Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 169

FIGURE 4.16 A snow avalanche in the Irene avalanche path, San Juan Mountains, Colorado. (Image
courtesy of R. Armstrong.)
Types of Avalanches
There are two principal types of avalanches: loose-snow avalanches and slab avalanches
(McClung and Schaerer 2006). Loose-snow avalanches are usually small and relatively
harmless, whereas slab avalanches may involve large amounts of snow and cause con-
siderable destruction. The distinction between the two types is based on the cohesive-
ness of the snow. Loose-snow avalanches have little internal cohesion and tend to initi-
ate at a point, growing wider as they move downhill and more snow becomes entrained
in the slide (Fig. 4.18a). Slab avalanches, on the other hand, consist of a more cohesive
snow slab overlying a weak layer or interface. These avalanches tend to fracture along
a broad front and begin sliding downward as a single unit, until they break into smaller
chunks (Fig. 4.18b). This latter characteristic makes slab avalanches especially danger-
ous for the people who trigger them, since the victim is often in the middle of the slope
that fractures (Tremper 2008). Types of avalanches are further subdivided according to
whether the snow is dry or wet; whether the slide takes place on a snow layer or ex-
tends all the way to the ground; and whether the motion is on the ground, in the air, or
both.