Mass Wasting
Frank Press, Raymond Siever,
John Grotzinger, and Thomas H. Jordan
Understanding Earth
Process by which material moves downslope under the force of gravity
Fourth Edition
Chapter 12:
Mass
Wasting
Copyright © 2004 by W. H. Freeman & Company
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Factors Influencing Mass Movement
Mass Movement is Classified on the Basis of:
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• Nature of material
• Velocity of movement
• Nature of movement
Nature of slope materials Steepness of slope
Water content
Slope stability
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Mass Movement Depends on Water Content
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Water Content
Lose of vegetations and root systems increases susceptibility of soils to erosion and mass movement
Steep slopes in damp sand maintained by moisture between grains
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Types of Rock Mass Movement
• Rock fall
• Rock slide
• Rock avalanche
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Rock Fall
Rockfall in Zion National Park
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Rockslide
Rockslide
The rapid movement of large blocks of detached bedrock sliding more or less as a unit
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Rockslide
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Rock Avalanche
The rapid* mass movement of broken rock material, often riding on a cushion of trapped air. Usually triggered b y an earthquake.
*10’s to 100’s of km/hr
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Rock Avalanches Triggered by an Earthquake (Nov. 3, 2002) in Alaska
Rock Avalanche
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Types of Unconsolidated Mass Movement
Creep
Unconsolidated Flows
The downhill movement of soil and other debris, typically at rates of about 1 to 10 mm/year
Creep
Earth Flow
Increased
velocity
Debris Flow Earth Flow
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Evidence of Creep
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Earthflow
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Earthflow
A fluid movement of relatively fine‐
grained material, e.g. soil, weathered shale, and clay.
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Debris Flow
A fluid mass movement of rock fragments supported by a muddy matrix. May move at speeds of up to 100 km/hr
Earthflow
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Debris Flow
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Debris Flow
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Mudflow
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Mudflow
A flowing mass of material (mostly finer than sand, along with some rock debris) containing a large amount of water. It may travel large distances and high speeds, and carry particles as large as a house!
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Debris Avalanche
Debris Avalanche
Fast* downhill movements of soil and rock, usually occurring in humid mountainous regions.
* Up to 280 km/hr
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Mt. Huascaran, Peru (before 1970)
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Mt. Huascaran, Peru (after 1970)
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Types of Unconsolidated Mass Movement
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Slump
Unconsolidated Slides and Falls
A slow slide of unconsolidated material that travels as a unit.
Slump
Increased
velocity
Debris slide
Debris avalanche
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Slump
Slump
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Debris Slide
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Debris Slide
A type of mass of material in which rock material and soil move largely as one or more units along planes of weakness.
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Debris Slide
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Ways to Reduce Losses Due to Landslides Include:
• Avoid construction in areas prone to mass movement
• Build in a way that does not make naturally stable slope unstable
• Engineer water drainage to prevent strata to become water saturated and prone to fail
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