A crevasse is a crack or some kind of wedge-shaped fissure in a glacier. These fractures in the ice are formed when the ice sheet or glacier shifts over the uneven landscape and essentially causes tears to appear.

It is not to be mistaken with a crevice which is a separation that appears in rock formations.

Generally a crevasse will have deep vertical walls although the depth is dependent on a number of factors, not least the depth of ice present in the glacier.

Whilst open and easily visible crevasses can present less danger simply because they are evident, crevasses can also be hidden. Snow bridges can form across crevasses due to the movement of surface snow by winds and the build up of new snow.

This effect does not necessarily fill the crevasse but serves only to mask it creating a potentially lethal hazard for mountaineers as often they cannot support the weight of a person.

A bergschrund is a type of crevasse that tends to open at the edge of a glacier, manifesting itself as a large crack between the moving ice and the static mountain.
Another feature created by crevasses is a serac. This is a large block or tower of ice created where more than one crevasse has converged.

Moving over glaciers and ensuring safe passage over crevasses requires experience and the appropriate gear, including rope. Tying into a rope with the other members of your party gives some security against falling into crevasses, or at least offers the possibility to your climbing partners to arrest your fall and perform a crevasse rescue should the need arise.