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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
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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