The Making of an Avalanche Natural Disaster: the Unstoppable Force of a Slab Avalanche
written by: Dr Mike C•edited by: Rebecca Scudder•updated: 11/23/2010
The mountains attract many people year around, but winter sports draw thousands of people to the slopes. Avalanche natural disasters can happen when a slab of snow is unstable and breaks away down the mountainside. This article looks at the types and causes of avalanches.
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Natural Disasters In The Mountains
Every year, a small number of holiday makers out to enjoy winter sports in the beauty of the mountains will get caught up in an avalanche natural disaster. Tragically, many people who get swept up by an avalanche will not survive, but others will be dug out of the snow alive. Survival depends on a range of different factors.
What causes the natural disaster of an avalanche?
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By definition, an avalanche is an event where a large mass of snow breaks away from a slope and cascades down the side of the mountain. Avalanches can be sub-divided into two further classes; slab and loose snow events. These classes are then both further divided to describe whether the snow involved is dry, damp or wet. For an avalanche to occur, two conditions need to be met; the slope on which the snow mass lies must be steep enough to permit the snow to slide down the slope, which usually requires an angle greater than 30°; and secondly, the snow mass must be unstable.
A large, slab type avalanche natural disaster may involve slopes anything from 200 to 1500 metres wide and can involve millions of tons of snow. The kinetic energy of this snow mass is sufficient to knock down trees and man-made structures in the path of the snow mass. Depending on the circumstances, this type of natural disaster can provoke winds (as the snow mass causes turbulence in the overlying air) which can cause substantial damage well beyond the track of the avalanche itself.
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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.