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Causes of Mass Wasting
Discussion 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. Examples of these mechanisms and processes are provided below.
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. Click on the USGS image to the left to see the effects of slope failure due to the Northridge, California earthquake of 1994.
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. Also, sometimes humans over-steepen slopes when constructing building sites, or roads in mountainous areas as shown in the image to the left.
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. The related photograph shows the effects of a fire that swept through the foothills of the San Bernardino Mountains, California in October of 2003. Intense rainfall two months later produced a debris flow that killed 13 people in Waterman Canyon down 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. The attached picture is of a campsite in Kings Canyon during a summer downpour. Although the rain ended before mass wasting occurred, a lot of damage resulted from the water flowing through the campground.
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. This photograph was taken in a Rocky Mountains canyon during the wintertime, when ice not only forms in rock fractures, but also coats rock surfaces.
6) biological activity - Animals moving along steep slopes may loosen rocks, sending them crashing down slope. Some animals are more destructive than others, rolling rocks down slope on purpose. Look out below!
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.
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