An avalanche occurs when a large mass of snow slides down the side of a mountain. Avalanches are "slabs" or plates of snow that shatter like a pane of glass and slide as a unit off the mountainside, like a magazine sliding off a table. Natural avalanches occur because new or windblown snow overloads the weak layers of snow underneath it. The weak layers underneath give way and an avalanche occurs. A bunch of loose snow falling down a mountainside is referred to as a "sluff."
For more detailed information about avalanches, you can visit www.avalanche.org.
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