mass wasting

mass movement

or mass wasting

Bulk movements of soil and rock debris down slopes, or the sinking of confined areas of the Earth's ground surface. The term mass wasting refers only to gravity-driven processes that move large masses of earthen material from one place to another. The term mass movement includes the sinking of confined areas.

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Mass wasting, also known as 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 both terrestrial and submarine slopes, and has been observed on Earth, Mars, and Venus.

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, both immediate and delayed, e.g., resulting from the formation of landslide dams.

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.

The importance of water in mass wasting

Water can increase or decrease the stability of a slope depending on the amount present. Small amounts of water can strengthen soils because the surface tension of water gives the soil a lot of cohesion. This allows the soil to resist erosion better than if it were dry. If too much water is present the water may act as a lubricant, accelerating the erosion process and resulting in different types of mass wasting (i.e. mudflows, landslides, etc). A good example of this is to think of a sand castle. Water must be mixed with sand in order for the castle to keep its shape. If too much water is added the sand washes away, if not enough water is added the sand falls and can not keep its shape.

Types of mass movement

Types of mass movement are distinguished based on how the soil, regolith or rock moves downslope as a whole.

Creeps

Downhill 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. The creep makes trees and other shrubs curve to reach the sun light. These often trigger land slides because the dirt underneath is not very strong. The trees most of the time die out because of lack of water and sun, and these rarely happen in wet climates. Caused by freezing then thawing, or hot then cold temperature, it causes surface soils to move up then down, inching its way towards the bottom of the slope forming terracettes. This happens at a rate that is not noticeable to the naked eye. and it also happens in the tropical regions.

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.

It is also one of the common classification of mass wasting.

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 sturzstroms. Water, air and ice are often involved in enabling fluidlike motion of the material.

Topples

Topples are instances when blocks of rock pivot and fall away from a slope.

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. Falls are promoted in rocks which are characterised by presence of vertical cracks. Falls are a result of undercutting of water as well as undercutting of waves. They usually occur at very steep slopes such as a cliff face. The rock material may be loosened by earthquakes, rain, plant-root wedging, expanding ice, among other things. The accumulation of rock material that has fallen resides at the base of the structure and is know as talus....

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

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.

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