The good news is […] 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. The pressure may locally damage the weak layer. Over time soft slabs can transform into hard slabs as crystals bind together.
However backcountry travellers should avoid focussing on this one instance. The slab itself may deform elastically under stress, such as the load from a skier but eventually fail in a brittle way if sufficient load is supplied. An avalanche starts when a layer in the snowpack becomes overloaded and fails.
Vol 24 no 3, Febrary 2006 The next section will discuss in detail how slab avalanches originate and how to predict them.But before getting into that, it's important to know some of the other types of avalanches that could be encountered. Slab density in hard slabs is typically at least 300 kg/m3. Videos, graphics, guidance, other information, or user generated content (the “Content”) on this site is presented for general educational and information purposes only and to increase overall backcountry safety awareness. noun.
Duclos and Louchet warn against trying to predict whether sub or supercritical (favoured on large 35 degree slopes) will occur. They are usually small slides which break directly under a skier or snowboarder but are never the less deadly, especially where there is a flat runout, terrain trap, trees or cliffs.
The weight of the skier or snowboarder is relatively insignificant (80kg) compared to the snow slab (1000's of tonnes). Avalanches can also be triggered remotely, even where the traveller is on flat ground, either below or above the slopes at risk, as the basal crack can propagate onto surrounding slopes often resulting in several slides.
A typical slab avalanche is 70 meters wide, 200 meters long and a has an initial fracture line—or crown face—50 centimetres deep.
A reminder that tracks are no guarantee of safety. In their Model the snow slab is symbolized as a series of cohesive blocks linked by springs. Slab avalanches are usually not that big compared to a catastrophic avalanche. Not exactly the dramatic avalanche Aunt Jenny saw on TV. Soft slabs are comprised of fresh snow, usually with less than 30% water. A typical slab avalanche is 70 meters wide, 200 meters long and a has an initial fracture line—or crown face—50 centimetres deep. Group spacing when climbing and skiing can play an important role where strong slabs are present. Not exactly the dramatic avalanche Aunt Jenny saw on TV. But just as deadly. Slab avalanches are the most dangerous type of avalanche. However for thicker slabs repeated traverses at the same point penetrate the snow pack deeply enough to cause the buried weak layer to collapse. Gilles Brunot, avalanche forecaster for Meteo France in Chamonix, says that the majority of avalanche accidents are not caused by wind slab but that most slab avalanches are due to weaknesses in the snow pack, whether they have been formed under the action of wind or not. It is held in place by cohesive resistance with the surrounding snow and shear resistance with the weak layer.
Soft slab avalanches can form anywhere on a slope where there is sufficient snowfall and are undetectable from powder snow. The user of this site should complete a ground course from a qualified avalanche center before engaging in any backcountry activities. Hard slabs can also be formed under the action of wind where the fine crystal structures are broken down under a sintering action. Avalanche Danger Scale; Avalanche Course Progression; Find a Course Near You; Get Listed as a Course Provider; Avalanche Encyclopedia; Avalanche Safety Tutorial;
In other words triggering goes unexpectedly from sub to supercritical. Although slabby snow is relatively easy to spot, especially close to the top of couloirs or other terrain features hard slabs may also get buried by fresh snowfall making them very difficult to detect. The avalanche will release under the skier. Louchet has calculated that this type of triggering is favoured by large slabs on slopes around 35.3 degrees rather than 45 degrees that may be expected with using a purely mechanical model. All the knowledge for all the skiers and snowboarders who want to ride out of bounds or in the backcountry. It is not unheard of for skiers to be pulled off ridgelines as the basal crack propagates upwards onto flatter ground. Education. As the remaining slabs gain strength we observe larger but less frequent hard slab releases. A great foundation for beginners and the perfect refresher for experts.
Advice might be to follow existing tracks, either when skiing or traversing a slope at risk in order to limit basal crack propagation. The Content is neither intended to be expert advice or a substitute for expert advice, nor is it a substitute for a ground course offered by qualified avalanche educational/certification centers. ‘A massive slab avalanche had broken off … There are two main types: loose-snow (or sluff) avalanches and slab avalanches. Snow cover on slopes is rarely consistent. This avalanche generally consists of dry snow, happens mostly on slopes on shadowed aspects (between west-northwest and east-southeast (WNW-N-ESE)), is an average of 70 meters wide and 50 centimeters thick where the first fracture took place and the angle is usually 38 degrees or more.
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