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How does an avalanche start?

To make an avalanche, all you need is a mountain, snow, and a little bit of gravity. Every year, there are about a million avalanches that occur throughout the world. Most of them are minor and occur in remote locations, but how do they begin?

What is an avalanche?

Aerial view of snowy cliff showing huge snow chunks breaking away from mountainside
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An avalanche is a rapid flow of snow down a mountainside. Avalanches can vary greatly in size from around 100 cubic meters (minor avalanches are called sluffs), to well over 100,000 cubic meters of snow. Some of the largest North American avalanches release enough snow to fill over 20 football fields with 10 feet of snow!

The most avalanches occur in the Alps of Italy, Switzerland, Austria, and France. Unfortunately, that’s also where the loss of life is highest because it has some of the best skiing in the world. The U.S. experiences the fifth-greatest number of avalanches in the world, occurring mostly in the mountains of Colorado, Utah, and Alaska.

How to make an avalanche

Aerial view of distant avalanche rolling down the side of a snowy mountain
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There are three things that make an avalanche: slope, snowpack and weather conditions. Avalanches mostly occur on slopes between 25 and 60 degrees. Slopes less than 25 degrees are too flat for the snow to flow, and slopes over 60 degrees are too steep for the snow to lie on the surface and stay there. There has to be enough of a slope for the snow to slide downhill, but not so much that the snow can’t pile up. If you’re one of those strange people who doesn’t carry a protractor, here’s the general rule: If the mountain is flat enough to hold snow and steep enough to ski on, it has the potential for an avalanche.

Next, you need snow. Avalanches prefer fluffy, unpacked snow. Tightly bonded snow and ice stick to the mountainside and typically don’t become avalanches. The deeper the snow, the more unstable it is, which leads to a higher risk of becoming an avalanche.

Most avalanches occur just after heavy snowstorms, when the snow hasn’t had enough time to freeze to the lower layers. Once the snow has gotten deep enough, it becomes unstable and cascades down the mountain. Temperature also plays a role in starting an avalanche. Warmer weather can cause ice to melt, destabilizing the snow on the mountainside.

The push

Aerial view of skiers flying down snowy mountain, kicking up snow behind them
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Now that the snow has been sufficiently destabilized, all you need is a push. Many small-to-medium avalanches are triggered by outdoor recreationists as they ski or snowmobile across the fresh powder. They’ll knock some snow loose, that snow will knock more loose, and so on and so on, until you get an avalanche. Avalanches caused by people are often the deadliest because they’re usually in more populated areas.

Major avalanches are caused when a large sheet of snow or ice is dislodged from the mountainside and comes crashing down the mountainside. These avalanches can happen in an instant and without any warning. With such incredible momentum, major avalanches can reach speeds of more than 100 miles-per-hour.

Avalanche zones

Avalanche on mountain in Caucasus, Kabardino-Balkaria, Bezengi region, Russia
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There are three parts to every avalanche. The “starting zone” is typically at the top of the mountain where slopes are the steepest. This is the most volatile part of the mountain where the snow layers are most likely to fracture.

After the snow fractures it moves into the “track.” The track is where the avalanche “runs.” These regions are typically clear from where previous avalanches have removed all the trees and barriers in their path. If you see a large clearing on a mountain that’s otherwise covered in foliage, there’s a good chance that avalanches have occurred there in the past.

Finally, there’s the “runout zone.” After thousands and thousands of cubic meters of snow come rushing down the mountain as fast as a car on the freeway, this is where it all comes to rest. The runout zone is where the mountain levels out and the avalanche loses its momentum. If you find deep piles of snow at the bottom of a mountain, there’s a good chance that there have been avalanches there before.

The largest avalanche

Aerial view of snowy, desolate mountain with yellow avalanche warning sign
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The largest recorded avalanche occurred on May 31, 1970 on Mt. Huascaran in Peru. An earthquake triggered a massive glacial sheet measuring about 3,000 feet wide and over a mile long to come loose from the mountain. Approximately 80 million cubic yards of ice, water, snow, mud, and rock sped down the mountainside at more than 100 miles-per-hour. The momentum was so great that the avalanche ran for a total of 11 miles, and leaving some areas buried under 300 feet of debris.

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