Geology AS: GL3-Mass Movement

Contains: Mass Movements

Stabilising Slopes

Dover Cliff Collapse


Mass Movement

General features of mass movement:

a. Downslope movement due to gravity.

b. Can occur on slopes on land or under water.

c. The rate of movement varies from very rapid to imperceptibly slow.

Controlling factors of mass movements are:

i. Slope angle.

ii. Nature of slope material (e.g. consolidated/unconsolidated).

iii. Water: -Affects internal cohesion.

-Water pressure reduces friction.

-Water-logged slope material adds to load wieght/pressure.

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Types of Mass Movement

There are two main categories of mass movement:

1. Slumps, slides, falls: The moving mass moves mainly as a single entity.

2. Flows: A large amount of internal disruption in the moving material.

Slumps: i. Outward rotational movement.

ii. Concave slope.

iii. Backward tilt on upper surfaces.

iv. Common along river banks, coasts and over-steepened slopes (e.g. road cuts).

v. Set off by heavy rain, earthquakes and human activity (e.g. quarry blasting).

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Types of Mass Movement

Falls & Slides: i. Commonly caused by undercutting of steep slopes.

ii. Aided by planes of weakness e.g. joints, bedding planes, cleavage.

iii. Resulting deposits are known as talus or scree.

iv. Angle of repose: Steepest angle at which slopes remain stable- 30-->37degrees.

Flows: a. Slurry flows: A moving mass of water-saturated sediment.

b. Granular flows: An unsaturated mass of sediment (air and water).

**Each category can be subdivided on the basis of flow velocity.

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Types of Flows

Slurry Flows: i. Solifluction: Very slow - rarely exceeds 30cm/yr.

Requires saturation for long periods of time.

Example: Periglacial deposits-Chaotic, unsorted material at the base of the slope (head).

ii. Debris flows: Speed ranges from a few metres/yr to >100km/hr.

Poorly sorted but mainly coarse grained.

iii. Mud flows: Rapidly moving (>1km/hr).

Mainly composed of finer sediment.

Example: Volcanic lahars. 

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Types of Flows

Slurry Flows: i. Creep: Extremely slow.

Aided by frost, wetting and drying, plant growth and decay, animal activity, snow.

ii. Earth flows: Speed - 1cm/day to >500m/hour.

Occur in wet, weak regolith.

Thickened at the toe of the slope.

Subject to liquefaction in earthquakes.

iii. Debris avalanches: Very rapid - upto >200km/hr.

Composed of huge masses.

Extremely destructive.

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