pyroclastic flow

A fast-moving, ground-hugging avalanche of superheated gas, volcanic ash, pumice, and rock fragments that races down the slopes of an erupting volcano at speeds up to 450 miles per hour and temperatures up to 1,300°F. A pyroclastic flow is unsurvivable. It incinerates and buries everything in its path. The destruction of Pompeii in 79 AD, of Saint-Pierre in 1902, and the devastation around Mount St. Helens in 1980 were all caused by pyroclastic flows.