Large mass of rock debris or snow that moves rapidly down a mountain slope, sweeping and grinding everything in its path.
Avalanches begin when a mass of material overcomes the frictional resistance of the sloping surface, often after its foundation is loosened by spring rains or is partially melted by a warm, dry wind. Vibrations caused by loud noises, such as gunfire or thunder, can start the mass moving. Some avalanches develop during heavy snowstorms and slide while the snow is still falling, but more often they occur after snow has accumulated at a given site. Avalanche control consists largely of detonating explosives in the upper reaches of avalanche zones, intentionally causing the snow to slide before accumulations become very great.