Avalanche

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An avalanche is a rapid downslope movement of some combination of rock, regolith, snow, slush, and ice. The movement can occur by any combination of sliding, falling, and rolling of pieces within the avalanche mass, but is generally very rapid. Avalanche velocities can reach tens to hundreds of kilometers per hour.

This avalanche in the Swiss ski resort of Evolene left at least two dead and more than a dozen missing. © AFP/Corbis Bettman. Reproduced by permission.

The term avalanche is generally associated with snow and ice. In its most general form, however, it can refer to the cascading of sand grains down the leeward face of a dune or the rapid downslope movement of largely disaggregated rock without snow or ice. Rock avalanches, for example, are very rapid and catastrophic mass movements of bedrock that has been broken into innumerable pieces either before or during movement.

Snow avalanches, hereafter referred to simply as avalanches, are classified according to whether they move across existing snow layers (surface avalanche) or the ground (ground avalanche), whether they are dry or wet, whether they move through the air or over ground and snow, and whether they consist of loose snow or intact slabs. Like landslides, avalanches begin when the weight of snow above some potential sliding surface exceeds the shear strength along that surface. In many cases, sliding occurs along a former snow surface that is quickly buried by new snow during a storm. The physics of slip surface formation, however, are more complicated for avalanches than most landslides because the snow and ice in an avalanche prone slope are near their melting points. Thus, phase transitions and metamorphosis of snow and ice crystals can alter the strength of snow and ice slopes in a way that does not occur in soil or rock slopes. Melting can also trigger avalanches. Although it is not proven that loud noises such as shouting can trigger avalanches, the vibrations caused by explosives can do so—and explosives are often used to deliberately trigger avalanches under controlled conditions as a safety measure.

The aftermath of an avalanche is an avalanche track or chute, which is commonly marked by bent or broken trees and significant amounts of erosion. The track can be either a channel-like or open feature. The rock and debris carried by an avalanche can be deposited as an avalanche cone when the avalanche comes to rest, and the rock debris deposited at the base of a cliff or other steep slope by an avalanche is known as avalanche talus.

Catastrophic Mass Movements; Freezing and Melting; Ice; Landslide; Mass Movement; Mass Wasting; Phase State Changes