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Lava Flows
Lava flows from andesitic and rhyolitic magmas rarely cover more than a few square kilometers because of their high viscosity. More viscous lava comes to the surface fairly slowly, so lava flows don't travel very far before cooling and slowing. If large quantities of lava are extruded quickly, then areas of up to a few hundred square kilometers can be covered. The most hazardous lava flows, then, are not from andesitic or rhyolitic magmas, but from more fluid basaltic eruptions.

A helicopter lands
near Mt. St. Helens.
Volcanic ash blanketed the
area after the volcano
erupted in 1980.

Tephra deposits cause damage in a variety of ways. Large fragments of tephra can cause significant damage on impact, colliding with structures or setting things on fire. Accumulations of tephra do their damage by burial: roofs collapse, for example, or crops are killed.

Very fine particles of tephra cause breathing difficulties and interfere with machinery. Thin layers of very fine tephra can accumulate thousands of kilometers from the source. Heavier layers of several centimeters or more can cover tens of thousands of square kilometers.

Pyroclastic Flows
Pyroclastic flows cause damage by burial and by incineration, and because of their speed and gas content can also cause impact damage and asphyxiation. These flows are common at stratovolcanoes, since they are associated with andesitic and rhyolitic magmas.

Pyroclastic flows can travel at speeds of up to 200 meters per second and can extend as far as 200 kilometers from their source. The heat of the flows can reach several hundred degrees Celsius. With the capacity to sweep over barriers as high as a hundred meters or more, they are a serious danger.

Debris Flows
Debris flows can be triggered by a volcanic eruption, by the normal rumblings and small emissions leading up to an eruption, by volcanic earthquakes, or simply by gravity acting on a weakened and overly steep part of the volcano. Some types of volcanic debris flows are very similar to rock avalanches. These rarely extend farther than 100 kilometers from the volcano but can spread debris over an area of 1,000 square kilometers.

Another common type of debris flow is a lahar, or volcanic mudflow. This mixture of mud (mainly volcanic ash from tephra deposits) and water flows quickly down stream valleys that drain the volcano's slopes. Large lahars can travel hundreds of kilometers down valleys. Because of their high density, these mudflows can cause great destruction.

[Back to JUDGING HAZARDS]     [Next: Forecasting]


"Volcanoes" is inspired by programs from Earth Revealed.

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