Ice fall avalanche in action, Minarets, New Zealand.
Snow is a unique material; it can flow like honey or fracture like glass.
(Fredston 2005)
Three types of avalanche
- slab avalanches
- loose snow or point release avalanches
- ice or serac falls
Each type of avalanche has different properties, different mechanisms of release and presents different problems in the field.
Even a small avalanche can have very serious consequences if you are in the wrong place, at the wrong time e.g. standing on a sleep slope above an ice cliff.
Sometimes avalanches combine. For example a serac fall which lands on a pillow of snow and initiates a slab avalanche below.
slab avalanches
Slab avalanche, Murchison Glacier, New Zealand.
Though the word slab conjures images of something solid like a side of beef, slab snow is simply one or more layers of snow that are more cohesive than the layer below.
- Most large and dangerous avalanches that bury snow enthusiasts are slabs.
- Layering is the most important property of the snowpack with respect to the formation of slabs.
- They start from a fracture line.
- Are set into motion at once over a wide area.
- Require a relatively strong snow layer above a weaker one (or a weak bond). The relatively strong snow layer may range from soft new snow (soft slab) to hard, wind-packed snow (hard slab).
- May start in a shallow layer at the surface or in deep snow layers.
- May start in dry or wet snow.
- Occur most frequently on 30 - 45 degree slopes.
A slab avalanche starts when there is a closely balanced stress - strength relationship in the snowpack and a trigger upsets the balance.
Natural triggers include: Snow or rain fall, wind-deposited snow, a rise in temperature, cornice collapse, earthquake . . .
Human triggers include: Skiers and snowboarders, mountaineers and hikers, machinery, snow mobiles, explosives . . .
loose snow avalanche
Small loose snow avalanches, Les Aiguilles Rouges, France.
- Start from a point and hence are also refered to as point release avalanches.
- Are set into motion progressively.
- Require snow with poor cohesion (similar to that of dry sand or granulated sugar).
- Are usually confined to surface layers, and are therefore often small.
- May start in dry of wet snow.
- Occur most frequently on 40 to 60 degree slopes.
ice or serac falls
It cannot be predicted when ice falls collapse or the jutting tower of a serac tumbles, setting off an avalanche. They are best given a wide berth.