Volcanic avalanches may be more destructive than previously thought

High-pressure pulses in these flows act like a jackhammer on obstacles

Pyroclastic flows are volcanic avalanches of gas, ash and rock. They possess rhythmic, high-pressure pulses as a result of turbulence, a new study suggests.

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Avalanches of ash, gas and rock can cascade downhill during volcanic eruptions. Known as pyroclastic flows, they may be even more dangerous than scientists had suspected.

Those pulses are the result of turbulence, lab and field measurements now show. Within these slides, pulses of high pressure form. Those pressures can be far stronger, and more destructive, than hazard assessments typically assume. Researchers shared their new finding this past December 15 in Nature Communications.

“It’s not a small difference,” says Gert Lube. He’s a volcanologist at Massey University. That’s in Palmerston North, New Zealand. A conventional hazard assessment might suggest a certain flow will only burst windows. But, he says, the pressures might be so strong “they knock down the walls of the building.”

Pyroclastic flows are the deadliest volcanic hazard. That’s due in part to the pressures they generate. Because these flows are so violent, studying them directly can be hard. Researchers often have to estimate average obstacle-smashing pressures in the flows. They do this with computer models. Those are based on studies of geologic deposits left by past flows.

Lube and his colleagues wanted to directly study the inner workings of these flows. So they reproduced smaller versions of the flows in experiments. They measured destructive factors. These included particle velocities and temperatures. They also measured flow densities. That let the team calculate pressures inside the flows. The researchers also analyzed the first measurements of pressures in natural flows. Those measurements were from 2019. That’s when pyroclastic flows burst from the Whakaari volcano in New Zealand. They engulfed a set of infrasound sensors.

To the researchers’ surprise, flow pressures rose and fell in a rhythmic pattern. This happened as volcanic particles clustered into cascading waves and trains of rolling eddies. These pulses would successively damage obstacles like blows from a jackhammer, Lube says. Sometimes the pulses smashed more than three times as hard as the average pressure estimates.

Many hazard assessments are probably underestimating the destructiveness of pyroclastic flows by a lot, Lube says. “It’s a big wake-up call.”

Researchers at Massey University in Palmerston North, New Zealand, released mixtures of hot rock, ash and gas down a channel to replicate volcanic avalanches known as pyroclastic flows. These flows have an internal rhythm that makes them unexpectedly destructive, a new study suggests.

Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

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