A mysterious, previously undiscovered supervolcano may be lurking beneath Alaska’s Aleutian Islands.

A new study suggests a wide crater, created when the supervolcano exploded, connects at least four existing volcanoes. It’s so big that if the supervolcano erupted during the last few thousand years, it could have disrupted civilizations around the world, says John Power, a geophysicist at the U.S. Geological Survey’s Alaska Volcano Observatory. Power presents the findings at the annual meeting of the American Geophysical Union on December 7.

The discovery, not yet confirmed, emerged from several pieces of evidence that at first glance seem unrelated, says Diana Roman, a volcanologist at Carnegie Institution for Science in Washington, D.C.  “There’s no one smoking gun,” she says. And in fact, the mythical-sounding Islands of the Four Mountains, actually six volcanoes located near the center of the island chain, look like an ordinary volcanic cluster.

But taken together, the data point convincingly to the existence of a caldera about 20 kilometers across. The volcanoes’ peaks are arranged in a ring and bathymetric seafloor mapping, mostly from the 1950s, shows arc-shaped ridges and a 130-meter-deep depression in the center of the ring.  Both are clues that the volcanoes are connected by one big caldera, a massive crater that forms when a very large magma chamber in a volcano explodes and empties.

Gravity data from satellites echo the look of other calderas. And analysis of such volcanic gases such as sulfur dioxide, as well as patterns of microearthquakes also suggest the presence of a caldera.

Searching for a supervolcano

Some of the clues that a supervolcano may lurk under the Islands of the Four Mountains, in the Aleutian Islands chain in southwest Alaska, come from seafloor topography mapping, like this bathymetry map compiled by NOAA. Gray areas mark the existing volcanoes. The orange zones show shallow volcanic areas apparently connected below the surface in a roughly circular pattern.

Seafloor mapping around Mount Cleveland

“We weren’t surprised there were microearthquakes,” says Roman, considering one of the volcanoes, Mount Cleveland, is one of the most active volcanoes in the Aleutians. But, she says, those microearthquakes extended farther east and north than they would expect just based on the volcanoes seen at the surface. “That makes more sense in the context of the caldera.” 

One hallmark of many calderas is still-active volcanoes on their rims that tap into the same magma chamber, even long after the caldera itself formed. Mount Cleveland fits that scenario. It has “erupted 60 or 70 times since 2001,” says Power. Besides blasting out sky-high ash plumes that disrupt air travel (SN: 11/27/18), this level of constant activity is typical  of volcanoes rimming other known calderas, he says. One such volcano is Indonesia’s Rinjani, whose eruption around the year 1257 dumped enough sulfur particles into the atmosphere to cool the entire planet (SN: 6/14/12).

Piecing the evidence together has been challenging, thanks to the extremely remote location, a largely underwater setting and newer volcanic deposits which obscure older ones. In addition, separate studies provided different lines of evidence for a supervolcano caldera, but none connected the dots. Roman likens the team’s approach to “looking under the couch cushions.”

“It’s a neat example of how lots of threads come together to make a bigger story,” says Michael Poland, a volcanologist with the USGS’s Yellowstone Volcano Observatory who was not involved in the study. “We’re starting to get the datasets we need to make these sorts of discoveries.”

The Aleutians site is accessible only a short time each year, Poland says, so “it’s a mad rush to collect data.” But that’s exactly what the team hopes to do to confirm the caldera’s existence. It also plans to search for matching ash in ice cores collected in other parts of the world to determine when the supervolcano would have erupted. “These very large calderas have very large impacts globally,” says Power. “This potential identification helps us understand what we might expect, why Cleveland is so active, and understand the hazards.”