The spreading of a crack within the snowpack. 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 up to 200 mph making it possible for huge slabs of snow to 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.

The tricky part is that these factors change through space and time. Here is an example that we notice in many areas of North America: Typically, during and right after the storm we see widespread soft slab natural avalanche activity on all kinds of slopes, even small slopes with a lot of compressive support and even in relatively thick trees, but cracks typically don't propagate very far. Since the slab is soft, we can also trigger avalanches easily, and they usually fracture at our feet or snowmobile instead of above us

But as time passes, the slab settles and gains strength. When a persistent weak layer is involved, the slab typically gains strength much more quickly than the persistent weak layer beneath it. Thus, over time, we usually see fewer avalanches and they are less sensitive to triggers, but the ones we do see will occur on large, open slopes without anchors, the fractures will occur above us instead of at our feet, and because of the stiffer slab, the fractures will propagate much longer distances. With very fragile, persistent, weak layers such as surface hoar and faceted snow, it's not unusual for fractures to propagate around corners and over ridges, or to trigger the slopes "remotely" or “sympathetically” meaning from a distance away.