Anticracks: a New Theory of Fracture Initiation and Fracture Propagation in Snow

Shear failure models have traditionally dominated the research on slab avalanche initiation in snow science, but evidence has accumulated that they tell us only part of the story. In these models weak snowpack layers are regarded as compounds of zero thickness, but in reality they often consist of ice grain aggregates of finite thickness and high porosity. When such granular aggregates fail, the remaining fragments can rearrange in a tighter packing and the weak layer undergoes a reduction in volume. This compaction has fundamental consequences for understanding slab avalanche release. We present a new theory of fracture initiation and propagation in weak layers, based on the mathematical concept of mixed-mode anticracks. Energy criteria for crack propagation and skier triggering are obtained from the theory. Initiation and propagation of anticrack-mediated fracture are illustrated on field measurements.