A crevasse is a large crack in the ice on a glacier. These form from the motion of the ice. They range in size from narrow enough to step across to large, deep gaping holes.
Crevasses present an additional avalanche danger in the form of a terrain trap. Even a small avalanche can have serious consequences if a person is swept into a crevasse. Especially if snow flows in on top of them, burying them deeply inside the crevasse.
During winter crevasses are usually (but not always) bridged with new snow across the top. This can make them difficult or impossible to identify or spot. If these bridges are thin they can easily break under the weight of a person. Thin bridges are common in early winter, late spring and summer, and in places where the wind may allow them to form but not thicken.
The large crevasse at the top of a glacier where it separates from the snow/ice above is called a bergschrund.
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