What Are the 3 Basic Types of Rocks

7/30/2019 What Are the 3 Basic Types of Rocks

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What are the 3 basic types of rocks?

Just as any person can be put into one of two main categories of human being, allrocks can be put into one of three fundamentally different types of rocks. They are

as follows:

Igneous Rocks

Igneous rocks are crystalline solids which form directly from the cooling ofmagma. This is an exothermic process (it loses heat) and involves aphase

changefrom the liquid to the solid state. The earth is made of igneous rock - atleast at the surface where our planet is exposed to the coldness of space. Igneous

rocks are given names based upon two things: composition (what they are made of)

and texture (how big the crystals are)

How do composition and texture relate to igneous rocks?

Igneous rocks are crystalline solids which cool from magma: theliquid phaseofsolid rock. Magmas occur at depth in the crust, and are said to exist in "magma

chambers," a rather loose term indicating an area where the temperature is great

enough to melt the rock, and the pressure is low enough to allow the material toexpand and exist in the liquid state. Many different types of igneous rocks can be

produced. The key factors to use in determining which rock you have are the rock's

texture and composition.

Texture

Texture relates to how large the individual mineral grains are in the final, solidrock. In most cases, the resulting grain size depends on how quickly the magma

cooled. In general, the slower the cooling, the larger the crystals in the final rock.Because of this, we assume that coarse grained igneous rocks are "intrusive," inthat they cooled at depth in the crust where they were insulated by layers of rock

and sediment. Fine grained rocks are called "extrusive" and are generally produced

through volcanic eruptions.

Grain size can vary greatly, from extremely coarse grained rocks with crystals the

size of your fist, down to glassy material which cooled so quickly that there are no

mineral grains at all. Coarse grain varieties (with mineral grains large enough tosee without a magnifying glass) are called phaneritic. Granite and gabbro are

examples of phaneritic igneous rocks. Fine grained rocks, where the individual

grains are too small to see, are called aphanitic. Basalt is an example. The mostcommon glassy rock is obsidian. Obviously, there are innumerable intermediate

stages to confuse the issue.

Composition
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The other factor is composition: the elements in the magma directly affect whichminerals are formed when the magma cools. Again, we will describe the extremes,

but there are countless intermediate compositions. (Composition relates tothemafic and felsicterms discussed in another question. If these terms are

confusing, pleaserefer to that discussionbefore continuing.)

The composition of igneous magmas is directly related to where the magma is

formed. Magmas associated withcrustal spreadingare generally mafic, andproduce basalt if the magma erupts at the surface, or gabbro if the magma never

makes it out of the magma chamber. It is important to remember that basalt and

gabbro are two different rocks based purely on textural differences - they arecompositionally the same.

Intermediate and felsic magmas are associated withcrustal compression and

subduction. In these areas, mafic seafloor basalt and continental sediments aresubducted back into the crust, where they re-melt. This allows

thedifferentiationprocess to continue, and the resulting magma is enriched in the

lighter elements. Intermediate magmas produce diorite (intrusive) and andesite(extrusive). Felsic magmas, the final purified result of the differentiation process,

lead to the formation of granite (intrusive) or rhyolite (extrusive).

What do the terms mafic and felsic mean?

These are both made up words used to indicate the chemical composition

ofsilicate minerals, magmas, andigneous rocks.

Mafic is used for silicate minerals, magmas, and rocks which are relatively high inthe heavier elements. The term is derived from using the MA from magnesium and

the FIC from the Latin word for iron, but mafic magmas also are relativelyenriched in calcium and sodium. Mafic minerals are usually dark in color and have

relatively highspecific gravities(greater than 3.0).Common rock-forming mafic

mineralsinclude olivine, pyroxene, amphibole, biotite mica, and theplagioclasefeldspars. Mafic magmas are usually produced at spreading centers,

and represent material which is newlydifferentiatedfrom the uppermantle.Common mafic rocks includebasaltand gabbro. (Please note that some geologists

with questionable motives switch the order of the magnesium and iron and come

up with the term "femag." This term is not to be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)

Felsic, on the other hand, is used for silicate minerals, magmas, and rocks whichhave a lower percentage of the heavier elements, and are correspondingly enriched

in the lighter elements, such assilicon and oxygen, aluminum, and potassium. The

term comes from FEL for feldspar (in this case the potassium-rich variety) and

SIC, which indicates the higher percentage of silica. Felsic minerals are usuallylight in color and have specific gravities less than 3.0. Common felsic minerals
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include quartz, muscovite mica, and the orthoclasefeldspars. The most commonfelsic rock isgranite, which represents the purified end product of the earth's

internal differentiation process.

CRUSTAL SPREADING

There are really only two processes: one that forms the physical earth, and anotherthat beats up the surface and tears it apart through weathering and erosion. The

formational process is called tectonics, and is manifested to those of us living on

earth by earthquakes, volcanos, and mountain building in general.

The earth is really just a sphere of liquid rock (magma) which has cooled to

thesolid statewhere exposed to the coldness of space. We call this cold and rigidouter shell thecrust, and it is actually rather thin in comparison to the overall

diameter of our planet. Because of the heat and pressure beneath the surface, thiscrust is constantly being subjected to stresses which break it up.

The earth's crustal sections are called plates, and they vary from small tocontinental in size. Immense forces cause these rigid plates to slowly move about

the surface, where they are constantly running into each other. Tectonic activity is

common at these plate boundaries, of which there arethree basic types:

Spreading centers occur where two plates are

moving away from each other, and deep cracks areopened through the crust. This lengthening of the

crust allows magma from the upper mantle to rise to

the surface and cool, commonly formingbasalt. Anexcellent example is the Mid-Atlantic Ridge. The

crust at these "zones of divergence" is thin and has a high heat flow, so volcanic

activity is persistent and earthquakes are relatively small. Clickherefor moreinformation on zones of extensional tectonics.

