Avalanches occur because of the unstable nature of snow masses in mountainous areas. Change in temperature, sudden sounds or earth borne vibrations may trigger an avalanche. They are a particular danger when the snow begins to thaw.
Then one snow layer can slide over another and crash down the steep slopes. Snow compacts into ice as it moves along the slopes. Down the way it carries along the rocks and boulders that come across its path making its final impact even more devastating. Avalanches take place generally on slopes that are more than 35 degrees. Slopes that are bar with no trees to hold back the snow are equally hazardous.
The largest avalanches take place in the Himalayas. Avalanches are a constant danger in mountainous areas especially in ski resorts such as the French Alps.
In 1991, a massive avalanche considerably changed the shape of Mount Cook in New Zealand. Some countries have begun large scale planting of trees on the mountain slopes to reduce the danger of avalanches. Snow bridges have built over roads and railways to protect them. Snow patrols have been posted to keep a watch in winter sports areas. At the slightest hint of an avalanche, warnings are sent out and these areas are closed.
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