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Chapter 2 Lecture McKnight's Physical

Geography

Chapter 15Preliminaries to Erosion:

Weathering andMass Wasting

Modified by AJ Allred for Geography 1000

Salt Lake Community College

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Original by Andrew MercerMississippi State University

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Learning Goals of Chapter 15• Define denudation, weathering, mass wasting, and

erosion.

• Differentiate between mechanical, chemical, and biotic weathering.

• Explain frost wedging, salt wedging, temperature changes, exfoliation, angle of repose, and oxidation.

• Identify mass wasting, glaciation, landslide, slump, earthflow, mudflow, creep and solifluction.

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The sun uses water & gravity to wear down landforms

– Weathering

– Mass wasting

– Erosion

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Weathering and Rock Openings

Notice how water gradually opens joints by gravity flow, freeze/expansion and chemically dissolving rocks.

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More examples of weathering into joints

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Weathering Agents

• Oxygen, carbon dioxide, and water

• Temperature changes – freeze/thaw, expand/contract

• Water penetration in bedrock openings

• Biotic agents – Burrowing and/or plant roots

– Chemical alteration

• Three principal categories– Mechanical

– Chemical

– Biotic

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Weathering Agents

• Mechanical Weathering– Physical disintegration of rock without changes to its

chemical composition

• Frost wedging– Freeze–thaw action of water

– Ice wedges downward in openings

– Ice melts and water falls farther into larger opening

– Process repeats

– Frost shattering

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Weathering Agents

• Salt wedging– Salt left behind from evaporated water collects and pries

apart rock openings

• Temperature changes– Diurnal and seasonal temperature fluctuations modify

volumes slightly

– Fracturing of rock over long time scales

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Weathering Agents

• Exfoliation– Curved layers peel off of bedrock

– Exfoliation dome

– Unloading through erosion

– Hydration

• Other mechanical weathering processes– Chemical and

biotic impacts on mechanical weathering

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Biological Weathering Agents

– Plant roots break apart rocks, and may also hold soils together.

– Burrowing animals promote water entry and chemical change to soil and rock

– Lichens slowly consumes rock, turning it into soil and dust.

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Mass Wasting

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Mass Wasting

• Fall– Rockfall

– Talus/Scree

– Uniform accumulation of rockfall material – talus apron

– Material tends to collect in cone shaped heaps – talus cones

– Talus cones grow up the mountain

– Slow talus flow in glaciers – rock glaciers

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Slumping – semi-wet material

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Very wet soil can flow

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Slow Creep

Walls, poles, head stones and other structures slowly move downhill over many years.