Mass Wasting
Today:
• Definition
• Factors Influencing
Mass Wasting
• Classification &
Examples
Slide La Conchita CA
Definition of Mass Wasting
• Down slope
displacement of soil
and rock under direct
influence of gravity
– Rapid (e.g., avalanche)
– Slow (e.g., creep)
• Result:
– Landscape evolution
– Hazard
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Landscape evolution:
• Mass wasting one of main processes of sediment transport
• Results in change in shape of slopes (hills, cliffs, mountains, river
banks etc.)
• Other processes that move sediment and shape landscape Rivers,
Glaciers, Wave, Wind
Mass Wasting: Examples
Slump and Earth flow
Rock Falls
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More Examples:
slide
Creep
Simple model for MW
• MW occurs when
Stabilizing
– force of gravity
exceeds stabilizing
forces
– Friction, cohesion &
strength
• Consider block on
inclined plane...
Normal
Gravity
Down slope
If Down slope force > Stabilizing force then...
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Factors influencing Mass Wasting: Slope
• Increase Slope
– Increase down slope forces
– Movement occurs when down-slope forces exceed
stabilizing forces
• Rives and waves cut steep cliffs. (so does man)
Factors influencing mass wasting: Water
• Loading:
– Add water = add mass
– Increases down slope forces
• Lubrication:
– Decrease friction
• Cohesion
– Add water and increase
cohesion due to surface
tension (e.g. sand castles)
– Too much water decreases
cohesion when sediment
grains no longer touch (e.g.
wet sand castle falls).
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Factors influencing mass wasting:
• Deforestation
– Root systems bind soil
– Remove it, and cohesion
decreases
• Earthquakes
– Ground shaking can
trigger mass wasting
Types of Mass Wasting
Falls
Slides
Flows
Falls = movement through air
Slides = block maintains contact with surface
Flows = fluid movement in water or air flowing down slope
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Rock Falls (frost wedging)
Slides
• Block slides
– down planar surface
– Maintains contact
– Surfaces can be bedding, fracture,
jointing etc.
• Examples: Rockslides...
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• Turtle Mt. Slide
(Alberta, Can)
– 1903
– 40 x 106 yrd
– < 2 min
• EQ Lake - Madison R.
– 1959
– 80 x 106 ton Block
disaggregated
– 28 killed
Vaiont Italy, 1963
• Bedrock dips into canyon
• Reservoir filled, water
wicked up bedding
(lubrication)
• 240 x 106 m3
• 90 m wave
• 7 minutes 2600 dead
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Slump (another type of slide)
• Slide along curved
surface
• Slump block rotates
• Usually caused by
erosion at base of
slope
• Common hazard to
homes
Slump examples
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Flows: Fluid movement of rock in water or air
• Creep
– Caused by frost heave (or expansion of
clay)
– Volumetrically the most significant form
of mass waisting.
Examples of Creep
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Flow: Solifluction = liquefied soil over impermeable base
Mudflows
• Cause: rapid mixing of
water with
sediment/rock
• Snow melt on
volcanoes (Lahar)
– Mt. Rainer
• Deforested hills S. CA
– enhanced by forest
fires
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Mudflow in San Juan Mt. CO
Flow: Debris Flow & Avalanche
• Most Rapid
• Begins as slide
• Block breaks into
small pieced
• Cushioned by
air/ice/water
• Flow great distances
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Mt. Huascaran, vilages of Yungay & Ranrahirca Peru
• Before 1970:
• Village of Yungay, Peru
• Off - shore EQ (benioff)
M~7
• After event:
• Slide block of ice and rock
(>2.2 x 106 m3)
• Debris flow > 9 km
distance
• > 100 mph
• >20,000 killed
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