Avalanches are divided into slab avalanches, loose snow avalanches and slush avalanches.
The most dangerous avalanches occur when an entire slab of snow starts to slide on less compact layers of snow down towards the ground. A slab of snow can be quite large and, in a slab avalanche, all of the snow starts to slide at the same time. It is not uncommon for the slab that slides out to cover an entire mountainside.
Nearly all avalanches that cause injury or damage to people or settlements are slab avalanches.
Loose snow avalanches:
Loose snow avalanches start when powder-like snow starts to move from a point. From that point, the avalanche spreads like a fan down the mountainside. The amount of snow that comes loose is not normally very large, and loose snow avalanches rarely cause major damage. It is not uncommon for loose snow avalanches to be triggered by skiers.
Slush slides can occur when the snow is saturated with water, typically in areas with a lot of natural water, such as in the form of small lakes, streams, or in marshy terrain. Slush slides can also occur on gentle slopes, often following river courses or other natural paths in the terrain.
In the same manner as slab slides, the snow cover in an entire area often comes loose all at once. The mixture of snow, ice and water means that this type of avalanche can travel at very high velocities, causing substantial damage.