Geology 2.pdf

7/30/2019 Geology 2.pdf

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Geology 229Engineering Geology

Lecture 23

Rock Weathering

(West, Ch. 8)


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OutlineIntroduction of weatheringMechanical weatheringChemical weathering


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Comparison of surface and

subsurface conditionsSubsurface

High temperature butconstant at which

minerals reachequilibrium

high confining

pressure (stress) less water or no water

no oxygen

Surface

low temperature, andhighly variable

little or no confiningpressure (stress)

abundant of water

abundant of oxygen


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Conclusion can be drawn from the comparison:Rock at the surface will undergo changes---- This change is called Weathering

Weathering is the physical breakdown(disintegration) and chemical alteration(decomposition) of rocks to form soil or looseparticles at or near Earth's surface. Weatheringcauses deterioration of building materials. It alsoweakens rocks, a great concern when weatheredrocks are used for foundation.


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Two types of weatheringMechanical weathering:Physical disintegration or degradation ofrock pieces without a change in composition--size reductionChemical weathering:decomposition whereby one mineral species is

changed into another through various chemicalprocesses. Water plays a major role, through:1, provide oxygen,2 provide mobility for moving ions.


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Mechanical weathering always involvesfracturing--but that can occur by a wholehost of causes.Chemical weathering tends to weakenrock, thereby making it easier to break.Likewise, mechanical weathering createsadditional surface area that is prone tochemical attack. In this way, the two

processes work together.


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Weathering is controlled largely by climate.The more water available, the more likelythat chemical processes can proceed.Additionally, if temperatures are warm, thenchemical weathering can proceed evenfaster. Then, mechanical weathering canmove more quickly also. In arid climates,however, weathering processes move veryslowly. Mechanical weathering will be thedominant process in arid climates; however,because of its reliance on chemicalweathering, it will also be quite slow.


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mechanical weathering:

With or without water makes distinguisheddifference in rock weathering process, in

arid region, since no water or little water

presence, even the carbonate rocks (e.g.,limestone) are not subject to solution but

persist as resistant rocks. The rock mass

may be sharp angular, and topographydeveloped.


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Mechanical Weathering (cont.):

Mechanical weathering processes include:

1) freezing & thawing (frost wedge)

Example:

Pottery container with water in winter time,when water is frozen and become ice it can

have 9% of relative volume change, i.e.,

dv/v = 9% = 0.09This is a very large strain! Similarly if there is

water in rock fractures it will force the fracture

to propagate into further depth.


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The stress generated by frozen water is

about 550 psi, check with Table 6.1 you can

find out that this is a value comparable tothe tensile strength of most rock types. Or

only one order of magnitude less. But

remember rocks are experiencing many

freezing-thawing cycles before they are

finally breaking down.


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2) differential expansion and contraction

As temperature changes (in deserts or from

forest fires), not all parts of a rock or all its

minerals expand or contract by the same amount.

So when rocks are heated or cooled, the mineralgrains are subjected to differential stresses,

which may be sufficient to make the rock spall, or

break off in sheet-like pieces.


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#

Thermal expansion -

* repeated daily heating and cooling of

rock;

* heat causes expansion; cooling causes

contraction.

* different minerals expand and contract

at different rates causing stresses along

mineral boundaries.


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The thermal expansion coefficient L

and V

characters how much a mineral change its

dimension in response to a unit degree

increase in temperature . Using a finiteexpression

L=LLT

PL

T

l

L)(

1

=

PV

T

V

V)(

1

0

=


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The linear thermal expansion coefficient L ison the order of 10-5/C, and it could beanisotropic, i.e.,

Lx Ly LzFor example, max is in the elongation axis (C-axis).


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Differential weathering


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Processes of mechanical weathering

unloading: jointing, exfoliation, and sheeting

Upon removal of overburden, the elastic component of rock

deformation is recovered and the rock expands. Theunloading may occur when the overlying rocks are eroded orrocks are removed from a quarry. The expansion caused by

unloading may be sufficient to fracture the rock. Suchnaturally formed cracks are known as joints.

Typically, large plutons (bodies of igneous rock) or

metamorphic bodies split into sheets that are parallel to themountain face, a process known as exfoliation. It is alsoknown as sheeting if the expansion from unloading occurs in

granite to form rock slabs.


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3), Sheeting and exfoliation

When subsurface confining stress isreleased fast, and the rock exposed to the

surface, if the rock has large residual stress,

it will be broken into blocks. The fracture invertical direction form exfoliation. If the

fracture is in horizontal direction, it calls

sheeting.The famous examples of exfoliation are the

North Dome and Half Dome in the Yosemite

National Park, CA.


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joints are parallel cracks in which rocks on eitherside are not offset; SheetingSheeting rock layers peel likerock layers peel likelayers of an onionlayers of an onion

Chapter 5: 2) Mechanical weatheringChapter 5: 2) Mechanical weathering ) Unloading


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Another sheeting example

See also T&L Figure 5.8

Chapter 5: 2) Mechanical weatheringChapter 5: 2) Mechanical weathering Unloading


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North Dome


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North Dome


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Half Dome

H lf D


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Half Dome


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Vertical columns frommagma cooling &shrinking

columnar jointing


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Glacially polished basalt columns(end view)

Columnar jointing in basalt


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4), Spheroidal weathering

Spheroidal weathering is caused by thecombination of any of the following causes:

pressure relief, frost wedging, expansion,wind, and chemical weathering.


