Ongoing research at the Alaska Volcano Observatory suggests that a group of volcanic islands in the central Aleutians are all part of a large previously unrecognized caldera that is largely hidden by recent deposits and the surrounding ocean. The research findings were presented at the 2020 meeting of the American Geophysical Union (AGU).

The group of six stratovolcanoes are Carlisle, Cleveland, Herbert, Kagamil, Tana and Uliaga. Of these, Cleveland is one of the most persistently active volcanoes in North America. Over the past 20 years, the volcano erupted continuously. Effusive events that form lava flows are often separated by explosions that produce ash clouds that rise 15,000 to 30,000 feet above sea level. These eruptions pose hazards to aircraft traveling the busy air routes between North America and Asia. As researchers work toward understanding what makes Cleveland so active, they have been uncovering evidence of an unforeseen and surprising past.

Analyses of seismicity, geologic deposits, structural trends, gravity, thermal and gas emissions suggest that all six volcanoes are connected to the same partially collapsed magma chamber, similar to Yellowstone Caldera and other volcanoes that have experienced super-eruptions in the past.

When large magma volumes erupt, a volcano may collapse downward into the emptied or partially emptied magma chamber, leaving a massive depression at the surface, from one to dozens of miles in diameter. This cauldron-like depression is appropriately called caldera by geologists.

Caldera-forming eruptions are the most explosive volcanic eruptions on Earth and they often have had global effects. The ash and gas they put into the atmosphere can affect Earth’s climate by partially blocking the sun. For example, the eruption of nearby Okmok volcano in the year 43 BCE has been recently implicated in the disruption of the Roman Republic. The proposed caldera underlying the six volcanoes would be even larger than Okmok. If confirmed, it would become the first in the Aleutians that is hidden underwater.

The caldera hypothesis might also help explain the frequent explosive activity seen at Mount Cleveland. The vast magma reservoir beneath the island provides an almost unlimited supply of lava and gas to the active volcano.