crevasse

crevasse, large crack in the upper surface of a glacier, formed by tension acting upon the brittle ice. Transverse crevasses occur where the grade of the glacier bed becomes suddenly steeper; longitudinal crevasses, where the glacier spreads over a wider valley or plain. Marginal crevasses are due to the strain built up when the central part moves faster than the sides.

Fissure or crack in a glacier resulting from stress produced by movement. Crevasses range up to 65 ft (20 m) wide, 150 ft (45 m) deep, and several hundred yards long. Crevasses may be bridged by snow and become hidden, and they may close up as the glacier moves.

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A crevasse is a fracture in a glacier caused by a large tensile stress at or near the glacier's surface. Accelerations in glacier speed cause extension and can initiate a crevasse. Crevasses often have vertical or near-vertical walls, which can then melt and create seracs, arches, etc.; these walls sometimes expose layers that represent the glacier's stratigraphy.

• Transverse crevasses are the most common crevasse type and they form in a zone of extension where the glacier is accelerating as it moves downslope. These crevasses stretch across the glacier transverse to the flow direction.
• Marginal crevasses extend diagonally from the edge of the glacier pointing upglacier. These crevasses form because of the higher speeds of the glacier near its centerline relative to its margin.
• Longitudinal crevasses form parallel to flow where the glacier width is expanding.
• A bergschrund is a crevasse that divides moving glacier ice below the bergschrund from the stagnant ice above it and may extend to bedrock below.

A crevasse may be covered, but not necessarily filled, by a snow bridge made of the previous year's snow. Falling into a hidden crevasse that is covered by a weak snow bridge is thus a danger for mountaineers. Anyone planning to travel on a glacier should be trained in crevasse rescue.

Fracture mechanics has been used to study the growth of crevasses on glaciers. The maximum depth of a dry crevasse, predicted using an estimate of the fracture toughness of ice, is about 30 m. However, crevasses deeper than 30 m have been observed. The presence of water in a crevasse can significantly increase its penetration. Water-filled crevasses may reach the bottom of glaciers or ice sheets and provide a direct hydrologic connection between the surface, where significant summer melting occurs, and the bed of the glacier, where additional water may lubricate the bed and accelerate ice flow.

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

See also

• Snow bridge

Gallery

References

Paterson, W.S.B., 1994, The Physics of Glaciers, 3rd edition, ISBN 0750647426.

van der Veen, C.J., 1998, Fracture mechanics approach to penetration of surface crevasses on glaciers, Cold Regions Science and Technology, 27, pp. 31-47.

Zwally, H.J., W. Abdalati, T. Herring, K. Larson, J. Saba, K. Steffen, 2002, Surface melt-induced acceleration of Greenland ice-sheet flow, Science, 297, pp. 218-222.

Mountaineering: The Freedom of the Hills, 5th edition. ISBN 0-89886-309-0.