Pyroclastic flow

An avalanche of hot lava fragments, which can cascade down at speeds of 50 mph or faster. The rock fragments range in size from ash to boulders. The hot temperatures of rocks and gas inside pyroclastic flows, generally between 400 and 1,300 degrees Fahrenheit, can cause plants and other combustible materials to catch fire. Most pyroclastic flows consist of two parts: a flow of coarse fragments that moves along the ground and a turbulent cloud of ash on top.