Slump is a form of mass wasting event that occurs when loosely consolidated materials or rock layers move a short distance down a slope. The landmass and the surface it slumps upon is called a failure surface. When the movement occurs in soil, there is often a distinctive rotational movement to the mass, that cuts vertically through bedding planes (landslides take place along a bedding plane or fault). This rotational movement moves along a curved slip surface of regolith (the failure surface) which overlies bedrock. This results in internal deformation of the moving mass consisting chiefly of overturned folds called "sheath folds." The surface of the mass often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near the toe of the slump . The cut which forms as the landmass breaks away from the slope is called the "scarp" and is often cliff-like and concave. Cracks at the head scarp drain water, killing trees. Power lines, fences, roads, houses, and other manmade structures are frequently damaged if in the path of a slump. Slumps frequently form due to removal of a slope base, either from natural or manmade processes. Stream or wave erosion, as well as road construction are common instigators for slumping. It is the removal of the slope's physical support which provokes this mass wasting event. Earthquakes also trigger massive slumps, such as the fatal slumps of Turnagain Heights Subdivision in Anchorage, Alaska. This particular slump was initiated by a magnitude 8.4 earthquake that resulted in liquifaction of the soil. Around 75 houses were destroyed by the Turnagain Slump.
The speed of slump varies widely, ranging from meters per second, to meters per year. Sudden slumps usually occur after earthquakes or heavy continuing rains. The rain provides lubrication for the material to slide, and increases the self-mass of the material. Both factors increase the rate of slumping. Slumps may also occur underwater along the margins of continents and islands. These submarine slumps can generate disastrous tsunamis. The underwater terrain which encompasses the Hawaiian Islands gains its unusual hummocky topography from the many slumps that have taken place for millions of years.