A video has emerged of an avalanche engulfing the South Base Camp on Mount Everest. The avalanche is one of a number triggered by the magnitude 7.8 earthquake that struck Nepal on Saturday.
Credit: Jost Kobusch
The video has a much grimmer association than the similarly fortuitous capturing of the Calbuco explosion. Although the people seen above survived relatively unscathed, 19 others at the camp site are known to have died in the avalanche, and the toll is likely to rise. Evacuations of the injured are underway. This makes it the deadliest day in the history of climbing on Everest, topping the 16 killed this time last year. However, the number is overshadowed by the 3,300 so far known to have died across Nepal and northern India, a number also almost certain to rise.
A group of climbers above the avalanche at camp I have had their descent cut off by the snow movement, and are being dropped equipment to assist them in finding a new way down.
Avalanches occur because the downward forces on a portion of snow outweigh the forces, including friction, holding it up. As the snow starts to fall, it applies pressure to everything below it, generating a heavier and heavier falling body. Depending on the shape of the mountain side, the snow can be channeled down a narrow gorge, or spread out over a wide slope. The processes are similar to rock and mudslides, but the seasonal growth of snow and the low associated strength make snow a medium particularly prone to slides.
In most cases, the trigger is simply the growing weight of snow, sometimes combined with melting from sunlight reducing the snowpack's strength. However, the earthquake, particularly one as large as Saturday's, will shake snow loose and make such events almost inevitable. Even loud noises can serve as triggers; some of the highest death tolls recorded were among soldiers in the First World War when artillery sounds created a sort of mutually assured destruction, bringing avalanches down on tens of thousands of soldiers from both sides.