mass wasting
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Transcript of mass wasting
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mass wasting
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11:37 am on August 17, 1959
magnitude 7.1? earthquake West Yellowstone, Montana
triggered landslide of85 million tons of rock
sped downslope at 150 km/hrand
produced hurricane force winds
• cars blown into air• valley floor covered by 45 m of rubble• 28 people (campers) were killed
produced waves in Hebgen Lakethat swept over dam
Madison Canyon slide
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mass movements occur everywhere……estimate damage annually in US at $1.5 billion…
…less than 1,000 deaths of 20,000 lost in natural disastersfrom 1925-1975 were from mass movements
not likely to be killed by mass movements, but likely to pay for effects
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classification of mass wasting
rate of movement
type of material
type of movement
cm/year to 100 km/hour
solid bedrock or unconsolidated debris
flow, slide, fall, creep
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flow: viscous fluid
slide: mass remains intact (2 types: landslide; slump)
fall: free-fall of material
types of movement
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fastest
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rock fall
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rock fall in action
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rock fall with talus slope
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hill gives way in coherent mass --large block moves
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surface of movementis
concave
scarp
(type of slide but with rotation)
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submarine landslides (Hawaii)QuickTime™ and a
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landslides on Mars
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move slowly (viscous)1-2 meter/hour
flows: earthflow
solifluction
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earth flowsand
solifluction
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may move quickly overgentle slopes
(1°-2°)
flows: mud flow (mixture of debris and water)
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mud flow at Nevada Huascaran, Peru: killed 18,000 people
before
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after
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dried mudflow
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mudflow on Toutle River from Mt. St. Helens
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flows: soil creep
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downslope motion for creep
freeze/thaw cycle
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permafrost: another example of freeze/thaw
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summary: rates and types of mass wasting
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controlling factors in mass wasting• gravity (friction and slope angle)
• slope composition
• vegetation
• water
• large relief
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gravity: 2 factors in balance1) gravity--pulls object to center of Earth
2) friction--resists block sliding downslope
• component perpendicular (normal) to surface(contributes to friction)
• component parallel (shear) to surface(contributes to sliding)
• depends on angle of slope; slipperiness of slope; and magnitude of normal component of gravity
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relief: change in elevationgreater difference in relief yields greater shear forces
along slopes
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1) small amounts of water
2) excessive amounts of water
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water: two effects
• counteracts normal component of gravity
…water pushed upward…
• reduces friction between surface material and underlying rock
• glues particles by surface tension--”sand castles”
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slope composition (amount of loose rock)….solid bedrock, unconsolidated bedrock (loose or weathered material)
solid rockvery stable even as cliffs…NOT stable if:
• has lots of fractures (cracks)
• is soluble (limestone) such that cavities form
• has layering of “wrong” orientation …bedding (sedimentary rocks) or foliation (metamorphic rocks)
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effect of bedding planes in sedimentary rock
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let’s be smart…recognize and prevent
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slope composition (continued)….solid bedrock, unconsolidated bedrock (loose or weathered material)
unconsolidated material
stability depends on frictional properties…
• is stable until maximum angle…angle of repose…
• is highly dependent on water content
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angle of repose: maximum angle where friction balances gravity
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roots stabilize loose, unconsolidated material…removal (by fire or clear-cutting) leads to mass movement
vegetation
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prevention
water, weight of house, road cut
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improve drainage -- leads to less creep
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modify slopes (where layering dips into roadway)