The islands of Alaska can be part of a single, huge volcano

A trail of volcanic islands off the coast of southern Alaska may actually be part of a single giant caldera, according to evidence presented next week at the American Geophysical Union’s autumn meeting. If so, it’s possible that the newly revealed volcanic giant once exploded in an explosion large enough to dwarf the catastrophic 1980 eruption of Mount St. Helens.

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

Even if the idea is confirmed, however, the results do not necessarily herald a future catastrophe.

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

Scientists did not set out to look for evidence for the powerful explosion when they first ascended the IFM in 2014, but instead focused on the region’s archeology. A second group came out over the next two years to examine the tectonic bases of volcanoes.

The researchers looked at the local geology and used a suite of technologies to study the region, including seismometers to detect tiny tremors and chemical analyzes to understand the composition of the gases emitting from the soil. As they were processing the data, however, perplexing elements kept surfacing that only recently they realized could belong to a huge, ancient eruption.

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

Calderas form when a huge reservoir of magma suddenly empties and the soil above collapses, creating a vast depression on the Earth’s surface anywhere from one to 30 miles in diameter. The formation of a caldera produces a series 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 IFM volcanoes could represent a series of linked geological structures around a potential 12-mile-wide caldera, which they think lies hundreds of feet below the surface of the icy Pacific waters.

‘This would be a simple problem if it were on land,’ says Diana Roman, a volcanologist at the Carnegie Institution for Science and one of the lead researchers on the project. “But it’s underwater, so it makes the order of magnitude more difficult.”

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

Driven by this baffling data, the team began “scraping the sofa cushions,” as Roman puts it, to find any other information that would help explain the phenomenon. They collected a range of evidence, including gravity anomalies from satellite data and bathymetric surveys conducted in the area shortly after World War II. While not high-resolution, the seafloor mapping hinted at several curved ridge structures and a depression more than 400 feet deep that could be part of a caldera.

If their suspicions are confirmed, the team believes the potential undersea basin may have been the result of a volcanic explosion that was just shy of earning the “super eruption” label.

“Any piece of this evidence is questionable,” says Power. “But as we get more and more of them in line, it becomes a stronger argument.”

The team warns that many unknowns remain about the structure. For one thing, they are still unsure of the caldera’s size and do not know whether it was produced by a large explosion or several minor eruptions.

Even if it had been a single event, it would have been a medium-sized explosion compared to others around the world through geological history, Roman notes. For example, a very rough calculation would place the MFI explosion about one-tenth the size of the one that shook Yellowstone about 640,000 years ago, says Adam Kent, an Oregon State University volcanologist who wasn’t part of the study team. “It would potentially change the world,” he says. “But not the end of the world.”

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

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