DEFINITION OF DRIVING FORCES OF MASS WASTING

Gravity is the main driving force behind mass wasting processes,

as it is constantly adding material and attempting to

force it 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 and results in mass wasting. When 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 gravity—

the greater the friction, the greater the resistance to

gravity’s pull. Friction can be greatly reduced by lubrication

of surfaces in contact, 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 in place, against the wishes of gravity.

Lubricating 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, as in this case, 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. This angle of repose is also a function

of the grain size of the regolith.

Driving forces for mass wasting can also be increased by

human activity. 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 make them unstable,

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