Subduction zones are associated with regions

where two plates are moving towards each other,and the crust of the earth is shortened. An exampleis where the western edge of South America meets

the Pacific Ocean. In this case, the collision is

between acontinental plateand an oceanic plate,and a subduction zone forms where the heavier

oceanic basalt is forced beneath the lighter continental materials along a deep

trench. This involves lots of rock being taken back down into the earth, where itcan melt. This leads to a very active volcanic environment. The crust is much

thicker here, and so earthquakes are also stronger. For these and a variety of other
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reasons, some of the most intense earthquake and volcanic activity is associatedwith these zones of compression

Elements & minerals common to various magmas

Ultramafic magmas

Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

Mafic (basaltic) magmas

Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Intermediate magmas

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Quartz - SiO2

Igneous Rock Classification

Texture vs. composition


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Felsic Intermediate Mafic Ultramafic

Aphanitic

fine grainRhyolite Andesite Basalt

Conditions needed to

produce ultramafic

flows do not exist innature at this time.

Intermediate Dacite Diabase

Phaneritic

coarse grainGranite Diorite Gabbro Peridotite

Glassy Obsidian

Frothy Pumice Scoria

It is important to note that there are many, many intermediate steps between these

main divisions. Geology is a science full of "shades of gray," and the naming ofigneous rocks is certainly no exception

hat are the most important types of rock in the crust?

Excluding the rocks between my ears, I'd have to say that basalt and granite have

the honor of being the most important rocks in the crust.

Basalt and granite actually have quite a bit in common. Both are igneous rocks,which means that they cooled from a magma (the earth gets very hot just below the

surface, and there is lots ofliquidrock available). Both are made up of mineralsfrom the silicate group, so both have large amounts ofsilicon and oxygen. Bothwill hurt if you drop a big piece on your toe. But there are several important

differences, too. These differences help define and explain how the earth works.

Granite is great stuff! Not only is it my personal favorite, it is without a doubt themost common rock type on the continental land masses. Yosemite Valley in the
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Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks. Butgranitic "basement rock" can be found just about everywhere east of the Rockies if

you're willing to dig through the dirt andsedimentary rocksat the surface. Graniteis intrusive, which means that the magma was trapped deep in the crust, and

probably tooka very long timeto cool down enough to crystallize into solid rock.This allows the minerals which form plenty of time to grow, and results in

acoarse-textured rockin which individual mineral grains are easily visible.

Granite is the ultimate silicate rock. As discussed elsewhere ingreater detail, on

average oxygen and silicon account for 75% of the earth's crust. The remaining

25% is split among several other elements, with aluminum and potassiumcontributing the most to the formation of the continental granitic rocks. Relatively

small amounts of iron and magnesium occur, but since they have generally

higherdensitiesit's not surprising that there isn't very much in the granite. Due to

the process ofdifferentiation, most of the heavier elements are moving towardsthecoreof the earth, allowing the silicon and oxygen to accumulate on the surface.And accumulate it has. Enough granitic "scum" has differentiated to the surface to

cover 25% to 30% of the earth with the good stuff. We call this purified

materialfelsicbecause of the relatively high percentage of silica and oxygen.

Basalt is extrusive. The magma from which it cools breaks through the crust of the

earth and erupts on the surface. We call these types of events volcanic eruptions,

and there are several main types. Thevolcanoes that make basaltare very common,and tend to form long and persistent zones of rifting in nearly all of the ocean

basins. We now believe that these undersea volcanic areas representhugespreading ridgeswhere the earth's crust is separating. It's a lot like a cut onyour arm, which will bleed until a scab forms. Basaltic magma is like the blood ofthe earth - it's what comes out when the earth's skin is cut the whole way through.

As an eruption ends, the basalt "scab" heals the wound in the crust, and the earthadds some new seafloor crust. Because the magma comes out of the earth (and

often into water) it cools very quickly, and the minerals have very little opportunityto grow. Basalt is commonly veryfine grained, and it is nearly impossible to see

individual minerals without magnification.

Basalt is considered amaficsilicate rock. Among other characteristics, mafic

minerals and rocks are generally dark in color and high inspecific gravity. This is

in large part due to the amount of iron, magnesium, and several other relativelyheavy elements which "contaminate" the silica and oxygen. But this heavy stuff

really isn't happy near the surface, and will take any opportunity it can to head

fordeeper levels. The trick is to heat the basalt back up again so it can melt andgive the iron another shot at the core. It wants to be there, and heat is the key

which unlocks the door.

As it turns out, most of the ocean floor is basalt, and most of the continents aregranite. Basaltic crust is dark and thin and heavy, while granite is light and
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accumulates into continent-sized rafts which bob about like corks in this "sea ofbasalt."When a continent runs into a piece of seafloor, it's much like a Mac truck

running into a Volkswagon. Not very pretty, but at least there's a clear winner. Andthe seafloor basalt ends up in pretty much the same position as does the VW -

under the truck (or continent, as the case may be). This may seem like a drag forthe basalt, but remember that it isn't all that happy on the surface anyway, and this

gives it the heat it needs to re-melt and try to complete the differentiation process

which was so rudely interrupted at the spreading ridge. If successful and allowed tocontinue, what's left behind is a "purified" magma with most of the iron,magnesium, and other heavy elements removed. When it cools, guess what forms?

