Crevasse

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Crossing a crevasse on the Easton Glacier in the North Cascades of Washington

A crevasse is a crack or fissure in a glacier or snow field, formed due to changes in glacier speed. An acceleration in glacier speed can cause extension and formation of a crevasse.

Transverse crevasses are the most common crevasse type, forming in a zone of extension where the glacier is accelerating as it moves downslope. They stretch across the glacier.

Marginal crevasses extend diagonally from the edge of the glacier pointing upglacier. These form because glaciers move slower at the sides than in the center because of the friction between the ice and the walls of the mountain. These crevasses form due to the increased speed of the glacier near its center compared to the margin.

Longitudinal crevasses form parallel to flow in an area where the glacier width is expanding. Crevasses open up with vertical walls. The walls can melt, creating seracs, arches etc. A special variant of a crevasse is a bergschrund which often marks the top border of a glacier and divides the moving glacier from the static ice, frozen to the steep surrounding mountain walls. A bergschrund extends to bedrock.

Crevasses in the accumulation zone expose layers denoting the differing snowfalls each year, rather like tree rings.

At the surface a crevasse can be covered by the most recent year's snow, creating a snow bridge that is hard to detect. Falling into a hidden crevasse (for example, one hidden by a layer of blown snow) is a danger for mountaineers. Anyone planning on walking on glaciers should be trained in crevasse rescue.

"Crevasse" is also a traditional term for a levee failure, such as those along the Mississippi River.