Your web browser (Safari 7) is out of date. For more security, comfort and the best experience on this site: Update your browser Ignore AVAL ANCHE snowslide For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia// During an, a mass of snow, rock, ice, soil, and other material slides swiftly down a mountainside. Avalanches of rocks or soil are often called landslides. Snowslides, the most common kind of, can sweep downhill faster than the fastest skier. A snow begins when an unstable mass of snow breaks away from a slope. 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. The moving mass picks up even more snow as it rushes downhill. A large, fully developed can weigh as much as a million tons. It can travel faster than 320 kilometers per hour (200 miles per hour). Avalanches occur as layers in a snowpack slide off. A snowpack is simply layers of snow that build up in an area, such as the side of a mountain. In winter, repeated snowfalls build a snowpack dozens of meters thick. The layers vary in thickness and texture. The bonds between the layers of a snowpack may be weak. Melted snow that refreezes may cause a slick coating of ice to form on the surface of a layer. A new snowfall may not stick to this slippery layer, and it may slide off. During spring thaw, melted snow can seep through a snowpack, making the surface of a lower layer slippery. Added weight or vibration can easily send the top layers of a snowpack hurtling downhill. 1 of 6
Sluffs and Slabs There are two main types of snow s sluffs and slabs. Sluff s occur when the weak layer of a snowpack is on the top. A sluff is a small slide of dry, powdery snow that moves as a formless mass. Sluffs are much less dangerous than slab s. A slab occurs when the weak layer lies lower down in a snowpack. This layer is covered with other layers of compressed snow. When the is triggered, the weak layer breaks off, pulling all the layers on top of it down the slope. These layers tumble and fall in a giant block, or slab. Once a slab starts, the slab shatters into many separate blocks. These snow blocks break up into ever-smaller pieces. Some of the pieces rise into the air as a moving cloud of icy particles. The cloud races downhill at very high speeds. The thickness and speed of slab s make them a threat to skiers, snowboarders, mountaineers, and hikers. In the mountains of the western United States, there are about 100,000 s each year. Avalanches kill more than 150 people worldwide each year. Most are snowmobilers, skiers, and snowboarders. Avalanche Control Storminess, temperature, wind, the steepness of the slope, terrain, vegetation, and general snowpack conditions are all factors that influence whether an happens and what type occurs. Snow s are most likely to occur after a fresh snowfall adds a new layer to a snowpack. If new snow piles up during a storm, the snowpack may become overloaded, setting off a slide. 2 of 6
Earthquakes can set off s, but much smaller vibrations can trigger them as well. A single skier can cause enough vibrations to set off a slide. In fact, 90 percent of incidents involving people are triggered by the victim or someone in the victim s party. Currently, scientists are not able to predict with certainty when and where s will happen. However, they can estimate hazard levels by checking on the snowpack, temperature, and wind conditions. Many ski areas employ control teams to lessen the danger by starting slides before skiers head for the slopes. At some ski areas, patrols use explosives to set off s. Or they may blast hazardous slopes with a cannon to shake loose any large, new accumulations of snow. In the high mountains of Canada and Switzerland, special military troops are in charge of control. Many Swiss mountain villages protect homes from snowslides by building large, sturdy structures to anchor snowpacks. Dangers of an Avalanche An is one of the most powerful events in nature. A fractured mass of snow may flow down a slope or become airborne. As a large 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. Avalanches strike suddenly and can be deadly. In 1970, a massive of rocks and ice destroyed the town of Yungay, Peru, killing 18,000 people. If you are caught in an, the first thing to do is try to get off the slab. Skiers and snowboarders can head straight downhill to gather speed, and then veer sideways out of the slide path. Snowmobilers can punch the throttle to 3 of 6
power out of harm s way. If this is not possible, reach for a tree. As a last resort, try to swim up out of the snow. The human body is three times denser than debris and will sink quickly. This makes finding and rescuing victims much more difficult. If buried in an, try to clear some space in front of you to breathe, then punch a hand skyward. Once the stops, it settles like concrete. Bodily movement is nearly impossible. Most victims are rescued, but those who aren t die of suffocation as the snow hardens and buries them. Avalanche beacons are the most common tools to help rescuers find victims. Avalanche beacons are beepers that emit consistent noise when activated. Beacons can help rescuers locate a buried victim more than 80 meters (262 feet) away. Vocabulary Term Part of Speech accumulation airborne beacon control a buildup of something. adjectivetransported by air currents. Definition large mass of snow and other material suddenly and quickly tumbling down a mountain. device attached to people on mountains that emits a noise signal able to be detected by other beacons, even when buried in layers of snow. Also called beeper. intentional triggering of s to produce frequent, smaller snowslides, when people can be cleared away to a safe distance. cannon very large gun used for firing heavy projectiles. compress verb to press together in a smaller space. 4 of 6
debris dense remains of something broken or destroyed; waste, or garbage. having parts or molecules that are packed closely adjective together. dozen a group of 12. earthquake the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity. emit verb to give off or send out. estimate verb to guess based on knowledge of the situation or object. explosive material that can quickly and violently expand due to a chemical change. hurtle verb to move very quickly, often with violence. ice water in its solid form. incident event or happening. landslide the fall of rocks, soil, and other materials from a mountain, hill, or slope. military armed forces. mountain landmass that forms as tectonic plates interact with each other. particle small piece of material. rock natural substance composed of solid mineral matter. slab sluff Term Part of Speech dangerous slide of snow that falls as a large, flat whole (slab). small slide of loose snow. Definition snow precipitation made of ice crystals. snowpack layers of snow that naturally build up during snowfalls. snowslide large mass of snow tumbling down a mountain. soil top layer of the Earth's surface where plants can grow. 5 of 6
Term Part of Speech Definition temperature degree of hotness or coldness measured by a thermometer with a numerical scale. terrain topographic features of an area. thaw verb to melt, or turn from ice to liquid. throttle lever or button controlling the amount of fluid, such as gas, that goes into an engine. troop a soldier. unstable adjectiveunsteady or likely to fall apart. veer verb to lean or change direction. vegetation all the plant life of a specific place. vibration rapid movement back and forth, usually of small particles. Articles & Profiles National Geographic Adventure: Survival Avalanche National Geographic Kids: Ski Patrol On Alert for Avalanche Safety Interactives National Geographic Environment: Unleash an Avalanche Websites National Geographic Environment: Avalanches 1996 2017 National Geographic Society. All rights reserved. 6 of 6