And thecontinental land massjust got a wee bit larger.

Felsic (granitic) magmas

Potash Feldspar - KAlSi3O8

Quartz - SiO2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

What is Differentiation?

It is my humble opinion that differentiation is one of the primary driving forces ofour planet, so pay attention. To understand the process we have to begin by

agreeing on several assumptions:

1. Different earth materials have differentdensities.

2. Given a chance, all materials will sort themselves by density, with the heaviermaterial sinking and the lighter material rising.

3. Theinterior of the earthis not a "solid" as we understand the term, but rather in

a semi-plastic state which allows ions to migrate (more or less) at will.

4. The earth's interior is zoned by density, with the heaviest material at the center(the core), and the lightest stuff floating about at the surface (the crust).
http://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Plate_Tectonics.html#Convergehttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Plate_Tectonics.html#Convergehttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Plate_Tectonics.html#Convergehttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry12.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry12.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry12.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry7.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry7.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry7.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry7.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry12.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Plate_Tectonics.html#Converge


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Chemical: many of these form when standing water evaporates, leaving dissolvedminerals behind. These are very common in arid lands, where seasonal "playa

lakes" occur in closed depressions. Thick deposits of salt and gypsum can form dueto repeated flooding and evaporation overlong periods of time.

Organic: any accumulation of sedimentary debris caused by organic processes.Many animals use calcium for shells, bones, and teeth. These bits of calcium can

pile up on the seafloor and accumulate into a thick enough layer to form an"organic" sedimentary rock.

Introduction

We've studiedigneous rocks& themineralsof which they are composed

Basement rocks

Most are covered by a thin veneer of debris

Consolidated into a "rock" through slow-acting processes

Usually involving pressure and fluid penetration

Relatively simple to understand

Relatively near-surface processes

As opposed toigneous & metamorphics, which usually occur at depth

Secondary (or derived) rocks

Several main categories

Clastic sedimentary rocks - The classic sedimentary rock

Accumulations of debris derived from the disintegration of pre-existing rocks

DIGRESS TO: Terrigenous sediments

Chemical sedimentary rocks - Chemical precipitates

Usually as the result of the evaporation of water

Ex. Salt (NaCl); Gypsum (CaSO4 2H2O)

Organic sedimentary rocks
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All hydrocarbons

Coal, peat, oil, etc.

The distinction between these three categories can get pretty fuzzy at times

Ex. Limestone, chert

Hard rocks vs. Soft rocks

Origin of Sedimentary Materials

DIGRESS TO: Physical vs. Chemical weathering

Click here for additional information onwater,weathering, anderosion(RCC)

Clickherefor additional information on surface processes (GPHS)

Clasts - derived from physical (and chemical) weathering processes

Smaller solid particles

Derived directly from the source area

Reflect lithology of the source area

Wide range of sizes, from silt to boulders

Chemical processes can result in the relative enrichment of more resistant (or inert)

minerals

Ex.quartzvs.feldspar

Clay minerals

I'm not a clay kind of guy

Extremely complex mineralogy

My understanding is minimal

Easy to get confused by the term

The term "clay" refers to both a size and a mineral family
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A clast can be clay size without being clay

DIGRESS TO: "clay the size" vs. "clay the mineral"

Clay formation forms small, sheet-like minerals (look like the micas)

Lots of different clay minerals

Which mineral is formed reflects primary lithology and environment

Can change to a different mineral if moved to a different environment

Downslope? Downstream?

Near-surface, low temperature environments

Hot and humid works best

Water is a universal solvent (HOH)

Tends to work parallel toBowen's Reaction Series

The higher temperature minerals are more susceptible to chemical weathering

Therefore, especially hard on the mafics and feldspar

To repeat what was mentioned above

Chemical processes can result in the relative enrichment of more resistant (or inert)

minerals

Ex.quartzvs.feldspar

Describe "decomposed granite"

Ions

Chemical weathering also results in "ions" which are "held in solution"

The solution is usually water

Remember:Wateris a universal solvent (HOH) and will play merry hell withanything "over the course of geologic time!"

SeeStrickler's 3rd and 4th Laws of GeoFantasy

Some elements will dissolve and be held in solution
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Ex. salt, sugar

DIGRESS TO: Solution (ions) vs. Suspension (clays)

Both make fundamentally different types of sed. rocks

Common ions include: Ca+2, Na+, CO3-2, Cl-

DIGRESS TO: What do the superscripts mean?

Atomic structure & the role of the electron

These ions are responsible for the "mineral taste" in some water

Therefore, we can tell that iron and sulfur must also be common

If the amount of ions increases relative to the amount of water, minerals can

precipitate

Ex. salt (Na+ + Cl- -> NaCl)

Saturation is the key

An undersaturated solution can become oversaturated in 2 ways

Increase the dissolved ions

Decrease the solvent (water)

This is more common (probably)

Can be initiated by the evaporation process

Organisms can also extract the ions directly from the water

Use them to build shell material

Ex.: Ca+2 + CO3-2 -> CaCO3

Can result in extensive deposition of calcium or silica sediments

Environments of Deposition

(Monroe; fig. 7-3, pg. 202)


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Water plays an important role in most aspects of sedimentary rocks

Fromweatheringanderosiontotransportation and deposition

DIGRESS TO:V=Q/A

Deposition occurs in a wide variety of locations

Basically, any low spot is a potential depositional environment

On bothregional and local level- expand

Three major divisions -Continentaldeposition,marinedeposition,

andtransitional(inter-tidal)

Infinite possible combinations of environments and materials

Results in infinite possible sedimentary rocks

Fortunately, most fall into one of several common environments

And as we already know from our study of igneous rocks, most of the rocks startwith asimilar chemistry

It can still be tough to recognize the depositional environment

DIGRESS TO: This is the ultimate goal of the study of sedimentary rocks

The names are important, but only insofar as they provide clues to how they got

there

The interpretation of earth's history is the purpose of any geological examination

In any event, this will usually take lots of field work

And the examination of lots of different rocks

As well as copious amounts of lubricant to make sense of the data!

