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Just east of the Sierra Nevada range, scientists have been keeping an eye
on a long-dormant supervolcano that has caused thousands of earthquakes in recent years.

Two theories could explain the unrest: either the volcano was at risk of
an eruption, or it was cooling down. Now, a group of researchers says new acoustic images prove it’s the latter.

The Long Valley Caldera, just under 200 miles east of the Bay Area, was
formed by a supereruption about 760,000 years ago, according to the U.S. Geological Survey. The eruption left a 20-by-10-mile depression in the
earth that hides a pit of magma, rock and gas, which scientists worried
could one day erupt again. The caldera’s most recent eruption happened
more than 16,000 years ago, according to the USGS.

On a normal day, the Long Valley Caldera erupting is about as likely as a magnitude 8 earthquake happening along the San Andreas fault, the USGS
says. But scientists believed that earthquakes, along with the ground
around the caldera rising up, could signal that magma is leaking into the
upper crust of the caldera, signifying a greater risk of eruptions.

In a new study, researchers with the California Institute of Technology’s Seismological Laboratory argue that the inflated ground and earthquakes
are actually the result of gas bubbling up as the magma inside the caldera cools down.

The researchers used a more than 60-mile-long fiber-optic cable to measure
the seismic waves sent off by earthquakes. By detecting how long the waves
took to travel through different material, the scientists determined what materials made up the different areas within the caldera.

They found that the volcano’s magma chamber has been cut off from the
crust of the caldera by a layer of crystallized rock.

The separation signified that magma wasn’t leaking upward — instead,
volatile gases such as carbon dioxide have been bubbling up from the
cooling magma and causing the unrest around the caldera, a phenomenon
known as “second boiling.”

Similar behavior has caused earthquakes near the dormant Mauna Kea volcano
in Hawaii, according to a 2020 study published in the same journal.

The researchers concluded the Long Valley Caldera is “still hazardous, but
not as dangerous.”

Next, the team hopes to use a similar technique with a much longer length
of cable to dive into the makeup of the cooling magma chamber deep within
the caldera.

“We’re excited to apply similar technology to other regions where we are curious about the subsurface environment,” Ettore Biondi, one of the scientists, said in a release.

https://www.sfchronicle.com/climate/article/supervolcano-near-yosemite- cooling-down-causing-18434687.php

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