Icefall

An icefall is a portion of certain glaciers characterized by rapid flow and a chaotic crevassed surface. The term icefall is formed by analogy with the word waterfall, a similar, but much higher speed, phenomenon. When ice movement is faster than elsewhere, because the glacier bed steepens or narrows, the flow cannot be accommodated by plastic deformation and the ice fractures, forming crevasses. Where two fractures meet, seracs (ice towers) can be formed. When the movement of the ice slows down, the crevasses can coalesce, resulting in the surface of the glacier becoming smoother.

Ice flow

Perhaps the most conspicuous consequence of glacier flow, icefalls occur where the glacier bed steepens and/or narrows. Most glacier ice flows at speeds of a few hundred metres per year or less. However, the flow of ice in an icefall may be measured in kilometres per year. Such rapid flow cannot be accommodated by plastic deformation of the ice. Instead, the ice fractures forming crevasses. Intersecting fractures form ice columns or seracs. These processes are imperceptible for the most part; however, a serac may collapse or topple abruptly and without warning. This behavior often poses the biggest risk to mountaineers climbing in an icefall.