Active volcanoes pose many hazards to life and property. Some hazards, like huge lava flows and explosive blasts associated with volcanic eruptions, are spectacular headline grabbers and recognized by everyone. Others, like glowing avalanches and ash falls, are much less flamboyant and less known by the general public, but they can be just as deadly. A few hazards, such as rockslides and mudflows, can occur even in the absence of an eruption. Short descriptions of many volcanic hazards are given here. If you wish more information on volcanic hazards, click here.
Lava flows are sheets and tongues of liquid rock expelled from the crown or flank of an effusively erupting volcano and are probably the best known volcanic hazard. They are usually depicted in books and movies as roaring down the erupting volcano's steep slopes to inundate houses, cars, trees, and expendable movie extras. Although some lava flows can travel at 50-60 mph, others move at human walking speeds or slower. The speed of a flow depends on the viscosity of the lava and the incline of the volcanoe's slope. The destructive power of lava flows lies in the high temperature of the rock, which can set structures aflame, and in the size and mass of the flow, which can engulf or crush even large buildings. Some lava flows are small enough for a person to step across and cause little damage; on the other hand, lava flows like the Columbia River Basalts are large enough to cover entire states and destroy everything in their path. Photo: Courtesy of NGDC/NOAA.
The explosive blast is the "feature presentation" of a (surprise!) explosively erupting volcano. It is an outburst of fragments of rock and lava driven by expanding gases that were dissolved in the erupting lava at great depths. These blasts may throw great blocks of rock many miles. However, the superheated blast cloud itself, which expands out from the volcano at hundreds of miles per hour, enveloping and searing anything in its path, is more destructive. The destructive power of the blasts lies in the high velocity winds (exceeding wind speeds in hurricanes) within the cloud and the very high temperatures of the gas. The blasts are capable of destroying all life within many miles of the volcano in a matter of minutes. The main blast at Mount St. Helens destroyed more than 230 square miles of forest in a few seconds. The destroyed area is pictured to the upper right of the shattered cone of the mountain in this shuttle image. Photo: NASA shuttle photograph #STS 47-73-056 (EarthRISE archive).
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