Tutorial:Gravity Erosion

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Mass Movement
Movements of soil, sediment and rock materials under the action of gravity are known as mass movements. The term mass wasting is also sometimes used but this has a broader meaning encompassing all processes leading to a lowering of the land surface.

Mass movement relies on gravity acting on unstable slopes, but can be exacerbated by other factors that act to destabilise slope material. This includes weathering of surface material, excess water content, freeze-thaw, and reduced vegetation coverage. Types of mass movement include creep, flow, slide, heave and fall.

Many of the terms used here such as slide and flow, solifluction, creep and soil creep are often used synonymously in the literature, and to confuse the situation further any one mass movement event may involve several processes.

Creep
Creep of soil and rock materials downslope solely through the actions of gravity can occur but are very slow. The effects can result in bulging valley sides known as cambering. This sort of creep is quite different from soil creep, which involves heave processes.

Slides and Flows
Slides involve movement of a cohesive block of material along a well defined failure plane. Failure occurs when the stresses due to the weight of overburden (often exacerbated by high moisture content) exceed the strength of material. The majority of slides are small and shallow and may involve soil or rock. This type of movement can be very rapid and very destructive.



A good example of land slip is the scar in the east side of Mam Tor, Derbyshire. This first failed over 3000 years ago on a steep slope in unstable shale. It is almost a kilometre long and half a kilometre wide. Smaller slides continue to affect the slip following periods of heavy rain, eventually leading to the closure of the main road in 1979.

In contrast, flows consist of non-cohesive materials which flow like fluids. Earth and rock flows are often initiated by falls or slides that become flows as the earth or rock breaks up, and they often involve a lot of water.

A common though less dramatic type of earth flow is solifluction, which is the slow downslope movement of saturated soil. Solifluction can occur in any environment and on slopes as low as 1o, however it is most prevalent in periglacial environments where permafrost creates an impermeable sub-surface layer. This is sometimes also known as gelifluction. The terms solifluction and soil creep (a form of heave) are often used synonymously and though the processes are different they are both exacerbated by freeze-thaw and hence often act together.

Fall
To be classed as a fall, the rock or block of earth must fall through air. Undercutting along riverbanks often results in earth falls.

Heave
Heave occurs due to the expansion and contraction of slope material; either soil (soil creep) or fractured rock (talus creep). This expansion and contraction is can be caused by wetting and drying, freezing and thawing (frost creep), temperature changes and burrowing animals. The downslope movement results from particles in the expanding soil being lifted normal to the slope angle and then dropping under the influence of gravity in a vertical direction. On steep grass covered slopes narrow terracettes can form through soil creep.

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