Multiple Working Hypotheses

Need to keepan open mind

Several working together with different ideas can be good

As can a "Devil's Advocate" to keep the group from getting cocky.
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It's far too easy to only see those units and/or features which support the currentlyfavorable model

Important factors include:

Sorting - key to interpreting the depositional environment

"The degree in similarity in particle size in a sediment"

Important in the clastic sediments

Particle size


Particle composition

Important in chemical and organic sediments

Sedimentary Rocks

General Statements

We've studied igneous rocks & the minerals of which they are composed

Basement rocks

Most are covered by a thin veneer of debris

Consolidated into a "rock" through slow-acting processes

Usually involving pressure and fluid penetration

Relatively simple to understand

Relatively near-surface processes

As opposed to igneous & metamorphics, which usually occur at depth

Secondary (or derived) rocks

Several main categories


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Clastic sedimentary rocks - The classic sedimentary rock

We will concentrate on this type

Chemical sedimentary rocks - Chemical precipitates

Usually as the result of the evaporation of water - Ex. Salt (NaCl)


Limestone, all hydrocarbons - Coal, peat, oil, etc.

Origin of Sedimentary Materials

DIGRESS TO: Physical vs. Chemical weathering

Clasts - derived from physical (and chemical) weathering processes

Smaller solid particles

Wide range of sizes, from silt to boulders

Clay minerals

Easy to get confused by the term

The term "clay" refers to both a size and a mineral family

A clast can be clay size without being clay

Clay formation forms small, sheet-like minerals (look like the micas)

Near-surface, low temperature environments

Hot and humid works best - chemical weathering!!

Note: chemical weathering also results in "ions" which are "held in solution"

Can result in chemical sedimentary rocks

Organisms can also extract the ions directly from the water

Use them to build shell material - Ex.: Ca+2 + CO3-2 -> CaCO3

Can result in deposition of organic sediments


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Environments of Deposition

Water plays an important role in most aspects of sedimentary rocks

From weathering and erosion to transportation and deposition

DIGRESS TO: Q=AV

Deposition occurs in a wide variety of locations

Basically, any low spot is a potential depositional environment

Two major divisions - Continental and marine

Also there are inter-tidal (transitional) environmnets

Important factors include:

Sorting - The degree in similarity in particle size in a sediment


Particle size


Particle composition

Important in chemical and organic sediments

Continental Deposition

Sediments trapped on land

Rivers and streams

Riverbed - size directly related to energy of the stream

Can be poorly sorted (all different sizes) or well sorted (all the same size)

Floodplain - Flat surfaces adjacent to a river

Represents sediments deposited during flooding


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Usually well sorted

Glaciers

Non-turbulent flow (unlike rivers)

Can and will carry all sizes of material

Commonly poorly sorted, but not always!

Lakebeds

By nature a temporary feature

A sure trap for sediments (because Q=AV)

Evaporites - common to arid regions with seasonal lakes (playas)

Ex.: Bonneville Salt Flats

Alluvial Fans

Generally arid and semi-arid climates

Deltas

Essentially an underwater alluvial fan

Eolian Deposition

Wind can also play a role in the erosion, transportation, and deposition of

sediments

Can affect wide areas

Not confined to a defined channel like a river is

Always well sorted (unless contaminated by other processes)

Small stuff only - no boulders!

Sand dunes

Marine Deposition


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The seafloor is the final resting place for the majority of weathered rock materials

Please refer toStrickler's 3rd Law of GeoFantasy

Remember - "The earth breaks what it makes and puts it in the ocean"

Factors affecting deposition include:

Distance from shore

Related to energy

Depth of the water

These result in 3 broad zones of deposition

Relatively good sorting within each zone

In general, the shore and shelf contain the majority of "terrigenous" sediments

Gravel ---> Sand ---> Silt ---> Clay ---> Carbonate Ooze

The Shore Zone

The shore acts like a channel and restricts the "flow" of the ocean

High energy zone

Coarse sand and gravel are deposited here

Smaller material stays in suspension/solution and moves offshore

The Continental Shelf

Much broader then the shore zone

Most terrigenous sediments end up here (sooner or later)

Mostly silt & clay

Locally coarser material related to times of higher energy

Carbonate deposits also common

Inorganic and organic deposits of CaCO3 - Limestone

Common to "shallow, warm water"
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The Abyss - much of this ends up being subducted

Mostly very fine grain sediments

Water depth important in which is deposited

Calcareous to siliceous to terrestrial clay ooze

As depth increases and/or temperature decreases

Features of Sedimentary Rocks

Stratification - the most common and distinctive

Most sedimentary rocks are composed of particles which settle through water (or

air)

Generally quiet water deposition results in nearly horizontal layers

Differences through time result in visible layering

Variation in clast size

Variation in clast composition/mineralization

Special enhancements to visible layering

Graded Bedding

Cross Bedding

Size and Roundness of the clasts

Usually reflects transport distance and/or time in transit

Long distance = smaller and rounder clasts

Color

Most igneous rocks are some shade of gray

Sedimentary rocks can be quite colorful

Different pigments can fill the void spaces between the clasts


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Iron - very common

Results in shades of red, brown, pink, or yellow

Dark to black color commonly the result of organic material

EXAMPLE: Black shale

Fossils - the classic sedimentary feature

Evidence of once-living organisms

Characteristic of many sedimentary rocks

Not igneous or metamorphic

Most relate to remains of "hard body parts" (bones, shells, teeth)

But any evidence is considered a fossil

Soft body molds

Footprints

Coprolites

Some amazing parts have been preserved

Jellyfish, compound eye parts, dragonfly wings

Clues to depositional environments

EXAMPLE: Clam fossils pretty much indicate marine deposition, etc.

