An avalanche is a very large slide of rapidly moving granular material, most commonly snow, down a mountainside caused by a build up of material. When a mass of material exceeds the static friction threshold, a cascading effect takes place and accumulates more material as it travels down the mountainside and causes massive, widespread destruction. There are many different types of avalanches including snow, ice, rock and soil.
There are three main factors that contribute to causing an avalanche. If the steepness of the terrain is between 35 to 45 degrees, is shady, has a convex shape and has a rock or slab base with little vegetation the chance of an avalanche is extremely high. Weather is another main factor where everything from temperature to wind and rain can loosen the material pack and cause an avalanche. For a snow avalanche, the snow itself can contribute to the probability of an avalanche. If there is a large amount of new, unbounded snow with little compaction and a large crystal size, the snow can cause an avalanche all by itself.