Ancient supervolcanic flow was actually rather slow, scientists say
Just how fast聽鈥撀爋r slow聽鈥撀爓ere pyroclastic flows from ancient supervolcanoes?
A new study investigated the question, using the eruption of the Silver Creek caldera in the western United States, 18.8 million years ago. Researchers from the United States Geological Survey, the University at Buffalo, and France鈥檚 Blaise Pascal University published their findings online Monday, in the scientific journal Nature Communications on Monday.
In the Silver Creek eruption, pyroclastic flows 鈥撀燿eadly rivers of gas, rocks, and ash, like the one that poured out of Mount Saint Helens in 1980聽鈥撀燿estroyed huge swaths of present-day California, Nevada, and Arizona, but probably聽traveled at the comparatively slow pace of , say scientists.
鈥淚ntuitively, most of us would think that for the pyroclastic flow , it would have to start off with a very high speed,鈥� lead author Oliver Roche told the University at Buffalo Reporter, 鈥淏ut this isn鈥檛 consistent with what we found.鈥�
Supervolcanoes are surprisingly common, says Dr. Roche, including several in the western United States alone. The , which last erupted 640,000 years ago.
The explosive eruption of the聽,聽located near the junction of present day California, Nevada, and Arizona, left ash layers as much as 450 feet thick close to the volcano, and up to 10 feet thick 100 miles away.
According to the USGS, 聽range from 50 miles per hour up to聽. While the enormous scale of supervolcano eruptions might suggest faster flows, Roche and his colleagues argue that they were likely much slower.
To understand ancient volcanic eruptions, the researchers simulated the Silver Creek explosion through experiments and utilized data gathered in the 1980s聽from the eruption site. They found that the river of ash had been powerful enough to lift rocks off the ground and transport them long distances, a process geologists call "entraining."
鈥淲e did laboratory experiments on gas-particle flows aimed to simulate pyroclastic flows entraining particles from the substrate,鈥� wrote Roche in an email to the 海角大神 Science Monitor.
They used the size of the entrained rocks as well as the size of the boulders left behind to place upper and lower limits on the flow speed. Based on Roche鈥檚 model, the ancient pyroclastic flows traveled between 5 to 20 meters per second (10 to 45 mph).
鈥淲e want to understand these pyroclastic flows so we can do a good job of forecasting the behavior of these flows when a volcano erupts,鈥� said co-author Greg Valentine of the University at Buffalo.
While modern cars could theoretically outpace a pyroclastic flow similar to the Silver Creek explosion, the best thing to do is evacuate as far ahead in advance as possible, said Dr. Valentine.
After all, pyroclastic flow temperatures often reach well over 1000 degrees Fahrenheit.聽
鈥淭he character and speed of the flows will affect how much time you might have to get out of the way," said Valentine, but "the only truly safe thing to do is to evacuate before a flow starts.鈥�
