Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 171

FIGURE 4.18 Typical forms displayed by (A) loose-snow avalanches and (B) slab avalanches. (Image
courtesy of K. Birkeland.)
Dangerous slab avalanches occur less frequently than loose-snow avalanches. Slab
avalanches originate in all types of snow, from old to newly fallen and from dry to wet.
The chief distinguishing characteristic is that the snow breaks away with enough intern-
al cohesion to act as a single unit until it disaggregates during its journey downslope.
The zone of release, or starting zone, is marked by fracture lines that are perpendic-
ular to the slope and extend to a well-defined basal-fracture plane (Figs. 4.18b, 4.20).
The size of a slab avalanche depends on many factors, but it is often confined to a spe-
cific area on the slope because of the nature of the terrain. However, during times of
extreme instability, whole mountainsides may become involved, with the fractures ra-
cing along for several kilometers, releasing snow in numerous slide paths. Though it
has been assumed that the entire mass of a slab avalanche is set in motion at once,
research shows that the mass of the slide often increases in a downhill direction, as
the avalanche erodes and entrains snow in the path (Sovilla et al. 2001). Avalanches
reach their maximum velocity quickly, so their destructive power is significant near the
point of origin (Atwater 1954: 27; McClung and Schaerer 2006). The exact behavior,
of course, depends on the nature of the snow and several other factors. If the snow is
dry, a powder-snow avalanche may develop. These move as much in the air as on the
ground, and their turbulent motion may create a dense dust cloud of ice crystals, which
behave like a body of heavy gas preceding the rapidly sliding snow. Such windblasts
may achieve a velocity of 320 km (200 mi) per hour and can cause damage well beyond
the normal avalanche zone (Seligman 1936; LaChapelle 1966). On the other hand, wet-
snow avalanches tend to slide at slower speeds with no particular dust cloud, but their
impressive mass can still cause great damage.