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Volcanic hazards

The latest info on Mount St. Helens from the USGS

Volcanoes, like most earthquakes, are related to tectonic plate motion. Since the eruption of Mount Lassen in 1919 and Mount St. Helens in 1980, most Northwesterners have accepted intermittent volcanic events as part of life here.

Volcanoes bring about a diversity of hazards to human culture, including clouds of hot gasses carrying rock and sand, blast effects, ash falls, and mud flows. On the positive side, it can be said that, unlike earthquakes, volcanoes generally give plenty of warning that they are awakening, although the actual moment of eruption may come as an unpleasant surprise.

Following an eruption, ash may take weeks to settle from the air. This fine powder is quite harmful to lungs and incredibly abrasive to moving parts of any machinery or engine. The weight of wet ash can collapse a building.

A most sensational aspect of a volcanic eruption is the nuée ardente or pyroclastic flow. In this event, superhot, burning gas is suddenly pumped into the air to fall back to earth as a heavy cloud and move across the landscape at hundreds of miles per hour, immolating everything in its path. Even though pyroclastic flows were known to geologists, they were only rarely witnessed and not filmed until the 1980s, when they were captured on videotape in Japan. In 1902 over 30,000 people in the village of St. Pierre on Martinique were incinerated by a pyroclastic flow, and more recently the island of Monserrat experienced the same phenomenon, fortunately without loss of life. In Oregon, deposits from pyroclastic flows are a frequent part of the geologic record east of the Cascades.

Before the 1980 Mount St. Helens episode, the incidence and impact of lateral eruptions was poorly understood. During this event, the northeastward blast knocked down trees and increased the damage significantly. Since then, it has been found that lateral blasts are not uncommon in Cascade volcanoes. An urban center in the path of such a force would be totally devastated. In Oregon, ther are very few populated areas that would be affected by a lateral blast.

More than any other single event in this century, the May 18, 1980, eruption of Mount St. Helens brought the reality of geologic hazards to the American public’s awareness.




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