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Physical Geography by Alan Arbogast

Chapter 14

Weathering and

Mass Movement

Lawrence McGlinn

Department of Geography

State University of New York - New Paltz

© 2007, John Wiley and Sons, Inc.

Weathering

• Process by which rocks break down into smaller

pieces

• Two Primary Types:

• Mechanical (or physical) – cold, wet/dry

environments

• Chemical – warm, moist environments

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Weathering Environments

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Mechanical Weathering • Destruction of rock through physical stresses

• Rocks break into small pieces – surface area of

rock increases – even greater weathering

• Types:

• Frost Wedging

• Impact of Roots

• Temperature Fluctuations

• Salt Crystal Growth

• Exfoliation

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Frost Wedging

• Most common form of mechanical weathering

• Water flows into joints (fractures) in rock

• Expands 9% when it freezes

• Stress cracks rock

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Impact of Roots

• Roots or trunks of vegetation, especially trees,

can grow through & expand cracks in rocks

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Temperature Fluctuations-

Temperature expansion • Warming & cooling – rocks expand & contract –

rocks break down – especially in arid regions

Death Valley, CA

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Salt Crystal Growth

• Salt in rocks weakens mineral bonds, loosening

sediments which erode through wind or water

Arid climate in

Southwestern US

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Exfoliation • Deep rock slowly uncovered by erosion of

overlying layers – pressure on rock reduced

• Under less pressure, rock layers flake off

• Most common in igneous or metamorphic rock

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Chemical Weathering

• Changes rock by altering its chemical

composition – water particularly important

• Types:

• Hydrolysis

• Oxidation

• Carbonation

• Acid Rain

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Hydrolysis

• Decomposes silicate minerals in rocks

• Hydrogen & hydroxyl ions from water added to

rock – silicate molecules split

• Rounds sharp edges and corners of rocks

Spheroidal

Weathering

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Oxidation

• Oxygen is added to chemical compounds,

causing electrons in compounds to be lost

• Happens in rocks high in iron (rust)

Oxidized Sandstone

in southwest US

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Carbonation • CO2 dissolves in atmospheric water vapor,

forming carbonic acid in precipitation

• This carbonic acid dissolves minerals, esp.

limestone (calcium carbonate)

Effects of Carbonation

in limestone

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Acid Rain

• Coal-burning power plants in Midwest emit sulfur dioxide (SO2) & nitrogen oxides (NOx)

• SO2 & NOx mix with water in clouds to form sulfuric or nitric acid

• Clouds move east where acid rain falls

• Acid rain accelerates chemical weathering

• Acid rain can also harm lakes & forests

• Regulation has cut acid emissions dramatically

Weathering

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Mass Wasting

• Large volumes of sediment moving down hill

slopes under force of gravity

• Types:

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Solifluction

• Form of soil creep in tundra landscapes

• Surface of permafrost melts & soil sags down

slope in uneven lobes

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Earth Flow

• Slow-to-rapid movement of wet soil & other

loose sediment over a broad surface

• One of 3 types of Flow – mass movement of wet

sediment

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Mudflow

• Fine textured sediments that move very quickly

down slope – occur after heavy rainfall

Mudflow deposit

Never-Sumner Range,

Colorado

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Debris Flow • Mud, boulders, trees, etc. flowing down

slope after heavy rainfall – very dangerous

La Conchita, CA

Debris Flow

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Slump • Slide where rock & sediment rotates & moves

down slope along plane concave to surface

• Happens on slopes w/ soft deposits rich in clay

and shale

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Debris Slide

• Type of Landslide – mass of rock, regolith & soil that flows downhill

• Slope failure along a plane roughly parallel to the slope

Madison Slide,

Montana

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Soil Creep

• Slowest mass wasting process

• Force of gravity slowly pulls soil particles down

hill – features of hill shift down hill, as well

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Rockfall • Process in which rocks break free from cliff

faces & rapidly tumble into valley below

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Avalanche • Large mass of snow or rock that suddenly slides

down a mountainside

Starting

Zone

Track

Runout

Weathering and Mass Movements