their long

directions perpendicular to the 'a=i'u' differential stress.

• Granulite - At the highest grades of 'eta'orphis' all of the hydrous 'inerals and sheet

silicates "eco'e unsta"le and thus there are few 'inerals present that would show a

preferred orientation. The resulting roc( will have a granulitic te=ture that is si'ilar to a phaneritic te=ture in igneous roc(s.

Metamorphism of &asalts and Ga))ros

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• Greenschist - ,livine! pyro=ene! and plagioclase in an original "asalt change to

a'phi"oles and chlorite #"oth co''only green$ as water in the pore spaces reacts with

the original 'inerals at te'peratures and pressures of low grade 'eta'orphis'.

• "mphi&olite - As pressure and te'perature increase to inter'ediate grades of'eta'orphis'! only dar( colored a'phi"oles and plagioclase survive and the resultingroc( is called an a'phi"olite.

• Granulite - At the highest grade of 'eta'orphis' the a'phi"oles are replaced "y

pyro=enes and garnets! the foliation is lost and a granulite that has a granulitic te=ture is

produced.

Metamorphism of #imestone and Sandstone

• ar&le - :ince li'estones are 'ade up of essentially one 'ineral! 9alcite! and calcite is

sta"le over a wide range of te'perature and pressure! 'eta'orphis' of li'estone only

causes the original calcite crystals to grow larger. :ince no sheet silicates are present theresulting roc(! a 'ar"le! does not show foliation.

• 4uart(ite - eta'orphis' of sandstone originally containing only uart8! results in

recrystalli8ation and growth of the uart8! producing a non foliated roc( called a

uart8ite.

ypes of Metamorphism

• Cataclastic etamorphism - This type of 'eta'orphis' is due to 'echanical

defor'ation! li(e when two "odies of roc( slide past one another along a fault 8one.

eat is generated "y the friction of sliding along the 8one! and the roc(s tend to crushed

and pulveri8ed due to the sliding. 9ataclastic 'eta'orphis' is not very co''on and isrestricted to a narrow 8one along which the sliding occurred.

• 0urial etamorphism - hen sedi'entary roc(s are "uried to depths of several

hundred 'eters! te'peratures greater than 300o9 'ay develop in the a"sence of

differential stress. ew 'inerals grow! "ut the roc( does not appear to "e

'eta'orphosed. The 'ain 'inerals produced are the Qeolites. 1urial 'eta'orphis'overlaps! to so'e e=tent! with diagenesis! and grades into regional 'eta'orphis' as

te'perature and pressure increase.

• Contact etamorphism - ,ccurs adacent to igneous intrusions and results fro' high

te'peratures associated with the igneous intrusion. :ince only a s'all area surroundingthe intrusion is heated "y the 'ag'a! 'eta'orphis' is restricted to a 8one surrounding

the intrusion! called a metamorphic aureole. ,utside of the contact aureole! the roc(s

are un'eta'orphosed. The grade of 'eta'orphis' increases in all directions towardthe intrusion. 1ecause te'perature differences "etween the surrounding roc( and the

intruded 'ag'a are larger at shallow levels in the crust! contact 'eta'orphis' is

usually referred to as high te'perature! low pressure 'eta'orphis'. The roc( producedis often a fine-grained roc( that shows no foliation! called a hornfels.

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• Regional etamorphism - This type of

'eta'orphis' occurs over large areas

that were su"ected to high degrees ofdefor'ation under differential stress.

Thus! it usually results in for'ing

'eta'orphic roc(s that are stronglyfoliated! such as slates! schists! and

gniesses. The differential stress usually

results fro' tectonic forces that producea co'pression of the roc(s! such as when

two continental 'asses collide with one

another. Thus! regionally

'eta'orphosed roc(s occur in the coresof 'ountain ranges or in eroded

'ountain ranges. 9o'pressive stresses

result in folding of the roc(! as shownhere! and results in thic(ening of the

crust which tends to push roc(s down to

deeper levels where they are su"ected tohigher te'peratures and pressures #:ee

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>igure B.* in your te=t$.

A 'ap of a hypothetical regionally 'eta'orphosed area is shown in the figure "elow. ost

regionally 'eta'orphosed areas can "e divided into 8ones where a particular 'ineral! called aninde/ mineral% is characteristic of the 8one. The 8ones are separated "y lines #surfaces in three

di'ensions$ that 'ar( the first appearance of the inde= 'ineral. These lines are called

isograds #'eaning eual grade$ and represent lines #really surfaces$ where the grade of'eta'orphis' is eual. Actual 'aps of regionally 'eta'orphosed areas are shown in figure

B.*+ of your te=t.

Metamorphic %acies

In general! 'eta'orphic roc(s do not undergo significant changes in che'ical co'position

during 'eta'orphis'. The changes in 'ineral asse'"lages are due to changes in thete'perature and pressure conditions of 'eta'orphis'. Thus! the 'ineral asse'"lages that are

o"served 'ust "e an indication of the te'perature and pressure environ'ent that the roc( was

su"ected to. This pressure and te'perature environ'ent is referred to as metamorphic 3acies)

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#This is si'ilar to the concept of sedi'entary facies! in that a sedi'entary facies is also a set of

environ'ental conditions present during deposition$.

The seuence of 'eta'orphicfacies o"served in any

'eta'orphic terrain! depends

on the geother'al gradient

that was present during'eta'orphis'. A high

geother'al gradient such as

the one la"eled OAO in thefigure shown here! 'ight "e

present around an igneous

intrusion! and would result in'eta'orphic roc(s "elonging

to the hornfels facies. Jnder

a nor'al geother'al gradient!

such as O1O in the figure!roc(s would progress fro'

8eolite facies to greenschist!

a'phi"olite! and eclogite

facies as the grade of'eta'orphis' #or depth of

"urial$ increased.

If a low geother'al gradient was present! such the one la"eled O9O in the diagra'! then roc(s

would progress fro' 8eolite facies to "lueschist facies to eclogite facies. Thus! if we (now the

facies of 'eta'orphic roc(s in the region! we can deter'ine what the geother'al gradient 'usthave "een li(e at the ti'e the 'eta'orphis' occurred.

Metamorphism and Plate ectonics

At present! the geother'al gradients o"served are strongly affected "y plate tectonics.

• Along 8ones where su"duction is occurring! 'ag'as are generated near the su"duction

8one and intrude into shallow levels of the crust. 1ecause high te'perature is "rought

near the surface! the geother'al gradient in these regions "eco'es high #geother'algradient OAO in the figure a"ove$! and contact 'eta'orphis' #hornfels facies$ results.

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• 1ecause co'pression occurs along a su"duction 'argin #the oceanic crust 'oves toward

the volcanic arc$ roc(s 'ay "e pushed down to depths along either a nor'al or slightly

higher than nor'al geother'al gradient #O1O in the figure a"ove$. Actually thegeother'al gradient is li(ely to "e slightly higher than 1! "ecause the passage of 'ag'a

through the crust will tend to heat the crust so'ewhat. In these regions we e=pect to see

greenschist! a'phi"olite! and granulite facies 'eta'orphic roc(s.• Along a su"duction 8one! relatively cool oceanic lithosphere is pushed down to great

depths. This results in producing a low geother'al gradient #te'perature increases

slowly with depth$. This low geother'al gradient #O9O$ in the diagra' a"ove! results in'eta'orphis' into the "lueschist and eclogite facies.


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Tulane University

Physical Geology

Deformation of Rock

This page last updated on *B-:ep-2003

ithin the Earth roc(s are continually "eing su"ected to forces that tend to "end the'! twist

the'! or fracture the'. hen roc(s "end! twist or fracture we say that they defor' #changeshape or si8e$. The forces that cause defor'ation of roc( are referred to as stresses #>orce4unit

area$. :o! to understand roc( defor'ation we 'ust first e=plore these forces or stresses.

Stress and Strain

:tress is a force applied over an area. ,ne type of stress that we are all used to is a unifor'stress! called pressure. A unifor' stress is a stress wherein the forces act eually fro' all

directions. In the Earth the pressure due to the weight of overlying roc(s is a unifor' stress! and

is so'eti'es referred to as confining stress.

If stress is not eual fro' all

directions then we say that the stress

is a differential stress. Three (inds of

differential stress occur.

*. 'ensional stress 1or

e/tensional stress2! which

stretches roc(R

2. Compressional stress! which

suee8es roc(R and

3. $hear stress! which result in

slippage and translation.

hen roc(s defor' they are said to strain. A strain is a change in si8e! shape! or volu'e of a

'aterial.

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Stages of $eformation

hen a roc( is su"ected to increasing stress it passes through 3 successive stages of

defor'ation.

Elastic .eformation -- wherein the strainis reversi"le.

.uctile .eformation -- wherein the strain

is irreversi"le.

3racture - irreversi"le strain wherein the

'aterial "rea(s )

e can divide 'aterials into two classes that depend on their relative "ehavior under stress.

• 1rittle 'aterials have a s'all or large region of elastic "ehavior "ut only a s'all region

of ductile "ehavior "efore they fracture.

• 6uctile 'aterials have a s'all region of elastic "ehavior and a large region of ductile

"ehavior "efore they fracture.

ow a 'aterial "ehaves will depend on several factors. A'ong the' are@

• Te'perature - At high te'perature 'olecules and their "onds can stretch and 'ove! thus

'aterials will "ehave in 'ore ductile 'anner. At low Te'perature! 'aterials are "rittle.

• 9onfining <ressure - At high confining pressure 'aterials are less li(ely to fracture

"ecause the pressure of the surroundings tends to hinder the for'ation of fractures. At

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low confining stress! 'aterial will "e "rittle and tend to fracture sooner.

• :train rate -- At high strain rates 'aterial tends to fracture. At low strain rates 'ore ti'e

is availa"le for individual ato's to 'ove and therefore ductile "ehavior is favored.

• 9o'position -- :o'e 'inerals! li(e uart8! olivine! and feldspars are very "rittle.,thers! li(e clay 'inerals! 'icas! and calcite are 'ore ductile This is due to the

che'ical "ond types that hold the' together. Thus! the 'ineralogical co'position of the

roc( will "e a factor in deter'ining the defor'ational "ehavior of the roc(. Anotheraspect is presence or a"sence of water. ater appears to wea(en the che'ical "onds and

for's fil's around 'ineral grains along which slippage can ta(e place. Thus wet roc(

tends to "ehave in ductile 'anner! while dry roc(s tend to "ehave in "rittle 'anner.

&rittle!$uctile Properties of the #ithosphere

e all (now that roc(s near the surface of theEarth "ehave in a "rittle 'anner. 9rustal roc(s areco'posed of 'inerals li(e uart8 and feldspar

which have high strength! particularly at low

pressure and te'perature. As we go deeper in theEarth the strength of these roc(s initially

increases. At a depth of a"out *+ (' we reach a

point called the "rittle-ductile transition 8one.

1elow this point roc( strength decreases "ecausefractures "eco'e closed and the te'perature is

higher! 'a(ing the roc(s "ehave in a ductile

'anner. At the "ase of the crust the roc( typechanges to peridotite which is rich in olivine.

,livine is stronger than the 'inerals that 'a(e up

'ost crustal roc(s! so the upper part of the 'antleis again strong. 1ut! ust as in the crust! increasing

te'perature eventually predo'inates and at a

depth of a"out %0 (' the "rittle-ductile transition8one in the 'antle occurs. 1elow this point roc(s

"ehave in an increasingly ductile 'anner.

$eformation in Progress

,nly in a few cases does defor'ation of roc(s occur at a rate that is o"serva"le on hu'an ti'escales. A"rupt defor'ation along faults! usually associated with earthua(es caused "y the

fracture of roc(s occurs on a ti'e scale of 'inutes or seconds. Gradual defor'ation along faults

or in areas of uplift or su"sidence can "e 'easured over periods of 'onths to years withsensitive 'easuring instru'ents.

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E1idence of %ormer $eformation

Evidence of defor'ation that has occurred in the past is very evident in crustal roc(s. >or

e=a'ple! sedi'entary strata and lava flows generally follow the law of original hori8ontality.

Thus! when we see such strata inclined instead of hori8ontal! evidence of an episode ofdefor'ation. In order to uniuely define the orientation of a planar feature we first need to

define two ter's - stri(e and dip.

>or an inclined plane the strike is the co'pass direction of any hori8ontal line on the plane. The

dip is the angle "etween a hori8ontal plane and the inclined plane! 'easured perpendicular to

the direction of stri(e.

In recording stri(e and dip 'easure'ents on a geologic 'ap! a sy'"ol is used that has a long

line oriented parallel to the co'pass direction of the stri(e. A short tic( 'ar( is placed in the

center of the line on the side to which the inclined plane dips! and the angle of dip is recordedne=t to the stri(e and dip sy'"ol as shown a"ove. >or "eds with a 00 dip #vertical$ the short

line crosses the stri(e line! and for "eds with no dip #hori8ontal$ a circle with a cross inside is

used as shown "elow..

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uplifted horst "loc(s.

Half-Gra&ens - A nor'al fault that has a curved fault plane with the dip decreasing with depth

can cause the down-dropped "loc( to rotate. In such a case a half-gra"en is produced! calledsuch "ecause it is "ounded "y only one fault instead of the two that for' a nor'al gra"en.

o Re+erse 3aults - are faults that result fro' hori8ontal co'pressional stresses in "rittle

roc(s! where the hanging-wall "loc( has 'oved up relative the footwall "loc(.

A 'hrust 3ault is a special case of a reverse fault where the dip of the fault is less than*+o. Thrust faults can have considera"le displace'ent! 'easuring hundreds of

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(ilo'eters! and can result in older strata overlying younger strata.

• $trike $lip 3aults - are faults where the relative 'otion on the fault has ta(en place

along a hori8ontal direction. :uch faults result fro' shear stresses acting in the crust.

:tri(e slip faults can "e of two varieties! depending on the sense of displace'ent. To an

o"server standing on one side of the fault and loo(ing across the fault! if the "loc( on the

other side has 'oved to the left! we say that the fault is a left-lateral strike-slip fault . Ifthe "loc( on the other side has 'oved to the right! we say that the fault is a right-lateral

strike-slip fault . The fa'ous :an Andreas >ault in 9alifornia is an e=a'ple of a right-

lateral stri(e-slip fault. 6isplace'ents on the :an Andreas fault are esti'ated at over &00('.

'ransform-3aults are a special class of stri(e-slip faults. These are plate "oundaries alongwhich two plates slide past one another in a hori8ontal 'anner. The 'ost co''on type of

transfor' faults occur where oceanic ridges are offset. ote that the transfor' fault only occurs

"etween the two seg'ents of the ridge. ,utside of this area there is no relative 'ove'ent

"ecause "loc(s are 'oving in the sa'e direction. These areas are called fracture 8ones. The :an

Andreas fault in 9alifornia is also a transfor' fault.

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• Geometry of %olds ! >olds are descri"ed "y their for' and orientation. The sides of a

fold are called lim&s. The li'"s intersect at the tightest part of the fold! called the hinge.A line connecting all points on the hinge is called the fold a/is. In the diagra's a"ove!

the fold a=es are hori8ontal! "ut if the fold a=is is not hori8ontal the fold is called a

plunging fold and the angle that the fold a=is 'a(es with a hori8ontal line is called the

plunge of the fold. An i'aginary plane that includes the fold a=is and divides the fold as

sy''etrically as possi"le is called the a/ial plane of the fold.

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ote that if a plunging fold intersects a hori8ontal surface! we will see the pattern of the fold onthe surface.

• "lassification of %olds

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>olds can "e classified "ased on their appearance.

o If the two li'"s of the fold dip away fro' the a=is with the sa'e angle! the fold

is said to "e a symmetrical fold .

o If the li'"s dip at different angles! the folds are said to "e asymmetrical folds.

o If the co'pressional stresses that cause the folding are intense! the fold can close

up and have li'"s that are parallel to each other. :uch a fold is called an

isoclinal fold #iso 'eans sa'e! and cline 'eans angle! so isoclinal 'eans theli'"s have the sa'e angle$. ote the isoclinal fold depicted in the diagra'

"elow is also a sy''etrical fold.

o If the folding is so intense that the strata on one li'" of the fold "eco'es nearly

upside down! the fold is called an o+erturned fold .

o An overturned fold with an a=ial plane that is nearly hori8ontal is called a

recum&ant fold .

o A fold that has no curvature in its hinge and straight-sided li'"s that for' a8ig8ag pattern is called a che+ron fold .

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• The ;elationship 1etween >olding and >aulting

1ecause different roc(s "ehave differently under stress! we e=pect that so'e roc(s when

su"ected to the sa'e stress will fracture or fault! while others will fold. hen such contrasting

roc(s occur in the sa'e area! such as ductile roc(s overlying "rittle roc(s! the "rittle roc(s 'ay

fault and the ductile roc(s 'ay "end or fold over the fault.

Also since even ductile roc(s can eventually fracture under high stress! roc(s 'ay fold up to a

certain point then fracture to for' a fault.

%olds and opography

:ince different roc(s have different resistance to erosion and weathering! erosion of folded areas

can lead to a topography that reflects the folding. ;esistant strata would for' ridges that havethe sa'e for' as the folds! while less resistant strata will for' valleys #see figure .2+ in youte=t$.

Mountain anges ! he esult of $eformation of the "rust

,ne of the 'ost spectacular results of defor'ation acting within the crust of the Earth is the

for'ation of 'ountain ranges. ountains originate "y three processes! two of which are

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directly related to defor'ation. Thus! there are three types of 'ountains@

*. 3ault 0lock ountains - As the na'e i'plies! fault "loc( 'ountains originate "y

faulting. As discussed previously! "oth nor'al and reverse faults can cause the uplift of "loc(s of crustal roc(s. The :ierra evada 'ountains of 9alifornia! and the 'ountains

in the 1asin and ;ange province of the western J.:.! as discussed previously! werefor'ed "y faulting processes and are thus fault "loc( 'ountains.

2. 3old 5 'hrust ountains - ?arge co'pressional stresses can "e generated in the crust

"y tectonic forces that cause continental crustal areas to collide. hen this occurs the

roc(s "etween the two continental "loc(s "eco'e folded and faulted under

co'pressional stresses and are pushed upward to for' fold and thrust 'ountains. Thei'alayan ountains #currently the highest on Earth$ are 'ountains of this type and

were for'ed as a result of the Indian <late colliding with the Eurasian plate. :i'ilarly

the Appalachian ountains of orth A'erica and the Alps of Europe were for'ed "ysuch processes.

3. ,olcanic ountains - The third type of 'ountains! volcanic 'ountains! are not for'ed "y defor'ational processes! "ut instead "y the outpouring of 'ag'a onto the surface of

the Earth. The 9ascade ountains of the western J.:.! and of course the 'ountains ofthe awaiian Islands and Iceland are volcanic 'ountains.



Earthquakes and the Earths Interior

This page last updated on 2%-,ct-2003

Earth5ua-es

Earthua(es occur when energy stored in elastically strained roc(s is suddenly released. This

release of energy causes intense ground sha(ing in the area near the source of the earthua(eand sends waves of elastic energy! called seis'ic waves! throughout the Earth. Earthua(es can

"e generated "y "o'" "lasts! volcanic eruptions! and sudden slippage along faults. Earthua(esare definitely a geologic ha8ard for those living in earthua(e prone areas! "ut the seis'ic

waves generated "y earthua(es are invalua"le for studying the interior of the Earth.

0rigin of Earth5ua-es

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ost natural earthua(es are caused "y sudden

slippage along a fault 8one. The elastic

re&ound theory suggests that if slippage along a

fault is hindered such that elastic strain energy "uilds up in the defor'ing roc(s on either sideof the fault! when the slippage does occur! the

energy released causes an earthua(e. This

theory was discovered "y 'a(ing

'easure'ents at a nu'"er of points across afault. <rior to an earthua(e it was noted that

the roc(s adacent to the fault were "ending.

These "ends disappeared after an earthua(esuggesting that the energy stored in "ending the

roc(s was suddenly released during the

earthua(e.

Seismology, he Study of Earth5ua-es

hen an earthua(e occurs! the elastic energy is released and sends out vi"rations that travel

throughout the Earth. These vi"rations are called seis'ic waves. The study of how seis'icwaves "ehave in the Earth is called seismology.

• :eis'ographs - :eis'ic

waves travel through the

Earth as vi"rations. A

seismometer is an

instru'ent used to record

these vi"rations and theresulting graph that

shows the vi"rations is

called a seismograph.The seis'o'eter 'ust "e

a"le to 'ove with the

vi"rations! yet part of it

'ust re'ain nearlystationary.

This is acco'plished "y isolating the recording device #li(e a pen$ fro' the rest of the

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Earth using the principal of inertia. >or e=a'ple! if the pen is attached to a large 'ass

suspended "y a spring! the spring and the large 'ass 'ove less than the paper which is

attached to the Earth! and on which the record of the vi"rations is 'ade.

• :eis'ic aves. The source of anearthua(e is called the focus! which

is an e=act location within the Earthwere seis'ic waves are generated "y

sudden release of stored elastic

energy. The epicenter is the point onthe surface of the Earth directly

a"ove the focus. :o'eti'es the

'edia get these two ter's confused.

:eis'ic waves e'anating fro' thefocus can travel in several ways! and

thus there are several different (indsof seis'ic waves.

o 0ody a+es - e'anate

fro' the focus and travel

in all directions throughthe "ody of the Earth.

