A weak layer buried inside
Why Does a Snowy Slope Suddenly Let Go?
An avalanche happens when a buried weak layer collapses and the whole slab above slides at once.
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Step 1 of 5
Snow builds up as separate layers.
Each snowfall lays down a distinct layer, and weather between falls changes each one differently before the next arrives.
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Step 2 of 5
Some layers bond badly to their neighbours.
A layer of large loose crystals, or a smooth ice crust, gives the snow above almost nothing to grip, creating a hidden weakness.
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Step 3 of 5
The slab above behaves as one sheet.
Wind-packed snow is stiff and holds together, so instead of grains sliding individually the whole slab acts as a single rigid plate.
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Step 4 of 5
A trigger collapses the weak layer.
Extra weight from new snow or a skier crushes the fragile crystals, and that collapse spreads outward through the layer very fast.
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Step 5 of 5
Then the whole slab releases at once.
With its support gone the slab breaks away along a clean fracture line and accelerates downhill, which is why it looks so sudden.
Most fatal avalanches happen on slopes between 35 and 45 degrees, steep enough to slide but gentle enough to hold deep snow.
The short version
An avalanche happens when a buried weak layer collapses beneath a rigid slab of snow, releasing the whole sheet at once.
Try it yourself
Stack a book on a sheet of paper over marbles, then tilt it. The load slides as one piece when the weak layer gives way.
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