mass wasting
DESCRIPTION
mass wasting. 11:37 am on August 17, 1959. magnitude 7.1? earthquake West Yellowstone, Montana. produced waves in Hebgen Lake that swept over dam. triggered landslide of 85 million tons of rock. sped downslope at 150 km/hr and produced hurricane force winds. • cars blown into air - PowerPoint PPT PresentationTRANSCRIPT
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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
flow: viscous fluid
slide: mass remains intact (2 types: landslide; slump)
fall: free-fall of material
types of movement
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fastest
rock fall
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rock fall in action
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)
mud flow at Nevada Huascaran, Peru: killed 18,000 people
before
after
dried mudflow
mudflow on Toutle River from Mt. St. Helens
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flows: soil creep
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
controlling factors in mass wasting• gravity (friction and slope angle)
• slope composition
• vegetation
• water
• large relief
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
relief: change in elevationgreater difference in relief yields greater shear forces
along slopes
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”
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)
effect of bedding planes in sedimentary rock
let’s be smart…recognize and prevent
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
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
improve drainage -- leads to less creep
modify slopes (where layering dips into roadway)