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Mass Movements. Mass movement - The down slope movement of earth material under the influence of gravity. Figure 10.3. Key role in mass movement is gravity. In order for the material to move down slope, it must overcome Friction –surface resistance to relative motion.

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Figure 10.3

Key role in mass movement is gravity


In order for the material to move down slope, it must overcome

  • Friction –surface resistance to relative motion.
  • Inertia – tendency of an object at rest to remain at rest.
internal causes of slope failure
Internal Causes of Slope Failure

Inherently weak earth materials

2 pyroclastic materials light colored tuff from santorini produced 3500 year ago in minoan eruption
2. Pyroclastic materials – Light colored tuff from Santorini produced 3500 year ago in Minoan eruption.

54 gal.


Internal problems caused by water

1. Adds weight to a slope

8.34 lb/gallon

54 gal. = 457.36 lb!

2 saturation increases the pore water pressure
2. Saturation increases the pore-water pressure

Low pore water pressure High pore water pressure

the vaiont dam failure
The Vaiont Dam Failure

Vaiont, Italy 1963

Setting: Dam on river in a glacially carved canyon.


Some Factors Leading to Slope Failure

  • Valley is a syncline
  • Cavernous weathering of limestone rock
  • Valley downcutting by river
  • Slope saturation - water from reservoir and heavy rains.

Rotational slide- slump

  • Curved failure plane.
  • Head moves down, toe moves up and out.

Translational slide – landslide

  • Mass moves down and out.
  • Moves far and fast.
  • Primary causes: heavy rains or earthquakes.

Earthflow- Downslope movement of a fluid layer of mainly soil and weathered rock material that is saturated with water.


Earthflows often begin as slumps.

Usually, like slumps, they do not move very fast.


Debris flow- Downslope movement of a fluid mixture of mud and larger rock debris.

Venezuela 1999


Key differences between earthflows and debris flows:

  • A debris flow moves faster; meters/second.
  • A debris flow carries larger rock material.
debris flows can carry large boulders because their density is 1 5 2 0 times that of water
Debris flows can carry large boulders because their density is 1.5 – 2.0 times that of water.


Causes of debris flows:

  • Most commonly heavy rains or rapid snow melt.
  • Volcanic eruptions – a lahar is a type of debris flow.
  • Earthquakes

La Conchita

1995 slump/earthflow

2005 debris avalanche


Debris avalanches


Small mass movements usually move horizontally less than twice their vertical distance of fall.

With very large mass movements they may move up to 25 times the distance of their vertical fall.


Sturzstroms- Long-run out debris flows.

Blackhawk sturzstrom 300 million m3

Drop .75 mi. Flow 5.6 mi.


1970 subduction zone earthquake

100 million m3 of rock, ice glacial sediments, and water.

Speed 175 - 210mph

Devastated area 8.7 mi2

18,000 deaths

what kind of movement is this
What kind of movement is this?