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6 Alaska island volcanoes may actually be part of a single giant volcano, scientists say

A vigorous steam plume rises from the summit of Mount Cleveland in the Islands of Four Mountains, Alaska, on Aug. 1, 2014. (John Lyons / USGS)

A cluster of mountains in Alaska’s central Aleutian Islands might not be six separate volcanoes but instead could be interconnected as one giant volcano.

A team of researchers uncovered evidence showing that six stratovolcanoes comprising the Islands of Four Mountains may be part of a volcanic caldera system. The area could be similar to other large volcanoes, including the Yellowstone Caldera, which have resulted in super eruptions with drastic global consequences.

But not to worry — super eruptions are infrequent, according to John Power, researcher with the Alaska Volcano Observatory. If the area is a caldera, it is smaller than Yellowstone’s, he said.

“We’re not expecting a big eruption here. Our hazard level hasn’t changed. We haven’t put out any additional warnings,” Power said in an interview Saturday. “Currently, this is a scientific result that does help us understand more about how volcanoes might erupt here in the future.”

A multidisciplinary team from several research institutions nationwide set out to uncover the true nature of the six volcanoes in the Islands of Four Mountains, Power said. The Alaska Volcano Observatory, a collaboration between the U.S. Geological Survey and the University of Alaska Fairbanks, is part of the network.

The volcanoes possibly connected by a caldera include Carlisle, Herbert, Kagamil, Tana, Uliaga and Mount Cleveland, which has been the most active volcano in North America over the last 20 years, Power said.

Mount Cleveland has erupted in clouds of ash as high as 30,000 feet above sea level, which is dangerous to aircraft traveling the busy air routes between North America and Asia, Power said. In 2001, an eruption brought ash all the way to Anchorage.

“The more we understand about the how the volcanoes work underground, the better job we can do of anticipating future eruptions and the hazards that they pose,” Power said.

The observatory was studying the often-astir Mount Cleveland when the researchers stumbled on evidence of a possible caldera, Power said.

A caldera is an expansive volcanic structure formed when a large reservoir of magma erupts out of the ground and causes a big circular collapse of the area.

“What often happens is around the ring of the caldera, after it forms, subsequent volcanoes kind of grow up around this ring,” Power said.

Pete Stelling, formerly of Western Washington University, assembles a seismic station on Cleveland Volcano in August 2015. (Diana Roman)

The volcanoes in the area are classified as stratovolcanoes: conical mountains, usually with cratered peaks, that erupt periodically in clouds of ash and streams of lava. Stratovolcanoes have smaller magma reservoirs than caldera systems.

The super eruptions that create calderas are much more violent and can have far-reaching consequences, sometimes causing the temperature of the Earth to drop. Subsequent large eruptions are possible.

“They can be responsible for global pandemics and crop failures and political instability,” Power said.

An eruption in the year 43 B.C. of Aleutian volcano Okmok has been shown to have caused extreme cold, crop failures, famine, disease and unrest in the Mediterranean, and researchers linked the eruption to the fall of the Roman Republic.

Pavel Izbekov, researcher with the University of Alaska Fairbanks, said calderas are a fairly common result of previous large eruptions. For example, Crater Lake in Oregon is a well-known North American caldera, as is Yellowstone’s caldera.

The possible caldera — hidden beneath the ocean waters surrounding the Islands of Four Mountains — would be even larger than the Okmok caldera, said researcher Diana Roman in a statement from UAF. Roman, a volcanologist with the Carnegie Institution for Science in Washington, D.C., helped author the study that the researchers presented this week at the American Geophysical Union’s annual fall meeting.

The crescent shape of the six stratovolcanoes making up the islands is one of the first clues that indicated the possible presence of a caldera to the researchers, Power said.

While researching in 2014 and 2015 on Chuginadak Island, Izbekov and his colleagues found another clue in the rocks: ignimbrites, a type of rock created by catastrophic, caldera-forming blasts.

Power said the researchers found more evidence while recording earthquake activity, sampling gases exuded by the volcanoes, examining imaging of the sea floor structure and using satellite-based gravity measurements. Though gravity is generally constant, gravity measurements change slightly for areas of lesser density, such as if a large pocket of magma lies beneath the Earth’s crust in a caldera instead of a denser area of rock.

“It’s one of those things where each of us, each of the various disciplines has a piece of the puzzle,” Power said. “It’s only when you consider all the pieces together that you really come away saying, ‘Boy, this probably lines up to be a big caldera here.’ ”

The islands are remote, difficult to get to and plagued with inclement weather, which has stymied in-depth research of the area for years, Power said.

“We’ve been scraping under the couch cushions for data,” Roman said in the UAF statement. “But everything we look at lines up with caldera in this region.”

Although the evidence is forming a clearer picture, Powers and Roman both say more study and data is needed before concluding that the area is indeed a caldera.

An underwater caldera of the Islands of Four Mountains may fall into the same classification as the Yellowstone Caldera, but don’t call it a supervolcano: Many scientists say that’s an improper term with no real definition.