How Does an Avalanche Start? A Weak Layer and a Trigger
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A snow slope does not slide because it is steep and heavy. It slides because somewhere inside the snowpack there is a weak layer, buried under a cohesive slab, and something applies enough stress to make that layer fail across an area large enough for the fracture to run. Almost all avalanche safety is the practice of finding out whether that structure exists.
The ingredients
A slab avalanche, which causes the overwhelming majority of deaths, requires several things at once and is unlikely without all of them:
- •A slope steep enough to slide, with most avalanches releasing between about thirty and forty-five degrees, and slopes below about thirty degrees rarely producing them
- •A cohesive slab, a layer of snow bonded well enough internally to move as a unit
- •A persistent weak layer beneath it, which is where the failure happens
- •A bed surface the slab can slide on, frequently an old crust or the ground itself
- •A trigger applying stress, which is usually the weight of a person, since human-triggered avalanches kill far more people than natural ones
- •A slope large enough for the fracture to propagate, since a weak layer that fails locally without spreading produces nothing
How weak layers form
Snow on the ground is not inert and changes continuously through metamorphism. When the temperature difference across the snowpack is small, grains round off and bond to each other, producing a strong, well-sintered pack. When the difference is large, which happens in shallow snowpacks under cold clear conditions, water vapour moves upward through the pack and deposits on grains, growing large angular faceted crystals with almost no bonds between them, sometimes developing into depth hoar that resembles loose sugar. That layer is weak and, crucially, persistent: it can sit buried for weeks or months and remain ready to fail long after the weather that made it has gone. Surface hoar, the feathery frost that forms on the snow surface on cold clear calm nights, is equally dangerous once buried, because it is beautifully formed and structurally useless. Wind builds slabs by scouring snow from one side of a ridge and depositing it on the lee side, which is why leeward slopes below ridgelines are so consistently implicated.
The kinds and what they do
Loose snow avalanches start at a point and fan outward as they entrain more snow, and are generally smaller and less deadly, though they can sweep a person over a cliff. Slab avalanches release as a block, with a sharp crown fracture across the top, and are the killers, because a person on the slab is on the moving object from the start. Wet avalanches occur when water percolates through the pack and destroys bonds, moving more slowly and with enormous density, and they set like concrete on stopping. A destructive size scale runs from small sluffs that could not bury a person up to events that destroy villages and forest. Most people caught in a slab avalanche trigger it themselves, and the statistics on survival are stark: someone completely buried has a good chance of survival if extracted within about fifteen minutes and a poor one after half an hour, with asphyxiation the dominant cause of death, which is why companion rescue by the people present matters far more than any organised response.
How risk is assessed
Forecasting combines weather data, field observations and stability tests. Snow pits dug through the pack expose the layering directly, and standardised tests, including the compression test and the extended column test, load an isolated column to see whether a layer fails and, importantly, whether the failure propagates across the column, since propagation is what turns a local collapse into an avalanche. Regional forecasts publish a danger rating and identify specific avalanche problems, naming the type, the aspect and elevation where it exists and the likely size, which is far more useful than a single number. The persistent problem is human rather than physical: a large proportion of fatal accidents involve experienced people who recognised the hazard and proceeded anyway, and analyses have identified recurring decision traps including familiarity with a slope, commitment to a plan, the presence of tracks suggesting safety, group size effects and the desire to be seen as capable. Equipment helps, with a transceiver, probe and shovel being the minimum for companion rescue and airbags improving survival odds, and none of it changes whether the slope will slide.
The takeaway
A slab avalanche needs a steep enough slope, a cohesive slab, a persistent weak layer beneath it, a surface to slide on and a trigger, which is usually the person caught in it. Weak layers form when a large temperature gradient grows faceted crystals with no bonds, or when surface hoar is buried, and both can stay dangerous for months. Survival falls sharply after about fifteen minutes of burial, so companion rescue decides outcomes, and most fatal accidents involve people who saw the hazard and continued.