Mass Movement - downslope transfer of material due DIRECTLY TO GRAVITY.
Mass movement may be very slow (mm/yr) or very fast (up to 300 km/hr!)

Three Styles of Mass Movement Processes
1.  Falls - rocks fall off cliffs
 
 
 

2.  Slides - blocks slide down the hill relatively intact
 
 
 

3.  Flows - wet material, flows like a slow liquid
 
 
 

Four Examples of Mass Movement Types (there are others)

A.  Creep:  gradual downslope movement of regolith (surficial materials - soil, sediment)
- contributing factors include frost heave, too much water
 
 

B.  Slumps:  rotational failure along a concave upward surface.
- common from oversteepening of roadcuts
- scarp and toe
 
 
 

C.  Rock or Debris Falls:
- form talus slopes, angle of repose ~ 30*
- rock glaciers
- may turn into an avalanche - a sediment flow riding on a carpet of air.
- to escape...
 

D.  Debris and Mud Flows:  unconsolidated sediment plus enough water to make it very fluid
- very high viscosity, can carry small boulders in suspension
- lahars are mud flows formed from volcanic ash - may be still very hot!
 
 
 
 

Roles of Slope and Friction.  Draw a 30* slope with a sliding block.
 
 
 
 
 

- Driving Forces: gravity acting parallel to the slope.  Flat = 0.  Steep = almost G.
- Resisting Forces: friction from Gravity measured perpendicular to slope.  Flat = G*f.  Steep = almost 0*f.
***Greater Slope => greater Driving Force, lower Resisting Force.
 
 
 
 
 

Role of Hillside Materials - strong materials are less likely to fall down!
- unconsolidated (sand, gravel, rocks) are weak
:: Angle of Repose (= 34° for sand).
 
 
 

- cohesive materials (clays) stick together and resist failure (to varying degrees)
- shales are weaker than sandstones or limestones.
- fractures (and bedding planes) in rocks make them much weaker, more easily Broken.
- tilted rocks are more dangerous - tilted bedding planes!
 
 
 
 
 

 Role of Water
- too much water saturates the material, the grains have less net gravitational force
 

- higher fluid pressure opens pores and fractures, reducing friction.
 

- a wee bit makes unconsolidated materials stick together - sand castles (capillary attraction)
 

Role of Earthquakes
- shake the ground where things are metastable and they all come tumbling down.
 
 
 
 

 Viaont Dam Disaster, Italy, 1963.
- big, deep reservoir with a high dam
- tilted sed rocks on both sides of valley
- limestones interbedded with (weak) shale
- lots of rain in September, 1963
- gradual creep up to a few cm/day, then 40 cm/day
- grazing animals walked away a week before the slide
- half the mountainside let loose and slid down into the reservoir
- Water displaced 250 meters up the far side of the valley, over/around the dam and down the valley killing 2600 people in about 7 minutes.
- Causes: