Mass wasting
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Mass wasting, also known as mass movement or slope movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with their own characteristic features, and take place over timescales from seconds to years. Mass wasting occurs on terrestrial and submarine slopes, and has been observed on Earth and Mars.
When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material's strength and cohesion and the amount of internal friction between material help maintain the slope's stability and are known collectively as the slope's shear strength. The steepest angle that a cohesionless slope can maintain without losing its stability is known as its angle of repose. When a slope possesses this angle, its shear strength perfectly counterbalances the force of gravity acting upon it.
Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilised the surface. It may also occur at very high speed, such as in rock slides or landslides, with disastrous consequences. Factors that change the potential of mass wasting include: change in slope angle; weakening of material by weathering; increased water content; changes in vegetation cover; and overloading.
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[edit] Types of mass movement
Types of mass movement are distinguished based on how the soil, regolith or rock moves downslope as a whole.
[edit] Creep
Creep is a long term process. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep.
[edit] Landslides
Where the mass movement has a well-defined zone or plane of sliding, it is called a landslide. This includes rock slides, slumps and sturzstroms.
[edit] Flows
Movement of soil and regolith that more resembles fluid behavior is called a flow. These include avalanches, mudflows, debris flows, earth flow, solifluction, lahars and sturztroms. Water, air and ice are often involved in enabling fluidlike motion of the material.
[edit] Topples
Topples are instances when blocks of rock pivot and fall away from a slope.
[edit] Falls
A fall, including rockfall, is where regolith cascades down a slope, but is not of sufficient volume or viscosity to behave as a flow.
[edit] Triggers of mass wasting
Soil and regolith remain on a hillslope only while the gravitational forces are unable to overcome the frictional forces keeping the material in place (see Slope stability). Factors that reduce the frictional resistance relative to the downslope forces, and thus initiate slope movement, can include:
- seismic shaking
- increased overburden from structures
- increased soil moisture
- reduction of roots holding the soil to bedrock
- undercutting of the slope by excavation or erosion
- weathering by frost heave
- bioturbation
[edit] References
- Monroe, Wicander (2005). The Changing Earth: Exploring Geology and Evolution. Thomson Brooks/Cole. ISBN 0-495-01020-0.
- Selby, M.J. (1993). Hillslope Materials and Processes, 2e. Oxford University Press. ISBN 0-19-874183-9.
