Lawrence LeBlond for redOrbit.com – Your Universe Online
New research has taken a closer look at how ash produced from supervolcanoes can turn back into lava once it falls back to Earth.
Supervolcanoes, such as the Yellowstone caldera, are capable of producing eruptions thousands of times stronger than normal volcanoes. Such a massive eruption can produce an ash cloud that is so hot it has the ability to re-form into lava once it hits the ground much farther away.
This evidence was previously revealed by California State University Bakersfield’s Graham Andrews.
Andrews discovered lava flows produced from an ancient Yellowstone super eruption that occurred some 8 million years ago were the result of ash that had traveled into the atmosphere and formed as lava only after returning to the ground tens of miles from the initial eruption.
During a typical eruption, lava will flow directly from the volcano until it cools enough to solidify. But during a supervolcanic eruption ash can remain superheated long after it leaves the volcano.
Now, Alan Whittington, an associate professor in the University of Missouri department of geological sciences in the College of Arts and Science, and lead author Genevieve Robert and Jiyang Ye, both doctoral students in the geological sciences department, reveal how this is possible.
“During a supervolcano eruption, pyroclastic flows, which are giant clouds of very hot ash and rock, travel away from the volcano at typically a hundred miles an hour,” Robert said. “We determined the ash must have been exceptionally hot so that it could actually turn into lava and flow before it eventually cooled.”
But the team believes another reaction was involved in this process, as ash should have cooled too much to turn into lava as it landed. They suggest that a process known as “viscous heating” – the degree to which a liquid resists flow – could have a lot to do with this type of event.
In explaining the process of viscous heating, Whittington likens it to stirring a pot of molasses.
“It is very hard to stir a pot of molasses and you have to use a lot of energy and strength to move your spoon around the pot,” Whittington said. “However, once you get the pot stirring, the energy you are using to move the spoon is transferred into the molasses, which actually heats up a little bit. This is viscous heating.
“So when you think about how fast the hot ash is traveling after a massive supervolcano eruption, once it hits the ground that energy is turned into heat, much like the energy from the spoon heating up the molasses. This extra heat created by viscous heating is enough to cause the ash to weld together and actually begin flowing as lava,” Whittington explained.
For this to happen, the team notes the ash produced by the eruption must be at least 1,500 degrees Fahrenheit. Since the ash should have lost some of this heat in the air, viscous heating likely accounted for anywhere between 200 and 400 degrees of additional heating for the ash to transform into lava.
A paper on this research is published in the journal Geology. The study was funded by the National Science Foundation.