Stability Of The Snow Mass
The stability of the snow depends on a number of factors which include the temperature and moisture content of the snow and the history of its deposition (how well “anchored" it is). The force that keeps the snow on the mountainside is friction between the upper layers and the snow mass on the rock surface itself. Simply put, when the gravitational force down the slope of the mountain exceeds the frictional force, then the snow mass will shear away and an avalanche will be produced.
The cohesion (or “stickiness") of the snow is a critical factor. If the cohesion is poor, a point release, or loose powder (sluff) avalanche can start. As snow moves down the mountainside, more low cohesion snow becomes involved as the avalanche develops. This type of avalanche generates a V pattern, but normally does not involve enough snow to bury victims deeply. Most fatalities occur because people get swept off the mountain over cliffs, or through collisions with rocks. This type of avalanche has the lowest associated fatality rate. In a sluff event, the snow layer involved is just the upper part of the snow mass as this is not strongly enough bound to the underlying snow mass (which stays in place).
In a slab avalanche, cohesive forces within the snow mass are enough for it to act as a single structure (hence the term “slab"), but the cohesion between the snow mass and the bedrock is not enough to overcome the gravitational force. The passage of a single skier over a slab (which is impossible to see) can be enough to disturb the balance of these forces and trigger an avalanche. When a slab avalanche takes place, it may involve many thousand tons of snow, can move down the mountainside with speeds as high as 200 km/hour and has the force to destroy anything in its path.
Probably the most devastating avalanche natural disaster in terms of loss of human life took place in Peru in 1962. A slab avalanche originating at 22000 feet on Mt Huascaran, an extinct volcano, destroyed two villages. The mass of snow associated with the tragedy was estimated to be in excess of six million tons. The wind associated with the avalanche also devastated other villages downhill, resulting in the loss of 4000 lives.