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Mass Wasting and landform development
2
Mass Wasting and landform development
Mass wasting
Def.: the downslope movement of rock and soil under the
direct influence of gravity
3
Role of mass wasting
The step that follows weathering
Transfers debris downslope to stream valleys
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Controls and triggers of mass wasting
Gravity is the controlling force
Important triggers include
1) Saturation of the material with water (rainfall)
– Diminishes particle cohesion
– Water adds weight
Controls and triggers of mass wasting
2) Oversteepening of slopes
– Oversteepening slopes are unstable
– Stable slope angle (angle of repose; 25-45o) is different
for various materials
Important triggers include:
3) Removal of anchoring vegetation
Root systems bind soil
7
“cut and fill” construction
8
Important triggers include:
4) Ground vibrations from earthquakes
– May trigger land slides and result in property damage
– Can cause liquefaction – water saturated surface
materials behave as fluid-like masses that flow
9
Other triggers
Freezing – thawing
Construction
1
Volcanic eruptions
Freeway traffic
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Landslides without triggers
Slope materials weaken over time
Random events that are unpredictable
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Classification
a) Type of material involved
•
•
•
•
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Debris
Mud
Earth
Rock
Classification
b) Type of motion
Fall (free-falling pieces)
Slide (moves along a surface as a coherent mass)
Flow (moves as a chaotic mixture)
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Classification
c) velocity
Fast (avalanche, ~200km/hour)
Slow (creep, mm or cm/year)
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Rock avalanche in Alaska triggered by the 1964 earthquake
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Types of mass wasting
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18
Rock fall, Oregon
Rock fall and Talus Slope, Banff National Park, Canada
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Types of mass wasting
Rockfall
Slump (rotational slide)
Movement of material as a unit along a rotational surface
Occurs along oversteepened slopes
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Slump and earth flow)
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Slump, southwest Montana
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Fig. 9.10, p. 252
Incipient Slump, Missouri River, North Dakota
Slump, Missouri River, North Dakota
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Types of mass wasting
Creep
Extremely slow downhill movement
Aided by soil moisture, ice wedging, and critters and plants
Fig. 9.17, p. 258
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Creep
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Some visible effects of creep
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Types of mass wasting
Fig. 9.18, p. 259
Earthflow
Def.: Downslope movement of clay-rich, water saturated materials
Fig. 9.6, p. 249
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29
An earthflow
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Types of mass wasting
Earthflow near San Francisco
Rockslide (planar slide)
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Blocks of bedrock slide down a slope
Generally very fast and destructive
Gros Ventre rockslide
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Earthquake Lake, Montana
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Types of mass wasting
Fig. 9.13, p. 254
Debris flow and/or mudflow
Soil and regolith with a large amount of water
Often confined to channels
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Lahar
35
Debris flow composed mostly of volcanic materials, on the flanks of volcanoes
Lahar
36
Types of mass wasting
Solifluction
Def.:Slow, downslope movement of water-saturated materials common to permafrost areas
Solifluction lobes, Tien Shan Mountains, Kyrgyztan
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Permafrost
regions
Ground subsidence due to thawing permafrost
Ground
subsidence
due to
permafrost,
Alaska
End of Chapter 9
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