type of natural disasters involving ground movements, often caused by slope instability triggered by specific event

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

Landslide in Pakistan

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:

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.

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