Used to establish the Relative Time Scale

Conversion into Rock

Lithification - "the process of converting soft, unconsolidated sediments into hard

rock"

Two major factors contribute to the lithification process

Remember: we are usually starting with a loose pile of debris, which is saturatedwith water


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Compaction

Weight of overlying sediments results in compaction

Reduction in pore space

Interstitial fluids (water) may be removed

Cementation - "The most significant process"

"The deposition from solution of a soluble substance"

Fills the interstitial pore spaces

Cements the grains together

Three common types of cement

Calcium- Probably the most common

Easily dissolved in groundwater

H20 + CO2 = H2CO3 (Carbonic Acid)

Will dissolve calcium and put it into solution

Silica - less soluble than calcite

Will form a much harder and stronger cement

Iron Oxide (Fe2O3)

Facies Changes

"Lateral change in the basic properties of a sedimentary horizon"

DIGRESS TO: Time-Stratigraphic Horizons

EXAMPLE: Conglomerate into sandstone into siltstone into shale

Reflect local variations in the depositional environment

DIAGRAM: on board

Transgression / Regression


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Unconformities

The sedimentary record is not complete

Long term gaps in the sedimentary record indicate periods of non-depositionand/or erosion

We actually can see only a small part of the earth's history in sedimentary rocks

The gaps clearly represent more time than do the beds themselves

Three major types of unconformities

Angular Unconformity

Easiest to recognize - describe

Non-parallel beds above and below

Represents: deposition, uplift, deformation, erosion, subsidence, and newdeposition

Disconformity

Parallel beds above and below

Can be real tough to recognize

Nonconformity

Sedimentary beds overlying igneous or metamorphic rocks

Represent immense time periods

Classification (types of sedimentary rocks)

As we said, there are 3 general categories

Clastic/fragmental; Chemical precipitates; and Organic

Distinction between different types often fuzzy in reality

Clastic Sedimentary Rocks - true secondary rocks


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Derived from the breakdown of pre-existing rock at the surface of the crust

Most sedimentary rocks are clastics

Quick review:

Surface weathering produces small clasts

Physical and chemical processes

As soon as a clast (at whatever size) is broken from bedrock, it is involved in the

erosion and transport process

Gravity is the ultimate driving force here

Clasts moved downslope to creek/river systems

Carried downstream to a suitable depositional environment

Weathering can continue during transport

Both physical and chemical

Its reasonable to assume that physical weathering dominates in the headwaters athigher elevations

Chemical weathering takes on a more active role at lower elevations

Smaller clast size results in greater surface area for chemical attack

Classification generally based on the size of the clasts

Conglomerate - cemented gravel

Usually poorly sorted, calcium or silica cement

Sandstone - Sand-sized clasts

Often interbedded with shale or conglomerate (facies changes)

Indicate near shore marine - your basic beach

Calcium or silica cement

Which one is present determines hardness

Friable - breaks up easily due to weak cement


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Compositional differences

Classic sandstone is generally quartz - final weathered product

Graywacke - "dirty sandstone"

Generally dark in color

Quartz, feldspar, mafics, lithic fragments all present

Indicates very short distance of transport

Silt & clay sized clasts

Lots of names based on size of clasts

Siltstone, claystone, mudstone

Shale works as a general descriptive name for most of them

Usually impossible to determine composition of clasts due to small clast size

Chemical sedimentary rocks

Evaporites

Result from the evaporation of water

Halite (salt), Gypsum (sheetrock)

Carbonates

Limestone - calcite (CaCO3)

Travertine

Hot springs deposits


Hydrocarbons

Coal - lithified plant and animal remains

Compacted swamps, etc.

Convert to coal in an anaerobic environment


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Calcium based rocks

Limestone the most common

Most limestone is organic as opposed to chemical in origin

Foraminifera

Microscopic plants & animals extract CaCO3 from seawater and use it to buildshells

These will settle to the seafloor and accumulate into Limestone deposits

Larger organisms also extract CaCO3 for shells which can accumulate on seafloor

Coquina - lithified shell debris

Can be reworked in the sea currents - broken and moved around

Are these then clastic sedimentary deposits?

Reefs

Made largely of corals and carbonate secreting algae

Like shallow, warm waters which are agitated by wave action

High in nutrients (for food)

Environment essentially free of terrigenous sediments

Can result in extremely pure limestone deposits

Commonly 30 of the equator

Silica based rocks

Chert - "general name used to cover many types of dense, hard, non-clastic,microcrystalline siliceous rocks"

Flint - dark color from included organic remains

Uniform texture - conchoidal fracture

Jasper - reddish flint

Sinter - hot springs (like travertine)


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Thick beds of chert are found throughout the geologic record

Some may result from direct chemical precipitation

White smokers at spreading axes

Most are thought to be organic (like the carbonates)

Microscopic plants & animals extract silica from seawater and use it to build shells

These will settle to the seafloor and accumulate into chert deposits

Metamorphic Rocks

The metamorphics get their name from "meta" (change) and "morph" (form). Anyrock can become a metamorphic rock. All that is required is for the rock to be

moved into an environment in which the minerals which make up the rock become

unstable and out of equilibrium with the new environmental conditions. In mostcases, this involves burial which leads to a rise in temperature and pressure. Themetamorphic changes in the minerals always move in a direction designed to

restore equilibrium. Common metamorphic rocks include slate, schist, gneiss, and

marble.

