Movement of a volume of rock during failure of a rock slope is induced by forces acting on the rock making up the slipped mass. These driving forces have a none of different origins:

1. 1 The accelerations imposed by shaking during an earthquake produce a force on the rock mass. These sideways forces may help trigger the movement.

2. 2 Under saturated conditions the pressure of ground water along bedding planes, fractures and other discontinuities that bound the potential slide mass can exert a push that helps drive the movement.

3. 3 If the potential slide mass rests on a tilted discontinuity, the downward-acting gravitational force of the rock will have a down-slope component. This driving force operates even in dry conditions and in times of seismic tranquillity.

These driving forces are ubiquitous but are often insufficient to cause movement. Stable rock slopes exist where resisting forces are present that outweigh the driving forces. The main resisting forces arise from friction on the discontinuity that operates as the potential plane of sliding (Fig. 44a). The friction depends on the nature of the rock material above and below the discontinuity and the nature of the discontinuity (planarity, fill, etc.)

The magnitude of the frictional forces also depends on the component of the weight of the rock acting normal to the discontinuity surface. For steep discontinuities this frictional resistance therefore lessens whilst the down-slope force on the slide mass due to gravity increases.