1. Chapter 15 Mass Wasting

2. Mass wasting
The downslope movement of rock, regolith, soil, etc. under the direct influence of gravity
Does not require a transporting medium (water, air, ice)

3. Plays a role in developing landforms (surface features)
Weathering, by itself, doesn’t produce significant landforms
Landforms are developed as weathering products are removed from their original place

4. Figure 15.2

5. Weathering weakens & breaks rock apart
Mass wasting transports the debris downslope
Generally to a stream, which carries material away
This material may then be deposited anywhere downstream
Ultimate destination – the ocean

6. For mass wasting to occur, need slopes
that is, differences in elevation
Most rapid mass wasting occurs in rugged, geologically-young mountains
as mtn building subsides, mass wasting & erosion lowers the land
leads to a gentler terrain

7. Causes of mass wasting
Water
Slope
Vegetation
Other “triggers”

8. Water’s role a major “trigger”
heavy rains or snowmelt saturate surface materials
pores in sediment fill w/water, reducing cohesion between particles
particles can then slide apart easily
(ex.: use slightly wet sand to make a sand castle. Add more water to the sand, what happens?)

9. Figure 15.4

10. Clay
A “dry” clay is fairly rigid
Wet clay is very “slick”
Water also adds mass (“weight”), and helps to start movement of material downslope

11. Effects of slope
If slope is too steep, material will move (oversteepening)
Examples:
Stream undercutting a valley wall
Waves pounding against a cliff
Human activity (that is, stupidity)

12. Angle of repose
A characteristic of unconsolidated, granular materials (sand size or larger)
The steepest angle where a pile of the material is stable
Generally 25 – 40º from horizontal
Larger, more angular fragments form the steepest slopes

13. Vegetation Root systems bind soil materials together
Plants also shield underlying materials from erosional effects of raindrops
More important when the vegetation removed
Build a house on a scenic hillside, remove the natural vegetation so you have a view & a “normal” house – wait for a few heavy rains
Wildfires (or clear-cutting)
Removes vegetation
Land exposed for erosion

14. Other triggers
Earthquakes – can dislodge rock & unconsolidated materials that were semi-stable
Liquefaction – shaking during EQ causes water-saturated sediments to lose strength & behave as a fluid
Some movements occur without any obvious trigger

15. Mass Wasting Processes
4 main processes
Slump
Rockslide
Debris flow
Earthflow

16. These classes based on:
Type of material
Kind of motion
Rate of motion

17. Type of material Type of motion Unconsolidated or bedrock
Fall, slide, or flow

18. Fall: The freefall of detached, individual pieces (of any size)
“Watch for falling rock”

19. Figure 15.8B

20. Figure 15.8A

21. Slide
The material remains fairly coherent
Moves along a well-defined surface
This surface may be parallel to the slope, or curved
“Landslide” – geologically, no definition. Yet, we all use the term to describe much mass wasting.

22. Flow Material moves downslope as a thick fluid
Material almost always saturated w/water

23. Rate of movement Most movement is quite slow (more in a bit)
Very rapid movement – generally asso. with rock avalanches
Speeds >125 mph
As best we can tell, air becomes entrapped beneath a mass of debris, creating a “rock hovercraft”

24. Slumps
Downward sliding of a mass of rock/ unconsolidated material along a curved surface
Material generally doesn’t move very fast or travel very far
Often happens in multiple “blocks”

25. Figure 15.7A

26. Slumps often the result of oversteepening of a slope
Examples (again)
Valley wall cut by river
Waves
Overloading a slope (material deposited on top, humans build on edge of a slope)

27. Figure 15.12

28. La Conchita, California, 1995
U.S. Geological Survey photo by R. L. Schuster

29. Rockslide Blocks of bedrock break loose, slide downslope
If material relatively unconsolidated, called a debris slide
These tend to be the fastest & most destructive movements
In areas where rock strata are inclined, or has joints/fractures parallel to the slope

30. Figure 15.7B
Editor’s goof

31. Figure 15.14

32. Debris flow
Involves the flow of soil/regolith containing large amounts of water
Also called mudflows
Generally seen in semiarid mountainous regions, and slopes of some volcanoes
Flows often follow existing canyons & stream channels

33. Figure 15.7C

34. Flows in semi-arid areas
Heavy rain or rapid snowmelt results in sudden floods
Large amounts of soil, etc., washed into nearby streams
Rate of flow depends on area slopes & water content of material
If flow dense enough, can carry or push large boulders, trees, houses

35. Lahars
A type of debris flow – defined as having 40% or more by weight of volcanic debris
Mostly volcanic materials on the flanks of volcanoes
May occur during eruption or when volcano quiet
Unconsolidated layers of ash & debris become water saturated & flow downslope
Caused by heavy rains, or melting of snow/ice as a result of pre-eruption heatflow

36. Earthflow
Generally occurs on hillsides in humid areas as a result of heavy precipitation or snowmelt
Material involved often rich in clay/silt-sized particles
Movement generally slow (up to several meters per day)
And, this isn’t the slowest movement

37. Figure 15.7D

38. Creep The slowest of the movements
The gradual downhill movement of soil/regolith
Often results from the alternate expansion/contraction of surficial materials by freeze/thaw cycles or wet/dry periods

39. Figure 15.18

40. Other causes of creep: Impact of raindrops
Disturbance of material by plant roots and/or burrowing critters
Saturation of ground with water (that pesky fluid again!)

41. Creep is a very slow process
We can’t sit there and observe it happening (unless you have little else to do)
We can see the effects of it, after a bit of time

42. Figure 15.19

43. Soil Creep, Yosemite National Park, California (A.A. Webb, Oct 2000) 

44. Fenceposts and telephone poles, on the other hand, don't grow vertically and merely tilt on creeping slopes.

45. Another fence

46. Some other ideas Solifluction
The down-slope flow of water-saturated soil
Occurs where water can’t escape from a saturated surface layer
Usually due to an underlying dense clay layer or an impermeable rock layer
Lewis Hills, Gros Morne National Park, Newfoundland

47. Geologic dictionary defines this as occurring in high elevations in regions underlain by frozen ground, which acts as a downward barrier to water movement
Rate of movement 0.5 to 15 cm/yr

48. Permafrost
A layer of unconsolidated material containing water which is frozen
May be 30 cm to over 1000 m thick (1 to 3000 ft)
When surface thaws (for whatever reason), water cannot seep down due to frozen material deeper down
Surface becomes a mushy muck

49. Figure 15.G

50. Figure 15.I
Log cabin shown here; any structure which generates heat

51. Upper, thawed layer can slowly flow Alaska pipeline
A few years back
Pipeline from the North Slope to the coast
Oil needs to be warm to flow
The pipeline would wreck havoc to Alaska’s permafrost area, not to mention the wildlife (oh, that’s biology or environmental science)

53. Geological activity has damaged the pipeline on several occasions
Geological activity has damaged the pipeline on several occasions. Since its completion in 1977, the pipeline has transported over 15 billion barrels of oil.

54. View of the pipeline's underside, heat pipes, and radiators
View of the pipeline's underside, heat pipes, and radiators. Outside Fairbanks, Alaska August 1980.

55. Mass wasting also occurs below the ocean surface
These just aren’t seen as much
Slides
Along flanks of volcanic islands & seamounts
Along continental margins, especially near deltas
Contribute to tsunamis

56. A final thought on mass wasting…….

57. The depth of weathering is visible in the distant cut
The depth of weathering is visible in the distant cut. These homes (costing $200,000 and up, when the photo was taken) were built on landslide deposits. The landslide deposits contain the ruins of the previous housing development built here. (