A slump is a form of mass wasting that occurs when a large mass of loosely materials or rock layers moves a short distance down a slope. Movement is characterized by sliding along a curved surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope.
Rotational slumps occur when a slump block, composed of sediment or rock, slides along a concave-upward slip surface with rotation about an axis parallel to the slope. Rotational movement causes the original surface of the block to become less steep, and the top of the slump is rotated backward. This results in internal deformation of the moving mass consisting chiefly of overturned folds called sheath folds.
Slumps have several characteristic features. The cut, which forms as the landmass breaks away from the slope, is called the scarp and is often cliff-like and concave. In rotational slumps, the main slump block often breaks into a series of secondary slumps and associated scarps to form stair step pattern of displaced blocks. The upper surfaces of the blocks are rotated backwards, forming depressions that may accumulate water to create ponds or swampy areas. The surface of the detached mass often remains relatively undisturbed, especially at the top. However, hummocky ridges may form near the toe of the slump. Addition of water and loss of sediment cohesion at the toe may transform slumping material into an earthflow. Thorough wetting is a common cause, which explains why slumping is often associated with heavy rainfall, storm events and earthflows. Rain provides lubrication for the material to slide, and increases the self-mass of the material. Both factors increase the rate of slumping. Earthquakes also trigger massive slumps.
