Pyroclastic Flows May Spread Swift Death On A Cushion Of Air

Mayon Volcano in the Philippines. (Photo: C.G. Newhall, USGS)

Scientists think they’ve figured out how pyroclastic flows, fast-moving bringers of death during volcanic events, can travel such incredible distances and speeds despite the friction between the volcanic material and the ground.

Pyroclastic flows are searing-hot mixtures of volcanic material and gas that travel explosively down the slopes of volcanoes, sometimes traversing tens of miles at hundreds of miles per hour. They’re responsible for some of the deadliest volcanic events, like the 1883 Krakatoa eruption and the 79 AD Vesuvius eruption that destroyed Pompeii. Though the spewed rocks should slow due to friction, new research from scientists in New Zealand, Australia, and the United States seems to show that the flows travel on cushion of lubricating air.

Volcanic material has a static friction of around 35-45 degrees, meaning it wouldn’t slide until it were placed on a 35- to a 45-degree slope, according to the paper published today in Nature Geoscience. And yet, pyroclastic flows can travel for tens to hundreds of miles down slopes with angles of just 8 to 9 degrees. How the material can move so far, for so long, has remained a mystery—after all, it’s hard to mathematically model a speeding death column.

The researchers explored the question using an experiment called PELE, the Pyroclastic flow Eruption Large-scale Experiment. The experiment is essentially a large, heated container with a literal ton of aerated, natural volcanic particles which are then dropped onto a 12.19m-long, 0.46m-wide, high-friction chute that leads down to a concrete pad. It’s a sandpaper slide at the theme park of death.

During the experiments, which the researchers recorded with a high-speed camera, the mixture separates into a denser underflow below and a 6-to-15-foot ash cloud above. The cloud would run to 115 feet, while the underflow would run up to 87 feet. The researchers performed a number of calculations and numerical modelling with the data and reviewed the footage.

The saw that areas with fewer particles developed near the ground. They hypothesized that in these flows, faster-moving materials higher up generate higher-pressure regions closer to, but not quite on, the ground. This would cause gas particles to travel toward the lower-pressure region closer to the ground and inflate an air pocket on which the flow can slide nearly frictionlessly.

There’s more work to do, as Alain Burgisser, scientist at the Université Savoie Mont Blanc in France wrote in a Nature Geoscience commentary: “The exact conditions under which this layer appears remain to be determined with precision.” But the effect is an exciting one to discover experimentally, and the researchers’ numerical modelling demonstrates that air lubrication could explain these pyroclastic flows in nature.

But these air cushions might not just describe pyroclastic flows. Perhaps, the authors write, avalanches and landslides are also propelled by similar air cushions. That’s physics for you. It can help you understand the mysteries of the Universe, but it won’t stop your near-instant death if you’re standing in the way of a pyroclastic flow.

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