Slab avalanches begin with a crack in a weak layer beneath the slab. The crack propagates horizontally, detaching the slab from the slope. The speed of this crack determines how much of the slope releases at once — and therefore the size of the avalanche.
Measured crack propagation speeds in full-scale avalanches reach 130 meters per second. The theoretical maximum, derived from the elastic properties of snow and the geometry of weak-layer collapse, is approximately 80 meters per second. The cracks are exceeding their own speed limit.
The discrepancy comes from the theoretical model assuming a purely elastic failure. In reality, the weak layer doesn't just crack — it collapses. The weak layer, typically composed of faceted crystals or depth hoar, has a structure that is much taller than it is strong. When it fails, it compresses vertically — the slab drops. This vertical collapse releases gravitational potential energy that feeds back into the crack tip, driving it forward faster than the elastic energy alone could sustain.
The crack is not just propagating through a material; it is propagating through a structure that falls behind it. The falling slab adds energy to the system continuously. The crack outruns its elastic prediction because it has an energy source that the elastic model doesn't account for: gravity acting on the collapsing material behind the crack front.
This makes slab avalanches fundamentally different from cracks in conventional materials. In a beam or a plate, the crack tip is fed only by stored elastic energy. In a slab avalanche, the crack tip is fed by elastic energy plus gravitational collapse — an open energy supply that scales with the area already failed. The bigger the crack, the more energy it receives. The avalanche accelerates beyond what the material alone would permit because the geometry feeds it.