Through the process of convection, the hot gases of a pyroclastic flow expand and rise above the mass of denser and cooler materials on the ground. Heavier fragments roll downward along the ground, while smaller fragments float in a stream of hot gases. These solids have been heated and fragmented by an explosive eruption. The world pyroclast is derived from the Greek pyr, meaning “fire”, and klastos, meaning “broken in pieces.” A pyroclastic flow’s “broken pieces” consist of volcanic glass, crystals, and rocks such as pumice or scoria. Reaching speeds greater than 100 kilometers per hour (60 miles per hour) and temperatures between 200° and 700° Celsius (392°and 1292° Fahrenheit), pyroclastic flows are considered the most deadly of all volcano hazards. These heavier-than-air flows race down the sides of a volcano much like an avalanche.
A pyroclastic flow is a high- density mixture of hot, fragmented solids and expanding gases. Pyroclastic flows are volcanic phenomena.
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