How deadly pyroclastic flows cheat friction.



The discharge of literal tons of hot volcanic material through a laboratory conduit may have finally revealed how hot gas and rock gas blends travel so far from volcanic eruptions.

These pyroclastic flows can travel from tens to hundreds of kilometers over rugged terrain and even uphill (SN: 7/7/18, p. 32). Despite being made of sandy volcanic rock, "they seem to have as much friction with the ground as if they were made of water," says Alain Burgisser, a geologist at the Savoie Mont Blanc University in France, who does not participate in the new study. The driving force behind these flows "has always been a mystery."

Now, laboratory experiments and computer simulations suggest that within pyroclastic flows, a dense layer of volcanic material slides along the ground on a layer of low friction composed largely of air. These findings, reported online on April 8 in Geosciences of nature, can help create more accurate forecasts of the speed and propagation of these flows.

Gert Lube, a volcanologist at Mbadey University in Palmerston North, New Zealand, and his colleagues created mini pyroclastic flows in the laboratory, piling volcanic material into a giant hopper and heating the rock to 130 ° C. After using a similar device To an elevator to raise the hopper in the air, the researchers unloaded the hot volcanic material into a 12-meter socket and tracked the flow with high-speed video.

AVALANCHA VOLCANICA To study how rock and gas streams travel so far from volcanic eruption sites, researchers threw hot volcanic material through a conduit in the laboratory and tracked its flow with high-speed cameras.

G. lubricant et al/Geosciences of nature 2019

Lube's team discovered that a diluted layer emerged at the bottom of the flow, topped with a layer of volcanic material much more densely packed. This scattered underlying layer develops because, near the bottom of the flow, the material right next to the ground moves much more slowly than the material slightly above.

This difference in flow creates a region of very low air pressure close to the ground, so that the gas from the higher pressure areas migrates rapidly downwards, creating a base layer rich in air. The volcanic particles in the upper part of the flow slide along this cushion as ventilated as disks on an air hockey table to travel long distances.

Computer simulations of pyroclastic flows with varying velocities and heights indicated that this phenomenon that defies friction also occurs in life-size pyroclastic flows. Researchers suspect that air lubrication can also stimulate fast-moving snow avalanches or landslides.

"Pyroclastic flows are the most devastating and deadly aspect of volcanic eruptions," says Michael Manga, a geologist at the University of California at Berkeley who does not participate in the work. This new understanding of how pyroclastic currents deceive friction can lead to better risk models that inform where people build infrastructure and plan evacuation routes, he says.


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