A volcano that slept for more than 100,000 years before erupting is blowing up our understanding of when volcanoes should be defined as active or extinct.

Volcanologists officially classify sleeping volcanoes as extinct if they haven't erupted in the past 10,000 years or so, or in the past 11,700 years since the start of the current geological epoch, the Holocene.

And yet, an analysis of the active volcano Methana, near Athens, Greece, recently found that it once slept for nearly 110,000 years before bursting back to life. The finding could be the final straw for volcanologists, many of whom were already questioning the current definition of what makes a volcano extinct.

"The take-home message," said study co-author Răzvan-Gabriel Popa, a volcanologist at ETH Zürich in Switzerland, "is that extinct volcanoes around us might not really be extinct."

Changing the way we define whether volcanoes are extinct or active is important because the misclassification of volcanoes leaves people exposed to the risk of an unexpected eruption. And many of these sleeping giants should be monitored more closely, experts told Live Science.

A sleeping giant wakes up angry

In a study published April 22 in the journal Science Advances, Popa and his colleagues reconstructed the history of Methana by dating and analyzing the chemistry of zircon crystals in the rocks around the volcano, which had been ejected in various eruptions.

This revealed 31 eruptions over roughly 700,000 years of volcanic activity and a long period in which it lay quiet. During this time, the volcano was building up its magma reservoir, and afterward, it erupted energetically about 168,000 years ago. "We discovered that it was surprisingly able to restart after 110,000 years of inactivity and erupted quite intensely afterwards," Popa told Live Science. "What's important from my point of view is to start looking at other volcanoes that we currently consider extinct, because they might be going through growth periods. Maybe they're going to wake up in this catastrophic stage."

Methana isn't the only "extinct" volcano that has roused after a long period, Popa said.

Last year, Pablo González, a volcanologist at the Spanish National Research Council's Institute of Natural Products and Agrobiology, and his colleagues published a study showing that Taftan volcano in Iran had woken up and that its summit was rising, probably due to a buildup of gas pressure below the volcano's surface. It's not clear when it erupted last, but researchers estimate it was around 700,000 years ago.

Another volcano, Ciomadul in Romania, was thought to be extinct because it last erupted 30,000 years ago, but a 2019 study found magma still lurking beneath the volcano.

"It has a magma chamber which is brewing; it's very active," Popa said. "It just hasn't erupted. Ciomadul is definitely going through a growth stage, and maybe there are more such volcanoes."

Hayli Gubbi volcano, in the Afar region of Ethiopia, also erupted unexpectedly in 2025, after having lain dormant for an estimated 12,000 years. However, due in part to the volcano's remote setting, geologists are unsure of precisely how long ago it last erupted.

What is clear is that 10,000 years of silence is no guarantee a volcano is dead. "We have to look at a much longer time scale to be more certain about the level of activity of a volcanic area," González told Live Science.

Expanded monitoring

Fortunately, volcanoes don't usually erupt without warning —as long as scientists are monitoring them.

For example, González said, thousands of small eruptions heralded the 1991 eruption of Mount Pinatubo in the Philippines, which had been quiet for about 500 years, and thorough seismic monitoring helps scientists anticipate eruptions at Mount Etna in Italy, one of the world's most active volcanoes.

But obvious seismic activity ramps up mainly in advance of imminent threats.

Other geophysical signs help reveal which volcanoes are quietly building up their magma stores — and those need to be monitored too, experts told Live Science. For example, satellites can detect a growing magma chamber if they pick up bulging in the ground of just an inch a year, Popa said. Scientists can also use magnetotellurics, a method that leverages Earth's magnetic and electric fields to give CT-scan-like images that reveal how much rock is being melted to create more magma.

Such assessments should be extended to many volcanoes assumed to have a lower risk level, the experts said. "I think it's important that as much as possible we are monitoring all potentially active volcanoes for signs of subsurface unrest," Jenni Barclay, a volcanologist at the University of Bristol in the U.K., told Live Science. This could reveal magma that is actively melting and building up to potentially fuel future eruptions.

The monitoring of quiet volcanoes should first focus on volcanoes in relatively populous regions, including Eifel in western Germany and La Garrotxa in northeastern Spain, the experts said.

Some such volcanoes are already monitored. Yellowstone, which is considered dormant even though it last erupted 70,000 years ago, is watched closely because it sits on a hotspot of magma production and the high flux of heat means the rocks in the crust behave in a plastic way, deforming to create space for even more magma. This means Yellowstone is likely to erupt one day, Popa said, and it is unlikely to be a complete surprise.

"When a giant like Yellowstone wakes up, there will be lots of signs," he said

New classification system needed

The studies on reawakening volcanoes are leading ever more volcanologists to question the time-linked definition of an extinct volcano. "The definition is more or less an arbitrary date," Popa said. "It just means that we can find evidence of eruptions easily in the geological record and we have experienced them."

Inspired by the issue, volcanologists Luca Caricchi at the University of Geneva in Switzerland and Ben Kennedy at the University of Canterbury in New Zealand surveyed attendees at an international meeting of volcanologists in Geneva last year. Caricchi told Live Science that about 70 to 80 people there agreed that the 10,000-year definition was flawed, but consensus hasn't coalesced around a new definition.

"The big challenge that we have as volcanologists is that thinking about the state of the volcano really depends on the time scale that you're thinking of," Barclay said. "These gaps of 100,000 years are all very well, but most people are wanting to know if it is going to erupt during their lifetime, and given unrest, is it going to erupt over the next few days or weeks?"

All of the experts Live Science spoke to suggested that a new classification system should rely on measures of volcanic or magmatic activity, rather than on time passed. Some also don't like the term "dormant," which implies a volcano that has magma beneath it but isn’t active, because even if a volcano isn't erupting, an active magma chamber is never sleeping.

Classification should probably be done on a case-by-case basis by looking at the system to see whether the magma chamber is alive, Popa said, "because that's telling us if the volcano is really dead."

González agreed. As long as the conditions for magma to erupt exist below the surface, "that's a good indication that a volcanic area can have a future eruption," he said.

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