Mass movement is defined as the movement of surface material from a vulnerable slope, under the force of gravity. The ultimate cause of these events is slope failure, which is initiated primarily by the topography of the land. Where a slope is present, it is possible for mass movements to occur. However, many factors combine in order to trigger such an occurrence or worsen the landslide or mudslide, by either reducing the shear strength of the slope, or increasing the shear stress. Ultimately, the main problem occurs when mass movement presents a hazard to the livelihoods of people. This issue arises because humans develop the area around slopes. It is therefore necessary to consider both the human and physical factors which contribute to impacts from mass movements.
Slope failure can be triggered by many physical factors. It is the nature of the slope which determines whether it will be vulnerable to mass movements. For example, the geology of the rock which composes the slope is an important physical factor. If the particles of rock and soil are un-cohesive and unconsolidated, it means that they are more likely to break away and fall down under the force of gravity. As well as this, the absorbing capacity of the rock is significant, as when a rock that is easily saturated is combined with a heavy rainfall event, mass movement becomes very likely. This can be seen to be an essential factor which contributed to the Holbeck Hall landslide in June 1993. The rock that comprised the cliffs at Start Bay in Scarborough was mainly Boulder Clay. This rock type is easily saturated, so when it rained the material flowed down slope. Similarly, The Philippine mudslide of December 2006 was from hill slopes of very unstable volcanic rock. The geology of the rock in both these areas therefore worsened the impacts, as it made the event more damaging due to the fact that more