Greece’s Forgotten Volcano Is Four Million Years Older Than Anyone Thought

Twenty kilometres southwest of the postcard-perfect caldera of Santorini lies a cluster of three tiny, uninhabited islands that most tourists sail past without a second glance. Christiani, Askania and Eschati are the weathered remnants of a once-mighty volcano, and according to new research they hide a secret that rewrites the volcanic history of the entire Aegean Sea. A team of Dutch and Greek geoscientists has now dated the lavas of the Christiana volcano directly for the first time, and the results are startling: the volcano erupted around 2.6 million years ago, roughly four million years earlier than scientists had long assumed. The finding, published in the journal Geochronology, pushes back the birth of the famous Christiana-Santorini-Kolumbo volcanic field by millions of years and reveals that the whole South Aegean Volcanic Arc was already ablaze three million years ago.

For decades, the Christiana Archipelago has been the neglected sibling of the volcanic field that contains Santorini, one of the most intensively studied volcanoes on the planet. Only two scientific papers on the islands had appeared in the past half century. Because the chemistry of Christiana’s lavas resembles the oldest exposed rocks on Santorini’s Akrotiri peninsula, dated to about 600,000 years ago, researchers simply assumed the two were contemporaneous. Other studies took a different route, correlating submarine volcanic deposits with Pliocene sediment layers in seismic reflection profiles, which suggested ages of roughly 1.6 to 1.7 million years. Without isotopic ages from the rocks themselves, the true age of the archipelago remained an open question.

To settle the matter, Pieter Z. Vroon of VU University Amsterdam and colleagues collected twenty-two rock samples weighing one to three kilograms each from sixteen outcrops along the coasts and higher ground of Christiani island. Weathered rinds were sliced away with diamond saws, and after careful petrographic screening twelve samples had to be discarded because their groundmass had been too heavily altered by time and seawater. The remaining ten samples were crushed, sieved and purified using heavy-liquid centrifuge separation and magnetic sorting until only fresh, inclusion-free groundmass grains remained, ready for one of the most precise dating techniques in geology.

The method of choice was argon-argon dating, a refinement of the classic potassium-argon clock. Volcanic rocks contain potassium-40, which decays slowly into argon-40, a gas that becomes trapped in the rock’s crystal lattice the moment the lava cools. In the argon-argon variant, samples are bombarded with neutrons in a reactor, converting potassium-39 into argon-39 so that both parent and daughter isotopes can be measured from the same grain. The team irradiated their samples at Oregon State University’s TRIGA reactor alongside the Fish Canyon Tuff sanidine standard, then step-heated them with a carbon dioxide laser through ten to eighteen incremental temperature steps, analysing the released gas on a high-sensitivity multi-collector mass spectrometer.

The ages that emerged were remarkably consistent. Five reliable samples from the island’s Upper Lava formation clustered tightly between 2.57 and 2.69 million years old, with analytical uncertainties of as little as 20,000 to 30,000 years, and a weighted mean of 2.63 plus or minus 0.05 million years. Inverse isochron calculations, which correct for any trapped atmospheric or excess argon, agreed with the plateau ages within error. One sample gave a slightly older and less precise age of about 2.23 million years, possibly reflecting minor argon loss, while three samples were rejected outright on statistical grounds following established quality-control protocols for argon-argon data.

Then came the surprise. A single obsidian sample collected from the island’s east coast yielded an age of just 133,000 plus or minus 14,000 years, a million times younger than its neighbours. The researchers concluded that this glassy clast, a lithic fragment embedded in a pyroclastic deposit, almost certainly did not erupt on Christiana at all. The volcano has likely been extinct for some 1.5 million years. Instead, the obsidian was probably ripped from older rock, completely reset by the searing heat of a passing pyroclastic flow, and carried to Christiani by one of Santorini’s great Plinian eruptions. Its age overlaps with the Middle Pumice eruption of approximately 145,000 years ago, one of at least eleven large explosive events that Santorini produced between 350,000 years ago and the famous Late Bronze Age catastrophe.

The new dates transform our picture of how the Christiana-Santorini-Kolumbo volcanic field evolved. The Christiana volcano, a conical edifice rising some 500 metres from the seafloor with an estimated minimum volume of 25 cubic kilometres, was built during a period when the regional stress field was stretching the crust in a north-northeast to south-southwest direction. When that stress field rotated toward a northwest-southeast orientation between roughly 2.5 and 1.5 million years ago, Christiana died. What followed was an extraordinarily long volcanic silence of more than a million years, until the submarine Poseidon volcano and the Kolumbo seamount chain, along with the ancestors of modern Santorini, finally reawakened the rift around 1.2 million years ago.

Perhaps the most consequential implication reaches far beyond one forgotten island. Comparing the new ages with published data from the other four volcanic fields of the South Aegean Volcanic Arc, from Sousaki and Aegina-Poros-Methana in the west to Milos and Kos-Nisyros-Yali in the east, the authors show that all five fields were simultaneously active around 3 million years ago. This synchronised ignition coincides with the moment that subducted oceanic crust of the African plate first reached a depth of about 100 kilometres beneath the arc, the depth at which dehydration of the slab triggers melting in the overlying mantle wedge. Before that, the subduction zone had been grinding down continental material for tens of millions of years without producing the modern arc volcanoes.

The study also reveals a striking arc-wide pattern of volcanic famine. Between roughly 2 and 1 million years ago, every volcanic field along the arc experienced quiescence or sharply reduced output, even as the orientation of the tectonic stress field was changing in each of them. The authors argue that this synchronised lull reflects the difficulty parental magmas faced in reaching the surface while the stress regime was reorganising, rather than any fundamental change in the subduction system itself. Where volcanoes erupt, and when they fall silent, appears to be dictated by the local architecture of faults and extension, a reminder that even the largest volcanic systems are prisoners of the crust above them.

For a volcanic field that includes one of the most notorious caldera-formers in human history, pinning down its deep past is more than an academic exercise. The work demonstrates that the Christiana-Santorini-Kolumbo rift has hosted at least two distinct volcanic eras separated by a million-year gap, and that the tectonic switch between them set the stage for everything from the giant 89-cubic-kilometre Archaeos Tuff of 520,000 years ago to the Minoan eruption that may have helped end Minoan civilisation. The three lonely islands that started it all, meanwhile, now hold a secure place in the story: not young cousins of Santorini, but the ancient founding fathers of one of the Mediterranean’s most dangerous volcanic neighbourhoods.

Subject of Research: Argon-argon geochronology of the Christiana volcano in the South Aegean Volcanic Arc, Greece

Article Title: 40Ar ∕ 39Ar constraints on the eruption history of the Christiana Volcano of the Christiana-Santorini-Kolumbo volcanic field, Greece

Article References: Vroon, P. Z., Beemster, T., Zhou, X., Nomikou, P., Klaver, M., Wijbrans, J. R., & Kuiper, K. F. (2026). 40 Ar ∕ 39 Ar constraints on the eruption history of the Christiana Volcano of the Christiana-Santorini-Kolumbo volcanic field, Greece. Geochronology, 8(2), 279-295. https://doi.org/10.5194/gchron-8-279-2026

Image Credits: AI Generated

DOI: 10.5194/gchron-8-279-2026

Keywords: Christiana volcano, Santorini, argon-argon dating, geochronology, South Aegean Volcanic Arc, volcanology, Kolumbo, pyroclastic deposits, subduction, Aegean Sea, obsidian, tectonics