slump

A slump (geology) is a mass movement process of slope failure, in which a mass of rock or unconsolidated material drops along a concave slip surface. Slump units move downslope as an intact block (without internal deformation of the landslide material) and frequently rotate backwards.

Slumps appear as discrete block movements, whereas slides usually break up and travel downslope. The term 'slump' is also used to refer to the material that breaks off in a slumping slide.

(images - click to enlarge - above left, diagram of slumping; below right, slump features; bottom right, cross-section of a Toreva block.)

Slumps are sometimes caused by clear cutting on unstable soils, and the sagging and rotational movement of the mass of soil and rock is due in part to water infiltration and lubrication of clay-rich soils below. Coastal cliffs are subject to slumping when wave action undercuts lower layers. A submarine slope slump movement may be result from tidal forces acting on an unstable slope, or from a large seismic event near the affected body of water.

Toreva block slumping is a distinctive form of landslide in which huge backrotated blocks of resistent strata such as sandstones and limestones collapsed when weaker underlying beds such as shales were undercut. The existence of Toreva blocks suggests that an earlier warmer or wetter earlier climate existed in the region. (The Transantarctic Mountains have Toreva blocks, implying temperate conditions in Cenozoic Antarctica)

[links: fresh slumping: slumping in unconsolidated sediments, the Naze; recent slumping: Norfolk coastal slumping; slump folds in shale, Chisholm Creek, outside Wichita; slumping, 2, Milpas; old slumping: slump structures in friable sandstones of the Bournemouth Marine Beds (Eocene), channeling and slump structures, close-up of slump structures in friable sandstone, sandstone with slumps, truncated at reactivation surface; slump in cliff, Italy; slump; slump block, Clifton Creek, 2; Toreva blocks abound in the Grand Canyon area where the Bright Angel Shale has failed: two massive backrotated Toreva blocks of the Cogswell East Landslide, close-up view of the upper slide block of the East Cogswell Landslide; multiple slump blocks; Vermillion Cliffs; Surprise Valley Toreva block; webpages: 218-Mile Toreva Block; Cogswell Butte Landslides And River Diversions]