Not every snow pack can produce a slab avalanche.  Structurally there are three factors that must be present for a slab avalanche to be triggered.

         1.    A slab must be present (cohesive layer of snow)

         2.   A weak layer or poor bond must exist below the slab

         3.  There must be a bed surface to support the weak layer and slab above it  

SLAB

A slab is a layer of snow that is cohesive enough (there are enough bonds between the snow crystals) to store energy.  This layer of snow no longer acts as collection of individual crystals but as a single unite. We can now apply forces to this layer of snow and find the resultant force on it just like it was a block on an inclined plan.

WEAK LAYER

A slab gets most of its strength (ability to stay adhered to the mountain) from its bound to the layer of snow below it.  A weak layer is a layer of snow that is not as structurally strong as the slab above it.  A slab could also be bonded underneath to a slippery surface(ice layer).  This would result in a weak bound between the to layers.

BED SURFACE

The bed surface is the consolidated snow beneath both the slab and the weak layer.  The bed surface supports the rest of the snow pack.  It is the bed surface that an avalanche runs on top of once it has been triggered