Landslide

A landslide includes a wide range of ground movements, such as rock falls, deep failure of slopes and shallow debris flows.

The biggest reason for a landslide is that there is a slope and material goes down the slope because of gravity.

But other things also contribute to landslides: 
 erosion by rivers, glaciers, or ocean waves makes slopes too steep
 rock and soil slopes made weak through saturation by snowmelt or heavy rains 
 earthquakes that make weak slopes fail or crack the rocks that will eventually fall down
 volcanic eruptions produce loose ash deposits, heavy rain, and debris flows
 vibrations from machinery, traffic, blasting and even thunder may trigger failure of weak slopes
 weight from much rain or snow, stockpiling of rock or ore, from waste piles, or from buildings may stress weak slopes to failure and other structures 
 groundwater pressure making the slope unstable
 in shallow soils, the removal of deep-rooted plants that bind the colluvium to bedrock

An underlying reason is the nature of the material. For example, a thixotropic material such as mud or sand and water, can change from a gel to a sol by adding water or pressure, or shaking. It goes from being stable one minute to fluid the next minute. The greatest disasters, like lahars or mudslides, happen suddenly. It is possible for much of a volcano to just slide down suddenly, as with Toba, Krakatoa and Mount St Helens.

Other websites

 United States Geological Survey site
 British Columbia government landslide information 
 Slide! , a program on B.C.'s Knowledge Network, with video clips
 Geoscience Australia Fact Sheet 

Category:Geology
Category:Landforms