6_weathering.ppt

7/28/2019 6_weathering.ppt

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Weathering profile of volcanic tuff in a road cut after five years of exposure to

weathering processes.


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Weathering profile of volcanic tuff in a road cut after fifteen years of exposure to

weathering processes. Note that rills have developed over time where surface runoffhas flowed down the face of the road cut.


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Weathering profile of volcanic tuff in a road cut after twenty-five years of exposure

to weathering processes. Note that sharp edges are now rounded and the profileis stained red.


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Exposed bedrock is subject to both physical and chemical weathering processes.

Natural joints within the bedrock facilitate weathering by allowing water to penetrateat depth and exposing a greater surface area of the rock to weathering processes.


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Note how the natural joint pattern facilitates weathering by providing an environment

conducive to vegetation growth by trapping soil and collecting water.


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Jointed bedrock of El Capitan Yosemite

Park, California forms from expansion

during uplift and unloading of overburden

rock.

Note how jointed grainitc bedrock in

this New Hampshire quarry permits

ground water to reach great depths.


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Joints can form during cooling and

contraction of lava flows such as

columnar basalts shown in theimage on the left.


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Physical or mechanical weathering

includes weathering processes that

cause rock or sediment to break down

into smaller pieces without changing

the chemistry (mineralogy) of the rock.

The image on the left demonstrates

how freezing water can exert high

stresses as it expands, causing theglass jar to break.

Water trapped in micro-cracks within

rock can expand during freezing

cycles and exert tremendous stressesto the crack wall and cause rock to

break apart.


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Freeze-thaw cycles in this alpine environment are responsible for the break-up of this

granitic bedrock by frost wedging. Freeze-thaw cycles are also important in subpolarenvironments where temperature fluctuate around the freezing isotherm (0 C).


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Spalling due to range or forest fires can cause the physical break-up of rock through

rapid expansion and contraction of hydrous mineral during heating (>900C) and cooling

of the rock as the fire passes over a given location.


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Root penetration, such as that

exhibited by this small lodgepole

pine, can exert great pressures

within joint cracks as the tree grows

and the roots begin to expand.


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Chemical weathering of an

Egyptian obelisk after arriving in

Central Park, New York.

An Egyptian obelisk survives

over 3000 year in the aridity of

the Sahara Desert.


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Chemical weathering processes cause changes in the mineralogy of rock. A

marble tombstone engraved in 1970 is subjected to chemical weathering.


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Over time the calcium carbonate (CaCO3) will dissolve by solution weathering to

form calcium (Ca+2)and bicarbonate (HCO3-1) ions causing the 1820 engraving to

disappear.


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A granitic tombstone engraved in 1820 is still well-preserved because its

constituent minerals are more resistant to chemical weathering processes.


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H d l i ti i l


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Hydrolysis reactions involve

hydrogen ions (H+1) derived from

carbonic or other acids within the

environment. Hydrolysis reactions

can convert primary feldspars in

rock to clay minerals, such askaolinite.

Eocene oxisol, Ione, CA

Iron oxide laterite (red) overlies

kaolinite clay (white). Formed onalluvium derived from eroded Sierra

Nevada volcanics in a tropical

climate 38 m.y. ago.


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Modern soil formed in laterite of

Eocene Ione Formation

A Horizon

B Horizon

(goethite yellow precipitate

and hematite red

precipitate)

4FeO + 2H2O + O2 2FeOOH

Goethite

Dehydration to form Hematite

2FeOOH Fe2O3+ H2O

Oxidation Reactions


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Rills form from solution weathering of limestone. Compare the depth of rilling

on the subsequent two slides.


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What factors may account for increasing depth of rilling on the limestone boulders.


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If environmental factors are held constant what factor will explain greater weathering?


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Terminal zone of melting glacier.


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Recessional moraine loop within Chiatovich Valley, CA. How

could you infer the relative age of glacial moraines from

weathering properties of surface boulders?


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Solution weathering of limestone results in hummocky topography (see inset slide).

Sinkholes form as acidic groundwater dissolves the underlying carbonate rock.

Roofs of caves can collapse when underlying support is removed by solutionweathering.


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The karst towers in southern China are the result of long-term solution weathering

the limestone bedrock by surface and groundwater. The top of the towers provide a

minimum estimate of downcutting and lowering of the surrounding landscape.


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The formation of stalactites and

stalagmites in limestone caves

demonstrates that dissolution of

calcium carbonate (calcite) is a

reversible process.

Environmental factors such as

changes in water temperature,

acidity, changes in pressure can all

influence the solubility of calciumcarbonate in solution.

The image was taken from

Carlsbad Caverns, New Mexico.


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Minerals weather at different rates. The plagioclase crystals stand in relief

because they are more resistant to chemical weathering processes than the mafic

minerals that are oxidized.


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Monuments and hoodoos form because of differential weathering. A resistant

cap rock, such as quartzite, protects the underlying weaker rock, in this case,sandtone and shale from weathering and erosion.


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Classic examples of differential weathering are prevalent throughout the southwestern

United States. These monuments and mesas are capped with resistant quartzite. The

weaker sandstone and shale are being eroded by fluvial and mass wasting processes.


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Frost-wedging and solution weathering of limestone (Claron Formation) have

worked in tandem to create the spectacular amphitheaters of Bryce National Park,

Utah.


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Water and wind worked in concert to form the arches within oxidized Entrada

sandstone in southern Utah.


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Spheroidal weathering patterns develop in bedrock that is exposed to weathering

processes for extended time periods.


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Strong spheroidal weathering pattern is exhibited in granitic bedtock in the

Sierra Nevada, California. Core stones are gradually exposed at the

surface as mafic minerals are oxidized and the more resistant quartz

grains accumulate as grus near the base of the boulder.


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Soils can form in residual bedrock.

Soils can be described as a weathering veneer covering the terrestrial surfaces of

the earth. Soils are composed of weathered minerals and decomposed organic

matter. Soil profiles can form in residual bedrock or unconsolidated sediment.


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Soil forming processes include: 1. accumulation (illuviation), 2. transformation, 3.

Removal (eluviation), and 4. translocation.


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(Partially weathered parent material)

B Horizon

A Horizon

C Horizon

Soil forming in granitic bedrock.


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A

Bwk

Bk

Formation of caliche soils

in arid climates. Calcium

carbonate accumulates in

the B (Bk) horizon in arid

climates.


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Eocene oxisol, Ione, CAModern soil formed in laterite of

Eocene Ione Formation

A Horizon

B Horizon

(goethite and hematite)


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Soil Forming Factors (Clorpt)

1. Climate

2. Organisms

3. Relief

4. Parent material

5. Time


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Note that climate plays a major role in the depth of the B horizon (zone of

accumulation) or illuvial zone.


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Vegetation can play an important role in soil nutrient replacement and pH.


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Unweathered parent material (glacial till)

Soil development versus position on a moraine slope. Compare solum depth at crest

versus the flank of the moraine

solum


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Bedding structure can influence the rate of soil development. Think about how

water penetration would differ between horizontal and vertical oriented strata.


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Soil development on a 13,000 year old moraine, Snoqualmie Pass, WA.


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Soil development on 75,000 and 150,000 year old moraines. As soils age

6_weathering.ppt

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