There are two types of

"ody waves@

P - wa+es - are <ri'ary waves. They travel with a velocity that depends

on the elastic properties of the roc( through which they travel.

, p 6 √ 718 9 ;< 2; =

here! L p is the velocity of the <-wave! F is the inco'pressi"ility of the

'aterial! µ is the rigidity of the 'aterial! and ρ is the density of the

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'aterial.

<-waves are the sa'e thing as sound waves. They 'ove through the

'aterial "y co'pressing it! "ut after it has "een co'pressed it e=pands!

so that the wave 'oves "y co'pressing and e=panding the 'aterial as ittravels. Thus the velocity of the <-wave depends on how easily the

'aterial can "e co'pressed #the inco'pressi"ility$! how rigid the

'aterial is #the rigidity$! and the density of the 'aterial. <-waves havethe highest velocity of all seis'ic waves and thus will reach all

seis'ographs first.

• $-a+es - :econdary waves! also called shear waves. They travel with a

velocity that depends only on the rigidity and density of the 'aterialthrough which they travel@

, s 6√

71 2;

=

:-waves travel through 'aterial "y shearing it or changing its shape in

the direction perpendicular to the direction of travel. The resistance to

shearing of a 'aterial is the property called the rigidity. It is nota"le thatliuids have no rigidity! so that the velocity of an :-wave is 8ero in a

liuid. #This point will "eco'e i'portant later$. ote that :-waves travel

slower than <-waves! so they will reach a seis'ograph after the <-wave.

o $urface a+es - :urface waves differ fro' "ody waves in that they do not travel

through the Earth! "ut instead travel along paths nearly parallel to the surface of

the Earth. :urface waves "ehave li(e :-waves in that they cause up and downand side to side 'ove'ent as they pass! "ut they travel slower than :-waves and

do not travel through the "ody of the Earth.

The record of anearthua(e! a

seis'ograph! as

recorded "y aseis'o'eter! will "e

a plot of vi"rations

versus ti'e. ,n theseis'ograph! ti'e is

'ar(ed at regularintervals! so that we

can deter'ine theti'e of arrival of the

first <-wave and the

ti'e of arrival of thefirst :-wave.

#ote again! that "ecause <-waves have a higher velocity than :-waves! the <-waves arrive at

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the seis'ographic station "efore the :-waves$.

• ?ocation of Earthua(es - In

order to deter'ine the location

of an earthua(e! we need to

have recorded a seis'ograph of the earthua(e fro' at leastthree seis'ographic stations at

different distances fro' the

epicenter of the ua(e. Inaddition! we need one further

piece of infor'ation - that is

the ti'e it ta(es for <-wavesand :-waves to travel through

the Earth and arrive at a

seis'ographic station. :uch

infor'ation has "een collectedover the last B0 or so years! and

is availa"le as travel ti'e

curves.

>ro' the seis'ographs at each

station one deter'ines the :-<

interval #the difference in theti'e of arrival of the first :-

wave and the ti'e of arrival of

the first <-wave. ote that onthe travel ti'e curves! the :-<

interval increases with

increasing distance fro' the

epicenter. Thus the :-< intervaltells us the distance to the

epicenter fro' the

seis'ographic station where theearthua(e was recorded. Thus!

at each station we can draw a

circle on a 'ap that has a radiuseual to the distance fro' the

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epicenter.

Three such circles will intersect in a point that locates the epicenter of the earthua(e.

• agnitude of Earthua(es - henever a large destructive earthua(e occurs in the

world the press i''ediately wants to (now where the earthua(e occurred and how "ig

the earthua(e was #in 9alifornia the uestion is usually - as this the 1ig ,ne$. Thesi8e of an earthua(e is usually given in ter's of a scale called the ;ichter agnitude.;ichter agnitude is a scale of earthua(e si8e developed "y a seis'ologist na'ed

9harles >. ;ichter. The ;ichter agnitude involves 'easuring the a'plitude #height$ of

the largest recorded wave at a specific distance fro' the earthua(e. hile it is correctto say that for each increase in * in the ;ichter agnitude! there is a tenfold increase in

a'plitude of the wave! it is incorrect to say that each increase of * in ;ichter agnitude

represents a tenfold increase in the si8e of the Earthua(e #as is co''only incorrectlystated "y the <ress$.

A "etter 'easure of the si8e of an earthua(e is the a'ount of energy released "y the

earthua(e. The a'ount of energy released is related to the ;ichter :cale "y the followingeuation@

?og E C **.B *.+

here ?og refers to the logarith' to the "ase *0! E is the energy released in ergs! and is the

;ichter agnitude.

Anyone with a hand calculator can solve this euation "y plugging in various values of and

solving for E! the energy released. Ive done the calculation for you in the following ta"le@

;ichter agnitude Energy#ergs$

>actor

* 2.0 = *0*3

3* =

2 &.3 = *0*%

3 2.0 = *0*&

3* =

% &.3 = *0*7

+ 2.0 = *0* 3* =

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6a'age fro' earthua(es can "e classified as follows@

• Ground :ha(ing - :ha(ing of the ground caused "y the passage of seis'ic waves near

the epicenter of the earthua(e is responsi"le for the collapse of 'ost structures. The

intensity of ground sha(ing depends on distance fro' the epicenter and on the type of "edroc( underlying the area.

o In general! loose unconsolidated sedi'ent is su"ect to 'ore intense sha(ing than

solid "edroc(.

o 6a'age to structures fro' sha(ing depends on the type of construction.

9oncrete and 'asonry structures! "ecause they are "rittle are 'ore suscepti"le to

da'age than wood and steel structures! which are 'ore fle=i"le.

• Ground ;upture - Ground rupture only occurs along the fault 8one that 'oves during the

earthua(e. Thus structures that are "uilt across fault 8ones 'ay collapse! whereasstructures "uilt adacent to! "ut not crossing the fault 'ay survive.

• >ire - >ire is a secondary effect of earthua(es. 1ecause power lines 'ay "e (noc(ed

down and "ecause natural gas lines 'ay rupture due to an earthua(e! fires are often

started closely following an earthua(e. The pro"le' is co'pounded if water lines arealso "ro(en during the earthua(e since there will not "e a supply of water to e=tinguish

the fires once they have started. In the *0& earthua(e in :an >rancisco 'ore than 0)

of the da'age to "uildings was caused "y fire.

• ;apid ass-asting <rocesses - In 'ountainous regions su"ected to earthua(es

ground sha(ing 'ay trigger rapid 'ass-wasting events li(e roc( and de"ris falls! roc(and de"ris slides! slu'ps! and de"ris avalanches.

• ?iuefaction -

!i>uefaction is a processes that occurs in

water-saturated

unconsolidated sedi'entdue to sha(ing. In areas

underlain "y such

'aterial! the

groundsha(ing causes

the grains to loose grainto grain contact! and thus

the 'aterial tends toflow.

/ou can de'onstrate this process to yourself ne=t ti'e your go the "each. :tand on the

sand ust after an inco'ing wave has passed. The sand will easily support your weight

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oceanic lithosphere is pushed "eneath either oceanic or continental lithosphereR and #2$

collision "oundaries where two plates with continental lithosphere collide.

o :u"duction "oundaries -At su"duction "oundaries cold oceanic lithosphere is

pushed "ac( down into the 'antle where two plates converge at an oceanic

trench. 1ecause the su"ducted lithosphere is cold it re'ains "rittle as it descendsand thus can fracture under the co'pressional stress. hen it fractures! itgenerates earthua(es that define a 8one of earthua(es with increasing focal

depths "eneath the overriding plate. This 8one of earthua(es is called the

0enioff ?one. >ocal depths of earthua(es in the 1enioff Qone can reach downto 700 ('.

o 9ollision "oundaries - At collisional "oundaries two plates of continental

lithosphere collide resulting in fold-thrust 'ountain "elts. Earthua(es occur due

to the thrust faulting and range in depth fro' shallow to a"out 200 ('.

he Earth3s Internal Structure

uch of what we (now a"out the interior of the Earth co'es fro' (nowledge of seis'ic wave

velocities and their variation with depth in the Earth. ;ecall that "ody wave velocities are asfollows@

, p 6 √ 718 9 ;< 2; =

, s 6 √ 71 2; =

here F C inco'pressi"ility

µ C rigidity

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ρ C density

If the properties of the earth! i.e. F! µ! and ρ where the sa'e throughout! then L p and Ls would

"e constant throughout the Earth and seis'ic waves would travel along straight line paths

through the Earth. e (now however that density 'ust change with depth in the Earth! "ecausethe density of the Earth is +!200 (g4cu"ic 'eter and density of crustal roc(s is a"out 2!+00

(g4cu"ic 'eter. If the density were the only property to change! then we could 'a(e esti'ates

of the density! and predict the arrival ti'es or velocities of seis'ic waves at any point away

fro' an earthua(e. ,"servations do not follow the predictions! so! so'ething else 'ust "e

happening. In fact we (now that F! µ! and ρ change due to changing te'peratures! pressures

and co'positions of 'aterial. The o" of seis'ology is! therefore! to use the o"served seis'ic

wave velocities to deter'ine how F! µ! and ρ change with depth in the Earth! and then infer

how <! T! and co'position change with depth in the Earth. In other words to tell us so'ethinga"out the internal structure of the Earth.

eflection and efraction of Seismic Wa1es.

If co'position #or physical properties$ change a"ruptly at so'e interface! then seis'ic wavewill "oth reflect off the interface and refract #or "end$ as they pass through the interface. Two

cases of wave refraction can "e recogni8ed.

*. If the seis'ic wavevelocity in the roc( a"ove

an interface is less thanthe seis'ic wave velocity

in the roc( "elow the

interface! the waves will "e refracted or "ent

upward relative to their

original path.

• If the seis'ic wave velocity decreases when passing into the roc( "elow the interface!

the waves will "e refracted down relative to their original path.

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• This discovery wasfollowed "y the discovery

of an :-wave shadow

8one. The :-wave shadow8one occurs "ecause no :-

waves reach the area on

the opposite side of the

Earth fro' the focus.:ince no direct :-waves

arrive in this 8one! it

i'plies that no :-waves

pass through the core.This further i'plies the

velocity of :-wave in the

core is 0. In liuids µ C 0!

so :-wave velocity is also

eual to 0. >ro' this it is

deduced that the core! orat least part of the core is

in the liuid state! since

no :-waves aretrans'itted through

liuids. Thus! the :-waveshadow 8one is "este=plained "y a liuid

outer core.

Seismic Wa1e /elocities in the Earth

,ver the years seis'ologists have collected data on how seis'ic wave velocities vary withdepth in the Earth. 6istinct "oundaries! called discontinuities are o"served when there is sudden

change in physical properties or che'ical co'position of the Earth. >ro' these discontinuities!

we can deduce so'ething a"out the nature of the various layers in the Earth. As we discussed

way "ac( at the "eginning of the course! we can loo( at the Earth in ter's of layers of differing

che'ical co'position! and layers of differing physical properties.

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• #ayers of $iffering "omposition

The 9rust - ohorovicic discovered "oundary the "oundary "etween crust and 'antle!

thus it is na'ed the ohoro+icic .iscontinuity or oho! for short. The co'position ofthe crust can "e deter'ined fro' seis'ic waves "y co'paring seis'ic wave velocities'easured on roc(s in the la"oratory with seis'ic wave velocities o"served in the crust.

Then fro' travel ti'es of waves on 'any earthua(es and fro' 'any seis'ic stations!

the thic(ness and co'position of the crust can "e inferred.

o In the ocean "asins crust is a"out B to *0 (' thic(! and has a co'position that is

"asaltic.

• 9ontinental crust varies "etween 20 and &0 (' thic(. The thic(est

continental crust occurs "eneath 'ountain ranges and the thinnest

"eneath lowlands. The co'position of continental crust varies fro'granitic near the top to ga""roic near the oho.

o The antle - :eis'ic wave velocities increase a"ruptly at the oho. In the

'antle wave velocities are consistent with a roc( co'position of peridotite

which consists of olivine! pyro=ene! and garnet.

o The 9ore - At a depth of 200 F' <-wave velocities suddenly decrease and :-

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wave velocities go to 8ero. This is the top of the outer core. As discussed a"ove!

the outer core 'ust "e liuid since :-wave velocities are 0. At a depth of a"out%B00 (' the sudden increase in <-wave velocities indicate a solid inner core. The

core appears to have a co'position consistent with 'ostly Iron with s'all

a'ounts of ic(el.

• #ayers of $ifferent Physical Properties

o At a depth of a"out *00 (' there is a sudden decrease in "oth < and :-wave

velocities. This "oundary 'ar(s the "ase of the lithosphere and the top of the

asthenosphere. The lithosphere is co'posed of "oth crust and part of the upper

'antle. It is a "rittle layer that 'a(es up the plates in plate tectonics! and appearsto float and 'ove around on top of the 'ore ductile asthenosphere.

o At the top of the asthenosphere is a 8one where "oth <- and :-wave velocities are

low. This 8one is called the !ow-,elocity ?one #?LQ$. It is thought that the lowvelocities of seis'ic waves in this 8one are caused "y te'peratures approaching

the partial 'elting te'perature of the 'antle! causing the 'antle in this 8one to

"ehave in a very ductile 'anner.

o At a depth of %00 (' there is an a"rupt increase in the velocities of seis'ic

waves! thus this "oundary is (nown as the @@ - 8m .iscontinuity. E=peri'ents

on 'antle roc(s indicate that this represents a te'perature and pressure where

there is a poly'orphic phase transition! involving a change in the crystalstructure of ,livine! one of the 'ost a"undant 'inerals in the 'antle.

o Another a"rupt increase in seis'ic wave velocities occurs at a depth of &70 ('.It is uncertain whether this discontinuity! (nown as the AB@ 8m .iscontinuity! is

the result of a poly'orphic phase transition involving other 'antle 'inerals or aco'positional change in the 'antle! or "oth.

Prof. Stephen A. Nelson EENS 111

Tulane University Physical Geology

!eologic "ime

This page last updated on 2+-:ep-2003

In order to understand how geologists deal with ti'e we first need to understand the concepts of relati+e age and a&solute age)

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• ;elative age - ;elative 'eans that we can deter'ine if so'ething is younger than or

older than so'ething else. ;elative ti'e does not tell how old so'ething is! all we

(now is the seuence of events. >or e=a'ple@ the sandstone in this area is older than

the li'estone.

• A"solute age- A"solute age 'eans that we can 'ore or less precisely assign a nu'"er

#in years! 'inutes! seconds! or so'e other units of ti'e$ to the a'ount of ti'e that has

passed. Thus we can say how old so'ething is. >or e=a'ple@ The sandstone is 300

'illion years old.

To "etter understand these concepts! lets loo( at an archeological e=a'ple@ I'agine we are agroup of archeologists studying two different trash pits recently discovered on the Tulane

Jniversity ca'pus and the Jniversity of ew ,rleans ca'pus. 1y carefully digging! we have

found that each trash pit shows a seuence of layers. Although the types of trash in each pit isuite varia"le! each layer has a distinctive (ind of trash that distinguishes it fro' other layers in

the pits.

hat can we say and learn fro' these e=cavations

• ;elative age of trash layers - 1ecause of the shape of the pits the oldest layers of trash

occur "elow younger layers i.e. the inha"itants of the area li(ely deposited the trash "y

throwing it in fro' the top! eventually filling the pits. Thus the relati+e age of the trashlayers is! in order fro' youngest to oldest.@

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o +.2+O 6is( ?ayer - /oungest

o Al 9ans ?ayer

o Tin 9ans ?ayer

o 9era'ic 9ups ?ayer

o :tone Tools ?ayer - ,ldest

otice that at this point we do not (now e=actly how old any layer really is.

Thus we do not (now the a&solute age of any given layer.

• The civili8ations that deposited the trash had a culture and industrial capa"ilities that

evolved through ti'e. The oldest inha"itants used pri'itive stone tools! later inha"itantsused cups 'ade of cera'ics! even later inha"itants eventually used tin cans and then

changed to Alu'inu' cans! and then they developed a technology that used co'puters.

• :i'ilar cultures 'ust have e=isted in "oth areas and lived at the sa'e ti'e. Thus we

can 'a(e correlations "etween the layers found at the different sites! "y reasoning thatlayers containing si'ilar discarded ite's #artifacts$ were deposited during the sa'e ti'e

period.

• 1ecause the 9era'ic 9ups layer is found at the Tulane site! "ut not at the J, site! the

civili8ation that produced the 9era'ic cups pro"a"ly did not live in the J, area.Thus! we can recogni8e a hiatus! or "rea( in the depositional seuence at the J, site.

The surface 'ar(ing in the "rea( in deposition would "e called an unconformity in

geologic ter's! and represents ti'e 'issing fro' the depositional record.

• The trash pits contain so'e clues to a&solute age@

o The Tulane trash pit has an old license plate in the Tin 9ans layer. This plate

shows a date of *+2! thus the Tin 9ans layer is a"out +* years old.

o The J, trash pit has an old newspaper in the Al 9ans layer. The date on the

newspaper is Sune %! *B3. Thus the Al 9ans layer is a"out 20 years old.

In geology! we use si'ilar principles to deter'ine relative ages! correlations! and a"solute ages.

• ;elative ages - <rinciples of :tratigraphy

• 9orrelations - >ossils! (ey "eds! physical criteria

• A"solute ages - ;adio'etric dating.

Principles of Stratigraphy

$tratigraphy C the study of strata #layers$ in the Earths crust.

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#a2s of Stratigraphy

• #riginal Hori(ontality - sedi'entary strata are deposited in layers that are hori8ontal or

nearly hori8ontal! parallel to or nearly parallel to the Earths surface. Thus roc(s that we

now see inclined or folded have "een distur"ed since their original deposition.

• $tratigraphic $uperposition - 1ecause of Earths gravity! deposition of sedi'ent will

occur depositing older layers first followed "y successively younger layers. Thus! in a

seuence of layers that have not "een overturned "y a later defor'ational event! the

oldest layers will "e on the "otto'. This is the sa'e principle used to deter'ine relativeage in the trash pits discussed previously. In fact! sedi'entary roc(s are! in a sense!

trash fro' the Earths surface deposited in "asins.

&rea-s in the Stratigraphic ecord

1ecause the Earths crust is continually changing! i.e. due to uplift! su"sidence! anddefor'ation! erosion is acting in so'e places and deposition of sedi'ent is occurring in other

places. hen sedi'ent is not "eing deposited! or when erosion is re'oving previously

deposited sedi'ent! there will not "e a continuous record of sedi'entation preserved in theroc(s. e call such a "rea( in the stratigraphic record a hiatus #a hiatus was identified in our

trash pit e=a'ple "y the non-occurrence of the 9era'ic 9ups layer at the J, site$. hen we

find evidence of a hiatus in the stratigraphic record we call it an unconfor'ity. An

unconformity is a surface of erosion or non-deposition. Three types of unconfor'ities arerecogni8ed.

• "ngular Unconformity

1ecause of the ?aws of :tratigraphy! if we see a cross section li(e this in a road cut or

canyon wall where we can recogni8e an angular unconfor'ity! then we (now thegeologic history or seuence of events that 'ust have occurred in the area to produce the

angular unconfor'ity. Angular unconfor'ities are easy to recogni8e in the field "ecause

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of the angular relationship of layers that were originally deposited hori8ontally.

• .isconformity

6isconfor'ities #called parallel unconfor'ities in your la" "oo($ are 'uch harder to

recogni8e in the field! "ecause often there is no angular relationship "etween sets oflayers. 6isconfor'ities are usually recogni8ed "y correlating fro' one area to another

and finding that so'e strata is 'issing in one of the areas. The unconfor'ity recogni8ed

in the J, trash pit is a disconfor'ity.

• *onconformity

onconfor'ities occur where roc(s that for'ed deep in the Earth! such as intrusive

igneous roc(s or 'eta'orphic roc(s! are overlain "y sedi'entary roc(s for'ed at the

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()solute Geologic ime

Although geologists can easily esta"lish relative ages of roc(s "ased on the principles of

stratigraphy! (nowing how 'uch ti'e a geologic Eon! Era! <eriod! or Epoch represents is a

'ore difficult pro"le' without having (nowledge of a"solute ages of roc(s. In the early years

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of geology! 'any atte'pts were 'ade to esta"lish so'e 'easure of a"solute geologic ti'e.

• Age of Earth esti'ated on the "asis of how long it would ta(e the oceans to o"tain their

present salt content. Assu'es that we (now the rate at which the salts #a! 9l! 9a! and

9,3 ions$ are input into the oceans "y rivers! and assu'es that we (now the rate atwhich these salts are re'oved "y che'ical precipitation. 9alculations in *BB gave

esti'ate for the age of the Earth of 0 'illion years.

• Age of Earth esti'ated fro' ti'e reuired to cool fro' an initially 'olten state.

Assu'ptions include! the initial te'perature of the Earth when it for'ed! the present

te'perature throughout the interior of the Earth! and that there are no internal sources ofheat. 9alculations gave esti'ate of *00 'illion years for the age of the Earth.

In *B& radioactivity was discovered! and it was soon learned that radioactive decay occurs at a

constant rate throughout ti'e. ith this discovery! ;adio'etric dating techniues "eca'e

possi"le! and gave us a 'eans of 'easuring a"solute geologic ti'e.

adiometric $ating

;adio'etric dating relies on the fact that there are different types of isotopes.

