The spontaneous spreading of a crack within the snowpack, without the addition of any external force. Slab avalanches occur when a crack propagates through a layer of snow underneath a slab sitting on a steep slope.
Propagation is the spread of a crack in a weak layer from an initial location. A crack can propagate extremely rapidly, making it possible for huge slabs of snow to seemingly release from a mountainside instantaneously. The propagation potential of a particular slab and weak layer dictates how large an avalanche may become once triggered, and also determines if it’s possible to trigger avalanches from flatter terrain connected to steeper slopes.
Whether a localized crack propagates or not, or how far the propagation will proceed, depends on a complex interaction of many different snowpack properties. Further complicating this interaction is the everchanging nature of snow.
For instance, if a skilled avalanche worker digs several snow profiles on a test slope and finds easy compression tests and propagating extended column tests, high quality shears, a persistent weak layer with a critical combination grain type, grain size and hardness differences between the slab and the weak layer, plus they find those same conditions in several snow profiles on the same slope, they can safely conclude that the snowpack can both initiate and propagate a crack. In other words, avoid all similar slopes steep enough to slide.