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.

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.