The initial failure of a slab avalanche is usually caused by what?

• A. Moisture in the snow
• B. Shear in a weak layer or between two weakly bonded layers
• C. Excessive weight from snowfall
• D. Temperature fluctuation

Answer: (B)

The initial failure of a slab avalanche is predominantly triggered by shear in a weak layer or between two weakly bonded layers. In the context of snowpack stability, the concept of shear strength is crucial. A slab avalanche occurs when a cohesive layer of snow (the slab) becomes unstable and slides over a weaker layer. This weak layer may consist of less dense snow, surface hoar, or facets that create a poor bond, thus lowering the overall stability of the snowpack.

When stress is applied to the slab, such as from the weight of new snow or additional loading, the weak layer cannot withstand the shear forces, resulting in failure. This failure often propagates quickly, leading to the catastrophic release of the slab. Understanding this mechanism helps in assessing avalanche risks and adopting appropriate safety measures in mountainous terrains where slab avalanches are a concern.

While moisture in the snow, excessive weight from snowfall, and temperature fluctuations can contribute to avalanche risks, they are usually not the primary cause of initial failure in a slab avalanche, which predominantly hinges on the condition and stability of weak layers within the snowpack.