Landslide is the movement of rock, soil, and debris down a hillside or slope. Landslides take lives, destroy homes, businesses and public buildings, interrupt transportation, undermine bridges, derail train cars, cover marine habitat, and damage utilities.
The term landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows. Ground failures that result in landslides occur when gravity overcomes the strength of a slope. While gravity is the primary reason for a landslide, there can be other contributing factors, including:
- Saturation, by snow melt or heavy rains, weakening rock or soils on slopes.
- Erosion by rivers, glaciers, or ocean waves creating over-steepened slopes.
- Slope topography, including shape, size, degree of slope, and drainage.
- Stress from earthquakes magnitude 4.0 and greater can cause weak slopes to fail.
- Volcanic eruptions which produce loose ash deposits and debris flows.
- Excess weight, from accumulation of rain or snow, stockpiling of rock or ore, waste piles, or man-made structures, may cause weak slopes to fail.
- Human action such as construction, logging, or road building which disturbs soils and slopes.
Landslides can occur due to the geologic formations present. For example, groundwater can accumulate and zones of weakness can develop, when layers of sand and gravel lay above less permeable silt and clay layers.