Introduction

As usual in geology, take big words apart

meta = change

morph = form

ick = tough to study

Talking about a change inmineralogyhere

Considered an "iso-chemical" process

Essentially, nothing is added or lost at the elemental level

Except for a subtle to profound loss of water

Existing elements recombine into new minerals
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Mineralogy ALWAYS changes in an attempt to restore equilibrium

One of the only times in geology when you can use the word "always"

Even toss the1st Law of GeoFantasy?

Start with any rock

Subjected to different environment conditions

Commonly due to burial, or subsidence of the crust due totectonics

Heat and pressure usually involved

Difficult process to study

Generally occurs atdepth in the crust

Impossible to observe directly

Similar in this way tointrusive igneous rocks

But generally far more complex

But not too deep - usually "less than 20 kilometers"

Higher temperatures at depth lead to complete re-melting and the formation of

magma

As always, this is a highly variable depth

Subject tolocalirregularities

Metamorphism is also considered to be a "solid-state" process

All of this happens at temperatures below the melting point of the rocks!

There are several factors which directly affect the process

Rock chemistry

Contained fluids

Heat

Pressure
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Time

There are infinite variations of these factors

Results in a very complex suite of rocks!

The study of metamorphic rocks can only take place after uplift, weathering, anderosion

And long after the actual metamorphic processes have ended

Can be real tough to determine the metamorphic history of a rock

Including what it was originally!

The metamorphics are without a doubt the toughest to understand

We'll takea very broad lookat them and just discuss the main categories

Factors involved in the metamorphic process

Rock chemistry

Metamorphism is an iso-chemical process

Therefore, what you start with is extremely important

The chemistry of the parent rock largely determines the composition of theresulting metamorphic rock

This should be a real no-brainer

Cook eggs and you get an omelette, not meatloaf

Unless you add a bunch of new stuff

But this is an iso-chemical process, so not much is added or lost

Limestone alters to marble, not quartzite!

Contained fluids

Generally water and carbon dioxide
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Similar to how volatiles affect magmas

REVIEW:mafic to felsic

The high volatile minerals tend to react early

Release their volatile components

Two things happen:

The loose volatiles tend to act as a catalyst

The best metamorphics are commonly derived fromsedimentary rocks

The resulting rock is generally decreased in the volatile components

Heat

Considered "the principle factor in the metamorphic process"

If metamorphism requires that the elemental ions migrate and recombine...

Ions diffuse easier at higher temperatures

Therefore higher temperatures tend to increase both the speed and efficiency of themetamorphic process

The increased heat directly affects the "strength" of the rock

And locally affects theBrittle-Ductile Transition Zone(REVIEW)

The resulting metamorphic rocks can be highly contorted, folded, and otherwise

deformed plastically

As a general rule: the higher the metamorphic grade the greater the plasticdeformation

DIGRESS TO: Metamorphic grade

Obviously, there are all possible ranges of heat (metamorphic grade)

From "just barely warm" to "just below the melting point"

But, what is the melting point?

REVIEW:Bowen's Reaction Series
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The metamorphic process affects the low temperature (felsic) minerals first

This results in some VERY interesting effects at the higher grades (see below)

Pressure

Heat and pressure are definitely related

Pressure leads to increased heat

In general, the increased pressure associated with the metamorphic process results

in a rock with tighter packing at the atomic level

Therefore, generally higherdensitythan the parent rock

There are several sources of pressure...

Pore-fluid pressure

Release of volatiles supplies some pressure to the overall system

Litho-static pressure (REVIEW) (Monroe; fig. 8-7, pg. 241)

The load weight of overlying rock

Equal pressure in all directions

Results in non-foliated rocks (DEFINE)

Marble, quartzite common non-foliated varieties

Directed pressure (REVIEW)

Acts in a specific direction

Generally related totectonics

Results in foliated rocks (DEFINE)

New mineral grains grow with their long axis oriented normal to the

stress (Monroe; fig. 8-10, pg. 244)

EXAMPLE: Drop a deck of cards; gravity is the directed stress

Most common metamorphic rocks fall into this category
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Time

Some of the higher grade rocks clearly required aVERY long timeto form

We can duplicate all the other factors in the lab, but not this one

This is the fatal flaw in most studies of earth processes

Clickherefor a discussion of geologic time and metamorphic rocks

Metamorphic environments and rocks

There are several major categories

Basically related to the size of the system

And the relative importance of heat and pressure

Localmetamorphic terrains

Relatively small and isolated occurances of limited extent

Regionalmetamorphic terrains

Large, fully developed, and complex environments

Metamorphic terrains of limited extent

Contact metamorphism

Usually associated with increased heat

Without a corresponding increase in pressure

Litho-static or limited directed stress

Therefore commonly non-foliated

Common along the margins of small plutons (dikes, sills, etc.)

Localized heating of country rock as magma cools
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Results in a thin "halo" of metamorphism

Also called a metamorphic aureole (Monroe; fig. 8-5, pg. 240)

Usually very thin (millimeters to a few centimeters)

Chill margin vs. baked zone(DESCRIBE)

Clickherefor a discussion of cooling history and texture

Can be larger in special cases

Hornfels: derived from shale

Dense, fine-grained, non-foliated

Skarn: derived from limestone

Skarns can be VERY important to economic geology

Calcium carbonate is highly reactive

Will extract many different elements from the cooling magma

Can result in very high grade mineral occurrences

But usually disappointingly small

Remember, they form in a contact metamorphic environment

Hydrothermal metamorphism (EXPLAIN: hydro + thermal)

Heat and chemically active solutions

Usually related to residual fluids escaping from afelsicmagma chamber

Does not have to be felsic, but is probably most common

Cataclastic metamorphism

Localized near-surface fault zones (redundant?)

Rock is tectonically broken and shattered

Increases surface area
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Leads to increased fluid penetration and hydrothermal metamorphism

Can also occur locally at greater depths

The added heat and pressure can accentuate the metamorphic processes

Mylonite: Greek for "mill" (Monroe; fig. 8-8, pg. 242)

Nearly complete pulverization of the rock

Leads to partial to complete recrystallization

Very tightly inter-grown minerals

Extremely hard and durable rock

Regional metamorphism: an overview

Clickherefor online mineral and rock ID charts

Can result in bodies of great extent

Most (but not all) are the result of directed stress environments

Also called "dynamo-thermal" metamorphic rocks

Associated with continental mountain building processes

Combined withgranite, these form the cores of the continental land masses

Calledcratons

Shields where exposed

Platforms where obscured by sedimentary layers

Heat, pressure, and volatiles are all important

Usually results in prominent foliation (but not always)

And very complex mineral assemblages related to local variations in rockchemistry and metamorphic grade (more later)
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THE ROCKS ---


Non-foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)

Heat and litho-static pressure predominate

Results in a recrystallization of existing material

These factors are everywhere beneath the surface

Therefore, takinga very broad view, all rocks can be considered non-foliated

metamorphics to some degree

There are several common non-foliated rocks

Quartzite: derived from sandstone (Monroe; fig. 8-17, pg. 247)

Very hard and durable

Looks like sandstone

But, the rock will break through the quartz grains, not around them

Hornfels: derived from shale (usually)

Also very hard, dense, and durable

Marble: derived from limestone (Monroe; fig. 8-16, pg. 247; and "Marble,"pg.234)

In most cases, the parent limestone had impurities

Add color and pattern to the marble

Can be dense and compact, but softer than quartzite or hornfels

It's made from CaCO3 like calcite and limestone

Good for carving, building stone, facing stone

Josephine County Courthouse
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All three represent common marine sedimentary facies which are probablymetamorphosed by the weight of overlying debris

Foliated metamorphic rocks (Monroe; Table 8-2, pg. 243)


Result of increasing heat and directed pressure

Increasing metamorphic grade generally results in a coarsening of texture

As well as a concentration of felsic and mafic constituents

Increasing grade also results in a progression specific minerals (Monroe; fig. 8-18,pg. 248)

Obviously dependent upon original rock chemistry

Called a metamorphic facies (Monroe; fig. 8-20, pg. 250)(Monroe; fig. 8-21, pg.248)

Examples: staurolite facies, actinolite facies, greenschist facies

The same elements recombine to form different minerals at different temperature

and pressure environments

Each facies indicates temperature, pressure, and fluid conditions at the time of themetamorphism

Platy minerals: mica, chlorite, graphite

Common at lower metamorphic grade

Orientation results in "foliation"

Elongate minerals: hornblende, staurolite, pyroxene

Common at higher metamorphic grade

Orientation results in "lineation"

The resulting progression of metamorphic rocks is fairly specific

With infinite gradations and variations!
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Let's start with deep-water marine sediments and follow the process

Add heat and pressure between (and within) each step

Metamorphics are the ultimate "shades of gray" situation in geology

These are only the broadest of category names

The variations are endless

Shale

A commonsedimentary rock

Very fine grain

Toss in a little sandstone and limestone and you've got your basicmarine

sedimentary assemblage

Slate

Little or no significant visible change (Monroe; fig. 8-11, pg. 244)

Still microscopic grains

But the mineralogy has begun to change

Usually to mica, graphite, or chlorite

Low temperature minerals with one perfect cleavage

A very hard and durable rock

Commonly used as pool table tops, roofs, and chalkboards

Phyllite

Begin to see mineral grains

Commonly lots of mica - gives rock a shiney look

Can be up to 50% muscovite

But can also be graphite or chlorite
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Schist

A very broad category (Monroe; fig. 8-12, pg. 245)

Significant change in mineralogy, texture, and visible foliation

Well developed foliation of micaceous minerals (usually greater than 50%)

Also called schistosity

The characteristic wavy or undulating rock cleavage common to schist

May not parallel original bedding

Most primary textures and features are lost

Other minerals begin to form based on composition of original rock and new

environmental conditions

Use additional minerals as modifier of name

EX: mica schist, quartz schist, hornblende schist, quartz-mica-hornblende schist,etc.

Gneiss

High grade metamorphic rock(Monroe; fig. 8-13, pg. 245)

Color banding of light and dark minerals

DIGRESS TO: layers vs. lenses

Lineation: orientation of prismatic minerals

Hornblende, actinolite, tourmaline, staurolite

Migmatite

Almost there! (Monroe; fig. 8-15, pg. 246)

Partial melting and recrystallization of felsic minerals

REVIEW: Reverse order ofBowen's Reaction Series

Results in a rock with layers of felsic igneous rock and very high grade maficgneiss
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To summarize...


