Is Long Valley Caldera a Supervolcano?
Dr. Drew Coleman talks ancient magma chambers, the end of civilization, and sweet potatoes at White Mountain Research Station
Dr. Drew Coleman of the University of North Carolina at Chapel Hill has spent over twenty years studying the Sierra Nevada, searching for supervolcanoes.
On Tuesday, October 17, Dr. Coleman gave a presentation regarding super volcanic eruptions at White Mountain Research Center. His talk was timely, following a smattering of articles published in major media outlets this week reporting that the Yellowstone Caldera could erupt at any time.
“When most people hear the word supervolcano, they think of Yellowstone,” said Dr. Coleman, referring to the wealth of stories over the last twenty years that referred to Yellowstone as a geological catastrophe waiting to happen.
In fact, the Eastern Sierra is home to one of the world’s great supervolcanoes—the one that created the Long Valley Caldera and Bishop Tuff, the welded deposits of ash and pumice that characterize Bishop’s Volcanic Tablelands. According to Coleman, the eruption occurred some 760,000 years ago. When the explosion happened, it blasted 500 cubic kilometers worth of material across the region, and sent ash as far as Kansas and Alaska. The volcanic eruption that created the Bishop Tuff is reportedly at the bottom end of what qualifies as a supervolcano.
The largest volcanic eruption in earth’s history occurred in what is now Colorado, 28 million years ago. The eruption, called La Garita, scattered 5,000 cubic km of material.
When these eruptions occur, they do so because a mass of liquid magma below the earth’s surface is compressed by geological forces that cause it to burst. According to Dr. Coleman, the key to understanding the behavior of these magma chambers is to study the rate at which they grow and expand.
For his research, Dr. Coleman dates rocks in granite formations that previously comprised the molten cores of supervolcanoes. To do this, he collects samples rich in zircon, a mineral present in all Sierra granites, that also contains two particular isotopes of uranium that tend to decay into lead.
“Dating rocks is like a single elimination sporting event. Every time a round of the tournament is completed, half of the teams go from being winners to losers, and only simple math is required to calculate how many rounds have elapsed,” said Dr. Coleman on Tuesday. “The winners are parent isotopes (uranium). They decay into daughter isotopes (lead), the losers in our analogy. If I count the number of parent isotopes present in a rock, and the number of daughter isotopes, and I know the half life of uranium [the time it takes for half of the uranium isotopes in a given sample to decay into lead], I can calculate how much time has elapsed since the rock was formed.”
In 1992, Dr. Coleman visited Yosemite National Park to evaluate a prevailing theory of the time: that Tuolumne Meadows was a massive unexploded supervolcano. “There is so much exposed bedrock that you can look up, down, laterally at the historic magma chamber,” said Dr. Coleman. Using samples from all over the rock layers, Coleman and his colleagues were able to determine that the Tuolumne Intrusive Suite (granite formation) grew very slowly, over a period of about 9 million years. The same was found to be true for the John Muir Intrusive Suite, which extends down to Mount Whitney. In contrast, geological records indicate that the Bishop Tuff grew more rapidly, over the course of about 3 million years. For comparison, the Fish Canyon Tuff in Colorado formed over about 1 million years. In essence, the High Sierra was found to have little in common with known supervolcanoes.
“So we can think of these magma chambers like sweet potatoes,” said Dr. Coleman. “If you hand me a hot potato and tell me it took you eight minutes to prepare it, I know you microwaved it, but if you hand me a hot potato and tell me it took you fifty minutes to prepare it, I know immediately that you baked it in an oven… so if I understand that my magma chamber was a microwaved sweet potato, I know a lot more about what’s going on.”
When geologists set out to measure the mass of molten magma underneath Yellowstone, they found a huge magma chamber with an upper layer, or crust, that was only about 9 percent liquid. “In fact, this was not a microwaved sweet potato, but one that was accumulating melt very slowly,” said Dr. Coleman.
With regard to the Long Valley Magma Chamber (which still exists underneath the caldera), Coleman said a paper was published in 2011 that claimed at least 30 percent of the chamber was melted, which was cause for some concern. “That’s at least three times more than I’ve ever seen anyone claim under any caldera anywhere in the world,” said Dr. Coleman. In 2017, a subsequent paper was published which indicated that a false alarm was raised by hydrothermal fluid in the valley that registered as magma.
“That said, most of the volcanic activity in the valley is toward the western rim, around Mammoth Mountain,” said Coleman, who also confirmed a lecture attendee’s question about whether reports of earthquake swarms in Long Valley over the last fifteen years were caused by the movement of magma. “I didn’t say that there could never be a volcanic eruption,” said Dr. Coleman. It just probably wouldn’t be “a civilization-ending event. That said, a volcanic eruption at Mammoth Mountain would be very bad for a lot of people.”
