Gravity is the main driving force behind mass-wasting processes, as it is constantly attempting to force material downhill. On a slope, gravity can be resolved into two components, one perpendicular to the slope, and one parallel to the slope. The steeper the angle of the slope, the greater the influence of gravity. The effect of gravity reaches a maximum along vertical or overhanging cliffs.
The tangential component of gravity tends to pull material downhill, resulting in mass wasting. When gt (the tangential component of gravity) is great enough to overcome the force of friction at the base of the loose mass, it falls downhill. The friction is really a measure of the resistance to grav-ity—the greater the friction, the greater the resistance to gravity's pull. Lubrication of surfaces in contact greatly reduces friction, allowing the two materials to slide past one another more easily. Water is a common lubricating agent, so mass-wasting events tend to occur more frequently during times of heavy or prolonged rain. For a mass-wasting event or a mass movement to occur, the lubricating forces must be strong enough to overcome the resisting forces that tend to hold the boulder (for example) in place against the wishes of gravity. Resisting forces include the cohesion between similar particles (like one clay molecule to another) and the adhesion between different or unlike particles (like the boulder to the clay beneath it). When the resisting forces are greater than the driving force (tangential component of gravity), the slope is steady and the boulder stays in place. When lubricating components reduce the resisting forces so much that the driving forces are greater than the resisting forces, slope failure occurs.
The process of the movement of regolith downslope (or under water) may occur rapidly, or it may proceed slowly. In any case, slopes on mountainsides typically evolve toward steady state angles, known as the angle of repose, balanced by material moving in from upslope, and out from downslope. The angle of repose is also a function of the grain size of the regolith.
Human activity can also increase driving forces for mass wasting. Excavation for buildings, roads, or other cultural features along the lower portions of slopes may actually remove parts of the slopes, causing them to become steeper than they were before construction and to exceed the angle of repose. This will cause the slopes to be unstable (or metastable) and susceptible to collapse. Building structures on the tops of slopes will also destabilize them, as the extra weight of the building adds extra stresses to the slope that could initiate the collapse of the slope.
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