Pyroclastic flow
A ground-hugging avalanche of hot volcanic gas, ash and rock that can race downhill at hundreds of kilometres per hour.
A pyroclastic flow is a fast-moving, scorching mixture of volcanic gas and fragments of rock and ash that sweeps down a volcano close to the ground. The name comes from Greek words meaning "fire" and "broken". The French term nuée ardente, "glowing cloud", describes how they can look.
They form when an eruption column collapses under its own weight or when a lava dome crumbles. Because the mixture is much denser than air it does not drift like smoke but rushes downhill like an avalanche, following valleys and spreading over lowlands. Depending on its size and the slope, a flow can reach about 450 miles per hour, over 700 kilometres per hour, and it can be hundreds of degrees Celsius hot. More dilute, turbulent versions, called pyroclastic surges, can cross water and ride over ridges.
They are among the deadliest of volcanic hazards, because there is no outrunning them and their heat is lethal. In 1902 a nuée ardente from Mont Pelée destroyed the town of Saint-Pierre on Martinique, killing nearly 30,000 people. In AD 79, surges from Vesuvius overwhelmed Herculaneum, killing people sheltering by the shore and carbonising a library of papyrus scrolls.
A common misconception is that lava is the main danger at a volcano. Lava usually moves slowly enough to walk away from; pyroclastic flows do not.