• ;adioactive Isotopes - isotopes # parent isotopes$ that spontaneously decay at a constant

rate to another isotope.

• ;adiogenic Isotopes - isotopes that are for'ed "y radioactive decay #daughter isotopes$.

The rate at whichradioactive isotopes

decay is often stated asthe half-life of the

isotope #t*42$. The half-

life is the a'ount ofti'e it ta(es for one half

of the initial a'ount of

the parent! radioactive

isotope! to decay to thedaughter isotope. Thus!

if we start out with *gra' of the parentisotope! after the

passage of * half-life

there will "e 0.+ gra' of the parent isotope left.

After the passage of two half-lives only 0.2+ gra' will re'ain! and after 3 half lives only 0.*2+

will re'ain etc.

7/23/2019 GEOL-GEOM


:o'e e=a'ples of isotope syste's used to date geologic 'aterials.

<arent 6aughter t*42 Jseful ;ange Type of aterial

23BJ 20&<" %.+ ".y

N*0 'illion years

Igneous ;oc(s and inerals

23+J 207<" 7*0 '.y

232Th 20B<" *% ".y

%0F %0Ar K %09a *.3 ".y N*0!000 years

B7;" B7:r %7 ".y N*0 'illion years

*%9 *% +!730 y *00 - 70!000 years ,rganic aterial

Potassium ! (rgon *.!(r+ $ating

In nature there are three isotopes of potassiu'@

o3F - non-radioactive #sta"le$

o%0F - radioactive with a half life of *.3 "illion years! %0F decays to %0Ar and %09a!

only the F-Ar "ranch is used in dating.

o%*F - non-radioactive #sta"le$

• F is an ele'ent that goes into 'any 'inerals! li(e feldspars and "iotite. Ar! which is a

no"le gas! does not go into 'inerals when they first crystalli8e fro' a 'ag'a "ecause

Ar does not "ond with any other ato'.• hen a F-"earing 'ineral crystalli8es fro' a 'ag'a it will contain F! "ut will not

contain Ar. ith passage of ti'e! the %0F decays to %0Ar! "ut the %0Ar is now trapped in

the crystal structure where the %0F once was.

• Thus! "y 'easuring the a'ount of %0F and %0Ar now present in the 'ineral! we can

deter'ine how 'any half lives have passed since the igneous roc( crystalli8ed! and thus

(now the a"solute age of the roc(.

adiocar)on *67"+ $ating

;adiocar"on dating is different than the other 'ethods of dating "ecause it cannot "e used to

directly date roc(s! "ut can only "e used to date organic 'aterial produced "y once living

organis's.•

*%9 is continually "eing produced in the Earths upper at'osphere "y "o'"ard'ent of*% "y cos'ic rays. Thus the ratio of *%9 to *% in the Earths at'osphere is constant.

• ?iving organis's continually e=change 9ar"on and itrogen with the at'osphere "y

"reathing! feeding! and photosynthesis. Thus! so long as the organis' is alive! it willhave the sa'e ratio of *%9 to *% as the at'osphere.

• hen an organis' dies! the *%9 decays "ac( to *% ! with a half-life of +!730 years.

easuring the a'ount of *%9 in this dead 'aterial thus ena"les the deter'ination of the

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ti'e elapsed since the organis' died.

• ;adiocar"on dates are o"tained fro' such things as "ones! teeth! charcoal! fossili8ed

wood! and shells.

• 1ecause of the short half-life of *%9! it is only used to date 'aterials younger than a"out

70!000 years.

()solute $ating and Geologic ime Scale

Jsing the 'ethods of a"solute dating! and cross-cutting relationships of igneous roc(s!

geologists have "een a"le to esta"lish the a"solute ti'es for the geologic colu'n. >or e=a'ple!

i'agine so'e cross section such as that shown "elow.>ro' the cross-cutting relationships and stratigraphy we can deter'ine that@

o The ,ligocene roc(s are younger than the 30 '.y old lava flow and older than

the 20 '.y. old lava flow.

o The Eocene roc(s are older than the +7 '.y. old di(e and younger than the 3&

'.y. old di(e that cuts through the'.

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o The <aleocene roc(s are older than "oth the 3& '.y. old di(e and the +7 '.y. old

di(e #thus the <aleocene is younger than +7 '.y.

1y e=a'ining relationships li(e these all over the world! the Geologic Ti'e scalehas "een very precisely correlated with the Geologic 9olu'n. "ut! "ecause the

geologic colu'n was esta"lished "efore radio'etric dating techniues wereavaila"le! note that the lengths of the different <eriods and Epochs are varia"le.

he (ge of the Earth

Theoretically we should "e a"le to deter'ine the age of the Earth "y finding and dating the

oldest roc( that occurs. :o far! the oldest roc( found and dated has an age of 3.& "illion years.

1ut! is this the age of the Earth <ro"a"ly not! "ecause roc(s e=posed at the Earths surface arecontinually "eing eroded! and thus! it is unli(ely that the oldest roc( will ever "e found. 1ut! we

do have clues a"out the age of the Earth fro' other sources@

• eteorites - These are pieces of planetary 'aterial that fall fro' outer space to thesurface of the Earth. ost of these 'eteorites appear to have co'e fro' within oursolar syste' and either represent 'aterial that never condensed to for' a planet or was

once in a planet that has since disintegrated. The ages of the 'ost pri'itive 'eteorites

all cluster around %.& "illion years.

• oon ;oc(s - The only other planetary "ody in our solar syste' that we have sa'ples

of are 'oon roc(s #sa'ples of ars roc(s have never "een returned to Earth$. The ages

o"tained on oon roc(s are all within the range "etween %.0 and %.& "illion years. Thus

the solar syste' and the Earth 'ust "e at least %.& "illion years old.

4ote on Possi)le "onflicts )et2een Science and eligion

9onflicts should not e=ist unless one "elieves that the 1i"le is a"solute truth and the thatso'e hu'ans interpretation of the 1i"le is a"solutely perfect. /ou are free to "elieve whatever

you want! "ut for this course the evidence points to the age of the Earth at %.& "illion years! and

one of Gods days would "e a"out &+7 'illion years #%.& "illion divided "y 7$.

4ote on reading material

The last part of this chapter #9hapter **$ on the 'agnetic ti'e scale will "e covered and tested

on later in the course during our discussion of <late Tectonics.


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#eathering and Soils


1efore we discuss the other types of roc(s #:edi'entary and eta'orphic$ we need to have an

understanding of the processes that cause the "rea(down of roc(s! either to for' new 'inerals

that are sta"le on the surface of the Earth! or to "rea( the roc(s down into s'aller particles . This process is called weathering ! and is also the first step in a process that we call

erosion.

Geologists recogni8e two categories of weathering processes

*. Physical eathering - disintegration of roc(s and 'inerals "y a physical or 'echanical process.

2. Chemical eathering - che'ical alteration or deco'position of roc(s and 'inerals.

Although we separate these processes! as we will see! "oth wor( together to "rea( down roc(s

and 'inerals to s'aller frag'ents or to 'inerals 'ore sta"le near the Earths surface.

Physical Weathering

<hysical weathering ta(es place "y a variety of processes. A'ong the' are@

• 6evelop'ent of oints - Soints are regularly spaced fractures or crac(s in roc(s that

show no offset across the fracture #fractures that show an offset are called faults$.

o Soints for' as a result of e=pansion due to cooling or relief of pressure as

overlying roc(s are re'oved "y erosion.

o Soints for' free space in roc( "y which other agents of che'ical or physical

weathering can enter.

• 9rystal Growth - As water percolates through fractures and pore spaces it 'ay contain

ions that precipitate to for' crystals. As these crystals grow they 'ay e=ert an outward

force that can e=pand or wea(en roc(s.

• eat - Although daily heating and cooling of roc(s do not see' to have an effect!sudden e=posure to high te'perature! such as in a forest or grass fire 'ay cause

e=pansion and eventual "rea(age of roc(. 9a'pfire e=a'ple.

• <lant and Ani'al Activities -

o <lant roots can e=tend into fractures and grow! causing e=pansion of the fracture.

Growth of plants can "rea( roc( - loo( at the sidewal(s of ew ,rleans for

e=a'ple.

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o Ani'als "urrowing or 'oving through crac(s can "rea( roc(.

• 3rost edging - Jpon free8ing! there is an increase in the volu'e of the water #thats

why we use antifree8e in auto engines or why the pipes "rea( in ew ,rleans during therare free8e$. As the water free8es it e=pands and e=erts a force on its surroundings.

>rost wedging is 'ore prevalent at high altitudes where there 'ay "e 'any free8e-thawcycles.

"hemical Weathering

:ince 'any roc(s and 'inerals are for'ed under conditions present deep within the Earth!

when they arrive near the surface as a result of uplift and erosion! they encounter conditions

very different fro' those under which they originally for'ed. A'ong the conditions presentnear the Earths surface that are different fro' those deep within the Earth are@

• ?ower Te'perature #ear the surface T C 0-+0o9$

• ?ower <ressure #ear the surface < C * to several hundred at'ospheres$

• igher free water #there is a lot of liuid water near the surface! co'pared with deep in

the Earth$

• igher free o=ygen #although ,2 is the 'ost a"undant ele'ent in the crust! 'ost of it is

tied up "onded into silicate and o=ide 'inerals - at the surface there is 'uch 'ore free

o=ygen! particularly in the at'osphere$.

1ecause of these differing conditions! 'inerals in roc(s react with their new environ'ent to

produce new 'inerals that are sta"le under conditions near the surface. inerals that are sta"leunder <! T! 2,! and ,2 conditions near the surface are! in order of 'ost sta"le to least sta"le@

• Iron o=ides! Alu'inu' o=ides - such as he'atite >e2,3! and gi""site Al#,$3.

• Puart8

• 9lay inerals

• uscovite

• Al(ali >eldspar

• 1iotite

• A'phi"oles

• <yro=enes

• 9a-rich plagioclase

• ,livine

ote the 'inerals with . These are igneous 'inerals that crystalli8e fro' a liuid. ote the

'inerals that occur low on this list are the 'inerals that crystalli8e at high te'perature fro''ag'a. The higher the te'perature of crystalli8ation! the less sta"le are these 'inerals at the

low te'perature found near the Earths surface #see 1owens reaction series in the igneous roc(s

chapter$.

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The 'ain agent responsi"le for che'ical weathering reactions is water and wea( acids for'ed

in water.

• An acid is solution that has a"undant free

ions.

• The 'ost co''on wea( acid that occurs in surface waters is car"onic acid.

• 9ar"onic acid is produced in rainwater "y reaction of the water with car"on dio=ide

#9,2$ gas in the at'osphere.

is a s'all ion and can easily enter crystal structures! releasing other ions into thewater.

ypes of "hemical Weathering eactions

• Hydrolysis - or ,- replaces an ion in the 'ineral. E=a'ple@

• !eaching - ions are re'oved "y dissolution into water. In the e=a'ple a"ove

we say that the F ion was leached.

• #/idation - :ince free o=ygen #,2$ is 'ore co''on near the Earths surface! it'ay react with 'inerals to change the o=idation state of an ion. This is 'ore

co''on in >e #iron$ "earing 'inerals! since >e can have several o=idationstates! >e! >e2! >e3. 6eep in the Earth the 'ost co''on o=idation state of >e

is >e2.

• .ehydration - re'oval of 2, or ,-

ion fro' a 'ineral.

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• Complete .issolution - all of the 'ineral is co'pletely dissolved "y the water.

Weathering of "ommon oc-s

;oc( <ri'ary inerals ;esidual inerals ?eached Ions

Granite

>eldspars 9lay inerals a! F

icas 9lay inerals F

Puart8 Puart8 ---

>e-g inerals9lay inerals e'atite

Goethite

g2

1asalt

>eldspars 9lay inerals a! 9a2

>e-g inerals 9lay inerals g2

agnetite e'atite! Goethite ---

?i'estone 9alcite one 9a2! 9,3-2

;esidual inerals C inerals sta"le at the Earths surface and left in the roc( afterweathering.

Weathering inds, Exfoliation, and Spheroidal Weathering

hen roc( weathers! it usually does so "y wor(ing inward fro' a surface that is e=posed to the

weathering process. This 'ay result in@

• eathering Rinds - a roc( 'ay show an outer weathered 8one and an inner unweathered

8one in the initial stages of weathering. The outer 8one is (nown as a weathering rind.

As weathering continues the thic(ness of the weathering rind increases! and thus canso'eti'es "e used as an indicator of the a'ount of ti'e the roc( has "een e=posed to

the weathering process. #:ee figure 1&.2! on page *&* in your te=t$

• E/foliation - 9oncentrated shells of weathering 'ay for' on the outside of a roc( and

'ay "eco'e separated fro' the roc(. These thin shells of weathered roc( are separated "y stresses that result fro' changes in volu'e of the 'inerals that occur as a result of

the for'ation of new 'inerals. #:ee figure &.*0 in your te=t$.

• $pheroidal eathering - If oints and fractures in roc( "eneath the surface for' a 3-

di'ensional networ(! the roc( will "e "ro(en into cu"e li(e pieces separated "y thefractures. ater can penetrate 'ore easily along these fractures! and each of the cu"e-

li(e pieces will "egin to weather inward. The rate of weathering will "e greatest along

the corners of each cu"e! followed "y the edges! and finally the faces of the cu"es. As a

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Soils

:oils are an i'portant natural resource. They represent the interface "etween the lithosphereand the "iosphere - as soils provide nutrients for plants. :oils consist of weathered roc( plusorganic 'aterial that co'es fro' decaying plants and ani'als. The sa'e factors that control

weathering control soil for'ation with the e=ception! that soils also reuires the input of organic

'aterial as so'e for' of 9ar"on.

hen a soil develops on a roc(! a soil profile develops as shown "elow. These different layersare not the sa'e as "eds for'ed "y sedi'entation! instead each of the hori8ons for's and grows

in place "y weathering and the addition of organic 'aterial fro' decaying plants and plant

roots.

Although you will not "e e=pected to (now all of the soil ter'inology discussed on pages *&2

through *&% in your te=t! the following ter's are i'portant.

• Caliche - 9alciu' 9ar"onate #9alcite$ that for's in arid soils in the F-hori8on "y

che'ical precipitation of calcite. The 9a and 9ar"onate ions are dissolved fro' theupper soil hori8ons and precipitated at the F-hori8on. In arid cli'ates the a'ount of

water passing through the soil hori8ons is not enough to co'pletely dissolve this caliche!

and as result the thic(ness of the layer 'ay increase with ti'e.

• !aterites - In hu'id tropical cli'ates intense weathering involving leaching occurs!

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leaving "ehind a soil rich in >e and Al o=ides! and giving the soil a deep red color. This

e=tre'ely leached soil is called a laterite.

• Paleosols - If a soil is "uried rapidly! for e=a'ple "y a volcanic eruption! the soil 'ay "e

preserved in the geologic record as an ancient soil called a paleosol.

Soil Erosion In 'ost cli'ates it ta(es "etween B0 and %00 years to for' a"out one centi'eter of topsoil

#an organic and nutrient rich soil suita"le for agriculture$. Thus soil that is eroded "y poorfar'ing practices is essentially lost and cannot "e replaced in a reasona"le a'ount of ti'e. This

could "eco'e a critical factor in controlling world population.



Mass$#asting

This page last updated on *+-,ct-2003

ass-wasting is the down-slope 'ove'ent of Regolith #loose unce'ented 'i=ture of soil and

roc( particles that covers the Earths surface$ "y the force of gravity without the aid of a

transporting 'ediu' such as water! ice! or wind. :till! as we shall see! water plays a (ey role.

ass-wasting is part of a continuu' of erosional processes "etween weathering and strea'

transport. ass-wasting causes regolith to 'ove down-slope where sooner or later the loose

particles will "e pic(ed up "y another transporting agent and eventually 'oved to a site ofdeposition such as an ocean "asin or la(e "ed.

In order for regolith to 'ove in a 'ass wasting process it 'ust "e on a slope! since gravity will

only cause 'otion if the 'aterial is on a slope.

Gra1ity Gravity is a force that acts everywhere on the Earths

surface! pulling everything in a direction toward the

center of the Earth. ,n a flat surface! parallel to theEarths surface! the force of gravity acts downward. :o

long as the 'aterial re'ains on the flat surface it will

not 'ove under the force of gravity.

,n a slope! the force of gravity can "e resolved into two co'ponents@ a co'ponent acting perpendicular to the slope! and a co'ponent acting tangential to the slope.

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• The perpendicular co'ponent of gravity! g p! helps to hold the o"ect in place on the

slope.

• The tangential co'ponent of gravity! gt! causes a shear stress parallel to the slope and

helps to 'ove the o"ect in the down-slope direction.

• ,n a steeper slope! the shear stress or tangential co'ponent of gravity! gt! increases! and

the perpendicular co'ponent of gravity! g p! decreases.

• Another force resisting 'ove'ent down the slope is grouped under the ter' shear

strength and includes frictional resistance and cohesion a'ong the particles that 'a(e

up the o"ect.

• hen the sheer stress "eco'es greater than the co'"ination of forces holding the o"ect

on the slope! the o"ect will 'ove down-slope.

• Thus! down-slope 'ove'ent is favored "y steeper slope angles #increasing the shear

stress$ and anything that reduces the shear strength #such as lowering the cohesion

a'ong the particles or lowering the frictional resistance.

he ole of Water

Although water is not directly involved as the transporting 'ediu' in 'ass-wasting processes!

it does play an i'portant role. Thin( a"out "uilding a sandcastle on the "each. If the sand istotally dry! it is i'possi"le to "uild a pile of sand with a steep face li(e a castle wall. If the sand

is so'ewhat wet! however! one can "uild a vertical wall. If the sand is too wet! then it flows li(e

a fluid and cannot re'ain in position as a wall.

• 6ry unconsolidated grains will for' a pile with a slope angle deter'ined "y the angle

of repose. The angle of repose is the steepest angle at which a pile of unconsolidated

grains re'ains sta"le! and is controlled "y the frictional contact "etween the grains. In

general! for dry 'aterials the angle of repose increases with increasing grain si8e! "utusually lies "etween a"out 30 and 37o.

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• :lightly wet unconsolidated 'aterials e=hi"it a very high angle of repose "ecause

surface tension "etween the water and the grains tends to hold the grains in place.

• hen the 'aterial "eco'es saturated with water! the angle of repose is reduced to very

s'all values and the 'aterial tends to flow li(e a fluid. This is "ecause the water gets

"etween the grains and eli'inates grain to grain frictional contact.

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Mass!Wasting Processes

The down-slope 'ove'ent of 'aterial! whether it "e "edroc(! regolith! or a 'i=ture of these! is

co''only referred to as a landslide. All of these processes generally grade into one another! so

classification of 'ass-wasting processes is so'ewhat difficult. e will use the classificationused "y your te=t"oo(! which divides 'ass wasting processes into two "road categories and

further su"divides these categories.

*. $lope 3ailures - a sudden failure of the slope resulting in transport of de"ris down hill

"y sliding! rolling! falling! or slu'ping.

2. $ediment 3lows - 'aterial flows down hill 'i=ed with water or air.

Slope %ailures

• $lumps - types of slides wherein

downward rotation of roc( or regolith

occurs along a curved surface. The uppersurface of each slu'p "loc( re'ains

relatively undistur"ed! as do the

individual "loc(s. :lu'ps leave arcuate

scars or depressions on the hill slope.eavy rains or earthua(es usually trigger

slu'ps.

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• Rock 3alls and .e&ris 3alls - ;oc( falls

occur when a piece of roc( on a steep

slope "eco'es dislodged and falls downthe slope. 6e"ris falls are si'ilar! e=cept

they involve a 'i=ture of soil! regolith!

and roc(s. A roc( fall 'ay "e a singleroc(! or a 'ass of roc(s! and the falling

roc(s can dislodge other roc(s as they

collide with the cliff. At the "ase of 'ost

cliffs is an accu'ulation of fallen 'aterialter'ed talus. The slope of the talus is

controlled "y the angle of repose for the

si8e of the 'aterial. :ince talus resultsfro' the accu'ulation of large roc(s or

'asses of de"ris the angle of repose is

usually greater than it would "e for sand.

• Rock $lides and .e&ris $lides - ;oc(

slides and de"ris slides result when roc(s

or de"ris slide down a pre-e=isting

surface! such as a "edding plane or ointsurface. <iles of talus are co''on at the

"ase of a roc( slide or de"ris slide.

Sediment %lo2s

:edi'ent flows occur when sufficient force is applied to roc(s and regolith that they "egin toflow down slope. A sedi'ent flow is a 'i=ture of roc(! regolith with so'e water. They can "e

"ro(en into two types depending on the a'ount of water present.

*. $lurry 3lows- are sedi'ent flows that contain "etween a"out 20 and %0) water. As the

water content increases a"ove a"out %0) slurry flows grade into strea's.

2. Granular 3lows - are sedi'ent flows that contain "etween 20 and 0) water. ote thatgranular flows are possi"le with little or no water. >luid-li(e "ehavior is given these

flows "y 'i=ing with air.

Each of these classes of sedi'ent flows can "e further su"divided on the "asis of the velocity at

which flowage occurs.

