Geologists: Supervolcanoes Can Bury Huge Expanses, But Only in Slow-Motion

If you live in the western United States, the chances are very good that you live within the cradle of a supervolcano. Yellowstone is but the most famous example.

There are volcanoes, and then there are supervolcanoes. When a volcano erupts, it might take out some neighboring towns and mess with air traffic, but a supervolcano—a volcano that ejects material (ejecta) on the order of quadrillions of kilograms—can be expected to spread pyroclastic flows for hundreds of miles and dump ash even further out than that. These are climate-changing, extinction-causing blasts. The big ones of the big ones.

The closest we’ve come to seeing a supervolcano blow in relatively modern times is probably the eruption of Mount Krakatoa in 1883, which is thought to have killed over 100,000 people. But Earth is host to a rich population of dormant supervolcanoes, many of which happen to be located near population centers in the western United States. One of these, the Silver Creek caldera near the intersection of California, Nevada, and Arizona, is the subject of a new study published in Nature Communications exploring the likely movements of pyroclastic flows following a supervolcanic eruption. The authors reached some unexpected results: Supervolcanoes destroy slowly, oozing molten rock across the landscape rather than blasting it in pyroclastic storms.

How slowly? The new report suggests that in order to cover such huge expanses of land, magma would need to ooze outward from its source at a mere 10 to 45 miles per hour. This is what the USGS-, Blaise Pascal University-, and University at Buffalo-based group was able to extrapolate from laboratory experiments and field data relating to the Silver Creek caldera, which is thought to have last erupted some 18.8 million years ago.

The result of that event can be seen now as the Peach Spring Tuff, a 32,000 square-kilometer expanse of pyroclastic rock stretching across the southwestern United States from near Barstow, California to the Grand Canyon in Arizona. The Tuff’s origins were only recently traced to the Silver Creek Caldera, which is nowadays a relatively unassuming feature (but a very exciting one for geologists):

Image: Vanderbilt.edu

Despite their sluggish pace, the flows were able to travel hundreds of miles, scorching vast portions of California, Nevada, and Arizona. This is the extent of the Tuff:

Image: Vanderbilt.edu

It may be that it’s this slowness itself that allowed the flows to cover so much ground in the first place.

“Explosive volcanic super-eruptions expel magma volumes of several hundred cubic kilometres or more and generate particle-gas flows called pyroclastic density currents,” the geologists write. “There are many locations around the world where the deposits of these pumice-rich currents (ignimbrites) extend 100 km from their source vents. These surprisingly long run-out distances raise fundamental questions about the flow velocity and propagation mechanism.”

As the group explains, it’s often assumed that such a vast spread of material would require very fast pyroclastic currents, and flows of relatively low density. That would seem intuitive enough, anyway.

This assumption is the source of an ongoing debate between those that favor the intuitive model of quick, billowing pyroclastic flows and those that favor a model involving sluggish, very dense flows. Unfortunately, it’s difficult to make a determination based on the supervolcanic leftovers we have around now.

“This debate is not just academic; an understanding of the mechanisms by which pyroclastic flows propagate is essential to accurately forecast related hazards at active volcanoes,” the study cautions.

While the volcanic material itself may offer only scant clues, the researchers were able to look to somewhat indirect evidence in the form of dense rocks that got shoved around by the flows for short distances. The movement of these rocks indicates that the pyroclastic material must have been dense and fluid-like for the simple reason that a less dense, gaseous flow would have just whooshed around them. They were able to support this notion by both examining these rocks in the field and by simulating flows in laboratory experiments.

We probably shouldn’t take this as evidence that we can sit around and finish our coffees before evacuating in the event of a supervolcano eruption. But it does offer a new predictive power that may offer insights into how exactly that evacuation should transpire.