Slab Components

A slab forms when a denser, more cohesive layer of snow overlies a weaker, less cohesive layer of snow.  The slab itself, may not be very dense and may be only composed of light, new snowflakes. However, as long as this layer is more cohesive than the layer below it, a 'slab' has been formed (Tremper, 2002). 

When the stress from this overlying layer eventually overcomes the strength of the weak layer, failure occurs, and an avalanche is initiated. As the weak layer fails, the slab begins to move downslope. It slides on top of the bedsurface, which is composed of older, dense snow.

In some instances, the weakest layer in the snowpack may rest directly on top of the ground, in which case, the entire snowpack slides with the ground acting as the bedsurface.  This is called a climax avalanche.
slab layers
The above diagram illustrates the basic components of a slab.

As the top of the slab pulls away from the snowpack, a combination of shearing  and tensile forces are applied, and the crown is formed (McClung, 2006). 

The flanks define either side of the slab, and are formed exclusively as a function of the shearing forces present.

The stauchwall marks the lower limit of the slab and forms the boundary between where the weak layer has failed, and where it remains intact or is no longer present in the snowpack.  This region is dominated by compressive forces as the mass of the moving snow is forced against, and then over, this boundary. 

After the slab moves downslope, the layer on which it slid is exposed.  This region is known as the bedsurface.  The forces between the slab and bedsurface are predominantly in shear, and as a result, the bedsurface often exhibits a relatively smooth, and planar appearance.
slab diagram
When the slab is triggered, there is an instantaneous reaction of shearing, tensile, and compressive forces in action.  As a result of these forces, some common features that are synonymous with all slab avalanches are formed.
Location:  Chugach Mountains near Valdez, Alaska

What is an

Loose Snow



of a Slab


Works Cited