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• $lurry 3lows

o $olifluction - flowage at rates 'easured on the order of centi'eters per year of

regolith containing water. :olifluction produces distinctive lo"es on hill slopes#see figures. *3.B K *3. in your te=t$. These occur in areas where the soil

re'ains saturated with water for long periods of ti'e.

o .e&ris 3lows- these occur at higher velocities than solifluction! and often result

fro' heavy rains causing saturation of the soil and regolith with water. They

so'eti'es start with slu'ps and then flow down hill for'ing lo"es with an

irregular surface consisting of ridges and furrows.

o udflows- a highly fluid! high velocity 'i=ture of sedi'ent and water that has a

consistency of wet concrete. These usually result fro' heavy rains in areas wherethere is an a"undance of unconsolidated sedi'ent that can "e pic(ed up "y

strea's. Thus! after a heavy rain strea's can turn into 'udflows as they pic( up'ore and 'ore loose sedi'ent. udflows can travel for long distances over

gently sloping strea' "eds. 1ecause of their high velocity and long distance of

travel they are potentially very dangerous.

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• Granular 3lows

o Creep- the very slow! usually continuous 'ove'ent of regolith down slope.

9reep occurs on al'ost all slopes! "ut the rates vary. Evidence for creep is oftenseen in "ent trees! offsets in roads and fences! and inclined utility poles #see

figure *3.*3 in your te=t$.

o Earthflows - are usually associated with heavy rains and 'ove at velocities "etween several c'4yr and *00s of '4day. They usually re'ain active for long

periods of ti'e. They generally tend to "e narrow tongue-li(e features that "egin

at a scarp or s'all cliff #see figure *3.*& in your te=t$

o Grain 3lows - usually for' in relatively dry 'aterial! such as a sand dune! on a

steep slope. A s'all distur"ance sends the dry unconsolidated grains 'oving

rapidly down slope.

o .e&ris "+alanches - These are very high velocity flows of large volu'e

'i=tures of roc( and regolith that result fro' co'plete collapse of a

'ountainous slope. They 'ove down slope and then can travel for considera"le

distances along relatively gentle slopes. They are often triggered "y earthua(es

and volcanic eruptions.

Mass!Wasting in "old "limates

ass-wasting in cold cli'ates is governed "y the fact that water is fro8en as ice during long periods of the year. Ice! although it is solid! does have the a"ility to flow! and free8ing and

thawing cycles can also contri"ute to 'ove'ent.

• 3rost Hea+ing - this process is large contri"utor to creep in cold cli'ates. hen water

saturated soils free8e! they e=pand! pushing roc(s and "oulders on the surface upward

perpendicular to the slope. hen the soil thaws! the "oulders 'ove down verticallyresulting in a net down slope 'ove'ent.

• Gelifluction - :i'ilar to solifluction! this process occurs when the upper layers of soil

thaw during the war'er 'onths resulting in water saturated soil that 'oves down slope.

• Rock Glaciers - a lo"e of ice-ce'ented roc( de"ris #'ostly roc(s with ice "etween the

"loc(s$ that slowly 'oves downhill #see figure *3.2* in your te=t$.

Su)a5ueous Mass!Wasting

ass wasting processes also occur on steep slopes in the ocean "asins. A slope failure can occur

due to over-accu'ulation of sedi'ent on slope or in a su"'arine canyon! or could occur as aresult of a shoc( li(e an earthua(e. :lu'ps! de"ris flows! and landslides are co''on8

riggering of Mass!Wasting E1ents

A 'ass-wasting event can occur any ti'e a slope "eco'es unsta"le. :o'eti'es! as in the case

of creep or solifluction! the slope is unsta"le all of the ti'e! and the process is continuous. 1ut

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other ti'es! triggering events can occur that cause a sudden insta"ility to occur.

• :hoc(s - A sudden shoc(! such as an earthua(e 'ay trigger a slope insta"ility. inor

shoc(s li(e heavy truc(s ra'"ling down the road! trees "lowing in the wind! or 'an

'ade e=plosions can also trigger 'ass-wasting events.

• :lope odification - 'odification of slope either "y hu'ans or "y natural causes can

result in changing the slope angle so that it is no longer at the angle of repose. A 'ass-

wasting event can then restore the slope to its angle of repose.

• Jndercutting - strea's eroding their "an(s or surf action along a coast can undercut a

slope 'a(ing it unsta"le.

• E=ceptional <recipitation - heavy rains can saturate regolith reducing grain to grain

contact and reducing the angle of repose! thus triggering a 'ass-wasting event.

• Lolcanic Eruptions - produce shoc(s li(e e=plosions and earthua(es. They can also

cause snow to 'elt or e'pty crater la(es! rapidly releasing large a'ounts of water that

can "e 'i=ed with regolith to reduce grain to grain contact and result in de"ris flows!'udflows! and landslides.

• :u"'arine :lope >ailures - these can "e caused "y rapid deposition of sedi'ent that

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does not allow water trapped "etween grains to escape! or "y generation of 'ethane gas

fro' the decay of organic 'aterial! which increases pressure "etween unconsolidatedgrains and thus reduces grain to grain contact.



Streams and Drainage Systems

This page last updated on *+-,ct-2003

Streams

A stream is a "ody of water that carries roc( particles and dissolved ions and flows down slopealong a clearly defined path! called a channel . Thus! strea's 'ay vary in width fro' a fewcenti'eters to several (ilo'eters. :trea's are i'portant for several reasons@

• :trea's carry 'ost of the water that goes fro' the land to the sea! and thus are an

i'portant part of the water cycle.

• :trea's carry "illions of tons of sedi'ent to lower elevations! and thus are one of the

'ain transporting 'ediu's in the production of sedi'entary roc(s.

• :trea's carry dissolved ions! the products of che'ical weathering! into the oceans and

thus 'a(e the sea salty.

• :trea's are a 'aor part of the erosional process! wor(ing in conunction with

weathering and 'ass wasting. uch of the surface landscape is controlled "y strea'

erosion! evident to anyone loo(ing out of an airplane window.

• :trea's are a 'aor source of water and transportation for the worlds hu'an

population. ost population centers are located ne=t to strea's.

Geometry and $ynamics of Stream "hannels

The strea' channel is the conduit for water "eing carried "y the strea'. The strea' can

continually adust its channel shape and path as the a'ount of water passing through thechannel changes. The volu'e of water passing any point on a strea' is called the discharge.

6ischarge is 'easured in units of volu'e4ti'e #'34sec$.

• "ross Sectional Shape ! varies with position in the strea'! and discharge8 The deepest

part of channel occurs where the strea' velocity is the highest. 1oth width and depth

increase downstrea' "ecause discharge increases downstrea'. As discharge increases

the cross sectional shape will change! with the strea' "eco'ing deeper and wider.

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• #ong Profile ! a plot of elevation versus distance. Jsually shows a steep gradient nearthe source of the strea' and a gentle gradient as the strea' approaches its 'outh.

0ase !e+el - "ase level is defined as the li'iting level "elow which a strea' cannot

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erode its channel. >or strea's that e'pty into the oceans! "ase level is sea level. ?ocal

"ase levels can occur where the strea' 'eets a resistant "ody of roc(! where a natural or artificial da' i'pedes further channel erosion! or where the strea' e'pties into a la(e.

hen a natural or artificial

da' i'pedes strea' flow!

the strea' adusts to thenew "ase level "y adusting

its long profile. In the

e=a'ple here! the long profile a"ove and "elow the

da' are adusted. Erosion

ta(es place downstrea'

fro' the da' #especially if

it is a natural da' and water can flow over the top$. Sust

upstrea' fro' the da' thevelocity of the strea' is

lowered so that deposition

of sedi'ent occurs causingthe gradient to "eco'e

lower.

• /elocity ! A strea's velocity depends on position in the strea' channel! irregularities in

the strea' channel caused "y resistant roc(! and strea' gradient. The average velocity isthe ti'e it ta(es a given particle of water to traverse a given distance. :trea' flow can

"e either la'inar! in which all water 'olecules travel along si'ilar parallel paths! ortur"ulent! in which individual particles ta(e irregular paths. Tur"ulent flow can (eep

sedi'ent in suspension longer than la'inar flow and aids in erosion of the strea' "otto'. Average linear velocity is generally greater in la'inar flow than in tur"ulent

flow.

• .ischarge ! The discharge of a strea' is the a'ount of water passing any point in a

given ti'e.

P C A = L

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6ischarge #'34sec$ C 9ross-sectional Area Uwidth = average depthD #'2$ = Average Lelocity

#'4sec$.

As the a'ount of water in a strea' increases! the strea' 'ust adust its velocity and cross

sectional area in order to for' a "alance. 6ischarge increases as 'ore water is added through

rainfall! tri"utary strea's! or fro' groundwater seeping into the strea'. As discharge increases!generally width! depth! and velocity of the strea' also increase.

• !oad - The roc( particles and dissolved ions carried "y the strea' are the called the

strea's load) :trea' load is divided into three parts.

o $uspended !oad - particles that are carried along with the water in the 'ain part

of the strea's. The si8e of these particles depends on their density and the

velocity of the strea'. igher velocity currents in the strea' can carry larger and

denser particles.

o 0ed !oad - coarser and

denser particles that re'ainon the "ed of the strea' 'ostof the ti'e "ut 'ove "y a

process of saltation

#u'ping$ as a result ofcollisions "etween particles!

and tur"ulent eddies. ote

that sedi'ent can 'ove

"etween "ed load andsuspended load as the

velocity of the strea'

changes.

o .issol+ed !oad - ions that have "een introduced into the water "y che'ical

weathering of roc(s. This load is invisi"le "ecause the ions are dissolved in thewater. The dissolved load consists 'ainly of 9,3

- #"icar"onate ions$! 9a2! :,%-

2! 9l-! a2! g2! and F . These ions are eventually carried to the oceans and

give the oceans their salty character. :trea's that have a deep underground

source generally have higher dissolved load than those whose source is on theEarths surface.

"hanges $o2nstream

As one 'oves along a strea' in the downstrea' direction@

• 6ischarge increases! as noted a"ove! "ecause water is added to the strea' fro' tri"utary

strea's and groundwater.

• As discharge increases! the width! depth! and average velocity of the strea' increase.

• The gradient of the strea'! however! will decrease.

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It 'ay see' to "e counter to your o"servations that velocity increases in the downstrea'

direction! since when one o"serves a 'ountain strea' near the headwaters where the gradient ishigh! it appears to have a higher velocity than a strea' flowing along a gentle gradient. 1ut! the

water in the 'ountain strea' is li(ely flowing in a tur"ulent 'anner! due to the large "oulders

and co""les which 'a(e up the strea'"ed. If the flow is tur"ulent! then it ta(es longer for the

water to travel the sa'e linear distance! and thus the average velocity is lower.Also as one 'oves in the downstrea' direction!

• The si8e of particles that 'a(e up the "ed load of the strea' tends to decrease. Even

though the velocity of the strea' increases downstrea'! the "ed load particle si8e

decreases 'ainly "ecause the larger particles are left in the "ed load at higher elevationsand a"rasion of particles tends to reduce their si8e.

• The co'position of the particles in the "ed load tends to change along the strea' as

different "edroc( is eroded and added to the strea's load.

%loods

>loods occur when the discharge of the strea' "eco'es too high to "e acco''odated in thenor'al strea' channel. hen the discharge "eco'es too high! the strea' widens its channel "y

overtopping its "an(s and flooding the low-lying areas surrounding the strea'. The areas that

"eco'e flooded are called floodplains.

"hannel Patterns

• Straight "hannels ! :traight strea' channels are rare. here they do occur! the channel

is usually controlled "y a linear 8one of wea(ness in the underlying roc(! li(e a fault or oint syste'. Even in straight channel seg'ents water flows in a sinuous fashion! with

the deepest part of the channel changing fro' near one "an( to near the other. Lelocity is

highest in the 8one overlying the deepest part of the strea'. In these areas! sedi'ent is

transported readily resulting in pools. here the velocity of the strea' is low! sedi'entis deposited to for' &ars. The "an( closest to the 8one of highest velocity is usually

eroded and results in a cut&ank .

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• Meandering "hannels ! 1ecause of the velocity structure of a strea'! and especially in

strea's flowing over low gradients with easily eroded "an(s! straight channels will

eventually erode into meandering channels. Erosion will ta(e place on the outer parts of the 'eander "ends where the velocity of the strea' is highest. :edi'ent deposition will

occur along the inner 'eander "ends where the velocity is low. :uch deposition ofsedi'ent results in e=posed "ars! called point &ars. 1ecause 'eandering strea's are

continually eroding on the outer 'eander "ends and depositing sedi'ent along the inner

'eander "ends! 'eandering strea' channels tend to 'igrate "ac( and forth across their

flood plain.

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If erosion on the outside 'eander "ends continues to ta(e place! eventually a 'eander "end can "eco'e cut off fro' the rest of the strea'. hen this occurs! the cutoff

'eander "end! "ecause it is still a depression! will collect water and for' a type of la(e

called an o/&ow lake.

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• &raided "hannels - In strea's having

highly varia"le discharge and easilyeroded "an(s! sedi'ent gets deposited to

for' "ars and islands that are e=posedduring periods of low discharge. In sucha strea' the water flows in a "raided

pattern around the islands and "ars!

dividing and reuniting as it flows

downstrea'. :uch a channel is ter'ed a&raided channel . 6uring periods of high

discharge! the entire strea' channel 'ay

contain water and the islands are coveredto "eco'e su"'erged "ars. 6uring such

high discharge! so'e of the islands

could erode! "ut the sedi'ent would "ere-deposited as the discharge decreases!

for'ing new islands or su"'erged "ars.

Islands 'ay "eco'e resistant to erosionif they "eco'e inha"ited "y vegetation

Erosion )y Streams

:trea's erode "ecause they have the a"ility to pic( up roc( frag'ents and transport the' to anew location. The si8e of the frag'ents that can "e transported depends on the velocity of the

strea' and whether the flow is la'inar or tur"ulent. Tur"ulent flow can (eep frag'ents in

suspension longer than la'inar flow. :trea's can also eroded "y undercutting their "an(sresulting in 'ass-wasting processes li(e slu'ps or slides. hen the undercut 'aterial falls into

the strea'! the frag'ents can "e transported away "y the strea'. :trea's can cut deeper into

their channels if the region is uplifted or if there is a local change in "ase level. As they cutdeeper into their channels the strea' re'oves the 'aterial that once 'ade up the channel

"otto' and sides.

Stream $eposits

:udden changes in velocity can result in deposition "y strea's. ithin a strea' we have seenthat the velocity varies with position! and! if sedi'ent gets 'oved to the lower velocity part of

the strea' the sedi'ent will co'e out of suspension and "e deposited. ,ther sudden changes in

velocity that affect the whole strea' can also occur. >or e=a'ple if the discharge is suddenly

increased! as it 'ight "e during a flood! the strea' will overtop its "an(s and flow onto thefloodplain where the velocity will then suddenly decrease. This results in deposition of such

features as levees and floodplains. If the gradient of the strea' suddenly changes "y e'ptyinginto a flat-floored "asin! an ocean "asin! or a la(e! the velocity of the strea' will suddenly

decrease resulting in deposition of sedi'ent that can no longer "e transported. This can result in

deposition of such features as alluvial fans and deltas.

• 3loodplains and !e+ees - As a strea' overtops its "an(s during a flood! the velocity of

the flood will first "e high! "ut will suddenly decrease as the water flows out over the

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gentle gradient of the floodplain. 1ecause of the sudden decrease in velocity! the coarser

grained suspended sedi'ent will "e deposited along the river"an(! eventually "uildingup a natural levee. atural levees provide so'e protection fro' flooding "ecause with

each flood the levee is "uilt higher and therefore discharge 'ust "e higher for the ne=t

flood to occur. #ote that the levees we see along the ississippi ;iver here in ew

,rleans are not natural levees! "ut 'an 'ade levees! "uilt to protect the floodplain fro'floods$.

• 'erraces - Terraces are e=posed for'er floodplain deposits that result when the strea'

"egins down cutting into its flood plain #this is usually caused "y regional uplift or "y

lowering the regional "ase level! such as a drop in sea level$.

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• "llu+ial 3ans - hen a steep

'ountain strea' enters a flatvalley! there is a sudden

decrease in gradient and

velocity. :edi'ent transportedin the strea' will suddenly

"eco'e deposited along the

valley walls in an alluvial fan.

As the velocity of the 'ountainstrea' slows it "eco'es cho(ed

with sedi'ent and "rea(s up

into nu'erous distri"utarychannels.

• 6eltas - hen a strea'

enters a standing "ody of

water such as a la(e or

ocean! again there is asudden decrease in velocity

and the strea' deposits its

sedi'ent in a deposit called adelta. 6eltas "uild outward

fro' the coastline! "ut will

only survive if the oceancurrents are not strong

enough to re'ove the

sedi'ent. As the velocity of

a strea' decreases onentering the delta! the strea'

"eco'es cho(ed with

sedi'ent and conditions

"eco'e favora"le to those ofa "raided strea' channel! "ut

instead of "raiding! thestrea' "rea(s into 'any

s'aller strea's called

distri"utary strea's.

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$rainage Systems

• $rainage &asins and $i1ides - 6rainage syste's develop in such a way as to

efficiently 'ove water off the land. Each strea' in a drainage syste' drains a certain

area! called a drainage "asin. In a single drainage "asin! all water falling in the "asin

drains into the sa'e strea'. 6rainage "asins can range in si8e fro' a few ('2

! for s'allstrea's! to e=tre'ely large areas! such as the ississippi ;iver drainage "asin which

covers a"out %0) of the contiguous Jnited :tates #see figure *%.2 in your te=t$. Adivide separates each drainage "asin fro' other drainage "asins.

• Stream 0rder - The s'allest strea's in a drainage networ( have no tri"utary strea's.

These are called first order strea's. Two first order strea's unite to for' a second order

strea'. :econd order strea's only have first-order strea's as tri"utaries. Third orderstrea's only have second and first order strea's as tri"utaries! etc. As the order of the

strea' increases! the discharge increases! the gradient decreases! the velocity increases!

and the channel di'ensions #width and depth$ increase to acco''odate the increased

discharge.

• $rainage Patterns - 6rainages tend to develop along 8ones where roc( type and

structure are 'ost easily eroded. Thus various types of drainage patterns develop in a

region and these drainage patterns reflect the structure of the roc(. /ou study these

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drainage patterns in ?a"! and e=a'ples are shown in figure *%.32 of your te=t.

• "ontinental $i1ides - 9ontinents can "e divided into large drainage "asins that e'pty

into different ocean "asins. >or e=a'ple@ orth A'erica can "e divided into several "asins west of the ;oc(y ountains that e'pty into the <acific ,cean. :trea's in the

northern part of orth A'erica e'pty into the Arctic ,cean! and strea's East of the

;oc(y ountains e'pty into the Atlantic ,cean or Gulf of e=ico. ?ines separatingthese 'aor drainage "asins are ter'ed 9ontinental 6ivides. :uch divides usually run

along high 'ountain crests that for'ed recently enough that they have not "een eroded.

Thus 'aor continental divides and the drainage patterns in the 'aor "asins reflect the

recent geologic history of the continents



!round%ater

This page last updated on 2*-,ct-2003

Groundwater is water that e=ists in the pore spaces and fractures in roc( and sedi'ent "eneath

the Earths surface. It originates as rainfall or snow! and then 'oves through the soil into the

groundwater syste'! where it eventually 'a(es its way "ac( to surface strea's! la(es! oroceans.

• Groundwater 'a(es up a"out *) of the water on Earth #'ost water is in oceans$.

• 1ut! groundwater 'a(es up a"out 3+ ti'es the a'ount of water in la(es and strea's.

• Groundwater occurs everywhere "eneath the Earths surface! "ut is usually restricted to

depths less that a"out 7+0 'eters.

• The volu'e of groundwater is a euivalent to a ++ 'eter thic( layer spread out over the

entire surface of the Earth.

• The surface "elow which all roc(s are saturated with groundwater is the water ta&le.

he Water a)le

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;ain that falls on the surface

seeps down through the soil andinto a 8one called the (one of

aeration or unsaturated (one

where 'ost of the pore spaces

are filled with air. As it penetrates deeper it eventually

enters a 8one where all pore

spaces and fractures are filledwith water. This 8one is called

the saturated (one. The surface

"elow which all openings in theroc( are filled with water #the

top of the saturated 8one$ is

called the water ta&le.

The water ta"le occurs everywhere "eneath the Earths surface. In desert regions it is always present! "ut rarely intersects the surface.

In 'ore hu'id regions it

reaches the surface at strea'sand la(es! and generally tends

to follow surface topography.

The depth to the water ta"le'ay change! however! as thea'ount of water flowing into

and out of the saturated 8one

changes. 6uring dry seasons!the depth to the water ta"le

increases. 6uring wet seasons!

the depth to the water ta"ledecreases.

Mo1ement of Ground2ater

Groundwater is in constant 'otion! although the rate at which it 'oves is generally slower thanit would 'ove in a strea' "ecause it 'ust pass through the intricate passageways "etween freespace in the roc(. >irst the groundwater 'oves downward due to the pull of gravity. 1ut it can

also 'ove upward "ecause it will flow fro' higher pressure areas to lower pressure areas! as

can "e seen "y a si'ple e=peri'ent illustrated "elow. I'agine that we have a OJO-shaped tu"e

filled with water. If we put pressure on one side of the tu"e! the water level on the other siderises! thus the water 'oves fro' high pressure 8ones to low pressure 8ones.

