The cool beginnings of a volcano’s supereruption
The New York Times
A supervolcano’s underground ocean of magma is not the seething, red-hot molten lava you might imagine. Instead, it is likely at a low enough temperature to be solid.
That is according to a new analysis of volcanic leftovers from an ancient California supereruption, which shows that the magma melted shortly before the volcano erupted. The findings, published Monday in Proceedingsof the National Academy of Sciences, might help scientists forecast when such volcanoespose a threat.
The supereruption in question occurred 765,000 years ago, carving a vast volcanic depression that is 20 miles long and 10 miles wide, now known as the Long Valley Caldera near California’s Mammoth Mountain. In the process, it ejected a giant quantity of ash and hot gas over one nightmarish week, enough to leave a layer of debris that spread from the Pacific Ocean to Nebraska.
“It would have completely wiped out everything within 50 kilometers of the caldera,” said Brad Singer, a geologist at the University of Wisconsin in Madison and the study’s co-author. “All the vegetation and biota in that area would have been extinguished.”
Scientists do not expect Long Valley to erupt again, but given enough time another supervolcano will likely scar our planet. So, Nathan Andersen, a geologist now at Georgia Institute of Technology, analyzed 49 crystals from the Bishop Tuff — a fossilized ash deposit from Long Valley’s supereruption. These crystals — and the elements locked within — contain clues about the supervolcano before and after it erupted. Take the element argon as an example. Because hot crystals cannot retain argon, the team did not expect to see it until after the eruption. Instead, they found that roughly half of the crystals contained argon that is as much as 16,000 years older thanthe eruption.
The data — which Tiffany Rivera, a geologist at Westminster College in Utah who was not involved in the study, calls “exquisite” — have surprisingimplications.
They suggest that those crystalswith older argon were not stored in hot magma for very long before the eruption. In fact, argon can only be retained in crystals that are surroundedby magma that is less than 500 degrees Celsius. For magma, that is quite chilly — so chilly in fact, that it would havebeen solid.
And yet, scientists estimate that when the supereruption occurred, the magma was at 785 degrees Celsius. To explain the discrepancy, the team suspects that heat infiltrated the system so quickly that the argon did not have time to escape. This heat is likely what awoke the cool supervolcano, pushing it toward eruption in less than a few hundred years — perhaps within decades.
That is a far cry from the lethargic time scales that often define the field of geology. And yet, recent research at the Yellowstone supervolcano in Wyoming and the Taupo supervolcano in New Zealand also has suggested that the events leading up to supereruptions can occur on human time scales.
The findings suggest that the supervolcano had to wake up from an extremely cold state, raising questions about how a solid ocean of magma could melt and mobilize so rapidly.