An avalanche is a large mass of rock debris, snow, ice or soil that moves rapidly down a mountain slope, sweeping and grinding everything on its path. It takes place when a mass of material overcomes the frictional resistance of the sloping surface. It can be caused by spring rains, dry warm winds or violent vibrations produced by an artillery fire, thunder, man-made blasts or earthquakes. Changes in the depth, pressure and humidity of the snow itself can make one snow layer slide over another and result in an avalanche.
There have been instances where huge avalanches have blocked rivers and buried towns. They are commonly composed of bed-rock fragments, a few centimeters thick in diameter and include a lot of soil and dust. The rock avalanches are thought to ride on a cushion of compressed air that allows them to travel long distances. A debris avalanche usually occurs in unconsolidated earth materials when weakened by moisture.
Some snow avalanches occur during heavy snow storms. Sometimes they slide while the snow is still falling, but more often they occur after the snow has accumulated at a given site. One of the causes of snow avalanches is the slow formation of hexagonal cup-like ice crystals under the snow pack. The formation of these crystals creates a zone of weakness near the ground. These crystals act as a lubricant when the upper layers of the snow start sliding down the mountain. The wet avalanche is the most dangerous thing because of its massive weight, heavy texture and tendency to solidify as soon as it stops moving. The dry type is also equally dangerous because it’s entrapping of great quantity of air makes it act like a fluid.
Sometimes, explosives are used to break large avalanches to control their intensity. Other measures such as snow-fences, wall and snow-bridges are erected on hillsides to check the fury of avalanches.
Since the avalanches pose considerable threat to the mountain climbers, skiers, and travellers, they are being a subject of a lot of research and study. Geologists and scientists all over the world are working in close cooperation to identify the avalanche-prone areas and to monitor the danger of avalanches. Switzerland has established a Snow and Avalanche Research Institute to locate the avalanche-prone areas.