Lava might be the most iconic fixture of an erupting volcano, but its sluggish ooze pales in comparison to the blast of a pyroclastic flow—a roiling river of gas, ash, and rock that can hurtle down a mountainside at hundreds of miles per hour. These fast-moving floods, with searing temperatures up to 1,800 degrees Fahrenheit, have been known to engulf villages miles from their source, leveling buildings and lifeforms alike.
With their astonishing speed and reach, pyroclastic flows are often the most dangerous parts of volcanic explosions. They’re what consumed the city of Pompeii after Vesuvius exploded in 79 CE and, more recently, claimed the lives of at least 150 people when Guatemala’s Volcán de Fuego—literally, “Volcano of Fire”—blew its top last summer.
The very traits that make pyroclastic flows so deadly are also what make them mysterious. “With just a bunch of rocks going down a hill, you wouldn’t think of them going that fast,” says Janine Krippner, a volcanologist studying pyroclastic flows at the Smithsonian’s Global Volcanism Program who was not involved in the study. But a pyroclastic flow can transport thousands to millions of tons of volcanic material to regions dozens of miles from its source, bypassing barriers and surging up hilly terrain.
Volcanologists have long been baffled by the way these torrents of gravelly debris seem to defy friction. But in a study published today in the journal Nature Geoscience, a team of scientists has finally pulled back the curtain on these catastrophic currents: By manufacturing their own cushions of air, pyroclastic flows buoy themselves off the roughness of rocky slopes, allowing them to glide down mountainsides unencumbered.
“It’s basically like a hovercraft, where air is being blown down to support the weight of something heavy,” says study author Gert Lube, a volcanologist at Massey University in New Zealand. Of course, with pyroclastic flows, there’s no machinery doing the work. The entire process is DIY, spurring a self-sustaining cycle that ferries scorching devastation for miles on end.
“This is outstanding work that provides...critical information about how pyroclastic flows work,” says Patricia Gregg, a volcano geophysicist at the University of Illinois at Urbana-Champaign who was not involved in the study. A deeper understanding of these dynamics, she says, could help forecast the hazard zones of these devastating flows—and aid officials in safely evacuating those most at risk.
After all, pyroclastic flows are the most prolific killers from volcanic eruptions, accounting for about 60,000 deaths in the last 500 years. Unfortunately, they’re extremely difficult to study in their natural context. Not only is there danger posed by their inescapable speed—but even up close, these currents are so dense that it’s just about impossible to see how they interact with the surfaces they traverse.
So rather than trying to get intimate with a bona fide pyroclastic flow, a team led by Lube decided to create their own. Together, they converted an abandoned boiler house into something of a makeshift log flume with a sloped, clear-sided 40-foot channel that spilled its contents outside the building. Just like on Splash Mountain, cameras were installed to capture the magic of the epic plunge.
Into the chute went nearly 3,000 pounds of piping hot volcanic material. Within milliseconds, the mixture began to move. The researchers barely had time to blink before a coarse torrent of rock, shrouded in a billowing cloud of ash, cascaded out the other end of the slide. Then, as the current exited the boiler house, it hit flat ground and gradually began to slow—but not before it blanketed about 100 feet of ground with a jumble of grit.
There was no volcano, and certainly no eruption. All the same, it was there: the inexplicable speed, the telltale gush of debris—a pyroclastic flow. But this time, it came with an explanation. With high-speed video footage on their side, the researchers could finally see directly into the belly of the beast.
“These observations are super precious,” says Arianna Soldati, a volcanologist at Ludwig Maximilian University of Munich who was not involved in the study. “There aren’t many places like this in the world that have this kind of large-scale capability to [model] pyroclastic flows.”
When Lube and his colleagues sifted through the recordings, they noticed a thin, air-rich layer, less than a millimeter thick, quickly developing between the flow and its chute. As the tangle of mixed material moved down the slope, it had naturally separated into layers, with the fastest moving ones on top. Gas, seeking the best route out of the high-pressure space created by jostling volcanic material, was drifting upwards into the atmosphere—but also downwards, below the current of rock itself.
It was as if the gas had found a hidden trapdoor into the basement of the flow, where it generated a gentle buffer of air. Boosted atop this plush pocket of gas, the tide of debris sailed easily down its chute, freed of the constraints of friction.
But eventually, as gas continued to escape the jumble, the mixture’s mojo petered out, finally grinding to a halt when the lubricant ran dry.
“This really sheds light on the physics of the [bottom] region of these flows,” Soldati says. “Before, we did not fully understand what happens in the very thin contact area between the ground and the pyroclastic flow, but now we have a much better idea of what’s going on.”
Of course, differences remain between pyroclastic flows in the lab and in nature, Krippner says. “With all models, no matter how big we make them, it’s always a challenge to scale it up to an actual flow,” she says. What’s more, a range of pyroclastic flows exists: Some are more gassy, others chock full of rock. Due to these differences in composition, there won’t be a one-size-fits-all when it comes to predicting their behavior. All the same, Krippner says, “this is one more step forward in understanding how pyroclastic flows move.”
Armed with this knowledge, researchers might be better equipped to predict the spread of these flows, upping the chances that civilians can make it to safety before disaster strikes. “Before, we might have put put people in harm’s way inadvertently because we didn’t understand the flow could go further,” Gregg says.
Lube thinks that the same dynamics might also apply to other natural disasters, including snow avalanches and fast-moving landslides. Those theories need testing, but in the meantime, “we have a hand on this process now, and the data to describe it,” he says. “We can use this for hazard models to save people—that’s really possible now. And that means there is light at the end of the tunnel.”