Crevasse

Crevasse is also a traditional term for a levee breach.

Crossing a crevasse on the Easton Glacier, Mount Baker, in the North Cascades, Washington

A crevasse is a deep crack in an ice sheet or glacier (as opposed to a crevice, which forms in rock). Crevasses form as a result of the movement and resulting stress associated with the sheer stress generated when two semi-rigid pieces above a plastic substrate have different rates of movement. The resulting intensity of the shear stress causes a breakage along the faces.

Crevasses often have vertical or near-vertical walls, which can then melt and create seracs, arches, and other ice formations. These walls sometimes expose layers that represent the glacier's stratigraphy.

Crevasses are more narrow at depth as it is here that pieces of the glacier may rub and break against each other. Crevasse size often depends upon the amount of liquid water present in the glacier. A crevasse may be as deep as 45 meters, as wide as 20 meters, and can be up to several hundred meters long.

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.

Types

Transverse crevasses are the most common crevasse type and they form in a zone of longitudinal extension where the principal stresses are normal to the direction of glacier flow, creating extensional tensile stress. These crevasses stretch across the glacier transverse to the flow direction, or cross-glacier. They generally form where a valley becomes steeper.

Splaying crevasses form as a result of shear stress from the margin of the glacier, and longitudinal compressive stress from lateral extension. They extend from the margin of the glacier and are concave up with respect to glacier flow, making an angle less than 45° with the margin. At the center line of the glacier, there is zero pure shear from the margins, so this area is typically crevasse free.

Longitudinal crevasses form parallel to flow where the glacier width is expanding. They develop in areas of compressive stress, such as where a valley widens or bends. They are typically concave down-glacier, and form an angle greater than 45° with the margin.

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.

An Ice Pinnacle is formed when multiple crevasses intersect at the end of a glacier.

A crevasse may be covered, but not necessarily filled, by a snow bridge made of the previous years' accumulation and snow drifts. The result is that crevasses are rendered invisible and thus lethal to anyone attempting to navigate their way across a glacier. Occasionally a snow bridge over an old crevasse may begin to sag providing some landscape relief, but this cannot be relied upon. Anyone planning to travel on a glacier should be trained in crevasse rescue.

Crevasse on the Gorner Glacier, Zermatt, Switzerland   Measuring snowpack in a crevasse on the Easton Glacier, Mount Baker, North Cascades, U.S.   Exploring the bottom of a crevasse in Antarctica   Crevasse on the Ross Ice Shelf, January 2001

Crevasses on the Upper Price Glacier of Mt. Shuksan, North Cascades, WA. Photo taken August 2011

Split-boarder skinning up past open crevasses on the Coleman Glacier of Mt. Baker. Photo taken October 2009

Looking down into a crevasse on Mt. Rainier, Cascade range, WA. Photo taken Mid August 2009

Crevasses on Mt. Rainier. Photo taken from the Disappointment Cleaver Route on Mt. Rainer. Photo taken August 2009

Mountaineers crossing a crevasse on Mt. Rainer. Photo taken August 2009

Ladder bridging a crevasse on Mt. Rainier. Photo taken Aug. 2009

Looking down on crevasses on the Hanging Glacier of Mt. Shuksan. Photo taken from Summit August 2011

See also

• Glaciology
• Snow bridge

References

• Paterson, W.S.B., 1994, The Physics of Glaciers, 3rd edition, ISBN 0750647426.
• Boon, S., M.J. Sharp, 2003, The role of hydrologically-driven ice fracture in drainage system evolution on an Arctic glacier, Geophysical Research Letters, 30, pp. 1916.
• Das, S.B., I. Joughin, M.D. Behn, I.M. Howat, M.A. King, D. Lizarralde, M.P. Bhatia, 2008, Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage, Science, 320, pp. 778.
• 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.
• "crevasse." Encyclopædia Britannica. 2010. Encyclopædia Britannica Online. 17 Oct. 2010.