Long Valley is one of the Earth’s largest calderas. Running east of the central Sierra Nevada Range, the caldera measures approximately 20 miles long, 11 miles wide, and at its lowest point, boasts a depth of 3,000 feet.
With the Glass Mountains to the north, Crowley Lake to the southeast, and Mammoth to the west, the Long Valley Caldera is what scientists call a depression—a sunken landform atop a huge magmatic system that, in this case, was formed by a volcanic eruption 760,000 years ago.
“It was formed because of what they call—well, the U.S. Geological Survey doesn’t like the term—but a supervolcano,” says Ettore Biondi, a research scientist at the California Institute of Technology’s Seismological Laboratory.
According to Biondi, the eruption that conceived the Long Valley Caldera erupted approximately 650 cubic kilometers of ash.
“To give you a sense of what that number means, if you spread those ashes on top of the Los Angeles area, you could cover the city in rhyolitic material 500 meters to one kilometer thick.”
In late October of this year, Biondi and his team of collaborators published their findings on the caldera—”An Upper-Crust Lid Over the Long Valley Magma Chamber”—in the peer-reviewed publication, Science Advances.
Biondi’s team is using a new kind of technology to monitor and analyze seismic activity called distributed acoustic sensing (DAS). The data that appeared in last month’s article used a 100-kilometer stretch of cable composed of 10,000 individual seismometers.
“The advantage of telecommunication fibers is that you get access to these nodes from telecom providers where you can put the instrumentation and get high speed internet,” says Biondi, whose team partners with the telecommunication network, Onward. “It’s not cheap, but you can download the data in real time.”
Deploying their DAS technology over the course of a year, the team measured approximately 2,000 seismic events to be entered as tomographic data, constructing the highest-resolution images of the volcanic system to date.
“The technology works this way: You have two instruments—the laser and the [fiber optic cable]. You can measure the deformation along the fiber, due to seismic waves. So when an earthquake arrives, it sends signals that pulse along the fiber that you can measure with a resolution.”
Those earthquakes have added up to what scientists call a period of “seismic unrest.”
Whether or not that unrestful state is an indication of imminent eruption is difficult to predict. And rarely does evidence of change in a volcanic system indicate the type, scale, or timeline of an eruption. Italy’s own supervolcano, Campi Flegrei, has been in a period of unrest for nearly 60 years.
“A super volcanic eruption is very unlikely to happen anytime soon,” says Biondi of the Long Valley Caldera.
According to Biondi, his team’s imaging of the caldera shows a hardening lid of crystalized rock atop the magmatic chamber of the volcano—what his study refers to as a “clear separation between the large magma body at depth and the upper-crust.” As the liquid magma cools, it crystallizes.
“And as it crystallizes,” says Biondi, “the fluid, like CO2 and water, can’t stay in the magma. They have to escape somewhere. They’re lighter than anything surrounding them … So they travel up.”
That cooling process, and the subsequent release of both liquid and gas, is enough to warrant earthquakes and small eruptions. It’s also the process, according to Biondi, responsible for the formation of the hot springs born out of the caldera’s landscape.
“So small eruptions may occur,” assured Biondi. “Very small. Maybe.”
In the meantime, Biondi’s team is working on refining their research tools, and enhancing their imaging technology to better understand the underlying structures of the Long Valley volcanic system.
Months ahead of publishing their research in Science Advances, Biondi and his team had already started using a 200-kilometer length of cable, in place of their previous length at half the size, to produce images even deeper from inside the caldera.
“The longer the cable, the deeper the structure you’re going to see,” says Biondi. “It’s possible we’ll be able to image the magma chamber better; Maybe even image some structures underneath.”
And as for that other, restless supervolcano a world away, Biondi is in communication with the team studying Campi Flegrei in his native Italy.
“We’re actually in contact with the National Institute of Volcanology and Geophysics in Italy. We’re trying to see if there’s an opportunity to use fibers there,” says Biondi.
In our conversation, Biondi stressed the importance of closely monitoring the ever-changing properties of volcanic systems. The Long Valley Caldera, according to the volcanologist, is not a simple structure to understand.
“It’s a beating heart,” says Biondi. “It’s not, of course, beating like a human heart is beating. But it’s pretty much saying the same thing: Oh, I’m still alive.”