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The sa'e thing happens "eneath the surface of the

Earth! where pressure is higher "eneath the hills

and lower "eneath the valleys

The rate of groundwater flow is controlled "y two properties of the roc(@ porosity and

permea&ility.

• Porosity is the percentage of the volu'e of the roc( that is open space #pore space$. This

deter'ines the a'ount of water that a roc( can contain.

o In sedi'ents or sedi'entary roc(s the porosity depends on grain si8e! the shapes

of the grains! and the degree of sorting! and the degree of ce'entation.

ell-rounded

coarse-grainedsedi'ents usually

have higher porosity than fine-grained sedi'ents!

"ecause the grains

do not fit togetherwell.

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<oorly sorted sedi'ents usually have lower

porosity "ecause the fine-grained frag'ents

tend to fill in the open space.

:ince ce'ents tend to fill in the

pore space! highly ce'ented

sedi'entary roc(s have lower porosity.

o In igneous and 'eta'orphic roc(s porosity is

usually low "ecause the 'inerals tend to "eintergrown! leaving little free space. ighly

fractured igneous and 'eta'orphic roc(s! however!

could have high porosity

• Permea&ility is a 'easure of the degree to which the pore spaces are interconnected! and

the si8e of the interconnections. ?ow porosity usually results in low per'ea"ility! "uthigh porosity does not necessarily i'ply high per'ea"ility. It is possi"le to have ahighly porous roc( with little or no interconnections "etween pores. A good e=a'ple of

a roc( with high porosity and low per'ea"ility is a vesicular volcanic roc(! where the

"u""les that once contained gas give the roc( a high porosity! "ut since these holes arenot connected to one another the roc( has low per'ea"ility.

A thin layer of water will always "e

attracted to 'ineral grains due tothe unsatisfied ionic charge on the

surface. This is called the force of

molecular attraction. If the si8e of

interconnections is not as large asthe 8one of 'olecular attraction!

the water cant 'ove. Thus! coarse-

grained roc(s are usually 'ore per'ea"le than fine-grained roc(s!

and sands are 'ore per'ea"le than

clays.

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Mo1ement in the 9one of (eration

;ainwater soa(s into the soil where so'e of it is evaporated! so'e of it adheres to grains in the

soil "y 'olecular attraction! so'e is a"sor"ed "y plant roots! and so'e seeps down into thesaturated 8one. 6uring long periods without rain the 8one of aeration 'ay re'ain dry.

Mo1ement in the Saturated 9one

In the saturated 8one #"elow the water ta"le$ water percolates through the interconnected pore

spaces! 'oving downward "y the force of gravity! and upward toward 8ones of lower pressure.here the water ta"le intersects the surface! such as at a surface strea'! la(e! or swa'p! the

groundwater returns to the surface.

echarge (reas and $ischarge (reas

The Earths surface can "e divided into areaswhere so'e of the water falling on the surface

seeps into the saturated 8one and other areas

where water flows out of the saturated 8oneonto the surface. Areas where water enters the

saturated 8one are called recharge areas!

"ecause the saturated 8one is recharged withgroundwater "eneath these areas. Areas where

groundwater reaches the surface #la(es!

strea's! swa'ps! K springs$ are called

discharge areas! "ecause the water isdischarged fro' the saturated 8one. Generally!

recharge areas are greater than discharge areas.

$ischarge and /elocity

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The rate at which groundwater

'oves through the saturated8one depends on the

per'ea"ility of the roc( and

the hydraulic gradient . The

hydraulic gradient is defined asthe difference in elevation

divided "y the distance

"etween two points on thewater ta"le.

Lelocity! L! is then@

L C F#h2 - h*$4?

where F is the coefficient of per'ea"ility.

If we 'ultiply this e=pression "y the area! A! through which the water is 'oving! then we get

the discharge! P.

P C AF#h2 - h*$4?!

which is .arcyDs !aw.

Springs and Wells

• A spring is an area on the surface of the Earth where the water ta"le intersects the

surface and water flows out of the ground. :prings occur when an i'per'ea"le roc(#called an a>uiclude2 intersects an per'ea"le roc( that contains groundwater #ana>uifer $. :uch u=taposition "etween per'ea"le and i'per'ea"le roc( can occur along

geological contacts #surfaces separating two "odies of roc($! and fault 8ones.

• A well is hu'an-'ade hole that is dug or drilled deep enough to intersect the water

ta"le. ells are usually used as a source for groundwater. If the well is dug "eneath the

water ta"le! water will fill the open space to the level of the water ta"le! and can "e

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drawn out "y a "uc(et or "y pu'ping. >racture syste's and perched water &odies can

often 'a(e it difficult to locate the "est site for a well.

(5uifers

An a>uifer is a large "ody of per'ea"le 'aterial where groundwater is present in the saturated

8one. Good auifers are those with high per'ea"ility such as poorly ce'ented sands! gravels!and sandstones or highly fractured roc(. ?arge auifers can "e e=cellent sources of water for

hu'an usage such as the igh <lains Auifer #in sands and gravels$ or the >loridian Auifer #in

porous li'estones$ as outlined in your te=t. Auifers can "e of two types@

• Unconfined ">uifers - the 'ost co''on type of auifer! where the water ta"le is

e=posed to the Earths at'osphere through the 8one of aeration. ost of the auifers

depicted in the drawings so far have "een unconfined auifers.

• Confined ">uifers - these are less co''on! "ut occur when an auifer is confined

"etween layers of i'per'ea"le strata. A special (ind of confined auifer is an artesian

syste'! shown "elow. Artesian syste's are desira"le "ecause they result in free flowingartesian springs and artesian wells)

"hanges in the Ground2ater System

hen discharge of groundwater e=ceeds recharge of the syste'! several adverse effects can

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occur. ost co''on is lowering of the water ta"le! resulting in springs drying up and wells

having to "e dug to deeper levels. If water is pu'ped out of an auifer! pore pressure can "ereduced in the auifer that could result in co'paction of the now dry auifer and result in land

su"sidence. In so'e cases withdrawal of groundwater e=ceeds recharge "y natural processes!

and thus groundwater should "e considered a non-renewa"le natural resource.

Water :uality and Ground2ater "ontamination

ater uality refers to such things as the te'perature of the water! the a'ount of dissolved

solids! and lac( of to=ic and "iological pollutants. ater that contains a high a'ount of

dissolved 'aterial through the action of che'ical weathering can have a "itter taste! and is

co''only referred to as hard water. ot water can occur if water co'es fro' a deep source orencounters a cooling 'ag'a "ody on its traverse through the groundwater syste'. :uch hot

water 'ay desira"le for "ath houses or geother'al energy! "ut is not usually desira"le for

hu'an consu'ption or agricultural purposes. ost pollution of groundwater is the result of "iological activity! 'uch of it hu'an. A'ong the sources of conta'ination are@

• :ewers and septic tan(s

• aste du'ps #"oth industrial and residential$

• Gasoline Tan(s #li(e occur "eneath all service stations$

• 1iological waste products - 1iological conta'inants can "e re'oved fro' the

groundwater "y natural processes if the auifer has interconnections "etween pores that

are s'aller than the 'icro"es. >or e=a'ple a sandy auifer 'ay act as a filter for "iological conta'inants.

• Agricultural pollutants such as fertili8ers and pesticides.

• :alt water conta'ination - results fro' e=cessive discharge of fresh groundwater in

coastal areas.

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Geologic (cti1ity of Ground2ater

• $issolution ! ;ecall that water is the 'ain agent of che'ical weathering. Groundwateris an active weathering agent and can leach ions fro' roc(! and! in the case of car"onateroc(s li(e li'estone! can co'pletely dissolve the roc(.

• "hemical "ementation and eplacement - ater is also the 'ain agent acting during

diagenesis. It carries in dissolved ions which can precipitate to for' che'ical ce'entsthat hold sedi'entary roc(s together. Groundwater can also replace other 'olecules in

'atter on a 'olecule "y 'olecule "asis! often preserving the original structure such as

in fossili8ation or petrified wood.

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• "a1es and "a1erns - If

large areas of li'estoneunderground are dissolved

"y the action of

groundwater these cavitiescan "eco'e caves or

caverns #caves with 'any

interconnected cha'"ers$

once the water ta"le islowered. ,nce a cave

for's! it is open to the

at'osphere and water percolating in can precipitate new 'aterial such as the co''oncave decorations li(e stalagtites #hang fro' the ceiling$! stalag'ites #grow fro' the

floor upward$! and dripstones! and flowstones.

• Sin-holes ! If the roof of a cave or cavern

collapses! this results in a sin(hole.

:in(holes! li(es caves! are co''on inareas underlain "y li'estones. >or

e=a'ple! in >lorida! which is underlain "y

li'estones! a new sin(hole for's a"out

once each year! go""ling up cars andhouses in process.

• .arst opography - In an area where the 'ain type of weathering is dissolution #li(e in

li'estone terrains$! the for'ation of caves and sin(holes! and their collapse and

coalescence 'ay result in a highly irregular topography called karst topography #see

pages %0% - %0& in your te=t$ )



!laciers and !laciation

This page last updated on 2B-,ct-2003

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Glaciers constitute 'uch of the Earth that 'a(es up the cryosphere! the part of the Earth that

re'ains "elow the free8ing point of water. ost glacial ice today is found in the polar regions!a"ove the Arctic and Antarctic 9ircles. hile glaciers are of relatively 'inor i'portance today!

evidence e=ists that the Earths cli'ate has undergone fluctuations in the past! and that the

a'ount of the Earths surface covered "y glaciers has "een 'uch larger in the past than in the

present. In fact! 'uch of the topography in the northern part of orth A'erica! as well as in thehigh 'ountain regions of the west! owe their for' to erosional and depositional processes of

glaciers. The latest glaciation ended only *0!000 years ago.

$efinition of a glacier

A glacier is a per'anent #on a hu'an ti'e scale! "ecause nothing on the Earth is really per'anent$ "ody of ice! consisting largely of recrystalli8ed snow! that shows evidence of

downslope or outward 'ove'ent due to the pull of gravity.

ypes of Glaciers

• ountain Glaciers - ;elatively s'all glaciers which occur at higher elevations in'ountainous regions.

o :'allest of these occupy hollows or "owl-shaped depressions on sides of

'ountains #cir>ue glaciers$.

o As cirue glaciers grow larger they 'ay spread into valleys and flow down the

valleys as +alley glaciers. <aths these valley glaciers ta(e are controlled "ye=isting topography.

o If a valley glacier e=tends down to sea level! it 'ay carve a narrow valley into

the coastline. These are called ford glaciers! and the narrow valleys they carveand later "eco'e filled with seawater after the ice has 'elted are fords.

o If a valley glacier e=tends down a valley and then covers a gentle slope "eyond

the 'ountain range! it is called a piedmont glacier .

o If all of the valleys in a 'ountain range "eco'e filled with glaciers! and the

glaciers cover then entire 'ountain range! they are called ice caps.

• Ice $heets@ #9ontinental glaciers$@ are the largest types of glaciers on Earth. They cover

large areas of the land surface! including 'ountain areas. odern ice sheets coverGreenland and Antarctica. These two ice sheets co'prise a"out +) of all glacial ice

currently on Earth. They have an esti'ated volu'e of a"out 2% 'illion ('3. If 'elted!

they contain enough water to raise sea level a"out &&' #2*& ft.$. This would cause

serious pro"le's for coastal cities #?.A.! /! ashington 69! ew ,rleans! ia'i! :>etc$. The Greenland ice sheet is in so'e places over 3000 ' #B00 ft$ thic( and the

weight of ice has depressed 'uch of the crust of Greenland "elow sea level. Antarctica

is covered "y two large ice sheets that 'eet in the central part along the Transantarctic

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ountains. These are the only truly polar ice sheet on earth #orth <ole lies in an ocean

covered "y thin layer of ice.

• Ice $hel+es@ Ice shelves are sheets of ice floating on water and attached to land. They

usually occupy coastal e'"ay'ents! 'ay e=tend hundreds of (' fro' land and reach

thic(nesses of *000 '.

Glaciers can also "e classified "y their internal te'perature.

• Te'perate glaciers - Ice in a te'perate glacier is at a te'perature near its 'elting point.

• <olar glaciers - Ice in a polar glacier always 'aintains a te'perature well "elow its

'elting point.

he %ormation of Glacial Ice

Glaciers can only for' at latitudes or elevations a"ove the snowline! which is the elevation

a"ove which snow can for' and re'ain present year round. The snowline! at present! lies at sealevel in polar latitudes and rises up to &000 ' in tropical areas. Glaciers for' in these areas if

the snow "eco'es co'pacted! forcing out the air "etween the snowfla(es. As co'paction

occurs! the weight of the overlying snow causes the snow to recrystalli8e and increase its grain-si8e! until it increases its density and "eco'es a solid "loc( of ice.

"hanges in Glacier Si;e

A glacier can change its si8e "y "ccumulation! which occurs "y addition of snowfall!

co'paction and recrystalli8ation! and "&lation! the loss of 'ass resulting fro' 'elting! usuallyat lower altitude! where te'peratures 'ay rise a"ove free8ing point in su''er. Thus! depending

on the "alance "etween accu'ulation and a"lation during a full season! the glacier can grow orshrin(.

Mo1ement of Glaciers

Glaciers 'ove to lower elevations under the force of gravity "y two different processes@

• Internal >low - called creep! results fro' defor'ation of the ice crystal structure - the

crystals slide over each other li(e dec( of cards. This type of 'ove'ent is the only type

that occurs in polar glaciers! "ut it also occurs in te'perate glaciers. The upper portionsof glaciers are "rittle! when the lower portion defor's "y internal flow! the upper

portions 'ay fracture to for' large crac(s called cre1asses. 9revasses occur where the

lower portion of a glacier flows over sudden change in topography #see figure *&.*2 on page %20of your te=t$.

• 1asal sliding - 'eltwater at "ase of glacier reduces friction "y lu"ricating the surface

and allowing the glacier to slide across its "ed. <olar glaciers are usually fro8en to their

"ed and are thus too cold for this 'echanis' to occur.

The velocity of glacial ice changes throughout the glacier. The velocity is low ne=t to the "aseof the glacier and where it is contact with valley walls. The velocity increases toward the center

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and upper parts of the glacier.

Glaciation

Glaciation@ is the 'odification of the land surface "y the action of glaciers. Glaciations haveoccurred so recently in . A'erica and Europe! that weathering! 'ass wasting! and strea'erosion have not had ti'e to alter the landscape. Thus! evidence of glacial erosion and

deposition are still present. :ince glaciers 'ove! they can pic( up and transport roc(s and thus

erode. :ince they transport 'aterial and can 'elt! they can also deposit 'aterial. Glaciatedlandscapes are the result of "oth glacial erosion and glacial deposition.

Glacial Erosion #note@ 'ost of this 'aterial will "e presented as slides in class$

• :'all scale erosional features

o Glacial striations - long parallel scratches and grooves that are produced at the

"otto' of te'perate glaciers "y roc(s e'"edded in the ice scraping against theroc( underlying the glacier #see figure *&.*7 in your te=t$.

o Glacial polish - roc( that has a s'ooth surface produced as a result of fined

grained 'aterial e'"edded in the glacier acting li(e sandpaper on the underlying

surface.

• ?andfor's produced "y 'ountain glaciers

o Cir>ues - "owl shaped depressions that occur at the heads of 'ountain glaciers

that result for' a co'"ination of frost wedging! glacial pluc(ing! and a"rasion.

:o'eti'es s'all la(es! called tarns occur in the "otto' of cirue.

o Glacial ,alleys - Lalleys that once contained glacial ice "eco'e eroded into a

OJO shape in cross section. :trea' erosion! on the other hand! produces valleys

that are OLO shaped in cross section #see figure *&.20 in your te=t$.

o "rFtes - If two adacent valleys are filled with glacial ice! the ridges "etween the

valleys can "e carved into a sharp (nife-edge ridge! called an arVte.

o Horns - here three or 'ore cirues are carved out of a 'ountain! they can

produce a sharp pea( called a horn #see figure *&.* in your te=t$.

o Hanging ,alleys - hen a glacier occupying a s'aller tri"utary valley 'eets thelarger valley! the tri"utary glacier usually does not have the a"ility to erode its "ase to the floor of the 'ain valley. Thus! when the glacial ice 'elts the floor of

the tri"utary valley hangs a"ove the floor of the 'ain valley and is called a

hanging valley. aterfalls generally occur where the hanging valley 'eets the'ain valley.

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o 3ords - >ords are narrow inlets along the seacoast that were once occupied "y a

valley glacier! called a ford glacier.

• ?andfor's produced "y Ice 9aps and Ice :heets

o A"rasional features - The sa'e s'all-scale a"rasional features such as striationsand glacial polish can occur "eneath ice caps and ice sheets! particularly inte'perate environ'ents.

o :trea'lined for's - The land surface "eneath a 'oving continental ice sheet can

"e 'olded into s'ooth elongated for's called drumlins #see figure *&.22 in

your te=t$.

Glacial $eposits

:ince glaciers are solid they can transport all si8es of sedi'ent! fro' huge house-si8ed "ouldersto fine-grained clay si8ed 'aterial. The glacier can carry this 'aterial on its surface ore'"edded within it. Thus! sedi'ent transportation in a glacier is very 'uch different than that

in a strea'. Thus! sedi'ents deposited directly fro' 'elting of a glacial can range fro' very

poorly sorted to "etter sorted! depending on how 'uch water transport ta(es place after the ice'elts. All sedi'ent deposited as a result of glacial erosion is called Glacial .rift .

• Ice ?aid 6eposits

o 'ill - nonsorted glacial drift deposited directly fro' ice. Till consists of a rando'

'i=ture of different si8ed frag'ents of angular roc(s in a 'atri= of fine grained!

sand- to clay-si8ed frag'ents that were produced "y a"rasion within the glacier.This fine-grained 'aterial is often called roc( flour "ecause it is really ground up

roc(. A till that has undergone diagenesis and has turned into a roc( is called a

tillite)

o Erratics - a glacially deposited roc( or frag'ent that now rests on a surface

'ade of different roc(. Erratics are often found 'any (ilo'eters fro' their

source! and "y 'apping the distri"ution pattern of erratics geologists can often

deter'ine the flow directions of the ice that carried the' to their presentlocations.

o oraines - are deposits of till that have a for' different fro' the underlying "edroc(. 6epending on where it for'ed in relation to the glacier 'oraines can

"e@ Ground oraines - these are deposited "eneath the glacier and result in a

hu''oc(y topography with lots of enclosed s'all "asins.

• End oraines and 'erminal oraines are deposited at the low elevation

end of a glacier as the ice retreats due to a"lation #'elting$ #see figure

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*&.22 in your te=t$.

• !ateral oraines are deposits of till that were deposited along the sides

of 'ountain glaciers.

• edial oraines - hen two valley glaciers 'eet to for' a largerglacier! the roc( de"ris along the sides of "oth glaciers 'erge to for' a

'edial 'oraine #see figures *&.*0 and *&.*& in your te=t$. These "lac(

strea(s in an active glacier! as well as the deposits left "ehind after the ice'elts are called 'edial 'oraines.

o Glacial arine drift - Glaciers that reach the oceans or even la(es! 'ay calve

off into large ice"ergs which then float on the water surface until they 'elt.

Jpon 'elting! the roc( de"ris that they contain "eco'es i''ediately deposited

on the sea floor or la(e"ed as an unsorted chaotic deposit. :o'eti'es singlelarge roc( frag'ents fall out on the floor of the water "ody! and these are called

dropstones.

• :tratified 6rift - Glacial drift can "e pic(ed up and 'oved "y 'eltwater strea's which

can then deposit that 'aterial as stratified drift.

o #utwash Plains - :trea's running off the end of a 'elting glacier are usually

cho(ed with sedi'ent and for' "raided strea's! which deposit poorly sorted

stratified sedi'ent in an outwash plain. These deposits are often referred to asoutwash.

o #utwash 'erraces - If the outwash strea's cut down into their outwash deposits!

the "an(s fro' river terraces called outwash terraces.

o 8ettle !akes - If depressions for' underneath a glacier and re'ain after the

glacier is 'elted then water filling these depressions "eco'e s'all la(es where

fine-grained sedi'ent is deposited. The state of innesota is called the land of athousand la(es! 'ost of which are (ettle la(es.

o 8ames and 8ame 'erraces. :trea's and la(es for'ing on top of stagnant ice

'ay deposit stratified sedi'ent on top of the glacier. hen the glacier 'elts

these deposits are set down on the ground surface. The for'er la(e deposits "eco'e (a'es! and the for'er strea' deposits "eco'e (a'e terraces #see figure

*&.2+ in your te=t$.

o Eskers - Es(ers are long sinuous ridges of sedi'ent deposited "y strea's than

ran under or within a glacier. The sedi'ent deposited "y these strea's "eco'es

an es(er after the ice has 'elted.

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Glacial (ges

The last glaciation ended a"out *0!000 years ago. 1ut the period "etween *0!000 years ago and

3 'y ago #<leistocene epoch$ was a ti'e of 'any glacial and interglacial ages. 6uring this period sea level fluctuated "ecause@

• during glaciations the continental land 'asses were depressed "y weight of ice.

• during glacial periods 'uch sea water was tied up in glaciers so sea level was lower.

