Slab Avalanches

A skier examining the crown face of a previous slab slide. (Dr. Bruce Jamieson)

Slab avalanches happen when a weak layer in the snowpack fails and the cohesive layer above, separate from the rest of the snowpack and flow down the mountain. The layer that separates remains intact as a unit, and resembles a slab of packed snow flowing down the mountain. As it travels downslope, collides with objects and rolls over the terrain, it generally breaks up and is crumpled into smaller, broken pieces of slab by the bottom of its runout.

The terminology associated with a slab avalanche. (Chuck O'Leary)

When either the weak layer fails, or the bond between the slab and the bedsurface releases, the force is drastically increased on all remaining bonds connecting the slab to the slope. The flanks and crown are the areas that feel the most stress in this situation. If their bonds alone are not strong enough, which that rarely are, the avalanche will begin its desent. The bed surface is the most critical bond, as its surface area can be "roughly 100 times greater than the surface area of all of the other boundary regions combined" (Fesler and Fredston). Slabs may be of nearly any thickness from a few inches to forty feet or more and can easily travel at over 100 m.p.h.

Hoar frost: a typical formation of a weak layer in the snowpack. This buried layer is 20mm thick.

(Dr. Bruce Jamieson)

*How An Avalanche Forms