Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 177

FIGURE 4.23 Large snow cornices overhang the main ridge of southwest Montana's Bridger Range.
The dominant wind direction is from right to left, and when the cornices break off they are effect-
ive avalanche triggers. (Photo by K. Birkeland.)
SNOWPACK
How the weather interacts with the existing snowpack determines whether the snow is
capable of producing avalanches, and we will touch briefly on some of the major points
regarding unstable snowpacks. Schweizer (1999) reviews some of the details, and Hei-
erli and others (2008) provide a model describing the current thinking on the fracture
mechanics behind avalanche release. Slab avalanches require three basic snowpack in-
gredients: a slab, a weak layer, and a bed surface. The slab is a relatively cohesive layer
of snow that overlies the weak layer. Slab densities can be quite variable, ranging from
50 to 450 kg/m 3 (3 to 28 lbs/ft 3 ) (McClung and Schaerer 2006). Slabs may be composed
of any type of snow, but new snow, equilibrium metamorphosed snow, and wind slabs
form the most common slab layers. The weak layer is simply a less cohesive layer under-
lying the slab, commonly composed of faceted crystals formed by kinetic-growth meta-
morphism like depth hoar, surface hoar, or near-surface facets. In some cases, the weak
layer may be no more than a weak interface between the slab and the underlying snow.
Bed surfaces are not critical for slab avalanches; in some cases (i.e., with a depth-hoar
avalanche) no bed surface is necessary. However, a hard bed surface, such as a frozen
rain crust, may create particularly unstable conditions when a weak layer and a slab are
deposited on top of it.