Greece: a volcano thought to be extinct has been accumulating magma for over 100,000 years

In the peaceful landscape of the Saronic Gulf, zircon crystals have recorded a magmatic life erased from the eyes of observers for tens of millennia.

In Methana, you'd expect the sound of the sea rather than deep geology. This Greek peninsula overlooks the Saronic Gulf, at the heart of the Hellenic volcanic system, and boasts an almost peaceful landscape: rocks, gentle slopes, thermal springs, remnants of ancient flows. According to the Global Volcanism Program, Methana last erupted in 258 BC, within the same geological complex as other, more famous Greek volcanoes.

For a very long time, however, the most interesting part of the Methana volcano lay elsewhere. Much deeper. The surface gave the impression of an almost extinct system: no flows, no columns of ash, no spectacular signs. Underneath, according to a new study published in Science Advances, magma continued to accumulate in deep chambers, even during a period of calm that lasted over 100,000 years.

To decipher such a long history, the researchers used tiny crystals: zircons. These minerals form in magmatic reservoirs as molten rock begins to cool. They have a precious quality: they preserve chemical traces and information about the time of their formation. A kind of natural archive, more patient than any human chronicle.

The research group has dated more than 1,250 zircon crystals spanning some 700,000 years of volcanic history. This work revealed a reality quite different from the apparent calm: Methana produced magma almost continuously. At certain periods, this magma reached the surface and triggered eruptions. During an extremely long interval, however, the volcano remained silent for over 100,000 years. Yet it was precisely during this period that zircon growth peaked: the sign of a still very active internal system.

The most disconcerting aspect of the volcano is to be found here, without the need for further explanation. The silence of the volcano, seen from the outside, looked like the end. Read from underground, it resembled a long phase of accumulation. In this case, the term "sleep" becomes less reassuring than it sounds. It describes a calm surface above a mechanism that continues to operate.

The magma, too rich in water, thickened during the ascent

The most intriguing part of the study concerns why this magma, although produced at depth, struggled to reach the surface. The answer lies in the water. The magma that fed Methana's upper chamber was much richer in water than expected, particularly during the resting phases. This water comes from subduction processes: a sinking plate carries with it oceanic sediments and fluids, modifying the mantle and making magma production more efficient.

It's almost a paradox, since a greater input from the depths should presage more eruptions. What's happening here is more viscous, slower and more obstinate. As the water-rich magma rises, it saturates, bubbles and crystallizes more readily. The number of crystals increases, the density increases and the material becomes less mobile. Magma thickens and slows down of its own accord. The physical and thermodynamic models used in the study indicate precisely this: part of the magma remains trapped in the crust, while the underground reservoir continues to grow.

The researchers associate this behavior with so-called "super-hydrated" magmas, whose water content is very high - over 6% by weight, according to the data linked to the study. A technical detail, to be sure, but one that changes the reading of many volcanoes located in subduction zones. Very "wet" magma can feed the system while, paradoxically, making immediate eruption more difficult.

The lesson of Methana also applies to other silent volcanoes

Methana thus becomes a textbook case that goes far beyond its own peninsula. The authors suggest that similar mechanisms could apply to other volcanoes linked to subduction, i.e. those systems where one tectonic plate sinks beneath another, fuelling magma production. In the statements relayed by ETH Zurich, the message is extended to the authorities responsible for assessing volcanic risk in regions such as Greece, Italy, Indonesia, the Philippines, Japan and the Americas.

Caution is still called for. The study does not predict an imminent eruption at Methana. It does not turn every silent volcano into a threat ready to explode. It says something more disturbing and more useful: a long absence of eruptions can lead to an incomplete perception of risk. Some systems receive little attention precisely because they have been silent for thousands or tens of thousands of years. The underground may have a very different memory from that of the surface.

That's why modern monitoring systems are essential. Earthquakes, ground deformation, gas emissions and high-resolution geophysical imaging can reveal movements and accumulations before they become visible to the naked eye. These are less spectacular tools than a lava flow, but far more useful when the problem lies below the level of the landscape.

Above all, the Methana volcano teaches us to be wary of too much orderly calm. A motionless slope, a silent coastline, an ancient rock exposed to the Greek sun may only tell half the story. The other half remains in the crust, between tiny crystals and a still, waiting magma. The surface was silent. The zircon was taking notes.

Source : ETH Zurich