Off Piste Awareness Part 1 — Types of Avalanche

Slab Avalanches

Most people that die in avalanches, die in slab avalanches. Slab avalanches occur when a more cohesive or harder layer of snow sets on top of a less cohesive or softer and weaker layer of snow. Slab avalanches are responsible for around 75% of fatalities and are triggered by the victim or a member of his group in 90% of cases.

Snow pit showing sugary (Faceted snow) weak lower snow pack

Snow pit showing sugary (Faceted snow) weak lower snow pack

A typical slab is about half the size of a football field, about one to two feet (30–60 cm) deep, and usually reaches speeds of 20 mph (32 km/h) within the first three seconds, quickly accelerating to around 80 mph (128 km/h) after the first, say, six seconds.

Notice the difference in the layers. We dug a snow pit in to review the snow pack, and found a strong upper bonded layer on top of a very sugary (Faceted) base. This allowed up to make a more informed decision on our ski touring route. Later in the day we witnessed the results of this weakened layer as shown in the photo at the top of the article.

Loose snow or sluff Avalanches

Is where the powdery surface slides. Typically are the least dangerous type of slide; however, sluffs can and often do injure skiers and boarders by pushing them over cliffs and rock bands in steep terrain.

Loose snow avalanches usually start from a point and fan outward as they descend, and because of this they are also called “point releases.” Very few people are killed by loose snow avalanches because they tend to be small and they tend to fracture beneath you as you cross a slope instead of above you as slab avalanches often do. The avalanche culture tends to minimize the danger of loose snow avalanches, sometimes calling them “harmless sluffs.” But, of course, this is not always the case. Sluffs can be a sign of stability within the deeper snow when new snow sluffs down without triggering deeper slabs. Sluffs are usually easy to deal with.

Wet slide Avalanches

Occur when warm temperatures melt the surface snow layers and saturate them with water. Wet avalanches usually occur when warm air temperatures, sun or rain cause water to percolate through the snowpack and decrease the strength of the snow, or in some cases, change the mechanical properties of the snow. Wet slides are harder for a person to trigger than a dry slide and they travel a lot slower.

We encountered a wet slide avalanche, late season in Hochfugen, Austria. Due to the conditions and temperatures our guide recommended we kept to a low angle of slope, yet the pressure of a hard turn on my board as I stopped by a tree (left side of photo) set off a small slide below me. This then triggered another slide (right side of photo). In this case there was plenty of time to ride out of the path of the snow as it was very slow moving and stopped within 50m.

A couple of low angle wet slide avalanches in Hochfugen, Austria

A couple of low angle wet slide avalanches in Hochfugen, Austria

Glide or Full Depth Wet Slides Avalanches

Full depth avalanche in Mayrhofen, Austria

Full depth avalanche in Mayrhofen, Austria

Occurs when the entire snowpack slowly slides as a unit on the ground, similar to a glacier. Don’t mistake glide for the catastrophic release of a slab avalanche that breaks to the ground. Glide is a slow process, that usually occurs over several days. Glide occurs because melt water lubricates the ground and allows the overlying snowpack to slowly “glide” downhill. The presence of glide cracks in the snow do not necessarily mean danger. It’s often difficult for a person to trigger a glide avalanche but at the same time it’s not smart to be mucking around on top of them.

This article forms part of a series that will highlight areas of off-piste knowledge.

Sources - 
http://pistehors.com/backcountry/wiki/Avalanches/Slab-Avalanches

http://www.fsavalanche.org/loose-snow-avalanche/

Just remember this information is not a substitute for an avalanche class. Get educated before heading into the winter backcountry.