Tech & Science

Pyroclastic Flows From Volcanic Eruptions Surf on Self-Generated Layer of Air

Scientists have discovered that the scorching material spewed from a volcano during eruptions generates a layer of air between it and the ground, allowing it to surf along at extreme speeds, destroying everything in its path.

Pyroclastic flows are made up of a mix of hot lava, pumice, ash and volcanic gases. They can reach temperatures of up to 1,000 degrees Celsius and can—in extreme cases—move down the slopes of volcanoes at over 400 miles per hour. They are responsible for around 50 percent of all deaths from volcanic eruptions globally. Pyroclastic flows were what destroyed the ancient cities of Pompeii and Herculaneum when Mount Vesuvius erupted in A.D. 79. 

Flows are normally split into two parts—a stream of hot rock fragments that move along the ground (known as the basal flow) and a hot cloud of ash that rises above. In a paper published in Nature Geoscience, a team of researchers look at how the lower level of material is able to move so fast.

To do this, the team, led by Gert Lube of New Zealand’s Massey University, carried out an experiment by releasing up to 6 tons of 400-degree Celsius pyroclastic material down a makeshift unit inside a disused boiler house. The researchers recorded the flow of the material with high-speed videos, allowing them to analyze exactly what was happening to it as it rolled down the 12-meter-long chute.

Results showed that the pyroclastic flows “generate their own air lubrication,” the team wrote. It found that an area of high-pressure volcanic material forms toward the base of the flow. The air is forced downward as a result of the pressure, creating a “near-frictionless” layer along which the material can flow quickly.

Their findings, the researchers said, could help authorities better understand the hazards posed by volcanoes—and how to plan for them. They also said the results could have implications for other events, including avalanches and fast-flowing landslides. "Currently, we are working on trying to understand how these volcanic flows generate enormous damage potential and how they maintain to do so during runouts over complex terrain," Lube told Newsweek. "Part of the results of the published work is used currently in an international exercise of volcanologists to intercompare and validate computational volcanic hazard models."

David Pyle, a professor of earth sciences at the University of Oxford, told Newsweek, “A long-standing puzzle for volcanologists has been the question of why pyroclastic flows are able to travel so far.”

He added, “We can find flow deposits hundreds of kilometers from the source volcano, and others that have crossed significant topographic or other barriers, such as mountain ranges or open bodies of water.

Mount Sinabung pyroclastic flow Pyroclastic smoke during an eruption of Mount Sinabung in Indonesia. Ulet Ifansasti/Getty Images

“The new large-scale experiments reported by Gert Lube and colleagues offer a physical explanation for the unusual mobility of pyroclastic flows: They travel on a self-generated cushion of air that reduces the friction at the base of the flow. This elegant analysis provides a unifying explanation for field observations [and] should open up new opportunities for the modeling of hazards from violent pyroclastic density currents.”

Volcanologist Rebecca Williams of Britain’s University of Hull said the research provides important mathematical information that should be incorporated into how we model pyroclastic density currents (PDCs).

“PDCs typically travel around 70 miles per hour but are known to have reached speeds up to 400 miles per hour,” she told Newsweek. “Their high mobility isn't just about speed—they can travel at these high speeds over rough terrain large distances from the volcano. They can even scale hills several hundred meters tall, running upslope. This research suggests that this high mobility is through air lubrication at the base of the flows.”

Williams added: “Basal friction values are poorly defined in our existing models, and this paper suggests we may be approaching them entirely incorrectly. By modeling the complex behavior of PDCs in as much detail as possible, we can look to improving our hazard assessments at volcanoes.”

This article has been updated to include quotes from Gert Lube. 

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