• during interglacial periods sea level was higher due to 'elting of the ice.

• during interglacial periods land that were covered with ice during a glaciation are

uplifted due to re'oval of the weight of the ice.

1ased on evidence fro' glacial deposits and glacial erosion features geologists have "een a"le

to docu'ent at least % glaciations during the <leistocene. 1ut recent studies of deep-seasedi'ents and dating of these deposits suggest that there were at least 30 glaciations that

occurred during the <leistocene. This evidence co'es fro' studies of fossils found in deep-seasedi'ent cores! and what they tell us a"out ocean surface te'peratures in the past. The results

co'e fro' studies of the isotopes of o=ygen.

• ,=ygen has two 'aor isotopes! *B,! which is considered heavy! and *&,! which is

considered light. 1oth of these isotopes are sta"le and non-radiogenic! so their ratio isconstant through ti'e.

• 1ecause *&, is lighter! it is preferentially evaporated with sea water fro' the oceans! and

thus gets concentrated in the water that eventually falls on the continents as rain or snow.1ecause of this! *B, gets concentrated in ocean water.

• 6uring constant cli'atic conditions the *&, lost to evaporation returns to the oceans "y

rain and strea's! so that the ratio of *B, to *&, #*B, 4 *&,$ is constant.

• 1ut! during a glaciation! so'e of the *&, gets tied up in glacial ice and does not return to

the oceans. Thus during glaciations the *B, 4 *&, ratio of sea water increases.

• 6uring an interglaciation! on the other hand! the *&, that was tied up in glacial ice

returns to the oceans causing a decrease in the *B, 4 *&, ratio of seawater.

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Thus! we e=pect that during glaciations the *B, 4 *&, ratioin seawater will "e high! and during interglaciations the*B, 4 *&, ratio in seawater will "e low.

:ince organis's that live in the oceans e=tract ,=ygenfro' seawater to for' their car"onate #9,3

-2$ shells!

'easuring the *B, 4 *&, ratio in the shells of dead

organis's gives a record of past ocean te'peratures. The

record for the past two 'illion years is shown here and infigure *&.30 on page %3% of your te=t. This suggests a"out

30 glaciations separated "y interglaciations during the

past 2 'illion years.

6uring the last * 'illion years it appears that each glacial - interglacial cycle has lasted a"out*00!000 years! "ut earlier cycles were a"out %0!000 years long.

,ther periods of glaciation are (nown fro' the geologic record! 'ainly fro' preserved glacial

striations and tillites #consolidated till$. The earliest recogni8ed glaciation occurred a"out 2.3

"illion years ago! "ut at least +0 other glaciations are recogni8ed to have occurred during the<aleo8oic era.

"auses of Glacial (ges

In order to understand what causes these cycles of glacial - interglacial episodes we need a

'uch "etter understanding of what causes glo"al cli'ate changes. 1ecause hu'an history is soshort co'pared to the ti'e scales on which glo"al cli'ate change occurs! we do not co'pletely

understand the causes. owever! we can suggest a few reasons why cli'ates fluctuate.

• ?ong ter' variations in cli'ate #tens of 'illions of years$ on a single continent are

li(ely caused "y drifting continents. If a continent drifts toward the euator! the cli'atewill "eco'e war'er. If the continent drifts toward the poles! glaciations can occur on

that continent.

• :hort-ter' variations in cli'ate are li(ely controlled "y the a'ount of solar radiation

reaching the Earth. A'ong these are astrono'ical factors and at'ospheric factors.

o Astrono'ical >actors -

Lariation in the eccentricity of the Earths or"it around the sun has

periods of a"out %00!0000 years and *00!000 years. Lariation in the tilt of the Earths a=is has a period of a"out %*!000 years.

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Lariation in the way the Earth wo""les on its a=is! called precession! has

a period of a"out 23!000 years.

The co'"ined effects of these astrono'ical variations results in

periodicities si'ilar to those o"served for glacial - interglacial cycles.

o At'ospheric >actors- the co'position of the Earths at'osphere can "e gleanedfro' air "u""les trapped in ice in the polar ice sheets. :tudying drill core

sa'ples of such glacial ice and their contained air "u""les reveals the following@

6uring past glaciations! the a'ount of 9,2 and 'ethane! "oth

greenhouse gasses that tend to cause glo"al war'ing! were lower than

during interglacial episodes.

6uring past glaciations! the a'ount of dust in the at'osphere was higher

than during interglacial periods! thus 'ore heat was li(ely reflected fro'the Earths at'osphere "ac( into space.

The pro"le' in unraveling what this 'eans co'es fro' not "eing a"le to

understand if low greenhouse gas concentration and high dust content in

the at'osphere caused the ice ages or if these conditions were caused "ythe ice ages.

o 9hanges in ,ceanic 9irculation - s'all changes in ocean circulation can a'plify

s'all changes in te'perature variation produced "y astrono'ical factors.

o

,ther factors The energy output fro' the sun 'ay fluctuate.

?arge e=plosive volcanic eruptions can add significant uantities of dust

to the at'osphere reflecting solar radiation and resulting in glo"al

cooling.

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Tulane University

Physical Geology

!lo&al "ectonics

This page last updated on *-ov-2003

<late Tectonics is a theory developed in the late *&0s! to e=plain how the outer layers of the

Earth 'ove and defor'. The theory has caused a revolution in the way we thin( a"out theEarth. :ince the develop'ent of the theory! geologists have had to ree=a'ine al'ost every

aspect of Geology. <late tectonics has proven to "e so useful that it can predict geologic events

and e=plain al'ost all aspects of what we see on the Earth. Although we have discussed platetectonics throughout the course! in this lecture we loo( at how the theory ca'e to "e discovered

and so'e of its i'plications for the evolution of the Earth.

ectonic heories

Tectonic theories atte'pt to e=plain why 'ountains! earthua(es! and volcanoes occur where

they do! the ages of defor'ational events! and the ages and shapes of continents and ocean "asins.

• ?ate *th 9entury Theories

o 9ontraction of the Earth due to cooling. This is analogous to what happens to the

s(in of an apple as the interior shrin(s as it dehydrates. It could e=plainco'pressional features! li(e fold4thrust 'ountain "elts! "ut could not e=plain

e=tensional features! such as rift valleys and ocean "asins. or could it e=plain

the shapes and positions of the continents.

o E=pansion of the Earth due to heating. This was suggested after radioactivity was

discovered. This could e=plain why the continents are "ro(en up! and could

easily e=plain e=tensional features! "ut did not do well at e=plaining

co'pressional features.

• egners Theory of 9ontinental 6rift

Alfred egner was a Ger'an eteorologist in the early *00s who studied ancient

cli'ates. ?i(e 'ost people! the igsaw pu88le appearance of the Atlantic continental

'argins caught his attention. e put together the evidence of ancient glaciations and thedistri"ution of fossil to for'ulate a theory that the continents have 'oved over the

surface of the Earth! so'eti'es for'ing large supercontinents and other ti'es for'ing

separate continental 'asses. e proposed that prior to a"out 200 'illion years ago all of

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the continents for'ed one large land 'ass that he called <angea #see figure 20.* in your

te=t$.

The wea(ness of egners theory! and the reason it was not readily accepted "y

geologists was that he proposed that the continents slide over ocean floor. Geophysicistsdisagreed! stating the ocean floor did not have enough strength to hold the continents

and too 'uch frictional resistance would "e encountered.

In *+0s and *&0s! studies of the Earths 'agnetic field and how it varied through ti'e# paleomagnetism2 provided new evidence that would prove that the continents do

indeed drift. In order to understand these develop'ents! we 'ust first discuss the Earths

'agnetic field and the study of <aleo'agnetis'.

he Earth3s Magnetic %ield and Paleomagnetism

The Earth has a 'agneticfield that causes a co'pass

needle to always point

toward the orth 'agnetic pole! currently located near

the rotation pole. The Earths

'agnetic field is what would "e e=pected if there were a

large "ar 'agnet located at

the center of the Earth #we

now (now that this is not

what causes the 'agneticfield! "ut the analogy is still

good$. The 'agnetic field isco'posed of lines of force as

shown in the diagra' here.

A co'pass needle or a 'agnetic weight suspended fro' a string! points along these lines of

force. ote that the lines of force intersect the surface of the Earth at various angles that dependon position on the Earths surface. This angle is called the magnetic inclination. The inclination

is 0o at the 'agnetic euator and 0o at the 'agnetic poles. Thus! "y 'easuring the inclination

and the angle to the 'agnetic pole! one can tell position on the Earth relative to the 'agnetic poles.

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In the *+0s it wasdiscovered that when

'agnetic 'inerals cool

"elow a te'peraturecalled the Curie

'emperature% do'ainswithin the 'agnetic

'ineral ta(e on anorientation parallel to

any e=ternal 'agnetic

field present at the ti'ethey cooled "elow this

te'perature.

At te'peratures a"ove the 9urie Te'perature! per'anent 'agneti8ation of 'aterials is not

possi"le. :ince the 'agnetic 'inerals ta(e on the orientation of the 'agnetic field presentduring cooling! we can deter'ine the orientation of the 'agnetic field present at the ti'e the

roc( containing the 'ineral cooled "elow the 9urie Te'perature! and thus! "e a"le to deter'ine

the position of the 'agnetic pole at that ti'e. This 'ade possi"le the study of <aleo'agnetis'

#the history of the Earths 'agnetic field$. agnetite is the 'ost co''on 'agnetic 'ineral inthe Earths crust and has a 9urie Te'perature of +B0o9

Initial studies of thehow the position of the

Earths 'agnetic pole

varied with ti'e wereconducted in Europe.

These studies showedthat the 'agnetic polehad apparently 'oved

through ti'e. hen

si'ilar 'easure'ents

were 'ade on roc(s ofvarious ages in orth

A'erica! however! a

different path of the'agnetic pole was

found.

This either suggested that #*$ the Earth has had 'ore than one 'agnetic pole at various ti'es inthe past #not li(ely$! or #2$ that the different continents have 'oved relative to each other over

ti'e. :tudies of ancient pole positions for other continents confir'ed the latter hypothesis! and

see'ed to confir' the theory of 9ontinental 6rift.

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Sea!%loor Spreading

6uring orld ar II! geologists e'ployed "y the 'ilitary carried out studies of the sea floor! a

part of the Earth that had received little scientific study. The purpose of these studies was to

understand the topography of the sea floor to find hiding places for "oth Allied and ene'ysu"'arines. The topographic studies involved 'easuring the depth to the sea floor. These

studies revealed the presence of two i'portant topographic features of the ocean floor@

• #ceanic Ridges - long sinuous ridges that occupy the 'iddle of the Atlantic ,cean and

the eastern part of the <acific ,cean.

• #ceanic 'renches - deep trenches along the 'argins of continents! particularly

surrounding the <acific ,cean.

Another type of study involved towing a 'agneto'eter #for 'easuring 'agnetic 'aterials$

"ehind ships to detect su"'arines. The records fro' the 'agneto'eters! however! revealed thatthere were 'agnetic ano'alies on the sea floor! with 'agnetic high areas running along the

oceanic ridges! and parallel "ands of alternating high and low 'agnetis' on either side of the

oceanic ridges. 1efore these features can "e understood! we need to first discuss another

develop'ent in the field of <aleo'agnetis' - the discovery of reversals of the Earths 'agneticfield and the 'agnetic ti'e scale #covered in 9hapter **$.

• ;eversals of the Earths agnetic >ield. :tudying piles of lava flows on the continents

geophysicists found that over short ti'e scales the Earths 'agnetic field undergoes

polarity reversals #The north 'agnetic pole "eco'es the south 'agnetic pole$ 1y datingthe roc(s using radio'etric dating techniues and correlating the reversals throughout

the world they were a"le to esta"lish the 'agnetic ti'e scale.

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Line! atthews! and orely put thisinfor'ation together with the "ands

of 'agnetic stripes on the sea floor

and postulated that the "andsrepresents oppositely polari8ed roc(s

on either side of the oceanic ridges!

and that new oceanic crust and

lithosphere was created at the oceanicridge "y eruption and intrusion of

'ag'a. As this 'ag'a cooled it too( on the 'agnetis' of the 'agneticfield at the ti'e. hen the polarity of

the field changed new crust and

lithosphere created at the ridge wouldta(e on the different polarity. This

hypothesis led to the theory of sea

floor spreading.

If new oceanic crust and lithosphere is continually "eing created at the oceanic ridges! theoceans should "e e=panding indefinitely! unless there were a 'echanis' to destroy the oceanic

lithosphere. 1enioff 8ones and the oceanic trenches provided the answer@ ,ceanic lithosphere

returns to the 'antle "y sliding downward at the oceanic trenches #su"ducting$. 1ecauseoceanic lithosphere is cold and "rittle! it fractures as it descends "ac( into the 'antle. As it

fractures it produces earthua(es that get progressively deeper.

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Plate ectonics

1y co'"ining the sea floor

spreading theory with continentaldrift and infor'ation on glo"al

seis'icity! the new theory of

<late Tectonics "eca'e acoherent theory to e=plain crustal

'ove'ents.

<lates are co'posed of

lithosphere! a"out *00 (' thic(!that OfloatO on the ductile

asthenosphere.

hile the continents do indeed appear to drift! they do so only "ecause they are part of larger plates that float and 'ove hori8ontally on the upper 'antle asthenosphere. The plates "ehave as

rigid "odies with so'e a"ility to fle=! "ut defor'ation occurs 'ainly along the "oundaries

"etween plates.

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ypes of Plate &oundaries

• $i1ergent Plate &oundaries

These are oceanic ridges

where new oceanic lithosphere

is created "y upwelling 'antlethat 'elts! resulting in "asaltic

'ag'as which intrude and erupt

at the oceanic ridge to create

new oceanic lithosphere andcrust. As new oceanic

lithosphere is created! it is

pushed aside in oppositedirections. Thus! the age of the

oceanic crust "eco'es

progressively older in "othdirections away fro' the ridge.

• 1ecause oceanic lithosphere 'ay get su"ducted! the age of the ocean "asins is

relatively young. The oldest oceanic crust occurs farthest away fro' a ridge. Inthe Atlantic ,cean! the oldest oceanic crust occurs ne=t to the orth A'erican

and African continents and is a"out *B0 'illion years old #Surassic$ #see figure20.+ in your te=t$. In the <acific ,cean! the oldest crust is also Surassic in age!

and occurs off the coast of Sapan.

• 1ecause the oceanic ridges are areas of young crust! there is very little sedi'ent

accu'ulation on the ridges. :edi'ent thic(ness increases in "oth directions away

of the ridge! and is thic(est where the oceanic crust is the oldest.

o Fnowing the age of the crust and the distance fro' the ridge! the relative velocity of the

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plates can "e deter'ined. #A"solute velocity reuires further infor'ation to "e discussed

later$.

o ;elative plate velocities vary "oth for individual plates and for different plates.

• Lariations in individual plate velocities occur "ecause spreading of the sea floorta(es place on a spherical surface rather than on a flat surface. Lelocities are

greatest at large distances away fro' the spreading pole #see figures 20.7 K 20.Bin your te=t$.

• 6ifferent plates have different velocities depending on the a'ount of continental

lithosphere within the plate. <lates with continental lithosphere have lowerrelative velocities than plates with only oceanic lithosphere.

• :ea floor topography is controlled "y the age of the oceanic lithosphere and the

rate of spreading.

If the spreading rate #relative velocity$ is high! 'ag'a 'ust "e rising rapidly and

the lithosphere is relatively hot "eneath the ridge. Thus for fast spreading centersthe ridge stands at higher elevations than for slow spreading centers. The rift

valley at fast spreading centers is narrower than at slow spreading centers.

As oceanic lithosphere 'oves away fro' the ridge! it cools and sin(s deeper into

the asthenosphere. Thus! the depth to the sea floor increases with increasing age

away fro' the ridge.

• "on1ergent Plate &oundaries

o hen a plate of dense oceanic lithosphere 'oving in one direction collides with

a plate 'oving in the opposite direction! one of the plates su"ducts "eneath theother. here this occurs an oceanic trench for's on the sea floor and the sin(ing

plate "eco'es a su"duction 8one. The 1enioff 8one identifies a su"duction 8one.

The earthua(es 'ay e=tend down to depths of 700 (' "efore the su"ducting

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plate heats up and loses its a"ility to defor' in a "rittle fashion.

o As the oceanic plate su"ducts! it "egins to heat up and 'eta'orphose. As it does

so! dehydration reactions release water into the overlying 'antle asthenosphere!causing a reduction in the 'elting te'perature and the production of andesitic

'ag'as. These 'ag'as rise to the surface and create a volcanic arc parallel tothe trench.

If the su"duction occurs

"eneath oceanic lithosphere! anisland arc is produced at the

surface #such as the Sapanese

islands! the Aleutian Islands! the<hilippine islands! or the

9ari""ean islands

If the su"duction occurs

"eneath continental crust! a

continental volcanic arc is produced #such as the 9ascades

of the western J.:.! or the Andes

'ountains of the :outh A'erica$

o If one of the plates has continental lithosphere on its 'argin! the oceanic plate

will su"duct "ecause oceanic lithosphere has a higher density than continentallithosphere.

:edi'ent deposited along the convergent 'argin! and particularly that in the

trench will "e defor'ed "y thrust faulting. This will "rea( the roc(s up into a

chaotic 'i=ture of "ro(en! u'"led! and thrust faulted roc( (now as a mlange.

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o :ince the 'Wlange is relatively cool! as it is pushed deeper! the pressure increases

and it "eco'es 'eta'orphosed under conditions of high pressure and low

te'perature! resulting in "lueschist facies 'eta'orphis'. 1eneath the 'ag'aticarc! the passage of 'ag'a through the overlying plate results in increased

te'peratures and 'eta'orphis' at high pressures and high te'peratures

producing greenschist and a'phi"olite facies 'eta'orphic roc(s.

• ransform Plate &oundaries

o here lithospheric plates slide past one another in a hori8ontal 'anner! a

transfor' fault is created. Earthua(es along such transfor' faults are shallow

focus earthua(es.

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ost transfor' faults occur whereoceanic ridges are offset on the sea

floor. :uch offset occurs "ecause

spreading ta(es place on the spherical

surface of the Earth! and so'e parts of a plate 'ust "e 'oving at a higher

relative velocity than other parts #see

figure 20.B in your te=t$. ,ne of thelargest such transfor' "oundaries

occurs along the "oundary of the

orth A'erican and <acific plates andis (nown as the :an Andreas >ault.

ere the transfor' fault cuts through

continental lithosphere

Hot Spots and ()solute Plate /elocities

<late velocities deter'ined fro' the rate of sea floor spreading or "y 'a(ing 'easure'ents

across a plate "oundary are only relative velocities. That is we (now the velocity of one plateonly if we can assu'e that the adacent plate is not 'oving.

In order to deter'ine a"solute plate velocities! we need so'e fi=ed reference point that we

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(now is not 'oving.

,ne place where this 'ight "e possi"leis in the <acific ,cean! where theawaiian Islands are part of a chain of

islands! far re'oved fro' any plate

"oundary! where islands and

sea'ounts in the chain increase in agefro' the southeast to the northwest

#see figure 2.20 U9hapter 2D in your

te=t$. >urther'ore! the island at thesoutheast end of the chain! the 1ig

Island of awaii! is the only island

with currently active volcanoes. Theisland chain appears to have for'ed as

the <acific plate 'oved over a Hot

$pot ! an area in the Earths 'antlewhere hot 'aterial fro' the Earths

interior is 'oving upward. If we can

assu'e that such a hot spot is

stationary! then we can calculate thea"solute velocity of the <acific <late as

it has 'oved over the hot spot.

,ther hot spots have "een recogni8ed "eneath the Earths surface "ased on si'ilarreasoning #see figure 2.22 U9hapter 2D in your te=t$. 1y using these hot spots to

deter'ine a"solute velocities! we find that the African <late is al'ost stationary

#e=pected "ecause the African <late is surrounded "y oceanic ridges! and the id-

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Atlantic ;idge is 'oving toward the west. >urther'ore! the Atlantic ,cean is getting

"igger and the <acific ,cean is getting s'aller.

"ause of Plate ectonics

>ro' seis'ic wave velocities we (now that the asthenosphere "ehaves in ductile 'anner! that iseven though it is solid it can flow under stress and "ehave li(e a liuid. If this is the case! then it

can also convect. ;ecall that convection is a 'ode of heat transfer wherein the heat 'oves with

the 'aterial. 9onvection is caused when 'aterial that occurs at a deeper level is heated to the point where it e=pands and "eco'es less dense than the 'aterial a"ove it. hen this occurs! the

hot less dense 'aterial rises. In a confined space! rising hot 'aterial will eventually cool and

"eco'e denser than its surroundings. This cool dense 'aterial 'ust then sin(. This gives rise toconvection cells! with hot rising currents and cool descending currents.

If the asthenosphere is in fact 'oving as a result of convection! then convection could "e the

'echanis' responsi"le for plate tectonics. ot rising currents would occur "eneath oceanic

ridges.

ag'a intruding into the ridge would push lithosphere apart at the ridge. As the new

lithosphere cools! it will slide off the topographic high that results fro' the upwelling of the'antle and will eventually "eco'e cold and dense. This dense lithosphere will tend to pull the

rest of the lithosphere downward. A co'"ination of dragging the lithosphere along the top of the

convection cell! ridge push! sliding! and sla" pull all appear to "e contri"uting factors to thecause of plate tectonics.

Plate ectonics and "ontinental "rust

The continents can "e divided into two (inds of structural units

• Cratons for' the cores of the continents. These are portions of continental crust that

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have attained isostatic and tectonic sta"ility. They were for'ed and were defor'ed very

early in Earth history and are the oldest parts of the continents.

• #rogens are "road elongated "elts of defor'ed roc(s that are draped around the cratons.

They appear to "e the eroded roots of for'er 'ountain "elts that for'ed "y continent -

continent collisions. ,nly the youngest of these orogens still for' 'ountain ranges #see

figure 20.*+ in your te=t$.

The o"servation that the orogens are generally younger towards the outside of any continentsuggests that the continents were "uilt "y collisions of plates that added younger 'aterial to the

outside edges of the continents! and is further evidence that plate tectonics has operated for at

least the last 2 "illion years. 1ecause continents can oin "y collision and can split "y rifting! weneed to e=a'ine the various types of continental 'argins that can occur and see how they relate

to plate "oundaries.

• Passi+e Continental argins - A

passive continental 'argin occurs in theinterior of plate! far away fro' any

plate "oundary. <resent e=a'ples of passive continental 'argins are theAtlantic coast of orth and :outh

A'erica! Europe! and Africa. o

current defor'ation is ta(ing placealong these 'argins "ecause they are

not close to plate "oundaries. The

passive continental 'argins developed

as a result of rifting of a for'er largercontinent #see figure 20.*& in your

te=t$.

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Areas where rifting 'ay "e for'ing future

passive continental 'argins include the ;ed

:ea area of northern Africa! the East African;ift Lalley! and the 1asin and ;ange

<rovince of the estern J.:. 1loc( fault'ountains are co''on in the initial stagesof rifting.

• Continental Con+ergent argins- 9ontinental convergent 'argins occur where the

'argin of the continent coincides with a convergent plate "oundary.

:u"duction of the oceanic lithosphere

"eneath the continental lithosphere

produces continental volcanic arcs that

erupt 'ostly andesitic 'ag'a.:edi'ents along the 'argin are

defor'ed into 'Wlanges and a pair of'eta'orphic "elts develops "eneath the

continental 'argin! the one closest to

the plate 'argin shows "lueschist facies

'eta'orphis' and the one "eneath thevolcanic arc shows greenschist and

a'phi"olite facies 'eta'orphis'.

hen such paired 'eta'orphic "elts are o"served in ancient roc(s! this provides

evidence that the area was once a convergent continental 'argin. The "est e=a'ple of acurrent convergent continental 'argin occurs along the <acific coast of :outh A'erica

and in the 9ascade ountains of the western J.:.

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• Continental Collision argins - hen

two plates that have continental

lithosphere collide with one another the'argin "eco'es a continental collision

'argin. :uch 'argins are characteri8ed

"y fold - thrust 'ountain "elts thatdevelop along the 8one of collision #see

figure 20.20 in your te=t$. 9urrently the

highest 'ountains in the world! the

i'alayas represent this (ind of 'argin.

The i'alayas resulted fro' a collision of the plate containing India with the plate

containing Eurasia. This collision is still ta(ing place and results in oining the two

for'erly separate plates. The occurrence of ancient fold -thrust 'ountain "elts such as

the Appalachian ountains of the Eastern J.:.! the Jrals of 9entral ;ussia! and theAlps of southern Europe! are evidence of ancient continental collision 'argins.

• 'ransform 3ault argins - hen acontinental 'argin coincides with a transfor'fault plate "oundary the continental 'argin is

called a transfor' fault 'argin. As discussed

"efore! the "oundary "etween the <acific <lateand the orth A'erican <late in the western

J.:. is such a plate "oundary for'ed "y the

:an Andreas >ault. The :an Andreas >ault

developed "eginning in the eso8oic when aridge separating the two plates was su"ducted

#see figure 20.2* in your te=t$.

• "ccreted 'errane argins - A for'er convergent 'argin or transfor' fault 'argin that

has "een 'odified "y nu'erous additions of s'all "loc(s of crust that have accreted to

the continental 'argin is called an accreted terrane 'argin. A terrane is a pac(age ofroc(s with a distinctive stratigraphic and structural history that appears to have for'ed

at so'e location other than its present location. The northwestern 'argin of orth

A'erica is co'posed of 'any such terranes that appear to have "een added to thecontinent. An accreted terrane 'argin occurs close to a plate "oundary "ecause it oncewas a plate "oundary of the convergent or transfor' type.


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Wind Erosion

ind can "e effective agent of erosion anywhere that it is strong enough to act. ind can erode

"y deflation and a&rasion.• .eflation is the lowering of the land surface due to re'oval of fine-grained particles "y

the wind. 6eflation concentrates the coarser grained particles at the surface! eventually

resulting in a surface co'posed only of the coarser grained frag'ents that cannot "e

transported "y the wind. :uch a surface is called desert pa+ement .

• ,entifacts are any "edroc(

surface or stone that has "eena"raded or shaped "y wind-

"lown sedi'ent in a process

si'ilar to sand "lasting.

• ardangs are strea'lined wind-

eroded ridges co''only found in

deserts.

Wind $eposits

ind can deposit sedi'ent when its velocity decreases to the point where the particles can no

longer "e transported. This can happen when topographic "arriers slow the wind velocity on the

downwind side of the "arrier. As the air 'oves over the top of the "arrier! strea'lines convergeand the velocity increases.

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After passing over the

"arrier! the strea'linesdiverge and the velocity

decreases. As the

velocity decreases!

so'e of the sedi'ent insuspension can no

longer "e held insuspension! and thus

drops out to for' a

deposit.

Topographic "arriers can "e such things as roc(s! vegetation! and hu'an 'ade structures that

protrude a"ove the land surface.

• $and .unes ! :and dunes for' when there is #*$ a ready supply of sand! #2$ a steady

wind! and #3$ so'e (ind of o"stacle such as vegetation! roc(s! or fences! to trap so'e ofthe sand. :and dunes for' when 'oving air slows down on the downwind side of ano"stacle. The sand grains drop out and for' a 'ound that "eco'es a dune.

o :and dunes are asy''etrical 'ounds with a gentle slope in the upwind direction

and steep slope called a slip face on the downwind side. 6unes 'igrate "y

erosion of sand "y wind #saltation$ on the gentle upwind slope! and depositionand sliding on the slip face! and thus are cross-"edded deposits.

o 6unes 'ay cover large areas and reach heights up to +00'.

o Types of sand dunes #ote@ 'ost of this 'aterial will "e covered on slides in

lecture$@

o 0archan .unes - are crescent-shaped dunes with the points of the

crescents pointing in the downwind direction! and a curved slip

face on the downwind side of the dune. They for' in areas wherethere is a hard ground surface! a 'oderate supply of sand! and a

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constant wind direction.

o 'rans+erse dunes - are large fields of dunes that rese'"le sand

ripples on a large scale. They consist of ridges of sand with asteep face in the downwind side! and for' in areas where there is

a"undant supply of sand and a constant wind direction. 1archan

dunes 'erge into transverse dunes if the supply of sand increases.

!inear .unes - are long straight dunes that for' in areas with a

li'ited sand supply and converging wind directions.

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• Para&olic #also called &lowout $ .unes - are OJO shaped dunes

with an open end facing upwind. They are usually sta"ili8ed "yvegetation! and occur where there is a"undant vegetation! a

constant wind direction! and an a"undant sand supply. They are

co''on in coastal areas.

$tar .unes - are dunes with several ar's and varia"le slip face

directions that for' in areas where there is a"undant sand and

varia"le wind directions.

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• ind 1lown 6ust - 6ust consists of silt and clay si8ed particles that are often pac(ed

together with s'ooth surface. :uch pac(ed dust is difficult to re'ove "y wind erosion

alone! unless the surface is very dry or is distur"ed. hen dust it is distur"ed! duststor's 'ay develop! and dust 'ay "e transported "y the wind over large distances. ost

soil contains so'e silt and clay particles deposited "y the wind.

A large deposits of wind deposited dust is called loess. uch loess was derived fro'

de"ris left "y glacial erosion.

• 6ust in ,cean :edi'ents and Glacial Ice. - 6ust can "e transported "y the wind and "y

glacial ice onto the surface of the oceans. As a result! 'uch of the fine grained continent-

derived sedi'ent that reaches the a"yssal plains of the oceans was originally transported

"y winds or ice"ergs.

• Lolcanic Ash - 6uring e=plosive volcanic eruptions! large uantities of dust-si8ed tephra

can "e eected into the at'osphere. If eected high enough! such ash can "eco'e

suspended in the wind and carried for long distances. Eventually it will settle out to "eco'e wind-deposited sedi'ent.

$eserts

6eserts are areas where rainfall is less than 2+0 '' #*0 in.$4year! or where evaporation e=ceeds

precipitation. Thus! deserts are areas that we thin( of as arid.

0rigin of $eserts

6eserts originate "y several different 'echanis's that result in several different types ofdeserts.

• $u&tropical .eserts - the general at'ospheric circulation "rings dry! su"tropical air into

'id-latitudes. E=a'ples@ :ahara of orthern Africa! Falhari of :outhern Africa! and the

Great Australian 6esert.

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• Continental .eserts - Areas in the continental interiors! far fro' source of 'oisture

where hot su''ers and cold winters prevail. E=a'ples@ Go"i! Ta(la a(an

• Rainshadow .eserts - Areas where 'ountainous regions cause air to rise and condense!

dropping its 'oisture as it passes over the 'ountains. E=a'ples@ 6eserts east of the

:ierra evada ountains! 9alifornia K evada! East of the 9ascades of ,regon and

ashington! and East of the Andes ountains in :outh A'erica.

• Coastal .eserts - Areas where cold upwelling seawater cools the air and decreases its

a"ility to hold 'oisture. E=a'ples @ Ataca'a 6esert of coastal <eru! a'i" 6esert of

coastal :outh Africa.• Polar .eserts - 9old polar regions where cold dry air prevails and 'oisture availa"le

re'ains fro8en throughout the entire year. E=a'ples@ orthern Greenland! and ice-freeareas of Antarctica.

e will concentrate on the first four types of deserts! the ones which occur in hot arid cli'ates.

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Surface Processes in $eserts

The sa'e geologic processes operate in deserts as in other 'ore hu'id cli'ates. The difference

is the intensity to which the processes act.

• eathering and ass astingo 6eserts have little soil "ecause 'oisture is so low and the rate of che'ical

weathering is slow.

o ?ittle plant life "ecause of lac( of soils and water. <lants tend to hold soil and

fine-grained roc( frag'ents in place.

o The desert surface is do'inated "y 'echanical weathering processes. If we

co'pare the surface features of deserts with those in hu'id regions! we find that@

• deserts are do'inated "y roc( falls! roc( slides! and the accu'ulation of

coarse grained 'aterial! and generally have steeper slopes.

• hu'id regions have soil and fine-grained regolith covering slopes! with

creep "eing the do'inant 'ass-wasting process! resulting in curved

gentle slopes.

• :trea's and >luvial ?andfor's #ote@ these features will "e shown as slides in class$

:urface waters are rare in deserts. :trea's that do flow in deserts usually originate at

higher elevations and supply enough water for the strea' to pass through the desert

region. :trea's in deserts tend to "e inter'ittent! that is they flow only during rains. >or

this reason! flash floods and "raided strea's are co''on.

o Alluvial >ans and 1aadas - An alluvial fan for's where a 'ountain strea'

enters a "road flat valley and deposits sedi'ent as its velocity decreases on

entering the flatter valley #see chapter $. hen a linear 'ountain range hasseveral closely spaced valleys! the alluvial fans 'ay coalesce to for' a gentle

undulated slope on the sides of the "ounding lowlands. :uch coalesced alluvial

fans are (nown as 0aadas.

o Pediments - A pedi'ent is "road "edroc( surface with a gentle slope away fro'

highlands. ith distance away fro' the highlands the pedi'ent passes "eneath a

thin cover of alluvial sedi'ent derived fro' erosion of the pedi'ent. Thehighlands re'ain as residual hills as the pedi'ent 'atures.

o Playa !akes - :tanding "odies of water li(e la(es are rare in desert regions

"ecause rainfall and input fro' strea's occurs only inter'ittently. ?a(es that do

for' during the rare periods of rainfall! uic(ly evaporate! leaving a dry la(e "ed "ehind. <laya ?a(es #also called dry la(es$ are for'ed in "asins of internal

drainage. The la(e "eds often consist of salts #evaporites$ that were carried in "y

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strea's and precipitated during evaporation. These precipitated salts give the dry

la(e "ed a white color rese'"ling a "each #playa 'eans "each in :panish$.

o Insel&ergs - The word insel"erg 'eans island 'ountain in Ger'an. Insel"ergs

are steep sided hills that rise a"ove a surrounding relatively flat plain. They

appear to for' "ecause the roc( 'a(ing up the insel"erg is 'ore resistant toerosion than the roc(s that once 'ade up the surrounding plain. ,nce aninsel"erg for's! it sheds water due to its steep slopes! and its steep slopes tend to

not develop soil. The surrounding less resistant roc( collects this water and is

su"ected to 'ore rapid rates of che'ical weathering. Thus as the surrounding plain is reduced "y strea' erosion and weathering faster than the 'ore resistant

roc(. Insel"ergs are co''on in desert regions! although they can also occur in

other areas where differential erosion ta(es place.

$esertification

6esertification occurs as a result of cli'atic changes! such as changing positions of the

continents! or changes in ocean and air circulation patterns. u'an i'pacts! such asovergra8ing! draining of land! and lowering of the groundwater ta"le! can also contri"ute to

desertification. As vegetation dies out! the soil is 'ore easily eroded and 'ay "e lost so thatother vegetation "eco'es desta"ili8ed. :ince soil can hold 'oisture! if the soil erodes! the area

'ay "eco'e arid! and the desert e=pands.

Prof. Stephen A. Nelson

EENS 111


(ur Changing Planet

This page last updated on *2-ov-2003

e started this course "y stating that the Earth is a co'ple= syste'. 9hanges that ta(e place in

one part of the syste' have effects on other parts. The tectonic syste' is driven "y the heat inthe Earth. This drives the roc( cycle! which is also affected "y the at'osphere and "iosphere.

The at'osphere is in che'ical euili"riu' with the oceans and e=changes 'atter with the

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"iosphere. All process act on a variety of ti'e scales fro' hundreds of 'illions of years to

'icroseconds. e as hu'an "eings are only now reali8ing that! as part of the "iosphere! we

have an effect on the Earth. hat the effect is! we are only now "eginning to understand. erewe try to put this in perspective.

,ne of the reasons life e=ists on Earth is that the surface has a controlled te'perature in the

range "etween the free8ing and "oiling points of water. The Earth is the only planet in the solar

syste' where this is true. <art of the reason for this results fro' the distance fro' the :un. 1ut!the reason that te'perature re'ains fi=ed is controlled "y the at'osphere.

Solar adiation and the (tmosphere

;adiation reaching the Earth fro' the :un is electro'agnetic radiation. Electro'agneticradiation can "e divided into different regions depending on wavelength. ote that visi"le light

is the part of the electro'agnetic spectru' to which hu'an eyes are sensitive

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• Earth receives all wavelengths of solar radiation. 1ut certain gases and other

conta'inants in the at'osphere have different effects on different wavelengths ofradiation.

• 6ry air is co'posed of a"out 7) itrogen! 20) o=ygen! and *) Argon. It also

contains water! %) at saturation! "ut saturation depends on te'perature. In addition trace

gases have an effect. A'ong the trace gases are@o #(one 1#< 2

,8one is produced in the upper at'osphere #30 - 3+ (' a"ove surface$ "y

inco'ing ultraviolet radiation. Jltraviolet radiation causes ,2 to go to , ,.

:o'e of the , then reco'"ines with ,2 to 'a(e ,3. This o8one then a"sor"s'ore ultraviolet radiation and "rea(s down to ,2 ,. This , can then can

reco'"ine with ,2 to 'a(e 'ore ,8one. The process is self regulating and

results in less ultraviolet radiation reaching the Earths surface.

Jltraviolet radiation is har'ful to organis's "ecause it is high energy radiation

that da'ages cells. In hu'ans! e=cessive e=posure to ultraviolet light causessun"urns and s(in cancer.

'he Effect of Chlorofluorocar&ons 1C3Cs2 in the atmosphere

9>9s are produced to 'a(e refrigerants and styrofoa'. 9hlorine fro' thesehu'an 'ade products enters the at'osphere and cataly8es the "rea(down of

o8one. 9l co'"ines with o8one to 'a(e 9l, and ,2. Jltraviolet radiation then

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causes 9l, to react with , to 'a(e 9l and ,2. This 9l can then react with

,8one! and the process repeats. It is esti'ated that for every 9l 'olecule in

at'osphere! *00!000 o8one 'olecules can "e destroyed. It has "een o"servedthat the protecting o8one layer in the upper at'osphere has deteriorated over the

last +0 years! a result thought to "e produced "y hu'an introduction of 9>9sinto the at'osphere.

o Greenhouse Gases

Energy co'ing fro' the :un is carried "y electro'agnetic radiation. :o'e of

this radiation is reflected "ac( into space "y clouds and dust in the at'osphere.

The rest reaches the surface of the Earth! where again it is reflected "y water andice or a"sor"ed "y the at'osphere. Greenhouse gases in the at'osphere a"sor"

so'e of the longer wavelength #infrared$ radiation and (eep so'e of it in the

at'osphere. This (eeps the at'ospheric te'perature relatively sta"le so long asthe concentration of greenhouse gases re'ains relatively sta"le! and thus! the

greenhouse gases are necessary for life to e=ist on Earth )

The 'ost i'portant green house gases are 2, #water vapor$! 9,2 #9ar"on

6io=ide$! 9% #'ethane$! and ,8one. 2, is the 'ost a"undant greenhouse gas! "ut its concentration in the at'osphere varies with te'perature. Lenus! which

has 'ostly 9,2 in its at'osphere! has te'perature of a"out +00o9 #also partly

due to nearness to :un$.

• Lolcanic Effects

Lolcanoes produce several things that result in changing at'osphere and at'osphericte'peratures.

*. 9,2 produced "y volcanoes adds to the greenhouse gases and 'ay result in

war'ing of the at'osphere.

2. :ulfur gases produced "y volcanoes reflect low wavelength radiation "ac( into

space! and thus result in cooling of the at'osphere.3. 6ust particles inected into the at'osphere "y volcanoes reflect low wavelength

radiation "ac( into space! and thus can result in cooling of the at'osphere.

%. 9hlorine gases produced "y volcanoes can contri"ute to o8one depletion in the

upper at'osphere.

The t. <inatu"o eruption in ** and El 9hichXn eruption in *B* released largeuantities of dust and sulfur gases - resulted in short ter' cooling of at'osphere.

Lolcanis' in the 'iddle 9retaceous produced large uantities of "asalt on the seafloor

and released large a'ounts of 9,2. The 'iddle 9retaceous was 'uch war'er than present! resulting in 'uch higher sea level.

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"ar)on $ioxide in the (tmosphere

The 9,2 concentration in the at'osphere has "een increasing since the 'id *B00s. The increase

correlates well with "urning of fossil fuels. Thus! hu'ans appear to have an effect.

ethane concentration in the at'osphere has also "een increasing. aturally this occurs due to

decay of organic 'atter! the digestive processes of organis's! and lea(s fro' petroleu'reservoirs. u'ans have contri"uted through do'estication of ani'als! increased production of

rice! and lea(s fro' gas pipelines and gasoline.

The Carbon Cycle

In order to understand whether or not humans are having an efect onatmospheric carbon concentrations, we must look at how carbon movesthrough the environment. Carbon is stored in our main reservoirs.

1. In the atmosphere as CO2 gas. From here it echanges with seawateror water in the atmosphere to return to the oceans, or echanges withthe biosphere b! photos!nthesis, where it is etracted rom theatmosphere b! plants. CO2 returns to the atmosphere b! respiration

rom living organisms, rom deca! o dead organisms, romweathering o rocks, rom leakage o petroleum reservoirs, and romburning o ossil uels b! humans.

2. In the h!drosphere "oceans and surace waters# as dissolved CO2.From here it precipitates to orm chemical sedimentar! rocks, or istaken up b! organisms to enter the biosphere. CO2 returns to theh!drosphere b! dissolution o carbonate minerals in rocks and shells,

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will "e greatest at high latitudes #near the poles$ where yearly te'peratures could "e as 'uch as

*&o9 war'er than present.

Again! "ecause of the large nu'"er of uncertainties involved in the co'puter 'odels scientistsare reluctant to rely on the 'odels. :till! there is a consensus that average te'peratures have

increased over the last *00 years! and that if these increases are due to the added input ofgreenhouse gases into the at'osphere! then te'peratures will continue to increase at a rate ofa"out 0.3o9 per decade. This will lead to average te'peratures a"out * degree war'er "y the

year 202+ and a"out 3 degrees war'er "y the year 2*00.

Effects of Glo)al 2arming8

• Glo"al <recipitation changes - A war'er at'osphere will lead to increased evaporation

fro' surface waters and result in higher a'ounts of precipitation. The euatorial regions

will "e wetter than present! while the interior portions of continents will "eco'e war'er

and drier than present.

