<P> A crevasse is a deep crack, or fracture, found in an ice sheet or glacier, as opposed to a crevice that forms in rock . Crevasses form as a result of the movement and resulting stress associated with the shear 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 . </P> <P> 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 . Crevasse size often depends upon the amount of liquid water present in the glacier . A crevasse may be as deep as 100 metres, as wide as 20 metres, and up to several hundred metres long . </P> <P> 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 potentially 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 . </P> <P> 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 moisten and lubricate the bed and accelerate ice flow . </P>

How do mulan accelerate the melting of ice sheets