Glaciers are rivers of ice in continuous motion, that are formed by compaction and re-crystallisation of snows over hundred or thousand of years. The flow movement, irrespective of whether it is a few centimetres or tens of metres a day, differentiates a glacier from a dead ice body. Global glacier cover, at present, is reported to be about 15,000,000 sq km of which about 14,000,000 sq km is restricted to the two icy continents of Antarctica and Greenland. Rest of the ice cover is distributed within the mountain ranges of the northern hemisphere like in Alps, Rockies, Himalaya and also in the New Zealand Alps in southern hemisphere. There are about 9,500 glaciers in the Himalaya. Glaciological studies report that many glaciers in the Himalaya show retreat; however, some are stationary while few have advanced.
Avalanches, a sudden rapid flow of ice/snow down a slope, similar to landslides, occur in ice bound terrain. The frosty covering, precariously held to mountain slopes, coupled with excessive amounts of snow layering in a particularly severe winter and mixed with unpredictable weather patterns such as sudden warming are the makings of a disaster. Avalanches, as popular cartoons lead us to believe, are also instigated by human activity, especially mining and construction. While avalanches are sudden, the warning signs are numerous. Avalanches kill more than 150 people worldwide each year.
Avalanches range from small slides of dry powdery snow that move as a formless mass, ‘sluffs’, to disastrous snow surges that occur when massive slabs break loose from a glacial mountainside and shatter like broken glass (Fig.1). These moving masses can reach speeds of 80 miles (130 kms) per hour within about five seconds. Victims caught in these events seldom escape. Avalanches are most common right after (up to 24 hours) a storm that dumps 12 inches or more of fresh snow. The swift precipitation overloads the underlying snowpack, which causes a weak layer beneath the slab to fracture. Storminess, temperature, wind, slope steepness and orientation (the direction it faces), terrain, vegetation, and general snowpack conditions are all factors that create low, moderate, considerable, and high avalanche hazards. Loose snowpack conditions as well as temperature rise cause melting of the exposed surface layers, which re-freeze to create a smoother and less stable surface for the next snowfall.
The higher reaches of the Himalaya remain under a perpetual cover of snow and it is here that thousands of avalanches occur, involving the movement of thousands of tons of ice and vertical displacements of over 1,500 metres. These avalanches where there are no inhabitants, do not really affect human lives. The inhabited areas in the Himalaya, for example Kalpa, the old district headquarters in the Kinnaur Himalaya was under a constant threat of avalanches in winter and this was cited as one of the reasons for shifting the headquarters to Peo. Several villages are known to have been wiped out by avalanches. In 1838, Tunda village in Ladakh was completely destroyed by an avalanche, leaving many people dead. Avalanches also block the flow of rivers and streams, thereby creating temporary dams. When these dams breach, flash floods occur washing away human life and property. In recent memory, Himalayan avalanches wrecked havoc in the Pin Valley of Spiti in Himachal Pradesh in 1978.