How deadly, fast-moving flows of volcanic rock and gas cheat friction

Dumping literal tons of hot volcanic material down a lab flume may finally have revealed how searing mixtures of hot gas and rock travel so far from volcanic eruptions.

These pyroclastic flows can travel tens to hundreds of kilometers over rough terrain and even uphill (SN: 7/7/18, p. 32). Despite being made of gritty 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 University Savoie Mont Blanc in France not involved in the new study. The driving force behind these flows “has always been a mystery.”
Now, lab experiments and computer simulations suggest that inside pyroclastic flows, a dense layer of volcanic material glides across the ground atop a low-friction layer made largely of air. These findings, reported online April 8 in Nature Geoscience, may help create more accurate forecasts of the speed and spread of these flows.

Gert Lube, a volcanologist at Massey University in Palmerston North, New Zealand, and colleagues created mini pyroclastic flows in the lab by piling volcanic material into a giant hopper and heating the rock up to 130° Celsius. After using an elevator-like device to hoist the hopper into the air, the researchers dumped the hot volcanic material down a 12-meter chute and tracked the flow with high-speed video.
Lube’s team discovered that a dilute layer emerged at the bottom of the flow, topped with a layer of much more densely packed volcanic matter. This sparse underlying layer develops because, near the bottom of the flow, material right next to the ground moves much more slowly than the material slightly above.

This difference in flow rate creates a region of very low air pressure close to the ground, so gas from higher-pressure areas quickly migrates downward, creating an air-rich base layer. Volcanic particles in the top part of the flow glide along this airy cushion like disks on an air hockey table to travel long distances.

Computer simulations of pyroclastic flows with varying speeds and heights indicated that this friction-defying phenomenon occurs in life-size pyroclastic flows, too. The researchers suspect that air lubrication may similarly spur on 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, Berkeley not involved in the work. This new understanding of how pyroclastic currents cheat friction may lead to better hazard models that inform where people build infrastructure and plan evacuation routes, he says.