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What is an Avalanche?

Types of Avalanches

There are three main types of avalanches that backcountry users may encounter:

Slab avalanches

These are, by far, the most dangerous types of avalanches for people. They occur when a more dense layer of snow, known as a slab, slides along a weakness in the snowpack, known as a weak layer. A slab will move very quickly downhill, up to 130 km/h, starting off as a cohesive unit and shattering into smaller pieces as it descends.

A slab avalanche.
Slab avalanches happen when a thick layer of consolidated snow breaks away from the surrounding snowpack and slides downhill. It can deposit big chunks of snow when it comes to a stop.
Avalanche Canada

Loose snow avalanches

Loose snow avalanches happen when poorly bonded surface snow slides downhill under its own weight. Sometimes called sluffs, these types of avalanches are generally smaller and less dangerous than slab avalanches, but they can still pose a hazard in the wrong terrain.

A series of loose snow avalanches.
A series of loose snow avalanches.
Raven Eye Photography


Cornices are overhanging masses of wind-deposited snow that protrude from sharp terrain features like ridges or peaks. These beautiful structures can be deadly, breaking off in response to changing weather or the weight of a person or machine. When they fail, tonnes of snow falls on the slope below, forming an avalanche or potentially triggering an even bigger avalanche.

A line of captions along a ridge.
Cornices form on the leeward (downwind) side of ridges. They form an overhang, with a void underneath, making them very dangerous if you wander on top of one.
Raven Eye Photography