Avalanches

An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope.

Avalanches are primarily composed of flowing snow, and are distinct from mudslides, rock slides, and serac collapses on an icefall. In contrast to other natural events which can cause disasters, avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snow pack. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.

Causes of Avalanches:

Layers of snow build up and slide down the mountain. A vibration or movement like the voice of a person or a train can start and avalanche. Certain weather conditions like wind and rain. A rock or a piece of ice can shake the snow to slide down the mountain. Water under the snow could make the snow slide. A section of the snow could fall down or break apart and slide down the mountain.

Frequent occurrence:

About 90% of all the avalanches begin on slopes between 30 to 45 degrees; about 98% of all the avalanches occur on slopes between 25 to 50 degrees. Avalanches start most often on slopes above the timberline that face away from prevailing winds. Avalanches can run, however, on small slopes well below the timberline, such as gullies, road cuts, and small openings in the trees. Very dense trees can anchor the snow to steep slopes and prevent avalanches from starting; however avalanches can release and travel through a moderately dense forest. Most avalanches occur in the backcountry, outside of developed ski areas.

Major Avalanches:

The world's largest recorded avalanche occurred on May 18, 1980 when Mount St. Helens erupted. The eruption had the energy of 1000 Hiroshima-sized bombs and resulted 2/3 a cubic mile of the north slope sliding down into the valley at over 150 miles per hour. This is enough material to cover Washington, D.C. 14 feet deep. The avalanche resulted in deposits as deep as 600 feet and as far away as 15 miles, the 23 square mile valley was covered with an average of 150 feet of debris.