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What happens when a volcano erupts?

When you think of a volcanic eruption, you probably imagine red-hot lava flowing down the side of a volcano. Lava is a serious hazard, but there are others--some of them far more dangerous.

Lava Flows
Lava flows are a threat, but they are usually slow enough that people can get out of the way. Economic loss from lava flows--including burned buildings and ruined crops--is more common. Because lava flows normally don't cover very large areas, the damage is usually limited.

 A lava flow on Hawaii's Pu'u O'o volcano.

Falling Ash
In an explosive eruption, pent-up gases escape violently. Magma breaks into pieces and bursts from the volcano in a column of ash and fiery fragments. The cooled fragments that fall back to Earth are called tephra. In a large eruption, tephra can cover vast areas with a thick layer of ash, presenting a much greater hazard than lava flows.

Glowing Avalanches
Pyroclastic flows are mixtures of very hot gas and tephra that cascade down a volcano's sides at high speeds. A pyroclastic flow covered the city of Herculaneum in A.D. 79, killing many residents. Because pyroclastic flows can spread destruction over large areas and move at very high speeds, they are extremely hazardous. Most people are not aware that this danger exists.

Mud and Debris Flows
Debris flows, fragments of mud and other debris that flow down the sides of a volcano, are another serious and little-known hazard. Debris flows often form when part of the volcano collapses, breaking up and flowing downhill. If the collapse is a major one, the large flow that results can travel great distances, often burying everything in its path. These are particularly dangerous on volcanoes that have glaciers on top, as the eruption instantly melts the ice, causing a massive mud slide, much like what happened during the Mt. St. Helens eruption in 1980.

Other Dangers
Lava, ash, and debris flows are the most common and serious volcanic hazards, but others do exist. Severe eruptions can disrupt the climate for long periods or cause atmospheric shock waves. Eruptions can also cause tsunamis, volcanic earthquakes, or the release of suffocating gases. These hazards pose serious threats to both life and property.

Katmai video clip (Quicktime, 1:02 minutes, 6,272K)
View footage from a 1916 expedition to Katmai, an Alaskan volcano.

"Volcanoes" is inspired by programs from Earth Revealed.