These are cracks or fractures in glacial ice, as opposed to those in other materials which are then called crevices. They can reach sizes of 20 metres across, 45 metres deep and hundreds of metres long. Crevasses develop from stress within ice as a glacier flows downhill and encounters uneven topography. Several types occur and are named due to their placement in relation to the long axis of the glacier and the circumstances under which they were formed.
Longitudinal crevasses are chasms which run down the length of a glacier, the product of compression on either side.
Transverse crevasses lie perpendicular to flow in an area experiencing tensile stress; when the ice is stretched out during slopes where ice at the forefront moves faster than ice behind.
Marginal crevasses develop at when ice within the centre flows faster than ice at the boundaries of the glacier due to friction resulting from contact with the walls of the valley or mountain. Bergshrund crevasses (German, lit. ‘mountain crevice’) are concentrated at the head of a glacier. They contribute to the erosion of rock beneath, and the development of cirques.
The causes of seracs, jagged pinnacles of ice, are the intersections of numerous crevasses at a glacier’s extremity.
When a glacier moves onto land which is flatter and more level, its breaches may close up and refreeze.
Crevasses are always a hazard and obstacle to climbers trekking across glaciers, and can be inescapable if fallen into due to their narrowness and sheer sides. Additionally, previous snowfall may cover the fissure and render them essentially invisible, these are called snow bridges. When a person steps onto the bridge, it usually collapses and plunges them into a fall metres deep. This is why anyone navigating a glacier is recommended to travel harnessed with a party. Accompanying members can facilitate a rescue and retrieve the victim; however such attempts are not always successful. After a long journey, climbers are often extremely fatigued and may not have the strength to haul another up many metres. They themselves may fall into nearby unnoticed crevasses when moving about in preparation. Additionally, the victim may suffer from immediate injury such as a broken spine or limb, and may die soon after from secondary causes such as hypothermia and shock if unrecovered.
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