Long Valley Supervolcano: Huge Pool of Magma Was in Cold Storage Before Ancient Eruption

Mammoth Mountain in California is just one piece of the Long Valley site. David McNew/Newsmakers/Getty Images

The supervolcano below Long Valley, California, near Yosemite National Park hasn't erupted in 765,000 years—but when it did, if geologists had been around to watch it, they may have been surprised by how little warning they had of the impending eruption from the magma deep inside the volcano. That's according to a new paper published today in the journal Proceedings of the National Academy of Sciences.

The paper suggests we may be thinking about volcanoes and their eruptions in the wrong way. "The historical view of volcanoes as fed by basically these tanks of magma in the crust isn't really a useful way to think about the physical processes," said first author Nathan Andersen, a geologist at the Georgia Institute of Technology.

Instead, he said, scientists are realizing that underground magma chambers are much less exciting places, "in a pretty calm, pretty cool, more or less unremarkable state for most of their lifetime." Unfortunately, that means they don't show up very well with the tools scientists have to monitor magma underground.

And that might mean that once scientists do see magma moving underground, it may be time to worry. "One of the first things to understand about a large magma chamber like this is that its ability to erupt is really controlled thermally," Andersen said. That's because the magma can't go anywhere if it isn't warm enough to be fluid. "The magma is stored relatively cool, still containing some molten rock, but mostly crystalline—but generally unable to erupt unless you give it a kick" with more heat to get the magma moving again.

At Long Valley, he said, that seems to have happened much more subtly than scientists had previously believed. He and his co-authors used a chemical fingerprinting technique to calculate when rocks last became hot enough to lose their argon, which floats away when magma warms up enough. Andersen described the technique as "like a stopwatch that only starts when you hit a certain low temperature."

Applying the technique to rock produced by the Long Valley supereruption, Andersen and his colleagues found that there was a 16,000-year range in dates at which that stopwatch went off. That means large swaths of the magma under the volcano never became hot enough during the run-up to the eruption to lose its argon. "It must have cooled to the point that it was completely solid," Andersen said of the magma. "It had to be really a rock, at something more like 400 degrees [750 degrees Fahrenheit]."

Relicts of the Long Valley supervolcano eruption that happened 765,000 years ago. David McNew/Newsmakers/Getty Images

And when the magma did receive enough heat during the run-up to the eruption, it happened fast—likely within decades, or a couple centuries at most, Andersen said. That's the sort of timescale that is of great interest to humans today, and it means that the new research might help scientists better understand eruption risks.

"The better that we understand the processes that triggered eruptions in the past, the better we'll be able to look for those same processes in monitoring records that we're able to acquire today," Andersen said.

That doesn't mean scientists will utterly solve the problem of predicting when precisely a volcano will erupt, he added. "Aside from a few well behaved volcanoes," he said, "it's really hard to predict volcanic eruptions." But every piece of information helps.