1. Mass Movements/ Wasting

2. Erosion Agents Agents: Mass wasting Running water (streams) Glaciers
Wind
Ocean waves
Ground water
Video

3. Mass Movement Includes
• Landslides
• Rock falls
• Avalanches
• Mud flows
• Debris flows
• Creep

5. >100 km/year
<1 cm/year 0%
~40%

6. Mass Movements Material moves downslope due to the pull of gravity
Can happen almost anywhere
Commonly associated with other events (heavy rainfall or earthquakes, for example) and are therefore under-reported
Movements can either be catastrophic (slope failure) or slow and steady (creep)
The rate of the mass movement can be increased by various erosive agents (especially water)

7. Factors in Slope Stability
Gravity
Water
Earth Materials
Triggering Events

8. Risk factors to increase likelihood of mass movement
Gravity - hill slopes more vulnerable (on top of a hill, on the slope, or at the bottom of a hill), modified slopes (road cut, cut flat area to build on, coastal erosion, etc.)
Water - risk is higher when ground is saturated and/or during heavy rains, El Niño events
Earth Materials - loose soils (particularly clay-rich) or fractured rock, and old landslides pose greater risk
Triggering Events - heavy rain during storm, rain after big storms or fires, earthquakes (when ground is saturated?)…are all triggers

9. Gravity & steepening of a slope

11. Angle of Repose Varies for Different Materials

13. Internal Causes for Slope Failure
Water (weight & interaction with clay minerals)
Decreasing rock cohesion
Incompetent/weak material
Adverse geologic structures

14. Controlling Factors Water adds weight Vegetation
BUT a little water makes soil stronger (damp sand castle)
“the water holds the grains together”
Too much water makes soil weak
“floats the grains apart”
Vegetation
Absorbs excess water from soil (increasing strength)
Binds soil together (increasing strength)

15. The Weight of Water
Sedimentary rocks commonly have porosities of %
If pore spaces fill with water, the weight of the material is increased substantially, creating instability

16. Mudflow Flow of watery debris Occurs where lack of vegetation:
Dry climates
Volcanoes
After forest fires
Very fast up to 80 km/hr
Triggered by heavy rainfall
Los Angeles

17. La Conchita, CA
March 1995

18. in exactly the same place…
It happened again in 2004…
in exactly the same place…

20. La Conchita, CA

21. Debris flows or mud flows
Mass movements that behave like fluids
Unlike slides, flows are not controlled by a failure surface, but instead are dominated by internal movements

22. Landslides in the Bay Area

23. 1982 San Mateo County

24. Devil’s slide area on Highway 1 north of Half Moon Bay

25. Rock Falls

29. Classification of Mass Wasting
Rate of movement
Soil creep to slumps to mudflows
Extremely slow (~1mm/year) to very rapid (>100 km/hour)
Soil creep is slowest but ultimately most important - it happens everywhere
Others happen in wet and/or steep places

31. Controlling Factors Most important: Water Climatic controls Ice
Vegetation
Mr. Parr Song

32. Controlling Factors Overgrazing Overloading Reduces vegetation
Weakens soil
Allows water to enter soil
Overloading
Dumping of mine debris
Heavy buildings on steep, denuded hillsides

34. Rotational landslide

41. Creep Downslope movement of soil and uppermost bedrock
Creep happens at too slow of a rate to observe directly
Instead, creep can be identified by it’s effect on objects

42. Most Common Mass Wasting
Freeze
Thaw
Soil creep
gentle slopes
vegetation slows movement
very slow flow (< 1 cm/year)
facilitated by water in soil
by freeze-thaw in colder climates
Freezing of water expands soil. Thawing drops the grains downhill.

45. Weathering Breakdown of rocks at the Earth’s surface
Factors controlling weathering
Presence of water
Gasses
Changing temperatures
Biological organisms
Pressure changes

46. Definitions Parent material is removed by erosion
Transport by wind, water, glaciers
In-place weathering produces soil
Differential weathering for different rock types

47. Mechanical Disintegration
Physically break rocks into smaller pieces
Frost wedging and heaving
Pressure release
Thermal expansion
Biological organisms
Salt Crystal growth

48. Frost Action Water expands when it freezes Necessary conditions
Temps that go below freezing
Free water
Cracks for the water to seep into
Rocks spall off and form a talus slope

50. Figure 5.4, p. 113

51. Pressure Release Rocks that form under high pressure Deep in the earth
Subsequent uplift to the surface

54. Thermal Contraction/Expansion
Rocks expand when heated and shrink when cooled
Interior remains at a steady temperature
Resulting in fracturing
Important during fires
On the Moon

55. Biological Organisms Tree roots and trunks
Burrowing insects, worms, animals
Earthworms
Help to expose deeper layers to water
Mix soils and circulate them to the surface

57. Chemical Weathering Rainwater with carbon dioxide (CO2)
Forms a weak acid
Acid attacks the rock
Results depend on what type of rock
Temperature, acidity, water, etc.

58. Granite and Acids
Granite in the presence of carbonic acid will weather and break apart
The Iron (Fe) will rust (oxidize)
It usually remains in the soil
The Aluminum (Al), Potassium (K), and Silica (SiO2) turn into clays and remain in the soil

59. Some of the Products Wash Away
The Calcium (Ca), sodium (Na), and Magnesium (Mg) wash away
The Ca goes into the sea and is used by sea creatures to make their shells
The Na makes the sea salty
The Mg reacts with rocks at the spreading centers to make new rocks

60. Soil What is left behind is soil
More heat, more water --> more stuff is washed away and the soil has fewer minerals in it.
Very dry areas, the Ca, Mg, and even Na remain and the soil is very salty

61. Badlands Erosion from the Rockies, deposited in the great plains.
Soil is very clay rich
The clays create a hard surface that channels the water, washes away plants
The clays are unstable, slopes slump down, washing away plants
Water doesn’t soak in well.

62. Magic School Bus - Erosion
Bill Nye - Erosion
Magic School Bus - Erosion