Volcanic Ash Flows Target Name:   Earth's Volcanos Produced by:   International Association of Volcanology and Chemistry of Earth's Interior (IAVCEI) Copyright:  © 1995, International Association of Volcanology and Chemistry of Earth's Interior (IAVCEI) Date Released:  1995

• Terrestrial Volcanoes

This video clip was taken from "Understanding Volcanic Hazards", © 1995, International Association of Volcanology and Chemistry of Earth's Interior (IAVCEI). The following was extracted from the video:

In addition to clouds of ash that rise above a volcano, some explosive eruptions produce avalanches of hot ash, rock and gas that move at high speeds down the slopes of a volcano. Ash flows are the most destructive type of volcanic activity. They are extremely hot and can move across the ground as fast as several hundred kilometers per hour. Ash flows knock down and burn everything in their path. Few living things can survive an ash flow. The eruption of Mount St. Helens in 1980 generated an extremely large ash flow that completely devastated an area of 600 square kilometers.

Close to Mount St. Helens forests were striped completely from hills. Trees two meters in diameter were mowed down like blades of grass twenty five kilometers from the volcano. Logging equipment was tossed about like toys. Everything in the path of the ash flow was destroyed. Cars not mangled by the ash flow were buried with rock debris. Ash flows are difficult to observe because they are extremely dangerous and move quickly. An ash flow cannot be outrun. As an ash flow moves down the side of a volcano, it usually pours into canyons and river valleys, but at the bottom of the volcano it can spread out across the land into populated areas.

One of the most destructive ash flows this century swept through the city of St. Pierre only seven kilometers from Mount Pele volcano. The city was completely destroyed. Twenty nine thousand people died. The ash flows knocked down buildings and homes. The hot ash started fires that burned through the city for hours. Pictures of St. Pierre before and after the eruption show its complete destruction. Only two men survived and even they were badly burned. The deaths caused by the eruption of Mount Pele shocked the world and alerted people to the dangers of ash flows. But this type of tragedy is repeated at some other volcano every few years.