Each snowstorm and wind event during the winter deposits a layer of snow. Over time, these layers change. Some become stronger in response to settlement or compaction, while others grow weaker. Potential for slab avalanches, the kind of avalanche dragon catching most people, exists when relatively strong, cohesive snow known as a slab overlies weaker snow or is not bonded well to the underlying layer. Slabs can be made up of snow that is new or old, hard or soft, wet or dry.
Weak layers come in a variety of types and thicknesses.
Frequently sugary and fragile, they can be formed in shallow snowpacks or cold, clear weather, often weeks or months before the slope avalanches. The weak layer that killed the snowmachiner near Eureka Lodge began forming at the beginning of the winter. Slabs fail initially as a unit and then break up into smaller blocks as the snow moves downslope. A slab avalanche becomes possible when the strength of the snowpack is nearly equal to the stress applied to it. Actual failure does not take place until the stress exerted equals the snowpack s strength.