|Detachment of soil bed from rock bed|
Landslides are a form of mass movement, a term used to describe any sort of gravity-induced movement of sediment down a slope. Mass movements can occur slowly over a period of years, or they can happen in a matter of minutes. A mass movement can be as small as some rocks and debris you kick down a small incline or as big as the 1980 landslide set off by the eruption of Mount St. Helens.
There are many different kinds of mass movements categorized by the type of material involved, the way it is moved and how fast it moves. However, with any mass movement, a soil layer is separated to some degree from the underlying bedrock. Soil is the relatively loose mixture of worn-down rock, minerals, air, water and decayed organic matter that covers the ground. Bedrock is the more stable, solid layer of rock underneath.