The Alaskan Islands may be part of a single massive volcano

A volcanic island trail off the coast of southern Alaska could actually be part of a single giant caldera, according to evidence presented next week at the American Geophysical Union’s fall meeting. If so, it’s possible that the newly revealed volcanic giant has already erupted in an explosion large enough to eclipse the cataclysmic eruption of Mount St. Helens in 1980.

The mastodon in question is marked by a semicircular cluster of peaks in the Aleutian Islands known as the Islands of the Four Mountains (IFM). Long considered independent volcanoes, the Six Peaks – including Herbert, Carlisle, Cleveland, Tana, Uliaga, and Kagamil – may in fact be a series of vents connected along the edge of a much larger volcanic caldera.

Even if the idea is confirmed, the results do not necessarily herald future disaster.

“This new research result does not change the dangers,” said John Power, a geophysicist at the US Geological Survey and the Alaska Volcano Observatory, who will present the work at the AGU meeting on Monday. “We are not planning anything dangerous here.” (Learn more about supervolcanoes and why this term can be misleading.)

Scientists did not seek to find evidence of the powerful explosion when they first sailed to the IFM in 2014, but instead focused on the archeology of the area. A second group set off over the next two years to examine the tectonic foundations of volcanoes.

Researchers looked at the local geology and used a suite of technologies to study the area, including seismometers to detect tiny tremors and chemical analyzes to understand the composition of gases escaping from the ground. As they worked through the data, however, puzzling features continued to crop up that they only recently realized may belong to a huge, old eruption.

The first piece of the puzzle was the curious half-ring shape of the tightly clustered IFM volcanoes. One explanation could be a caldera.

Calderas form when a massive reservoir of magma suddenly empties and the overlying ground collapses, creating a vast depression on the Earth’s surface one to 30 miles in diameter. The formation of a caldera produces a multitude of fractures through which magma can then seep to the surface, so volcanic clusters are common around their edges or centers.

In this case, the researchers suspected that the IFM volcanoes could represent a series of connected geologic structures around a potential 12-mile-wide caldera, which they say lies hundreds of feet below the surface of the frigid waters of the Peaceful.

“It would be a simple problem if it were on earth,” says Diana Roman, a volcanologist at the Carnegie Institution for Science and one of the project’s principal investigators. “But it’s underwater, so that makes it a lot more difficult.”

Another piece of the puzzle was the discovery of rocks known as welded ignimbrites. These materials are formed when a large eruption deposits burning volcanic ash so thick that the grains weld together into solid rock, says Pete Stelling, who participated in the 2015 research season but is not part of the new analysis.

Spurred on by this confusing data, the team began “scratching the cushions of the sofa,” as Roman puts it, to find other information that might help explain the phenomenon. They gathered a range of evidence, including gravity anomalies from satellite data and bathymetric studies that were conducted in the area shortly after World War II. Although not high resolution, the seabed mapping has hinted at several curved ridge structures and a depression over 400 feet deep that could be part of a caldera.

If their suspicions are confirmed, the team believes that the potential underwater basin may have resulted from a volcanic explosion that was just timid to earn the “super eruption” label.

“Any piece of this evidence is questionable,” Power says. “But as we line up it becomes a stronger argument.”

The team warns that many unknowns about the structure remain. On the one hand, they are still unsure of the size of the caldera and whether it was caused by a large explosion or several smaller eruptions.

Even if this was a single event, it would have been an explosion of medium size compared to others in the world throughout geological history, notes Roman. For example, a very rough calculation would place the IFM explosion about a tenth the size of the one that rocked Yellowstone about 640,000 years ago, says Adam Kent, a volcanologist at Oregon State University who did not part of the study team. “It would potentially change the world,” he says. “But not the end of the world.”

Yet preliminary research provides tantalizing clues to help scientists better understand current and potential future dangers in this region.

“This poses a very nice story for a future investigation,” says Jackie Caplan-Auerbach, a volcanologist and seismologist at Western Washington University who was not part of the research team.