In a slab avalanche, a cohesive mass of snow breaks free from the surrounding snowpack and starts to slide downhill. The small wall that remains when the slab breaks off is called the crown. The side walls are called the flanks. The bed surface is the surface the slab slid on. Typically, this was the weak layer that created the instability. When the slab breaks away, for a few seconds it stays together and slides as a unit. Then it breaks up into chunks. Snow mixes with the air and the avalanche begins to flow downslope, behaving like a liquid. Dry slab avalanches can flow downhill as fast as 100 meters per second. The impact forces from a large, high speed avalanche can take out trees and destroy buildings. Once the avalanche comes to rest, the debris sets up like concrete. During the avalanche snow grains break up into smaller pieces, so when the avalanche stops the debris is much denser than the original slab. Also, its easier for these grains to sinter (or bond) together. This makes avalanche debris very dense, hard, and difficult to dig through.
Crown
Flanks
Bed surface
Debris
Photo: Mike Jenkins
Mt. Ogden Flank
Copyright 2008,
by the Contributing Authors.
Cite/attribute Resource.
factcouraud. (2007, May 08). Slab Avalanche Anatomy. Retrieved November 23, 2009, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/avalanche-and-snow-dynamics/AvClass/AvalancheBasics/Slab%20Avalanche%20Anatomy.html.
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