Mass wasting or mass movement -I

What is mass wasting?

Mass wasting, is the down-slope movement of a mass of sediment and/or rock mainly due to the force of gravity.

What is mass in mass wasting?

The “mass” part of the name implies that a somewhat coherent grouping of sediment/rock begins moving downward due to the force of gravity, and usually in combination with some triggering mechanism such as an earthquake or rapid erosion of the base of a slope.

What is wasting in mass wasting?

The “wasting” part of mass wasting means that a cliff or mountain slope is diminishing in size, or wasting away. This can occur suddenly with tremendous destructive force, or very slowly with only a gradual alteration of Earth’s surface over a period of many years. Given enough time and repetition, the different types of mass wasting can play significant roles in reducing a tall mountain to a mound of low rolling hills, or in widening a narrow canyon into a broad stream valley.

How is gravity a factor in mass wasting?

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The force of gravity is downward, towards Earth’s center. As gravity pulls downward on material comprising a tilted or sloping portion of Earth’s surface, a translational force is formed within the slope sediment/rock. This force creates shear stress within the slope’s material, reducing the slope’s strength and making it more prone to mass wasting.

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So, since gravity is always in effect, there is always the possibility of mass wasting of a sloped surface. Note that the steeper the slope, the more in line its material components (sediment and/or rock) are with gravity, so the more likely is mass wasting of that slope.

What are the causes of mass wasting?

Since gravity is always exerting a downward force on a slope, producing shear stress within the slope, slopes are predisposed to move downward (to fail or mass waste). Sudden slope failures are usually associated with a singular triggering mechanism which overcomes a slope’s internal resistance to shear stress, its shear strength. Slower, long-duration types of mass wasting are associated with gradual, repetitive processes resulting from low levels of shear stress within the slope.

What are the triggering mechanisms for sudden mass-wasting events?

1) Earthquake – The vigorous shaking of an already-unstable slope by seismic waves may cause it to fail. Typically, the higher the magnitude of an earthquake, the more mass wasting will occur.

2) Over-steepening of a slope – A slope whose material is stable at a fairly gentle slope angle may become unstable if its slope angle becomes steeper. This can occur where a stream cuts into a valley slope, or where ocean waves remove the base (toe) of a slope.

3) Removal of slope vegetation – A slope denuded of vegetation loses surface protection from the impacts of raindrops, which can mobilize sediment grains with water flowing down slope. The roots of plants on a slope can play a significant role in binding sediment together, reducing the likelihood of rapid or sudden mass wasting of a slope. Removal of the vegetation, due to human cutting or harvesting, or due to fire, reduces strength of the slope.

4) Introduction of water into slope material – An excessive amount of water within a slope increases its mass, increasing shear stress within parts of the slope, especially along rock fractures tilted in the same direction as the slope surface. If the slope is composed of sediment where grains are not cemented together, excess water can float the grains apart, reducing friction (and shear strength). Both of these situations, often associated with heavy rainfall or rapid snowmelt, can lead to mass wasting.

5) Ice wedging – Water can flow into even the narrowest of rock fractures. If the temperature then drops below freezing, ice crystals will form, expanding in volume by 9 %. This is a very powerful force that can wedge apart rocks, often causing them to fall from steep slopes in mountains and canyons.

6) Biological activity – Animals moving along steep slopes may loosen rocks, sending them crashing down slope.

What are the repetitive processes for gradual, long-term mass wasting?

1) Freeze-thaw cycle – Freezing of water within surface sediment on a slope causes the sediment to expand upward, perpendicular to the slope surface. As the ice melts, gravity pulls the sediment grains downward causing progressive down-slope movement (creep) of the upper few inches of the slope sediment.

2) Wet-dry cycle – Clay rich sediment expands when it is moistened, and contracts when it dries. Over a period of years, this expansion and contraction causes the surface sediment of a slope to creep down slope, much like the freeze-thaw cycle described above.

TO BE CONTINUED IN  –  Mass wasting or mass movement -II