Can ‘extinct’ volcanoes still erupt? A Greek peak holds surprising clues

For more than 100,000 years, a Greek volcano lay silent. But deep underground, it was still growing. Tiny zircon crystals show magma was quietly brewing between eruptions, researchers report April 22 in Science Advances. The finding suggests that some volcanoes scientists think are dead may not be dead at all, and it could help identify quiet volcanoes that might still erupt in the future.

“I think that we definitely have to start reevaluating how we classify extinct volcanoes,” says Razvan-Gabriel Popa, a volcanologist at ETH Zurich in Switzerland.

Small volcanoes are usually considered extinct if they haven’t erupted in the last 10,000 years or so. Larger volcanoes can go much longer between eruptions, because it takes longer to build up enough magma to feed an eruption.

Popa and colleagues discovered there can be a lot more going on under the surface when they studied the history of Methana, an active volcano about 50 kilometers from Athens. The team collected volcanic rock samples and searched for zircon because the crystals form in magma chambers deep underground. Eruptions carry the zircon crystals to the surface. By dating crystals in more than 1,250 rock samples, the team put together a 700,000-year timeline of Methana’s eruptions.

The volcano went through two main periods of eruptions, the first ending around 280,000 years ago and the second beginning around 168,000 years ago. If the volcano had truly gone silent, the team wouldn’t have found any zircon crystals between the two eruptions. Instead, the team found the highest zircon generation during that period of quiescence. This suggests that even if a volcano looks dormant, it might be active deep below.

Water-saturated magma may have been responsible for kicking off the long break in eruptions, the team says. At high pressures deep underground, water helps keep magma molten. But as that magma rises and the pressure decreases, the water starts to bubble out.

“It’s like a fizzy drink,” Popa says. “We open the bottle, and — pssshht — all the gas comes out.” As water vapor escapes, the magma begins to crystallize and solidify. It eventually becomes so viscous that it can no longer rise, and it stalls before reaching the surface and erupting.

This magma behavior could help explain why volcanoes erupt when they do, says Kari Cooper, a geochemist at the University of California, Davis. Most magmas that enter Earth’s crust don’t erupt, Cooper says, so if other volcanoes exhibit similar behavior to Methana, it could support the idea that the water content of magma could, in part, determine when a volcano does or does not erupt.

Linking magma chemistry to the long-term life cycles of volcanic systems could also help researchers forecast volcanic hazards and decide which volcanoes to monitor more closely.

“Part of that calculation is how recently they have erupted,” says Adam Kent, who studies igneous rocks and volcanoes at Oregon State University in Corvallis but was not involved in the study. “In that sense, there are probably volcanoes out there that are threatening but not evaluated as such because they haven’t erupted for a while.”