mass wasting

11:37 am on August 17, 1959 magnitude 7.1? earthquake West Yellowstone, Montana triggered landslide of 85 million tons of rock sped downslope at 150 km/hr and 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 Lake that 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 disasters from 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 movement is concave scarp (type of slide but with rotation)

submarine landslides (Hawaii) landslides on Mars

move slowly (viscous) 1-2 meter/hour flows: earthflow solifluction

earth flows and solifluction

may move quickly over gentle 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 balance 1) 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 elevation greater 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 rock very 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)