An avalanche occurs when a mass of snow which
has built up on a mountain side begins to slip and finally to fall. There can
also be avalanches of earth, stones, rock and ice, but usually the word is used
to describe a rapid fall of snow.
Snow
builds up to great thickness on steep slopes, especially if the surface is not
smooth. Even a very small disturbance may set it in motion. The vibration of a
passing vehicle, the movement of a man or animal, the fall of a tree branch
or even a sound can cause thousands of tons of snow to crash down a mountainside.
The speed of
an avalanche varies enormously, but some have been estimated to move at about 200
miles an hour. A big avalanche hurtles down the side of the mountain with a
thunderous roar, crushing or sweeping away anything in its path.
The swiftly moving mass of snow
pushes the air in front of it with such violence that it fans out sideways as
well as driving directly ahead. This wind sometimes reaches a force almost
equal to that of a tornado. This great wind is often a more powerful force of
destruction than the avalanche itself.
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