Geology 111G/Lecture 14

Mass Wasting

Factors that Cause Mass Movement

Triggers or Causes

Types of Flows

Consolidated

Unconsolidated

I. Mass wasting: processes by which masses of soil and rock move down hill under the influence of gravity. Three factors influence mass movement:

A. Nature of Slope Materials. Unconsolidated vs consolidated

1. Unconsolidated: loose uncemented sediment is unstable, assumes an equilibrium slope called the angle of repose, which is 30-40° in dry sand. Angle increases with grain size.

2. Consolidated: Compactecd and cemented material. This type of material tends to form steeper slopes. May become unstable when oversteepened-either naturally or unnaturally-or denuded of vegetation.

B. Water Content: Water in pore spaces tends to decrease the internal friction among grains, permitting movement of particles.

C. Steepness and slope instability:

1. Soft rocks (shales, volcanic ashes) tend to form gentle slopes that become covered with unconsolidated material during weathering.

2. Hard rocks form steeper slopes, depending upon climate.

II. Triggers or causes for mass movements: Combinations of factors tend to render hill sides unstable and make movement probable.

A. Shocks, such as earthquakes.

B. Slope modification, particularly local steepening or removal of toe of slope. The latter is a natural result of undercutting by streams or wave erosion.

C. Saturation of slope material. Heavy or prolonged rain may saturate soil or upper layers of sediment

D. Volcanic eruptions. Rapid production of water-saturated, unconsolidated material on slopes creates a slumping and mudflow hazard.

III. Types of Mass Movements: these are classified by the nature of the material, the speed of movement and the flow mechanism (ie, sliding vs flowing).

A. Rock or Consolidated Mass Movements.

1. Rockfall: Free-falling bedrock material loosened from a steep face.

2. Rock slide: large masses of bedrock move as a loosely defined unit.

3. Rock avalanche: large masses of broken rocky material that flow rather than sliding. High velocity, they flow at `0's to 100's km/hour. Very turbulent, possibly cushioned by air trapped beneath the material.

B. Unconsolidated mass movements.

1. Creep: very slow movement of unconsolidated slope materials, 1-10 mm/yr. Promoted by freezing-thawing or wetting-drying cycles.

2. Earthflow: movement of fine-grained material at a few km/hour.

3. Debris flow: coarser-grained, travels a few km/hour.

4. Debris avalanche: soils and rock that have high water content and travel rapidly, to 70 km/hour, almost as fast as water.

5. Slump: slow slide of semi- to unconsolidated material above a curved, discrete failure plane.

6. Debris slide: slump-like feature composed of more than one unit, travels faster than a slump.