Increasing grade very common in sedimentary sequences

Layers of sediment pile up deeper and deeper

Leads tolithificationof the lower layers

As additional layers of sediments are added on top, the lowest portions begin to

metamorphose

Followed to its logical conclusion...

Imagine an unbroken transition from unconsolidated sediments to sedimentaryrock through increasing metamorphic grade to...

Migmatites and the Formation of Granitic Magmas

Migmatite - a very high temperature metamorphic rock

Because ofBowen's, thefelsicconstituents have reached theirmelting point

But themaficsstill have a way to go

So we end up with a highly contorted, mixed igneous and metamorphic rock

Called "roof pendants" because they usually grade into felsic intrusives at greater

depth

Several excellent examples

Kaweah River - Sierra Nevada foothills

Near south entrance to Sequoia National Park

Convict Lake area - eastern Sierra Nevada

Ashland pluton - Siskiyou Mountains, Oregon and California

Add more heat and the whole thing melts - aphase change

When I was in this class, most granitic magmas were "emplaced from below"
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Usually through "forceful injection"

Kind of an ominous thought

...and where did they come from?

Direct differentiation from the upper mantle is hard to believe

In almost every case, magmas we see coming out of deep rifts in the crust aremafic

Your basicoceanic spreading ridge

They begin to purify into felsic materials where they are re-worked along the

continental margins

Subduction zonesand the cores of volcanic arcs

Therefore, the ultimate source of most granitic magmas must be a metamorphicprocess

Clickherefor additional information on the formation of granitic magmas

Clickherefor additional thoughts on the directdifferentiationof granitic magmas

from theupper mantle

The realms of dynamo-thermal metamorphism

No clear-cut answers, but lots of circumstantial evidence

Commonly in elongate bodies

10's to 100's of miles wide

100's to 1000's of miles long

Associated with deep- seated plutonic rocks

Batholiths like the Sierra Nevada

Form the axes of many of the world's mountain ranges

Sierra Nevada, Alps, Rocky Mountains, etc.

Intermediate to high temperatures
http://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry8.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry9.htmlhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Newhttp://jersey.uoregon.edu/~mstrick/RogueComCollege/RCC_Lectures/MagmaFormation.html#Granitichttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Convergent.htmlhttp://jersey.uoregon.edu/~mstrick/geology/Geo_Lectures/Divergent.html


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Intermediate to high directed pressure

Clearly long and well developed crustal tectonic environments

Time spans measured in 100's of millions of years

Moderate to great depth - but still in the crust

All this adds up to subduction complexes as the most logical location

These metamorphic suites most likely form the cores of thesubduction zones

Metamorphic Rocks

Overview of the process

Start with any rock

Subjected to different environment conditions

Commonly due to burial or subsidence of the crust due to tectonics

Heat and pressure usually involved

Generally at depth in the crust

Mineralogy ALWAYS changes in an attempt to restore equilibrium

Solid-State process - explain the reality of what this means

Iso-chemical process - explain the reality of what this means

Contact vs. Regional metamorphism

Litho-static vs. directed stress environments

Foliated vs. non-foliated rocks

The metamorphic process
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry18.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry18.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry18.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry18.html


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Foliated metamorphic rocks

Usually associated with regional metamorphism

Result of heat and directed pressure

Therefore will generally exhibit a distinct layering

There is a fairly specific progression through the main metamorphic sequence

For example, starting with Shale - a common sedimentary rock

Very fine grain

Add HEAT and PRESSURE and it metamorphoses to...

Slate - little or no significant visible change

Still microscopic grains

Mineralogy begins to change

Usually to mica

Add more HEAT and PRESSURE and it metamorphoses to...

Phyllite - begin to see mineral grains

Commonly lots of mica - gives rock a shinny look


Schist - significant change

Foliation of micaceous minerals (muscovite and/or biotite)

Other minerals begin to form based on composition of original rock and newconditions

Use additional minerals as modifier of name

EX: Hornblende schist, quartz schist, etc.


Gneiss - high grade metamorphic rock


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Color banding of light and dark minerals


Migmatite - Partial melting of felsic minerals

RememberBowen's Reaction Series?

The felsic minerals will melt at lower temperatures

Results in a rock with layers of "granite" and high grade mafic gneiss

Add enough HEAT and PRESSURE and it ultimately melts to form magma...

Granite or ??? - a new igneous rock after complete melting

Non-Foliated metamorphic rocks

Usually associated with contact metamorphism

Result of heat and litho-static pressure

Therefore will generally not exhibit a distinct layering

Marble - metamorphosed limestone

Relatively soft and will pass the fizz test

Generally coarsely crystalline

But can also be fine grained

Can be quite beautiful - many colors

Commonly used in carving

Carrera Marble - Italy

Very vine grain and pure

Quartzite - metamorphosed sandstone

Generally very hard and resistant
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.htmlhttp://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.html


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Andesite

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite. The specimen shown isabout two inches (five centimeters) across.

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Basalt
http://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/andesite.shtmlhttp://geology.com/rocks/andesite.shtmlhttp://geology.com/rocks/igneous-rocks.shtml#top


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Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters)across.

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Diorite

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz. The specimen shown above is abouttwo inches (five centimeters) across.
http://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/igneous-rocks.shtml#tophttp://geology.com/rocks/diorite.shtmlhttp://geology.com/rocks/basa

Date post:	04-Apr-2018
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What Are the 3 Basic Types of Rocks

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