Geologists like categories.
They have a category for the volcanoes that form where two tectonic plates pull apart — the mid-ocean ridges, the East African Rift. They have a category for the volcanoes that form where one plate sinks beneath another — Mount Fuji, the Cascade Range, most of the Ring of Fire. And they have a category for the volcanoes that form in the middle of tectonic plates, above superheated pockets of mantle called hotspots — Hawaii, the Canary Islands, Yellowstone.
Between them, these three categories account for essentially every volcano on Earth.
Except one.
Mount Etna, sitting on the eastern coast of Sicily, three thousand metres tall and half a million years old, fits none of them. It sits above a subduction zone, but its lava has the chemical signature of a hotspot volcano. There is no hotspot beneath it. There has never been one. Geologists have known this for decades and have not, until very recently, been able to explain what is actually going on.
The specific mismatch
Etna’s location is not what should produce Etna’s lava.
Subduction-zone volcanoes have a specific chemistry. When an oceanic plate sinks beneath a continental one, water carried down inside the descending slab lowers the melting point of the surrounding mantle rock, producing magma with a particular composition. This magma is generally rich in water, gas-heavy, and prone to explosive eruptions. Mount Fuji is a subduction-zone volcano. So is Mount St Helens.
Hotspot volcanoes produce a chemically different lava. The magma comes from mantle plumes — narrow columns of unusually hot material rising from deep within the Earth, sometimes from the boundary with the outer core. Hotspot lava tends to be less water-rich, more effusive, and characterised by a specific signature of trace elements including certain ratios of iron, titanium, and rare earth elements. Hawaii’s eruptions produce hotspot lava. So do the eruptions of La Réunion in the Indian Ocean.
Etna is located above the collision between the African and Eurasian plates. The African plate is currently sinking beneath the Eurasian one in the Ionian Sea just east of Sicily. Etna should, by every ordinary rule of plate tectonics, be producing subduction-zone lava.
It doesn’t. Etna’s lava has the geochemical fingerprint of a hotspot volcano — high sodium, alkaline composition, specific trace element ratios — and this signature has remained stable for the entire 500,000 years of Etna’s recorded eruptive history.
The mystery is not why Etna erupts. The mystery is why Etna erupts what it does.
What a century of failed explanations looked like
The list of proposed solutions to the Etna problem, over the past several decades of geological research, is genuinely long.
Some scientists have proposed a small hidden mantle plume — a hotspot beneath the region that seismic imaging hasn’t yet detected. Others have proposed that mantle flow around the edge of the descending African plate is producing decompression melting in the upper mantle beneath Sicily. This was the essence of a 2010 model by the geophysicist Wouter Schellart, which offered a plausible mechanism but did not fully explain Etna’s specific chemistry.
Others have suggested that Etna’s magma comes from an unusually old and stable pocket of material in the upper mantle, unrelated to the current subduction geometry. Others still have argued that Etna is a genuine fourth type of volcano, requiring a new geological category to describe.
Each of these hypotheses had some support and some problems. None of them fully accounted for both Etna’s location (subduction) and its chemistry (hotspot-like). The volcano remained, quietly, one of the most-studied and least-understood in the world.
Then, in April 2026, a team of scientists from the University of Lausanne and the Istituto Nazionale di Geofisica e Vulcanologia in Catania published a paper that may have finally resolved it.
The petit-spot hypothesis
Led by Sebastien Pilet at the University of Lausanne, with Anna Rosa Corsaro of INGV Catania as a collaborator, the team published a study in the Journal of Geophysical Research: Solid Earth proposing that Etna belongs to a rare fourth category of volcano called petit-spot.
Petit-spot volcanoes were first identified in 2006 by Japanese geologists studying small submarine volcanic features on the Pacific seafloor east of Japan. Unlike the three standard volcano types, petit-spot volcanoes are not fed by freshly-generated magma at any of the usual sources. They are fed by small pockets of pre-existing magma that already exist in the upper mantle, stored at depths of around 80 kilometres beneath the surface, kept in place for very long periods before being pushed upward through fractures in the overlying tectonic plate.
The mechanism is unusual. Most volcanoes have a specific source of new magma generation, whether it’s a subducting slab, a hot spot, or a spreading centre. Petit-spot volcanoes don’t. Instead, they draw from a reservoir of ancient melt already lodged in the upper mantle — melt that has been sitting there, undisturbed, sometimes for very long periods before being extracted by tectonic stress.
The Pilet team’s proposal is that Etna is a petit-spot volcano on land. The pockets of ancient magma beneath the region are being squeezed upward by the specific mechanical stresses of the African-Eurasian plate collision. As the plate bends and fractures near the subduction zone, cracks open in the overlying crust, allowing the stored magma to escape upward through them. The researchers described the process as similar to “liquid being squeezed from a sponge.”
This mechanism would explain both puzzles at once. It explains the hotspot-like chemistry: the magma is old, mantle-derived, and geochemically similar to the material that feeds hotspot volcanoes elsewhere. It explains the volcano’s stable chemical signature over 500,000 years: if the magma has been sitting in the mantle for a very long time, its composition would not change with variations in current tectonic conditions. And it explains why standard subduction-zone chemistry is absent: the water-driven melting that feeds normal subduction volcanoes is not what’s producing Etna’s magma.
What this means for the science
If the Pilet team is correct, Etna is not a puzzle. It is the first identified terrestrial member of an entirely separate class of volcano.
Petit-spot volcanism was only recognised twenty years ago. All previous examples were small submarine features on the Pacific seafloor. If Etna belongs to the same category, it substantially expands the physical scale at which the mechanism operates. A submarine petit-spot volcano is typically a small feature, a few hundred metres tall at most. Etna is a stratovolcano more than three kilometres high, erupting continuously for half a million years. The petit-spot process, on this account, can produce full-scale continental volcanic systems given the right tectonic setting.
That is a substantial revision to volcanic geology. It suggests other unexplained volcanoes around the world — several of which have similar chemistry-location mismatches — may belong to the same fourth category. It also suggests that the standard three-category model of volcano formation, taught in every introductory geology textbook, needs a fourth line.
Etna itself, meanwhile, will continue erupting.
The volcano has been active for at least 500,000 years, has almost certainly been active on some level throughout most of recorded human history, and shows every sign of continuing. The people of eastern Sicily have lived on its slopes for thousands of years. They have watched it, learned to predict it as well as anyone can predict a volcano, built cities and vineyards and roads on the ash-rich soil at its base.
What no one has been able to fully explain, until now, is what has been feeding it all this time. If Pilet and his colleagues are right, the answer has been sitting quietly in the upper mantle since long before there was anyone above it wondering.