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Spheroidal Weathering. Granite illustrates weathering forms quite well.Chemical weathering attacks to granite along joints and makes rounded

boulders (Alabama Hills near Lone Pine) .


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Devils Marbles Australia


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5), Other mechanical weathering

erosion by wind and water(only the mechanical part);

forest fires;

plants rooting;

Fire and water quenching was used till 16th

century to do rock quarrying.


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Chemical weathering

Chemical weathering=chemical processes thatdissolve and decay earth materials;Chemical weathering needs the rock exposes toair and water. Mechanical weathering couldenhance chemical weathering by disintegration,i.e., increase the surface area of rock blocks anddebris and this will greatly accelerates chemicalweathering.


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Chemical weathering rate depends on1. Temperature

2. Amount of surface area

3. Availability of water or natural acid

Thus, rocks in tropical environment

experience most severe chemicalweathering.


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Because of its dipolar nature

water is able to dissolve many

chemical compounds. In

addition to the solution effect,

water aids decomposition

through acid action,

oxidation, and hydrolysis.


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Acidity of Natural Waters

Water is a good solvent.

Acidic water is better!

pH of most naturalwaters ranges from 4

to 9pH > 9 or < 4 occurs inextreme

environments

Chemical Weathering of silicate


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Chemical Weathering of silicate

minerals by carbonic acid

feldspar + water + carbonic acid = clayminerals + dissolved ions

2KAlSi3O8 + H2O + 2H2CO3 =

Al2Si2O5(OH)4 + 2K+

+ 4SiO2(aq) + 2HCO3-


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Why is rainwater naturally acidic?

Rainwater contains dissolved CO2 from

atmosphere.Dissolved CO2 reacts with water to form

carbonic acid (H2

CO3

)

CO2 + H2O H2CO3Carbonic acid dissociates to produce

hydrogen ion (H+) and bicarbonate

H2CO3 H++ HCO3

-


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Chemical weathering (cont.)

Common chemical weathering processes aresolution (dissolution), oxidation, and hydrolysis.Rock reacts with water, gases and

solutions (may be acidic); will add orremove elements from minerals.


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Solution (or dissolution)

* Several common minerals dissolve in

water

i), halite; ii), calcite

* Limestone and marble contain calcite

and are soluble in acidic water.

Marble tombstones


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and carvings are

particularlysusceptible tochemicalweathering bydissolution. Notethat the urn andtops of ledges are

heavily weathered,but the inscriptionsare somewhat

sheltered andremain legible.Photo taken in oneNew Orleansgraveyard.


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A 16th-century

monastery in Mexicoshows the ravages of

weathering mostly from

wind and wind-driven

rain. The rock is volcanic

tuff.

Karst landscape of Guilin, China, caused


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Karst landscape of Guilin, China, caused

dissolved Carbonate rocks.

Oxidation

O id i O bi i h i b i ili i l i i


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Oxidation - Oxygen combines with iron-bearing silicate minerals causing "rusting".Iron oxides are produced that are red, orange, or brown in color. Iron-bearing

silicate minerals that undergo oxidation include the following:

* olivine

* pyroxene

* amphibole

* biotite

Iron oxides are produced by oxidation of iron-bearing silicate minerals. These

minerals are iron oxide minerals:

* limonite

* hematite

* goethite

* Iron oxides are red, orange, or brown in color

* Mafic rocks such as basalt (which may contain olivine, pyroxene, or amphibole)

weather by oxidation to an orange color

Broken piece of fine-


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grained basalt from a dike

near Stone Mountain, GA.

Note the black color of the

unweathered rock, and the

weathering rind colored by

iron oxides. Theweathering rind has two

distinct layers, an inner

yellowish layer and an

outer orange layer. Sampleis about 10 cm in width

(below) .

Mafic rocks such as basalt (which

may contain olivine, pyroxene, or

amphibole) weather by oxidation

to an orange color (above)


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Hydrolysis: Hydration-reaction between mineral and water

Since many igneous rocks contain feldspar minerals (K-feldspar andplagioclase) as a major component in mafic, intermediate, and felsic rocks,clays are common products of chemical weathering of igneous rocks. Clays are

a group of minerals that are all sheet silicates with various amounts of water intheir crystal structures (added to feldspar structure, which is a frameworksilicate). An example is kaolinite (a chalky white mineral)

Feldspar + water -> kaolinite + silica + potassium ions

KAlSi3O8 + H2O -> Al2Si2O5(OH)4 + SiO2 + K+


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The difference between Weathering and Erosion

Weathering involves two processes Chemical weathering

and Mechanical weathering, that often work in concert to

decompose rocks. Both processes occur in place. Nomovement is involved in the weathering.

If a particle is loosened, chemically or mechanically, but

stays, call it weathering.

Once the particle starts moving, it is erosion.


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Gods Garden, Colorado


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Twin Tower, Gods Garden, Colorado


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Summary


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Mechanical weathering is accomplished by

physical forces that break rock into smaller

and smaller pieces without changing the

rock's mineral composition.Chemical weathering involves breaking down

rock components and internal structure and

forming new compounds.

Whereas weathering breaks rocks apart,

erosion removes rock debris by mobile agentssuch as water, wind, or ice.

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