Internal strength in these systems is derived from the effects of the solidification and crystallization of magma or the lithification of once loose particles. The stability of a hillslope depends on the relationship between the stresses applied to the materials that make up the slope and their internal strength. Mass movement occurs when the stresses exceed the internal strength. This condition is not always caused by an increase in stress. In some cases, the internal strength of the materials can be reduced over relatively short periods of time resulting in mass movement.
Many factors can act as triggers for hillslope failure.
One of the most common is prolonged or heavy rainfall. Rainfall can lead to mass movement through three different mechanisms. Often these mechanisms do not act alone. The saturation of soil materials increases the weight of slope materials which then leads to greater gravitational force. Saturation of soil materials can reduce the cohesive bonds between individual soil particles resulting in the reduction of the internal strength of the hillslope.