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Mount St. Helens

On May 18, 1980, in Washington state, Mount St. Helens volcano exploded leaving 57 people dead or missing. This was a Plinian eruption. It validated Pliny the Younger’s eyewitness description, often regarded as exaggeration, of the Vesuvius explosion that buried Pompeii and Heraculaneum.

Mount St. Helens is a stratovolcano in the Cascade Volcanic Arc that is part of the Pacific Ring of Fire of over 160 active volcanoes. Before the 18th, seismographs recorded earthquake swarms from deep magma chambers under stress from the slip of the fractured Juan de Fuca Plate beneath the North American Plate.

Instead of erupting through its crater, magma beneath Mount St. Helens found its way through less resistant material in its north wall, creating a very large bulge. Rock slid away beneath the bulge and, with the weight removed, the pressures of gas and magma blew through the wall above the slide releasing a massive, destructive blast. For over 9 hours, it released more than 1.5 metric tons of sulphur dioxide into the atmosphere as well as ash that, by next morning, was falling on cities over a thousand kilometers away. Its pyroclastic flow of superheated gas and rock covered 600 sq km (230 sq mi). Mudflows of glacier, snowmelt, and ash coursed down natural channels of valleys and rivers taking out trees, docks, bridges, and hatcheries in their paths.

The following year, 44,515 hectares (110,000 acres) around Mount St. Helens were set aside as the National Volcanic Monument that allows the site to recover naturally and to inform researchers.

Historically, Mount St. Helens has been one of the most active volcanoes in the Cascades. It continued blowing off steam and ash for another 28 years.

Volcanoes are the biggest recyclers on the planet!

B Bondar / Real World Content Advantage