Download - mass wasting

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

mass wasting



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







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



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



fastest

rock fall



rock fall in action

rock fall with talus slope



hill gives way in coherent mass --large block moves





surface of movementis

concave

scarp

(type of slide but with rotation)





submarine landslides (Hawaii)QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

landslides on Mars



move slowly (viscous)1-2 meter/hour

flows: earthflow

solifluction







earth flowsand

solifluction

QuickTime™ and aSorenson Video decompressorare needed to see this picture.



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



flows: soil creep

downslope motion for creep

freeze/thaw cycle







permafrost: another example of freeze/thaw





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





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



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



prevention

water, weight of house, road cut

improve drainage -- leads to less creep

modify slopes (where layering dips into roadway)