• 9hanges in vegetation patterns - "ecause rainfall will distri"uted differently! vegetation

will have to adust to the new conditions. id latitude regions are li(ely to "e 'ore

drought prone! while higher latitude regions will "e so'ewhat wetter and war'er thannor'al! resulting in a shift in agricultural patterns.

• Increased stor'iness - A war'er! wetter at'osphere will favor tropical stor'

develop'ent. urricanes will "e stronger and 'ore freuent.

• 9hanges in Ice patterns. - 6ue to higher te'peratures! ice in 'ountain glaciers will 'elt.

1ut! "ecause 'ore water will "e evaporated fro' the oceans! 'ore precipitation will

reach the polar ice sheets causing the' to grow.

• ;eduction of sea ice - :ea ice will "e greatly reduced to the increased te'peratures at

the high latitudes! particularly in the northern he'isphere where there is 'ore a"undant

sea ice. Ice has a high al&edo #reflectivity$! and thus reduction of ice will reduce theal"edo of the Earth and less solar radiation will "e reflected "ac( into space! thus

enhancing the war'ing effect.

• Thawing of fro8en ground - 9urrently 'uch of the ground at high latitudes re'ains

fro8en all year. Increased te'peratures will cause 'uch of this ground to thaw. ,rganic

co'pounds and gas hydrates in the fro8en ground will "e su"ect to decay! releasing

'ore 'ethane into the at'osphere and enhancing the greenhouse effect. Ecosyste's andhu'an structures currently "uilt on fro8en ground will have to adust.

• ;ise of sea level - ar'ing the oceans results in e=pansion of water and thus increases

the volu'e of water in the oceans. Along with 'elting of 'ountain glaciers and

reduction in sea ice! this will cause sea level to rise and flood coastal 8ones! where 'uch

of the worlds population currently resides.

• 9hanges in the hydrologic cycle - ith new patterns of precipitation changes in strea'

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flow and groundwater level will "e e=pected.

• 6eco'position of organic 'atter in soil - ith increasing te'peratures of the

at'osphere the rate of decay of organic 'aterial in soils will "e greatly accelerated. This

will result in release of 9,2 and 'ethane into the at'osphere and enhance the

greenhouse effect.

• 1rea(down of gas hydrates - This is "asically solid water with gas 'olecules li(e

'ethane loc(ed into the crystal structure. They occur in oceanic sedi'ents and "eneath

fro8en ground at the high latitudes. ar'ing of the oceans or war'ing of the soil at

high lattitudes could cause 'elting of the gas hydrates which would release 'ethane

into the at'osphere. :ince 'ethane is a greenhouse gas! this would cause further glo"alwar'ing.

Glo)al Warming in the Past.

>ro' our study of glaciations in the past we (now that cli'ate can change as result of natural

processes! "oth "eco'ing war'er and colder than present. Although these cli'atic fluctuationsappear to "e caused "y eccentricities in the Earths or"it! it is interesting to note that during

glaciations in the past the concentrations of greenhouse gas concentrations in the at'osphere

were lower! at'ospheric dust was higher! and the Earths al"edo was higher! all of these factors

could have contri"uted to cooler cli'ates. :i'ilarly! during past interglacial episodes! theat'osphere contained less dust! higher concentrations of greenhouse gases! and the Earth had a

lower al"edo! all of which contri"ute to war'er cli'ates. The uestions that re'ain to "eanswered are@

• Are there higher concentrations of greenhouse gases and lower dust concentrations in

the at'osphere due to the war'er te'peratures or did they cause the war'er

te'peratures

• Are these differences si'ply due to or"ital variations! or is there so'e other natural self

regulating process that allows for cycles

• ow do hu'an affect these cycles

,ver the past *00 'illion years! geologists have "een a"le to reconstruct 9, 2 concentrations inthe at'osphere and average at'ospheric te'perature "ased on a wide variety of geologic and

geoche'ical evidence. >ro' this reconstruction! it appears that te'perature was 'uch higher

than present during the id-9retaceous! during the Eocene! and during the <liocene. e willne=t loo( at what 'ight have caused these periods of glo"al war'ing.

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Mid!"retaceous

6uring this period we note the

following o"servations@

• The rate of production ofnew oceanic crust "etween

*20 and 0 'illion years

ago #'id 9retaceous$ wasnearly twice the rate prior

to and after that ti'e.

• ?arge volcanic plateaus were e'placed in the ocean "asins. The total volu'e of these

eruptions of "asalt are un(nown! as so'e 'ay have "een su"ducted! "ut 'any are

greater than *0 'illion ('3. #The ,ntong Sava plateau of the southwestern <acific alonehas a volu'e of Z ++ 'illion ('3.

• The ti'e interval during which these volcanic plateaus were e'placed correlate with@

• A long interval of nor'al 'agnetic polarity.

• A pea( in oceanic paleote'peratures.

• A pea( in world-wide for'ation of petroleu'.

• 6eposition of o=ygen depleted sedi'ents li(e "lac( shales.

• A pea( in sea level stands! which "eca'e *00 to 200 ' higher than present.

This infor'ation can "e interpreted in the following 'anner@

• agnetic polarity re'ained constant "ecause a superplu'e originated at outer

core4'antle "oundary ta(ing with it a large a'ount of heat. This resulted inincreasing the Te'perature gradient in the core and thus resulted in vigorous

convection in the core! which then "eca'e resistant to 'agnetic polarity

changes. #9onvection currents in the core are what are thought to cause theEarths 'agnetic field. If the rate of convection is high! then it is 'ore difficult to

change the polarity of the 'agnetic field$.

• 9,2 released fro' the 'ag'as erupted on the ocean floor "y these plu'es

resulted in a super green house effect! causing 'id 9retaceous cli'ates to

increase to *0 to *2o 9 a"ove current average glo"al te'peratures.

• Increased ocean te'peratures resulted in an increase in productivity of 'arine

life which resulted in the for'ation of increased for'ation of petroleu'.

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• Increased glo"al te'peratures resulted in sluggish circulation of ocean water

which resulted in o=ygen depleted waters and the deposition of 9ar"on-rich

"lac( shales. These shales were preserved "ecause shallow seas flooded thecontinents.

• The large volu'e of "asalts erupted on the ocean floor displaced sea water

resulting higher stands of the sea.

This e=a'ple serves to show how events deep within the Earth! #events ta(ing place atthe core - 'antle "oundary$ could have a drastic effect on conditions at the Earths

surface.

Eocene Glo)al Warming

6uring Eocene we note the following@

• >ossils of alligators are found on Els'ere Island at 7Bo orth ?atitude

• Tropical vegetation and tropical 'arine organis' fossils occur up to %+ to ++o orth and

:outh ?atitude! a"out *+o higher than today.

• Esti'ates of at'ospheric 9,2 concentrations show values "etween 2 and & ti'es current

values.

The increased 9,2 concentrations have "een attri"uted to a large scale 'eta'orphic event that

occurred as a result of the continent-continent collision that "egan to uplift the i'alayas! and

other 'eta'orphic events that occurred in the editerranean region and the circu'-<acificregion during the Eocene. :uch 'eta'orphic events! particularly in the upper parts of the

'eta'orphic areas where greenschist 'eta'orphis' would occur! would release large a'ounts

of 9,2 into the at'osphere.

This e=a'ple shows how the roc( cycle itself! aided "y tectonic processes could affectat'ospheric conditions.

opefully this will give you an idea a"out how hu'an "eings can effect the way the Earth

wor(s! and also give you an idea a"out the co'ple=ity of the interactions "etween various parts

of the Earth and processes that occur throughout the Earth.

Jnfortunately! the co'ple=ity of the processes are not co'pletely understood. This has 'aor political i'plications. >or e=a'ple! scientists are uncertain a"out the relia"ility of 'odels that

atte'pt to predict future conditions. This uncertainty is ta(en "y so'e political factions as adenial that an event li(e glo"al war'ing will ta(e place. ost scientists! however! agree that

glo"al war'ing is highly possi"le! "ut they are unwilling to say that it will definitely occur.

<oliticians want! or e=pect you to want! e=act answers. The real uestion! however! is whether

or not we should "e preparing for such events to avoid disaster if it does occur! or! since we cant "e certain! ust wait until the disaster has occurred and we can do nothing a"out it.

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"he (ceans and their Margins

This page last updated on 07-ov-2003

The ,ceans

• 9over a"out 7*) of the surface of the Earth.

• The greatest ocean depth of **!03+ ' occurs in the ariana Trench

• ave an average depth of 3!B00 '.

• ave a present volu'e of a"out *.3+ "illion cu"ic (ilo'eters! "ut the volu'e fluctuates

with the growth and 'elting of glacial ice.

• $alinity! a 'easure of a'ount of dissolved ions in the oceans! ranges "etween 33 and 37

parts per thousand.

o The dissolved ions have "een concentrated in seawater as a result of che'ical

weathering #a! 9a! g! :! F! 1r! and 9,3 $ and degassing of the 'antle "y

volcanic activity #9l K :$.o :eawater would contain higher concentrations of dissolved ions if so'e were not

re'oved "y che'ical precipitation! plants and ani'als! and a"sorption onto clay

'inerals.

o :alinity varies in the oceans #see figure *B.3a in your te=t$ "ecause@

:urface waters evaporate! rain and strea' water is added! and ice for's

or thaws.

:alinity is higher in 'id-latitude oceans "ecause evaporation e=ceeds

precipitation :alinity is higher in restricted areas of the oceans li(e the editerranean

and ;ed :eas #up to %* parts per thousand$. :alinity is lower near the euator "ecause precipitation is higher. :alinity is low near the 'ouths of 'aor rivers "ecause of input of fresh

water.

• The te'perature of surface seawater varies with latitude! fro' near 0o 9 near the poles

to 2o9 near the euator. 1ut restricted areas can have te'peratures up to 37o9. #:eefigure *B.3" in your te=t.$

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• <roperties of seawater also vary with depth.

o The density and salinity of seawater increase with depth.

o Te'perature decreases with depth.

0cean "irculation

:urface ,cean currents are result of drift of the upper +0 to *00 ' of the ocean due to drag "y

wind. Thus! surface ocean currents generally follow the sa'e patterns as at'ospheric

circulation with the e=ception that at'ospheric currents continue over the land surface whileocean currents are deflected "y the land. The surface currents have the following properties@

• 9irculation is cloc(wise in the northern he'isphere and countercloc(wise in the

southern he'isphere.

• In each he'isphere cooler waters fro' higher latitudes circulate toward the euator

where they are war'ed and circulate "ac( toward the poles.

In addition to surface circulation! seawater also circulates vertically as a result ofchanges in density controlled "y changing salinity and te'perature #see figures *B.+!

*B.&a! and *B.&" in your te=t$. :uch circulation! "ecause it controlled "y "oth

te'perature differences and differences in salinity of the water! is called thermohaline

circulation.

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0cean ides

Tides are due to the

gravitational attraction of'oon and to a lesser e=tent!

the sun on the Earth.1ecause the 'oon is closerto the Earth than the sun! it

has a larger effect and

causes the Earth to "ulge

toward the 'oon. At thesa'e ti'e! a "ulge occurs

on the opposite side of the

Earth due to inertial forces#this is not e=plained well in

the "oo(! "ut the

e=planation is "eyond thescope of this course$.

These "ulges re'ain stationary while Earth rotates. The tidal "ulges result in a rhyth'ic rise

and fall of ocean surface! which is not noticea"le to so'eone on a "oat at sea! "ut is 'agnified

along the coasts. Jsually there are two high tides and two low tides each day! and thus avariation in sea level as the tidal "ulge passes through each point on the Earths surface. Along

'ost coasts the range is a"out 2 '! "ut in narrow inlets tidal currents can "e strong and fast and

cause variations in sea level up to *& '.

1ecause the :un also e=erts a

gravitational attraction on the Earth!

there are also 'onthly tidal cycles

that are controlled "y the relative position of the sun and 'oon. The

highest high tides occur when the

:un and the 'oon are on the sa'eside of the Earth #new 'oon$ or on

opposite sides of the Earth #full

'oon$. The lowest high tides occurwhen the :un and the 'oon are not

opposed relative to the Earth

#uarter 'oons$. These highest hightides "eco'e i'portant to coastal

areas during hurricane season andyou always hear dire predications of

what 'ight happen if the stor'surge created "y the hurricane

arrives at the sa'e ti'e as the

highest high tides.

0cean Wa1es

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aves are generated "y winds that "low over the surface of oceans. In a wave! water travels in

loops. 1ut since surface is the area affected! the dia'eter of the loops decreases with depth. The

6ia'eters of loops at the surface is eual to wave height #h$.

avelength #?$ C distance to co'plete one cycle

ave <eriod #<$ C ti'e reuired to co'plete on cycle.

ave Lelocity #L$ C wavelength4wave period #?4<$.

a+e 0ase

otion of waves is only effective at 'oving water to depth eual to one half of the

avelength #?42$. ater deeper than ?42 does not 'ove. Thus! waves cannot erode the

"otto' or 'ove sedi'ent in water deeper than ?42. This depth is called wave "ase. Inthe <acific ,cean! wavelengths up to &00 ' have "een o"served! thus water deeper than

300' will not feel passage of wave. 1ut outer parts of continental shelves average 200

' depth! so considera"le erosion can ta(e place out to the edge of the continental shelf

with such long wavelength waves.

hen waves approach shore! the water depth decreases and the wave will start feeling

"otto'. 1ecause of friction! the wave velocity #C ?4<$ decreases! "ut its period #<$

re'ains the sa'e Thus! the wavelength #?$ will decrease. >urther'ore! as the wave

Ofeels the "otto'O! the circular loops of water 'otion change to elliptical shapes! asloops are defor'ed "y the "otto'. As the wavelength #?$ shortens! the wave height #h$

increases. Eventually the steep front portion of wave cannot support the water as the rear part 'oves over! and the wave "rea(s. This results in tur"ulent water of the surf! where

inco'ing waves 'eet "ac( flowing water.

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;ip currents for' where water is channeled "ac( into the ocean.

• ave Erosion- ;igorous erosion of sea floor ta(es place in surf 8one! i.e. "etween

shoreline and "rea(ers. aves "rea( at depths "etween * and *.+ ti'es wave height.

Thus for &' tall waves! rigorous erosion of sea floor can ta(e place in up to ' of

water.

aves can also erode "y a"rasion and flinging roc( particles against one another oragainst roc(s along the coastline.

• ave refraction- aves generally do not approach shoreline parallel to shore. Instead

so'e parts of waves feel the "otto' "efore other parts! resulting in wave refraction or

"ending.

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ave energy can thus "econcentrated on headlands! to for'

cliffs. eadlands erode faster than "ays "ecause the wave energy getsconcentrated at headlands

9oastal Erosion and :edi'ent Transport

• $ongs!ore urrents - :ince 'ost waves arrive at the shoreline at an angle even after

refraction. :uch waves have a velocity oriented in the direction perpendicular to thewave crests #Lw$! "ut this velocity can "e resolved into a co'ponent perpendicular to the

shore #Lp$ and a co'ponent parallel to the shore #L?$. The co'ponent parallel to the

shore can 'ove sedi'ent and is called the longshore current.

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• 0each drift ! is due to waves approaching at angles to "each! "ut retreating perpendicular to the shore line. This results in the swash of the inco'ing wave 'oving

the sand up the "each in a direction perpendicular to the inco'ing wave crests and the "ac(wash 'oving the sand down the "each perpendicular to the shoreline. Thus! with

successive waves! the sand will 'ove along a 8ig8ag path along the "each.

0ffshore ransport and Sorting

<articles pic(ed up "y wave 'otion 'ove down slope! "ut the deeper the water! the lessenergy is involved in wave 'otion! so s'aller and s'aller particles are 'oved farther

off shore. This results in si8e sorting of sedi'ent! with grain si8e decreasing away fro'coast.

Shaping of "oasts

9oast represents the "oundary "etween sea and land. hen waves hit the coast! they can erode

"y "rea(ing up roc(s into finer particles and a"rading other roc(s "y flinging roc(s! sand and

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water against the'. ,ver ti'e! the effects can "e large. :edi'ent is 'oved and redeposited to

increase the si8e of continental shelves. The effects on the land surface can "e seen "y

e=a'ining the shore profile.

• 1eaches occur where sand is deposited along the shoreline. A "each can "e divided intoa foreshore 8one! which is euivalent to the swash 8one! and "ac(shore 8one! which is

co''only separated fro' the foreshore "y a distinct ridge! called a &erm. 1ehind the

"ac(shore 'ay "e a 8one of cliffs! 'arshes! or sand dunes.

;oc(y 9oasts - here wave

action has not had ti'e to lowerthe coastline to sea level! a roc(y

coast 'ay occur. 1ecause of the

resistance to erosion! a wave cut "ench and wave cut cliff develops.

If su"seuent uplift of the wave-

cut "ench occurs! it 'ay "e

preserved a"ove sea level as amarine terrace.

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The cliff 'ay retreat "y undercutting and resulting 'ass-wasting processes. In areas

where differential erosion ta(es place! the undercutting 'ay initially produces sea caves.If sea caves fro' opposite sides of a roc(y headland 'eet! then a sea arch 'ay for'.

Eventual wea(ening of the sea arch 'ay result in its collapse to for' a sea stack 8

"oastal $eposits and #andforms

9oastlines represent a "alance "etween wave energy and sedi'ent supply. If wave energy and

sedi'ent supply are constant! then a steady state is reached. If anyone of these factors change!

then shoreline will adust. >or e=a'ple! winter stor's 'ay increase wave energy! if sedi'entsupply is constant! fine grained "each sand 'ay "e carried offshore resulting in pe""le "eaches

or co""le "eaches. 6ue to input of sedi'ent fro' rivers! 'arine deltas 'ay for'! due to "each

and longshore drift such features as spits! "ay "arriers! and to'"olos 'ay for'.

6epositional >eatures along coasts.

• .eltas -- 6eltas for' where sedi'ent supply is greater than a"ility of waves to re'ove

sedi'ent. An e=a'ple is the ississippi ;iver 6elta! which is co'posed of several

lo"es that were deposited within the last several thousand years. Erosion of the olderdelta lo"es has ta(en place due to su"sidence! sea level rise! and lac( of new sedi'ent

"eing supplied to the delta "ecause of the hu'an-'ade levee syste'.

• $pits - elongated deposits of sand or gravel that proects fro' the land into open water.

:pits usually for' at the 'outh of a "ay due to long shore current and "each drift.

Generally they curve inward towards the "ay due to refraction of the waves around the

'outh of the "ay.• 0ay 0arriers - if a spit e=tends across a "ay! it is called a "ay "arrier. E=change of water

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"etween the "ay and the ocean is acco'plished through the groundwater syste'.

• 'om&olos - a spit that connects the 'ainland to an offshore island is called a to'"olo.

• 0arrier Islands - A "arrier island is a long narrow ridge of sand ust offshore running

parallel to the coast. :eparating the island and coast is a narrow channel of water called a

lagoon. ost "arrier islands were "uilt during after the last glaciation as a result of sealevel rise. 1arrier islands are constantly changing. They grow parallel to the coast "y

"each drift and longshore drift! and they are eroded "y stor' surges that often cut the'

into s'aller islands.

• Reefs and "tolls - ;eefs consist of colonies of organis's! li(e corals! which secrete

calciu' car"onate. :ince these organis's can only live in war' waters and needsunlight to survive! reefs only for' in shallow tropical seas. In the deeper oceans reefs

can "uild up on the 'argins of volcanic islands! "ut only do so after the volcanoes have

"eco'e e=tinct. After the volcanis' ceases! the volcanic island "egins to erode and also "egins to su"side! due to the weight of newly added 'aterial. As the island su"sides! the

reefs continue to grow upward. Eventually! the original volcanic island su"sides and is

eroded "elow sea level. 1ut! the reefs trap sedi'ent and a circular or annular island!

called an atoll! for's #see figures *B.2+ and *B.2& in your te=t$.

"oastal E1olution

The shape of coast is controlled 'ainly "y tectonic forces and 'eteorological conditions.

E=a'ples@

• The west coast of the J.:. is a rugged coast with 'any sea cliffs and raised wave cut

"enches #'arine terraces$. This is what is called an emergent coastline and in this case

is due to a recent episode of uplift of the land relative to the sea "y tectonic forces. The

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coast of ew England is also an e'ergent case! "ut in the case! the rise of the land

surface has "een due to re'oval of glacial ice which had depressed the land during thelast glaciation. Jpon re'oval of the ice "y 'elting at the end of the last glaciation! the

land was uplifted.

• The rest of the east coast! on the other hand! is a su"'erged coast! and shows

su&merged +alleys! "arrier islands! and gentle shorelines! all due to rise of sea levelsince last glaciation age #during glacial ages! seawater is tied up in ice! and sea level islowerR when the ice 'elts sea level rises$.

"oastal Ha;ards

• :tor's - great stor's such as hurricanes or other winter stor's can cause erosion of the

coastline at 'uch higher rate than nor'al. 6uring such stor's "eaches can erode rapidlyand heavy wave action can cause rapid undercutting and 'ass-wasting events of cliffs

along the coast.

• Tsuna'is - a tsunami is a giant sea wave generated "y an earthua(e. :uch waves travelat speeds up to +0 ('4hr! have wavelengths up to 200 '! and upon approaching ashallow coastline! can have wave heights up to 30 '. :uch waves have great potential to

wipe out cities located along coastlines.

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