Slab Avalanches

A slab avalanche is an avalanche that loosens as a large, cohesive area of snow of snow and slides downwards.

These avalanches are triggered when a layer of snow bonds into a stable, continuous layer on top of a weaker layer. If the weak layer collapses due to external forces such as wind loading, or the force of a skier, then the released slab will slide on the surface of the snow below the weak layer, picking up snow and debris with it.

Slab avalanches are the most dangerous type of avalanche and they are responsible for most fatal accidents.

Wet Slab Avalanche

Wet snow is recognizable by its more transparent and bluish color. It can occur due to rain, sun, warm temperatures, or lack of refreezing of the snowpack at night. Wet slab avalanches take rock and dirt along with them down the slope, which will gather in the runout zone. When the slope moves downwards, most of the snow masses will move along the ground due to the high water content which makes it heavy. Another consequence of the high water content is a long runout length.

Dry Slab Avalanche

Dry snow is recognizable by its more natural white color. After the avalanche is released, a lot of snow often is thrown into the air because of its lighter crystal formations. For this reason, a dangerous snow cloud is often formed. The snow cloud follows the slope with the terrain, but since the snow cloud has less friction to the ground, it will not slow as much as the snow masses when the steepness declines. The snow cloud could therefore continue over long distances even after the snow masses have stopped. This could cause major damage far from the runout zone, as well as a cloud the view of the skier who is caught in the debris.

Release

Most slab avalanches are triggered by natural conditions, but some are triggered when there is an additional load on the snow, for example a skier. The largest slab avalanches occur when there is a continuous weak layer. Such snow layers are usually formed during cold periods and can remain unstable for a long time. These layers can therefore be buried so deep that they are difficult to influence, but very heavy loads, from a large snowfall, or melting of the snow due to heat can set off an avalanche. Slab avalanches can be remotely triggered, and large avalanches could be triggered from places where the snow layer is much thinner. Its therefore extremely important to be aware of your surroundings and avoid standing directly below avalanche terrain.

Other Avalanche Types

Loose Snow Avalanches

A loose snow avalanche is an avalanche that releases from a single point and spreads downwards in a cone shape.

Find an Avalanche Course

This information is meant to be an introduction to avalanche safety and a knowledge base for backcountry travelers.

But reading it cannot replace real experience in reading conditions and making decisions in the field. We strongly recommend that all backcountry travelers take instructor-led avalanche courses to get hands-on experience in wild snowpacks.