Skiers and snowboarders can head straight downhill to gather speed, and then veer sideways out of the slide path. If you are caught in an avalanche, the first thing to do is try to get off the slab. In 1970, a massive avalanche of rocks and ice destroyed the town of Yungay, Peru, killing 18,000 people. As a large avalanche speeds down a mountainside, it may compress the air below it, producing a powerful wind that can blow a house apart, breaking windows, splintering doors, and tearing off the roof.Īvalanches strike suddenly and can be deadly. A fractured mass of snow may flow down a slope or become airborne. Many Swiss mountain villages protect homes from snowslides by building large, sturdy structures to anchor snowpacks.Īn avalanche is one of the most powerful events in nature. In the high mountains of Canada and Switzerland, special military troops are in charge of avalanche control. Or they may blast hazardous slopes with a cannon to shake loose any large, new accumulations of snow. At some ski areas, patrols use explosives to set off avalanches. Many ski areas employ avalanche control teams to lessen the danger by starting slides before skiers head for the slopes. However, they can estimate hazard levels by checking on the snowpack, temperature, and wind conditions. In fact, 90 percent of avalanche incidents involving people are triggered by the victim or someone in the victim’s party.Ĭurrently, scientists are not able to predict with certainty when and where avalanches will happen. A single skier can cause enough vibrations to set off a slide. If new snow piles up during a storm, the snowpack may become overloaded, setting off a slide.Įarthquakes can set off avalanches, but much smaller vibrations can trigger them as well. Snow avalanches are most likely to occur after a fresh snowfall adds a new layer to a snowpack.
Storminess, temperature, wind, the steepness of the slope, terrain, vegetation, and general snowpack conditions are all factors that influence whether an avalanche happens and what type occurs. Most are snowmobilers, skiers, and snowboarders. Avalanches kill more than 150 people worldwide each year. In the mountains of the western United States, there are about 100,000 avalanches each year. The thickness and speed of slab avalanches make them a threat to skiers, snowboarders, mountaineers, and hikers. The cloud races downhill at very high speeds. Some of the pieces rise into the air as a moving cloud of icy particles. These snow blocks break up into ever-smaller pieces. Once a slab avalanche starts, the slab shatters into many separate blocks. These layers tumble and fall in a giant block, or slab. When the avalanche is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. This layer is covered with other layers of compressed snow. Sluffs are much less dangerous than slab avalanches.Ī slab avalanche occurs when the weak layer lies lower down in a snowpack. A sluff is a small slide of dry, powdery snow that moves as a formless mass. Sluff avalanches occur when the weak layer of a snowpack is on the top. There are two main types of snow avalanches-sluffs and slabs. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery.
A new snowfall may not stick to this slippery layer, and it may slide off. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer. The bonds between the layers of a snowpack may be weak. The layers vary in thickness and texture. In winter, repeated snowfalls build a snowpack dozens of meters thick. A snowpack is simply layers of snow that build up in an area, such as the side of a mountain. It can travel faster than 320 kilometers per hour (200 miles per hour).Īvalanches occur as layers in a snowpack slide off. A large, fully developed avalanche can weigh as much as a million tons. The moving mass picks up even more snow as it rushes downhill. The snow picks up speed as it moves downhill, producing a river of snow and a cloud of icy particles that rises high into the air. Snowslides, the most common kind of avalanche, can sweep downhill faster than the fastest skier.Ī snow avalanche begins when an unstable mass of snow breaks away from a slope. Avalanches of rocks or soil are often called landslides. During an avalanche, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside.