Mass Wasting Lecture Notes

GeoClassroom Physical Geology Historical Geology Structure Lab

Physical Geology

Mass Wasting

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Mass Movement:

- downslope movement of material under the force of gravity

- several factors influence this:

- angle of repose: steepest slope on which loose material will remain
- saturation of material with water
- vibrations from earthquakes
- undercutting
- freeze and thawing

1) Creep

- extremely slow downslope movment of soil and rock debris

- can be seen in titled signs and posts

- generally results from the freezing and thawing of the regolith, causing the soil above to heave.

- 10-15 degress slopes in humid climates: 1-2 mm/year

- 5-10 mm/year in semiarid regions w/cold winters

-SOLIFLUCTION: creep which occurs in polar regions

- permafost: permanently frozen ground
- in summers, meltwater accumulates on top of the permafrost and cannot penetrate it, this saturates the soil and can cause it to move.

2) Debris flow

- mix of rock, mud, and water which flows downslope as a viscous fluid.

- greater the amount of water, the faster the flow.

- mudflows are a type of debris flow w/ lots of silt & clay sized particles.

- water content as high as 30%

- common in arid and semiarid regions along steep sided gullies

- like the hills surrounding LA

- generally results from saturation of unconsolidated material with water through abnormal precip.

- angle of repose is much less for dry material than wet material.

- LAHARS are a type of volcanic mudflow, where loose unconsolidated & hot volcanic ash becomes wet and flows downhill.

- one of the most destructive things about Pinatubo: tropical storm hit the day after the main eruption, filled valleys ther with several hundred feet worth of mud.

3) Landslides

- movement along a well-defined slippage plane.

- landslides move as a unit, or block (called a slump block), unlike mudflows and creep.

- leaves behind a distinct incision, or scar, along the failure plane

- many started through earthquakes, loosining blocks of material along fractures or bedding planes.

Other types of mass movement:

Rockslides: rapid movement of large masses of rock downslope along beddingplane, joint, or fracture.

Rockfall: free-fall of single rock fragments down the face of a cliff.

- contributes to the formation of a talus slope at the bottom of the cliff

- frost wedging is the most important process here

- Both of these processes contribute in the mechanical weathering of a mountain from a grand peak to a slight hill, over time of course.

- Mass movement contributes as part of a slope system, from mountain to stream transport.

- slope can be steeper or more gradual, depending on the type of rock the stream is flowing on (whether the water can downcut into the rock at all), and what type of mass movement process dominates on the slope.