Volcanic avalanches of rock and gas may be more destructive than previously thought

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

A pyroclastic flow, or an avalanche of gas, ash and rock, flowing down a volcano

Pyroclastic flows — volcanic avalanches of gas, ash and rock — possess rhythmic, high-pressure pulses, a new study suggests.

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Avalanches of ash, gas and rock that cascade downhill during volcanic eruptions may be even more dangerous than scientists had realized.

Pulses of high pressure form within these slides, known as pyroclastic flows, as a result of turbulence, laboratory and field measurements show. Those pressures can be far stronger, and more destructive, than hazard assessments typically assume, researchers report December 15 in Nature Communications.

“It’s not a small difference,” says Gert Lube, a volcanologist at Massey University in Palmerston North, New Zealand. Conventional hazard assessments might suggest a certain flow will only burst windows, he says, “when actually, the pressures are so strong, they knock down the walls of the building.”

Pyroclastic flows are the deadliest volcanic hazard (SN: 6/5/18), in part because of the pressures they generate. Due to their violent nature, researchers often have to estimate average obstacle-smashing pressures in the flows using computer simulations based on measurements of geologic deposits left by past flows.

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

To directly study the inner workings of these forces of nature, Lube and colleagues reproduced smaller versions of the flows in experiments, measuring destructive factors like particle velocities and flow densities and temperatures. That let the team calculate the pressures inside the flows. The researchers also analyzed the first measurements of pressures in natural flows, collected in 2019, when pyroclastic flows burst from the Whakaari volcano and engulfed a set of infrasound sensors (SN: 6/18/21).

To the researchers’ surprise, pressures in the flows oscillated rhythmically, as volcanic particles clustered into cascading waves and trains of rolling eddies. These pressure pulses would successively damage obstacles like blows from a jackhammer, Lube says. The pulses sometimes smashed more than three times as hard as the average pressure estimates typically suggested by conventional hazard-gauging simulations.

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

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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