Lexicon

Pyroclastic flow

A hot (typically >800 degrees C), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent. Pyroclastic flows that form from an explosive eruption column contain a high proportion of fine ash and pumice and are sometimes called ash flows or ignimbrites. Pyroclastic flows that form by failure of the front of a cooling lava dome or flow are called block and ash flows.