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-made 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. 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.

Large mass of material, such as snow or rock debris, that moves rapidly down a mountain slope, sweeping everything in its path. Avalanches begin when a mass of material overcomes the frictional resistance of the sloping surface, often after the material's foundation has been weakened by rains or the snow has been partially melted by a warm, dry wind. Other weather conditions that can lead to avalanches are heavy snowfall and high winds. A common method of avalanche control consists of detonating explosives in the upper reaches of avalanche zones, which intentionally causes the snow to slide before accumulations have become very great.

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