Crevasses are cracks or fractures that open in the surface of a moving glacier in response to stress variations caused by glacier flow. Crevasses range in shape from linear to arcuate and in length from metres to kilometres. Their orientation may be in any direction with respect to the glacier flow. The deepest crevasses may exceed 30 m. Theoretically, the weight of the ice limits crevasse depth to about 30 m. Below that there is typically enough compressive force in the ice to prevent cracks from opening. However, if water enters a growing crevasse, it acts like a wedge to carve an extra deep fracture, sometimes hundreds of m through the entire glacier. The main types of crevasses are longitudinal, transverse and marginal. Crevasses are often hidden under the snow, but many become prominent in late summer. In the ablation area, crevasses are often highly visible, and some are transformed into moulins that extend to the glacier bed.
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Crevasses form due to stress in the fragile upper layers. Photo: Þorvarður Árnason, 2016.
Heavily crevassed surface of an outlet glacier. Ash layers influence melting of the surface layers. Photo: Snævarr Guðmundsson, 2014.
Snow filled crevasses in the caldera of Öræfajökull ice cap.
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Photo: Snævarr Guðmundsson, 2012.
Few years ago the Icelandic Association for Search and Rescue, issued crevasse maps for all the larger Icelandic glaciers by using the LiDAR digital elevation models and aerial images. The goal is to increase the safety for travellers. Three levels of risk were defined, 1small crevasses, 2heavily crevassed, 3hazardous. Way points have also been issued for experienced drivers. The maps and points can be downloaded from the website of safetravel, https://safetravel.is/crevasse-maps.
Crevasse map of safetravel.is, authors Snævarr Guðmundsson and Ágúst Þór Gunnlaugsson.