Crouched figures, a child with its mother, a dog writhing on its back – the harrowing plaster casts of the dead of Pompeii reveal that when the end came in AD79, it was as swift as it was final.

Caught in the eruption of Mount Vesuvius, many died when buildings collapsed under heavy ash and pumice. But for those still alive, another catastrophe was about to hit as deadly currents of superheated gases, ash and rock fragments raced through the streets.

In nearby Herculaneum, the heat from such flows is thought to have been so strong that some have argued it vaporised the bodily fluids of the town’s inhabitants and exploded their skulls.

A resin cast of a crouching man covering his mouth with a hooded cloak he was wearing before he was overcome by the eruption of Mount Vesuvius. Photograph: Stephen Morrison/EPA

Now scientists say they have discovered how such pyroclastic flows travel so fast: the volcanic material travels on a cushion of air, rather like a hovercraft.

“Volcanologists have known for almost a century that there must be a process operating in pyroclastic flow that outsmarts the high friction of the volcanic material and lets it flow like water for very long distances,” said Gert Lube, first author of the study from Massey University in New Zealand. But, he added, until now it was unclear what that process was.

Writing in the journal Nature Geoscience, researchers from the US and New Zealand report how they made their discovery by creating pyroclastic flows in a large-scale outdoor experimental setup and recording what happened on video.

Up to 1,300kg of heated volcanic particles – recovered from the eruption of the Taupo volcano in New Zealand in AD232 – were dropped into a 12-metre-long chute containing various sensors. This mimicked the collapse of a volcanic eruption column.

The team found the flows moved faster and further than might be expected.

The researchers propose that this happens when the velocity of the base of the flow is very different to that higher up. This results in a lowering of the air pressure at the base of the flow, while the air pressure increases above it. The upshot is that gas is pushed downwards, forming a thin film of air at the base of the flow.

Plaster casts of victims of the eruption of Mount Vesuvius, which destroyed the Roman city of Pompeii in AD79. Photograph: Mick Tsikas/Reuters

The team say this cushion of air effectively carries, or “lubricates”, the volcanic material down the slope, with the lack of friction meaning the flow moves much faster and further.

“Once it is established, and this happens in just a few milliseconds, this air film lubricates the pyroclastic flow somewhat in the same way as gas streaming through little holes in an air-hockey table lubricates the hockey puck,” said Lube.

Computer simulations run by the researchers further show that this process can occur in natural volcanic events, while the team say similar effects might be at play in other situations, such as snow avalanches and fast-flowing landslides.

Dr Rebecca Williams, an expert in pyroclastic flows from the University of Hull, who was not involved in the study, said that while the research gives only a snapshot of what might happen in a natural event, it helps to explain the behaviour of such flows, which have been known to reach speeds of 400mph.

“Their high mobility isn’t just about speed – they can travel at these high speeds, over rough terrain large distances from the volcano. They can even scale hills several hundred metres tall, running upslope,” she said. “This research suggests that this high mobility is through air lubrication at the base of the flows.”

Mike Burton, professor of volcanology at the University of Manchester, said the downward movement of air in the pyroclastic flow produces a lifting effect similar to that seen in a hovercraft, adding that the findings would prove useful in predicting the course of pyroclastic flows, aiding volcanic risk management.

Williams agreed: “The more accurately we can define [the behaviour of pyroclastic flows], the better chance we have of avoiding tragedies like Pompeii happening again.”