avalanche, rapidly descending large mass of snow, ice, soil, rock, or mixtures of these materials, sliding or falling in response to the force of gravity. Avalanches, which are natural forms of erosion and often seasonal, are usually classified by their content such as a debris or snow avalanche. Speeds can reach over 200 mi per hr (300 km per hr). They are triggered by such events as earthquake tremors, human-caused disturbances, or excessive rainfall on high gradient slopes, often where materials are loosely consolidated or weathered. Avalanches of snow result when weak layers within a snowpack fail to support the weight of the snow above it and collapse, causing the overlying snow to break free and flow downhill. A high proportion of people who die in snow avalanches trigger the event, typically when skiing or snowmobiling in backcountry areas where the risk of an avalanche is higher. Destruction from avalanches results both from the avalanche wind (the air pushed ahead of the mass) and from the actual impact of the avalanche material.
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