An avalanche is a massive slide of snow, ice, rock or debris down a mountainside. Provoked by an earth tremor, extreme precipitation or man-made disturbances (such as a loud noise or the heavy movement of a skier or snowboarder), an avalanche can reach speeds of over 200 m/h (300 km/h). The impact of the falling material and the winds produced by the flow can cause extensive damage to anything in its path. According to experts, there are some 1 million avalanches yearly.
In the case of a snow avalanche, the new snow that accumulates on top of another heavy layer of snow can begin to slide down the mountainside. The risk of an avalanche can be reduced by building a snow shed â€” a barrier made of rocks, soil and other materials â€” or by triggering a controlled avalanche at a time when no one is on the mountain.
The worst US avalanche occurred in 1910, when a snowslide swept two trains into a canyon in Wellington, WA, killing 96. In January 1962, an avalanche down an extinct volcano in Peru killed 3,000.