Types of Avalanches

An avalanche is a mass of snow that moves rapidly down a steep mountain slope. There are two main types to be concerned about: loose snow and slab avalanches.

A loose snow avalanche (a.k.a. a slough or sluff) is simply loose snow that originates at a single point on a slope and gathers cohesionless snow on the surface of the pack as it descends. (Cohesionless snow is loose, unconsolidated, individual snow crystals that are not well bonded to each other.)

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

Photograph of a large cornice formed by prevailing winds over a ridgetop, Alaska

Loose snow avalanches often appear as an inverted ‘V” pattern on the snow slope. They are capable of burying a person or carrying someone over a cliff but rarely are large enough to do significant property damage.

A slab avalanche occurs when a cohesive layer of snow slides down a slope. (Cohesive snow consists of very well bonded and consolidated snow crystals throughout the entire layer.) Since slab avalanches often form from new snow and wind, they are referred to as “wind slabs.”

Photograph of a slab avalanche

Slabs can consist of snow from a single storm or from multiple layers of snow from several storms. As you’d expect, slab avalanches cause the most fatalities and do the most property damage.

Slabs have several parts:

  • A fracture line at the upper limit on the slope (the “crown”)
  • Flanks, which are continuations of the fracture lines down both sides of the slab
  • A stauchwall, which is the bottom or lower limit of the slab; this is often obliterated as the avalanche moves downhill
  • A bed surface upon which the avalanche slides; this is usually smooth and planar, although it can be the ground itself

Photograph of a slab avalanche with the parts labeled

Slab avalanches are further categorized as “soft” or “hard.”

Soft slabs form when winds are relatively light and/or the snow has relatively low density. They break up easily and become more powdery as they run downhill.

Photograph of a skier-released, medium sized, soft slab avalanche

Hard slabs form when winds are relatively strong and/or the snow is of higher density. They maintain large blocks of snow as they descend to the bottom of the slope.

Photograph of a small, hard slab avalanche

Both loose snow and slab avalanches may consist of dry or wet snow.

Photo of a wet snow avalanche in early spring in Alaska

Question

Photograph of the Glory Bowl avalanche path

The Glory Bowl avalanche discussed earlier was a slab avalanche. What evidence supports this? (Choose all that apply.)

The correct answers are A, B, and D.

The Glory Bowl avalanche encompassed a large area, almost the entire right side of the bowl (when looking down from the top). The photograph from the road below shows a crown (partially visible on the upper left side of the Bowl) where the fracturing began. The exposed vegetation and rocks mark the bed surface.