The town of Oia on a clifftop of the Santorini caldera, Greece taken on February 11 2025 during the earthquake swarm. Credit: Savvas/Karmaniolas NurPhoto via Getty Images
Earlier this year, more than 28,000 earthquakes rocked the Greek island of Santorini in a crisis which lasted from late January to March.
Now, geological analysis reveals the swarm was caused by about 300 million cubic metres of magma, which rose from about 18km deep in the crust to a shallow reservoir about 4km below the seafloor.
Dr Marius Isken, a geophysicist at Germany’s GFZ Helmholtz Centre for Geosciences and one of the lead authors of the study, says the seismic activity was typical of magma ascending through the Earth’s crust.
“The migrating magma breaks the rock and forms pathways, which causes intense earthquake activity. Our analysis enabled us to trace the path and dynamics of the magma ascent with a high degree of accuracy.”
Map showing surface displacement between (a) 23 February and 3 March 2025 and (b) July 2024 and January 2025. It locates the deflating mid-crustal magma reservoir (blue circle) beneath Kolumbo at 7.6km depth and the vertical inflating magma dike (red line) which aligns with seismic activity (black dots). The volume increase in the dike is 0.313 cubic km. Credit Isken et al 2025, Nature
Isken and collaborators combined data from earthquake stations on Santorini and nearby islands, and ocean bottom instruments at the Kolumbo underwater volcano 7km away, to reconstruct the magma migration before and during the swarm.
They found magma initially rose into a shallow reservoir beneath the island in July 2024, uplifting the land by just a few centimetres. The source of the magma was the mid-crust reservoir beneath the neighbouring Kolumbo, indicating a previously unknown link between the volcanoes.
Evolution of the 2025 Santorini crisis showing (a) the preparatory phase from June 2024 to January 2025 and (b) the volcano-tectonic crisis from 27 January 2025. The influx of magma from a mid-crustal magma reservoir into the shallow magma reservoir beneath Santorini resulted in surface uplift and increased seismicity at the caldera. Credit Isken et al 2025, Nature
“Although the highly explosive volcanoes of Santorini and Kolumbo in the Greek Aegean Sea are just 7 km apart, their potentially coupled deep magmatic feeding systems are only poorly understood,” the authors write.
“The 2025 volcano–tectonic crisis of Santorini simultaneously affected both volcanic centres, providing insights into a complex, multistorage feeder system.”
Dr Jens Karstens, marine geophysicist at the Helmholtz Centre for Ocean Research Kiel (GEOMAR) and co-lead author of the study, adds: “We were able to follow the development of the seismic crisis in near real time and even learn something about the interaction between the 2 volcanoes. This will help us to improve the monitoring of both volcanoes in the future.”
The research is published in the journal Nature.