Large volcanic eruptions can trigger the formation of superheated gas-and-ash clouds. These pyroclastic density currents, or pyroclastic flows, are the most lethal volcanic threat. They're also the most common.

For the first time, scientists have discovered the driving force behind the deadly currents of volcanic particles and gas.

Measuring pyroclastic flows in real time is impossible, so scientists at Massey University in New Zealand used the Pyroclastic flow Eruption Large-scale Experiment eruption simulator to model the phenomena.

The PELE simulator precisely replicated the conditions of an erupting volcano, revealing the source of its super-mobility.

"With several tons of pumice and gas in motion, our large-scale eruption simulations uncovered the flow enigma that has been baffling researchers for decades," Massey volcanologist Gert Lube said in a news release. "We measured a low-friction air cushion that is self-generated in these flows and perpetuates their motion. We were able to mathematically describe the resulting flow behavior."

Scientists identified a unique process negates the flow's granular friction. As a layer of air lubrication develops, air is forced downwards and volcanic particles are displaced upward.

"This explains how the currents are able to propagate over slopes, bypass tortuous flow paths, and ignore rough substrates and flat and upsloping terrain, without slowing down," Lube said.

The findings, published this week in the journal Nature Geoscience, will help researchers improve hazard models and better predict the velocity, runout and spread of pyroclastic flows, allowing safety officials to get people out of harm's way.

"Discovery of this air-lubrication mechanism opens a new path towards reliable predictions of pyroclastic flow motion and the extreme runout potential of these lethal currents, thereby reducing future casualties," Lube said. "It will be used by hazard scientists, as well as decision makers, and is envisaged to lead to major revisions of volcanic hazard forecasts."

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