A sudden slide of snow and ice, usually in mountainous areas where there is heavy snow accumulation on moderate to steep slopes. Snow avalanches flow at an average speed of 80 mph (130 km/hr), and their length can range from less than 300 ft (100 m) to 2 mi (3.2 km) or more. Generally the term "avalanche" refers to sudden slides of snow and ice, but it can also be used to describe catastrophic debris slides consisting of mud and loose rock. Debris avalanches are especially associated with volcanic activity in which melted snow, earthquakes, and clouds of flowing ash can trigger movement of rock and mud. Snow avalanches generally consist either of loose, fresh snow or of slabs of accumulated snow and ice that move in large blocks. Snow avalanches occur most often where the snow surface has melted under the sun and then refreezes, forming a smooth surface of snow. Later snow falling on this smooth surface tends to adhere poorly, and it may slide off the slick plane of recrystallized snow when it is shaken by any form of vibration—including sound waves, earthquakes, or the movement of skiers.
Several factors contribute to snow avalanches, including snow accumulation, hill slope angle, slope shape (profile), and weather. Avalanches are most common where there is heavy snow accumulation on slopes of 25–65°, and they occur most often on slopes between 30° and 45°. On slopes steeper than 65° snow tends to sluff off rather than accumulate. On shallow slopes avalanches are likely to occur only in wet (melting) conditions, when accumulated snow may be heavy, and when snow melt collecting along a hardened old snow surface within the snowpack can loosen upper layers, allowing them to release easily. Slab avalanches may be more likely to start on convex slopes, where snow masses can be fractured into loose blocks, but they rarely begin on tree-covered slopes. However, loose snow avalanches often start among trees, gathering speed and snow as they cross open slopes. Weather can influence avalanche probability by changing the stability and cohesiveness of the snow pack. Many avalanches occur during storms when snow accumulates rapidly, or during sustained periods of cold weather when new snow remains loose. Like snow melt, rainfall, can increase chances of avalanche by lubricating the surface of hardened layers within the snow pack. Sustained winds increase snow accumulation on the leeward side of slopes, producing snow masses susceptible to slab movement. When conditions are favorable, an avalanche can be triggered by the weight of a person or by loud noises, earthquakes, or other sources of vibration. Avalanches tend to be most common in mid-winter, when snow accumulation is high, and in late spring, when melting causes instability in the snow pack.
Avalanches play an ecological role by keeping slopes clear of trees, thus maintaining openings vegetated by grasses, forbs, and low brush. They are also a geomorphologic force, since they maintain bare rock surfaces, which are susceptible to erosion.
Most research into the dynamics and causes of avalanches has occurred in populous mountain regions such as the Alps, the Cascades, and the Rocky Mountains, where avalanches cause damage and fatalities by crushing buildings and vehicles. Avalanches are very powerful: they can crush buildings, remove full-grown trees from hillsides, and even sweep railroad trains from their tracks. One of the greatest avalanche disasters on record occurred in 1910 in the Cascades near Seattle, Washington, when a passenger train, trapped in a narrow valley in a snow storm for several days, was caught in an avalanche and swept to the bottom of the valley. Ninety-six passengers died as the cars were crushed with snow. Although avalanches are among the more dangerous natural hazards, they have caused fewer than 200 recorded mortalities in North America, and most avalanche victims in North America are caught in slides they triggered themselves by walking or skiing across open slopes with accumulated snow.
