A mass of black ash acres-wide crosses the sky toward the horizon, where it hits ground, sprays across the landscape and is transformed into tumbling sheets of glowing lava.
During almost all volcanic eruptions, lava is resigned to spilling down a volcano’s sides, then cooling and hardening. But that’s not the case with super eruptions. Eight million years ago, in what is now Yellowstone National Park, a volcano belched ash for miles and became molten upon striking ground. The evidence is in the lava swirls that hardened into rock. In Grey’s Landing, Idaho, scientists have discovered several swirls as a result of the Yellowstone eruption.
Now, researchers at the University of Missouri have discovered how lava could flow miles from a volcanic eruption. Alan Whittington, an associate professor in MU’s geological sciences department in the College of Arts and Science, along with doctoral students Genevieve Robert and Jiyang Ye, published their thesis this month in the journal Geology.
“During a super volcano eruption, pyroclastic flows, which are giant clouds of very hot ash and rock, travel away from the volcano at typically 100 miles an hour,” Robert said. “We determined that the ash must have been exceptionally hot so that it could actually turn into lava and flow before it eventually cooled.”
Volcanic ash and rock needs to be at least 1,500 degrees Fahrenheit to transform into lava. But as detritus sails through the sky, it loses hundreds of degrees. The researchers postulate that the combination of the already super hot materials from the mega explosion and “viscous heating” when it strikes the ground accounts for the phenomenon.
Viscosity is the degree to which a liquid resists flow. The higher the viscosity, the less the substance can flow. For example, water has a low viscosity, so it flows easily, while molasses has a higher viscosity and flows much slower. Whittington likens the process of viscous heating to stirring a pot of molasses.
Stirring molasses is difficult, but it gets easier during the process. “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,” Whittington said. “This is viscous heating.”
When the volcanic detritus lands, its energy “is turned into heat, much like the energy from the spoon heating up the molasses,” Robert said. This could add 400 degrees Fahrenheit to the materials. “This extra heat created by viscous heating is enough to cause the ash to weld together and actually begin flowing as lava.”