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Rocky debris and superheated steam that erupts out of volcanoes can travel over land at around 200 kilometres per hour. A study shows that these pyroclastic flows are so speedy because the rubble surfs on a pocket of air that builds up underneath the flowing rocks.

“We know that the material inside these things, ash and pumice, does not like to move. It’s extremely frictional. But when it comes out of a volcano, it flows like water,” says Gert Lube at Massey University in New Zealand.

To simulate these flows, Lube and his team dropped volcanic rock and ash – 2 nanometres to 64 millimetres in size – into a chute with heat-resistant glass sides, and measured their movement using high-speed cameras and pressure sensors (see video below). The rocks at the top of the chute passed through a fiery portal heated to between 15°C and 130°C, so the debris simulated a volcano’s column of superheated ash.

The team found a feedback loop that determines the speed of the flow. When a pyroclastic flow moves slowly, the air between the ash and rock has the highest pressure near the ground, which shoves rock particles together and pushes gas upwards, slowing down the rubble. But if it is moving quickly, the region of highest pressure within the flow lifts off the ground.

As a result, the low pressure at the bottom of the flow draws air downward, and the volcanic material packs together and traps the gas beneath it. This makes an air pocket that the rocks coast on as they cover uneven ground or climb inclines, which makes them more dangerous. “We see this in rare footage of these flows. We can see after an eruption how it wipes through a town and climbs a series of hills,” says Lube.

This work helps us understand how pyroclastic flows transport and deposit particles over such long distances, says Benjamin Andrews at the Smithsonian Institution Global Volcanism Program in Washington DC. “The air that the current is riding on is not necessarily coming from the front of the current – it isn’t just air that the flow has ‘rolled over’ – but also air that is getting pulled down through the current,” he says.