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 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 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!

Rock Fall and Landslide in Yosemite at Curry Village – Oct. 8, 2008

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 or slope angle!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)!