Four ways to be killed by a volcano

Pyroclastic flow boiled the brains and vapourised the flesh of Herculaneum's inhabitants

Active volcanoes are dangerous places. They can wipe out whole cities and kill large numbers of people.

The ghost-like casts from the Roman city of Pompeii are a reminder of the lethal eruption of Mount Vesuvius in AD79, which killed thousands and preserved their bodies in the position of their death. But it wasn't red-hot lava or suffocating clouds of ash that killed them, it was something far more unusual. Lava flows, or the molten rock that oozes from shield volcanoes moves far too slowly to be truly deadly. The real killers are much more frightening.

1: Cooked by super-hot waves of gas

Volcanologist Brittany Brand explains how a volcanic eruption can produce lethal waves of hot gas and ash

The Roman cities of Pompeii and Herculaneum were annihilated on 24 August AD79 when Mount Vesuvius erupted explosively, sending fast-moving waves of superheated gas down the sides of the volcano at hurricane speeds. These 'pyroclastic flows' contain gas, ash and rock and can travel up to 450mph (700km/h).
The first wave hit the nearby Herculaneum with temperatures as high as 500 degrees Celsius. The searing heat was enough to boil the brains and instantly vaporise the flesh of its victims so that only blackened skeletons remained.
But how the people of Pompeii died has remained a mystery for many centuries. Volcanologists have now discovered they were killed by a later wave of pyroclastic flow.

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Pompeii's wave was significantly cooler than the one that swept through Herculaneum, so although the victims bodies remained intact, the heat 'cooked' their flesh instantly. They were preserved by the falling volcanic ash and some of these can still be seen in Pompeii today.

Pyroclastic flows are arguably the most deadly volcanic event because they can travel for miles and are impossible to outrun.
They are produced by explosive 'composite volcanoes', which are made up of alternating layers of lava, ash and rock. When a composite volcano erupts, the rock layer is smashed into tiny dust particles. These particles mix with the hot ash and gases to form a giant mushroom cloud.
As the eruption weakens, this cloud can collapse under its own weight. It then cascades down the side of the volcano as a pyroclastic flow - destroying everything in its path. But that's not the only way they can cause big problems...