Supervolcanic eruptions are rare events on Earth that are difficult to study, and much of what we know about them comes from the deposits left behind. Now, on the basis of such deposits, scientists have managed to reconstruct the flow speed of the enormous eruption at the Silver Creek caldera site in the southwestern U.S. 18.8 million years ago. The dense flows traveled relatively slow, scientists say. The new research was published in Nature Communications on March 7, 2016.
Pyroclastic flows consist of hot gas, ash, and rock that are released after a volcanic eruption, and flows are generally thought to travel at rapid speeds following the eruption of a supervolcano, where greater than 1,000 cubic kilometers of material is ejected. However, new data on the enormous eruption at Silver Creek 18.8 million years ago suggest that the flows there were relatively slow—on the order of only 5 to 20 meters per second (11 to 45 miles per hour).
During this eruption, more than 1,300 cubic kilometers of material was ejected, and it blanketed an area of about 32,000 square kilometers in the region where the borders of Arizona, California, and Nevada are now located. The deposits were 5 to 40 meters deep in many locations surrounding the caldera, and they reached distances of more 170 kilometers (106 miles) away from the vent.
The eruption at Silver Creek was about half as big as the largest known eruption at the Yellowstone supervolcano 2.1 million years ago, which released 2,450 cubic kilometers of material, and it was about 260 times as big as the large eruption at Mount Pinatubo in 1991.
Scientists used the size of rocks that were carried by the flow to distant locations to estimate the speed of the pyroclastic flows from the supervolcanic eruption. They also relied on a model of how steel beads moved within fluid material in the laboratory to calculate their estimates of the flow speed.
Olivier Roche, lead author of the new paper who completed this work at Blaise Pascal University in France, commented on their findings in a press release. He said:
Intuitively, most of us would think that for the pyroclastic flow to go such an extreme distance, it would have to start off with a very high speed. But this isn’t consistent with what we found.
They suspect that the slow flows were able to travel far distances because of both the sustained high discharge rates of the eruption and the abundant ash concentrations in the flow, which changed the gas pressure dynamics and promoted flows with a long duration.
Greg Valentine, co-author of the new paper and director of the Center for GeoHazards Studies at the University at Buffalo, said that these results are important for informing current disaster preparedness efforts in population centers surrounding potentially dangerous volcanoes:
We want to understand these pyroclastic flows so we can do a good job of forecasting the behavior of these flows when a volcano erupts. The character and speed of the flows will affect how much time you might have to get out of the way, although the only truly safe thing to do is to evacuate before a flow starts.
David Buesch, who is a U.S. Geological Survey scientist, was also a co-author of the new paper. Funding for the work was provided in part by the French Institute for Research and Development.
Bottom line: Scientists have estimated that slow, dense pyroclastic flows blanketed the southwestern U.S. after the enormous eruption at Silver Creek 18.8 million years ago. The new research was published in Nature Communications on March 7, 2016.