Slow-moving super-eruptions still travel great distances

by Mary Caperton Morton
Thursday, June 16, 2016

When Hollywood movies depict the destruction unleashed by volcanic eruptions, they usually focus on red-hot lava, but even more dangerous are pyroclastic flows: mixtures of rocky debris and searing hot ash and gas that move as fast as 700 kilometers per hour and can bulldoze, incinerate and suffocate anything in their paths. In a new study looking at pyroclastic flow deposits associated with the Silver Creek Caldera in the southwestern U.S., researchers have found that not all pyroclastic flows are so swift. Dense, slow-moving flows can still wreak havoc over vast distances.

The Silver Creek Caldera is the remnant of a supervolcano sitting at the intersection of Arizona, California and Nevada. During the Peach Spring super-eruption about 18.8 million years ago, the volcano unleashed massive pyroclastic flows into all three states. Some arms of the flow extended more than 170 kilometers away from the caldera — all the way to what’s now Barstow, Calif., for example. The sheer distance traveled by the flows seems to point to an extraordinarily fast-moving pyroclastic flow. But large boulders that appear to have been carried by the current suggest a denser, slower-moving mixture.

“We have two models for pyroclastic flows: They can be either dilute and fast, or dense and slow,” says Olivier Roche, a volcanologist at Blaise Pascal University in France and lead author of the new study, published in Nature Communications. Examples of each model can be found at different volcanic systems around the world, but the Peach Spring eruption is unique because it shows that dense, slow-moving flows can still travel great distances. “Intuitively, most of us would think that for the pyroclastic flow to [travel] such an extreme distance, it would have to start off with a very high speed,” he says. “But this isn’t consistent with what we found.”

Roche, David Buesch of the U.S. Geological Survey in Menlo Park, Calif., and Greg Valentine of the University at Buffalo used a combination of field observations collected in the 1980s and recent lab experiments to study the compositions of the pyroclastic flow deposits originating from Silver Creek Caldera and to calculate the velocity and density of ash currents in the flows. Based on the meter-sized boulders that were carried by the eruption, they determined that the Peach Spring flows likely traveled at somewhat modest speeds of 15 to 70 kilometers per hour.

“We say this is a slow flow, but at 70 kilometers per hour, nobody could escape [it]. It’s only slow when compared to fast-moving pyroclastic flow models, which are believed to have velocities … as fast as 700 kilometers per hour,” Roche says. The laboratory models, which simulated particle physics of pyroclastic flows, suggested that the flows were able to travel up to 170 kilometers from the caldera because the internal gas pressure among ash particles in the flows was high enough to overcome the friction among the particles. “This allows the material to flow like water, with minimal resistance, for very long distances,” Roche says.

Given how widespread the Peach Spring deposit is, “it’s somewhat surprising that it could have been created by a slow-moving flow,” says Calvin Miller, a volcanologist at Vanderbilt University who was not involved in the new study. “Then again, when you see the size of some of the blocks that were carried by the eruption, it makes sense. A fast-moving, low-density cloud couldn’t have carried those big blocks.”

The new study’s findings could have implications for continuing efforts to model pyroclastic flows, both from rare supervolcanic eruptions and from more modest but more frequent smaller eruptions. This behavior may not be confined only to super-eruptions, Roche says. “We think that slow, dense currents may also [result from] smaller-volume eruptions that occur around the world on a regular basis.”