Slab avalanches involve tremendous amounts of snow and are potentially hazardous. As the name indicates, a slab of snow breaks loose from the slope and travels downslope under the forces of gravity. The slab itself is formed as snow is packed down and redistributed by wind.
The snow in the starting zones of slab avalanches is typified by a cohesive dense snow layer overlying less cohesive weak snow. The forces that cause a "slab" to fail occur when an external shear load is added parallel to the slope exceeds the shear strength of the weak snow layer. The loading causes fractures which can spread through the weak layer until collapse occurs - marking the beginning of an avalanche.
The mass of snow invloved in the initial slab is very large and has a tremendous amount a potential energy. The initial slab can be up to several meters thick and 100 meters wide (Perla and Martinelli, 1976). As the slab of sn ow moves downslope it picks up mass and energy, creating the potential for a lot of damage. In the end, the slab avalanche can increase its initial mass by 100 (Perla and Martinelli, 1976) times.
In slab avalanches, the moisture content of the snow indirectly affects the snowpack's stability. When snow melts the moisture content of the snow can increase dramatically, sometimes producing free flowing water within the snowpack. The flowing water percolates though the snow layers until it reaches a hard layer. Once on top of the hard layer, the water spreads out melting any attachment to snow layers above, which greatly decreases the strength of the snowpack. This lubrication of the snowpack is responsible for many spring avalanches (Perla and Martinelli, 1976).
Snow's moisture content also affects an avalanche's movement. For example, dry slab avalanches tend to move with a speed of 25 to 75 m/s (Perla and Martinelli, 1976). They travel in straight paths and will even jump over gully walls that lie in the way. Wet avalanches move much slower with speeds of 5 to 30 m/s (Perla and Martinelli, 1976) and tend to follow the c urved path of gullies.
Slab avalanches pose a much larger hazard than loose snow avalanches because of their large mass, large area, speed of travel, and unpredictable nature.