History’s largest and most destructive landslides have been caused by earthquakes that started the material moving. Excessive rainfall or snowmelt, however, is also known to saturate and lubricate soil on steep angles. Rapid temperature changes can also cause land to slide by alternately shrinking and expanding soil formations, or by forming ice heaves between layers of rock. Forest fires are indirectly responsible for landslides because they take away slope vegetation, making erosion easier. Man can also cause slides by mining the earth, underground excavation, pumping and draining groundwater levels, or overdeveloping hillsides.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/landslides/howdotheyhappen.shtml
All earth and material on a slope has an “angle of repose,” or an angle at which that material will remain stable. Loose dry rock remains in place at angles up to 30 degrees, but wet clay will start to slip at more than 1 or 2 degree inclinations. Landslides are the sudden downhill movements of earth or other solid material, and are usually caused by rain, thaws, or forces either increasing the top material weight, lubricating the material layers, or making the slope too steep. They can be triggered by earthquakes, saturation with heavy rain, or crashing waves.