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.

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