A crevasse is a deep crack in an ice sheet

Ice sheet

An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...

 rhys glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

 (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 serac

Serac

A serac is a block or column of ice formed by intersecting crevasses on a glacier. Often house-sized or larger, they are dangerous to mountaineers since they may topple with little warning...

s, arch

Arch

An arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...

es, and other ice formations . These walls sometimes expose layers that represent the glacier's stratigraphy

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

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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 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

Bergschrund

A bergschrund is a crevasse that forms where the moving glacier ice separates from the stagnant ice above. It is often a serious obstacle for mountaineers, who sometimes abbreviate "bergschrund" to "schrund"....

 is a crevasse that divides moving glacier ice below the bergschrund from the stagnant ice above it and may extend to bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 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 rescue

Crevasse rescue is the process of retrieving a climber from a crevasse in a glacier. Because of the frequency with which climbers break through the snow over a crevasse and fall in, crevasse rescue technique is a standard part of climbing education....

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