mass wasting

1. mass wasting

2. 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 • valley floor covered by 45 m of rubble • 28 people (campers) were killed Madison Canyon slide

6. 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

7. classification of mass wasting rate of movement cm/year to 100 km/hour type of material solid bedrock or unconsolidated debris type of movement flow, slide, fall, creep

8. types of movement slide: mass remains intact (2 types: landslide; slump) flow: viscous fluid fall: free-fall of material

9. fastest

10. rock fall

11. rock fall in action

12. rock fall with talus slope

13. hill gives way in coherent mass --large block moves

15. (type of slide but with rotation) surface of movement is concave scarp

17. submarine landslides (Hawaii) landslides on Mars

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

19. earth flows and solifluction

21. flows: mud flow (mixture of debris and water) may move quickly over gentle slopes (1°-2°)

22. mud flow at Nevada Huascaran, Peru: killed 18,000 people before

23. after

26. dried mudflow

27. mudflow on Toutle River from Mt. St. Helens

28. flows: soil creep

29. downslope motion for creep freeze/thaw cycle

34. permafrost: another example of freeze/thaw

35. summary: rates and types of mass wasting

36. controlling factors in mass wasting • gravity (friction and slope angle) • large relief • water • slope composition • vegetation

37. gravity: 2 factors in balance 1) gravity--pulls object to center of Earth • component perpendicular (normal) to surface (contributes to friction) • component parallel (shear) to surface (contributes to sliding) 2) friction--resists block sliding downslope • depends on angle of slope; slipperiness of slope; and magnitude of normal component of gravity

38. relief: change in elevation greater difference in relief yields greater shear forces along slopes

39. water: two effects 1) small amounts of water • glues particles by surface tension--”sand castles” 2) excessive amounts of water • reduces friction between surface material and underlying rock • counteracts normal component of gravity …water pushed upward…

40. 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)

41. effect of bedding planes in sedimentary rock

42. let’s be smart…recognize and prevent

43. slope composition (continued) ….solid bedrock, unconsolidated bedrock (loose or weathered material) unconsolidated material stability depends on frictional properties… • is highly dependent on water content • is stable until maximum angle …angle of repose…

44. angle of repose: maximum angle where friction balances gravity

45. vegetation roots stabilize loose, unconsolidated material …removal (by fire or clear-cutting) leads to mass movement

47. prevention water, weight of house, road cut

48. improve drainage -- leads to less creep

49. modify slopes (where layering dips into roadway)