Santorini caldera
Santorini caldera is a large, mostly submerged caldera, located in the southern Aegean Sea, 120 kilometers north of Crete in Greece. Visible above water is the circular Santorini island group, consisting of Santorini, the main island, Therasia and Aspronisi at the periphery, and the Kameni islands at the center. It has been designated a Decade Volcano.
Geography
The caldera measures about, with high steep cliffs on three sides, and a maximum depth of.There are two small volcanic islands at the center of the caldera, Nea ("New") Kameni and Palea ("Old") Kameni.
The main island, Santorini has an area of, Therasia, and the uninhabited islands of Nea Kameni, Palea Kameni and Aspronisi.
Santorini's high walls, draped by whitewashed villages, combined with a sunny climate and good observation conditions, have made it a magnet for volcanologists, as well as a highlight of tourism in the Aegean.
Geology
The volcanic complex of Santorini is the most active part of the South Aegean Volcanic Arc, which includes the active volcanoes of Methana on the mainland of Greece, Milos, Santorini and Nisyros. It is formed by the subduction of the African tectonic plate underneath the Aegean subplate of the Eurasian tectonic plate, which occurs at a rate of up to 5 cm per year in a northeasterly direction. This subduction causes earthquakes at depths of 150–170 km.Non-volcanic rocks are exposed on Santorini at Mikro Profititis Ilias, Mesa Vouno, the Gavrillos ridge, Pirgos, Monolithos and the inner side of the caldera wall between Cape Plaka and Athinios.
The Kameni islands at the center of the caldera are made of lava rocks.
Volcanology
The caldera of Santorini lies in the center of the Christiana-Santorini-Kolumbo volcanic field, which comprises the extinct Christiana Volcano, the Santorini Caldera, the polygenetic submarine Kolumbo Volcano, as well as the Kolumbo Volcanic Chain. This volcanic lineament evolved during four main phases of volcanic activity, which initiated in the Pliocene from several local centers that only recently matured to form the vast Santorini edifice. The present-day caldera is composed of overlapping shield volcanoes, cut by at least four partially overlapping calderas, of which the oldest southern caldera was formed about 180,000 years ago. The subsequent Skaros caldera was created about 70,000 years ago, and the Cape Riva caldera about 21,000 years ago. The current caldera was formed about 3,600 years ago during the Minoan eruption.Palea Kameni and Nea Kameni were formed as a result of multiple, initially submarine, smaller eruptions at the center of the caldera.
Eruptive history
Following is a list of the major eruptive events of Santorini beginning with the catastrophic Minoan eruption, most of them listed by the Smithsonian National Museum of Natural History's Global Volcanism Program:| Start date | Stop date | Characteristics of eruption |
| 518,000 BC | Unknown | Central vent eruption, regional fissure eruption, submarine eruption, explosive eruption, pyroclastic flows and pyroclastic surges, phreatic explosions, extensive physical damage, massive lava fountains and extensive lava flows, megatsunamis, caldera collapse. |
| 1610 BC ± 14 years | Unknown | Central vent eruption, regional fissure eruption, submarine eruption, explosive eruption, pyroclastic flows and pyroclastic surges, phreatic explosions, extensive physical damage, massive lava fountains and extensive lava flows, minor mudflows on the sea, megatsunamis, caldera collapse, fatalities, mass evacuation. |
| 197 BC | Unknown | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption. |
| 31 December 46 AD | 1 February 47 AD ± 30 days | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, lava dome extrusion, tsunami. |
| 15 July 726 AD ± 45 days | Unknown | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, lava dome extrusion, damage. |
| 1570 | 1573 | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, lava dome extrusion. |
| 27 September 1650 | 6 December 1650 | Flank vent, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, fatalities, damage, tsunami. |
| 23 May 1707 | 14 September 1711 | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, lava dome extrusion, damage. |
| 26 January 1866 | 15 October 1870 | Central vent eruption, regional fissure eruption, submarine eruption, new island formation, explosive eruption, lava flows, lava dome extrusion, fatalities, damage, evacuation. |
| 11 August 1925 | 17 March 1928 | Central vent eruption, regional fissure eruption, explosive eruption, phreatic explosions, lava flows, lava dome extrusion. |
| 20 August 1939 | 2 July 1941 ± 1 day | Central vent eruption, regional fissure eruption, submarine eruption, explosive eruption, phreatic explosions, lava flows, lava dome extrusion, damage. |
| 10 January 1950 | 2 February 1950 | Central vent eruption, regional fissure eruption, submarine eruption, explosive eruption, phreatic explosions, lava flows, lava dome extrusion. |
In 2024, researchers identified a massive, ancient eruption of Santorini that took place roughly 520,000 years ago, which dwarfed the 1,600 BC Minoan eruption by comparison and was the largest known eruption uncovered in geologic records. The ancient eruption produced pyroclastic flows 10 times larger than those of the 2022 Hunga Tonga–Hunga Haʻapai eruption, and the eruption breached the sea surface and covered three nearby islands in volcanic deposits. The eruption also covered the seafloor with pyroclastic flows from the volcanic vent, with pyroclastic deposits up to thick and in volume, which is more than six times bigger than the pyroclastic deposits of the Minoan eruption.
The huge Minoan eruption of Santorini in the 17th century BC may have inspired the legend of Atlantis. It was rated 7 in the Smithsonian Global Volcanism Program's Volcanic Explosivity Index.
Although dormant, Santorini is an active volcano. Numerous minor and medium-sized, mainly effusive, eruptions have built the dark-colored lava shields of Nea and Palea Kameni inside the caldera.
Their last eruption was in 1950, and now only fumarolic activity, primarily inside the recently active craters, takes place. GPS instruments registered renewed deformation around the caldera in 2011 and 2012.
Recent unrest
In July 2024, Santorini entered a new phase of volcanic unrest. Starting in mid-summer 2024, seismic activity increased significantly. The most intense volcano-tectonic seismic sequence occurred in the broader Anydros islet area, ~ to the northeast of Santorini since late January 2025. However, low-seismicity intra-caldera unrest since early July 2024 also occurred.Starting in the summer of 2024, slow deformation was recorded inside the Santorini Caldera. From 27 January – 24 February 2025, rapid inflation was observed with up to 7 mm/d moving vertically and westward. GNSS station and satellite InSar measurements also confirmed uplift was ongoing. The intense seismic activity led to the evacuation of 11,000 people from the island. While authorities closed schools and warned against large indoor gatherings, Prime Minister Kyriakos Mitsotakis urged residents to remain calm.
Another study interpreted the unrest as a time-evolving system that transitioned from a fluid-influenced swarm into a more regular tectonic sequence. The study used a manually revised public catalog, anomaly detection, and statistical modeling of time-dependent seismicity quantify diagnostic changes in event rate, magnitude–frequency behavior, and spatiotemporal clustering. They identify four stages:
- an initial, volcano-driven phase beginning in summer 2024 with slightly accelerating moment release and growing focus toward the Amorgos area
- A progressive onset in January 2025 marked by stronger clustering, a nearly steady b-value, and rapidly increasing magnitude variability was captured.
- A short, intense and chaotic episode in early February 2025, when those metrics signal peak irregularity
- a post-crisis interval in which the statistics reorganize toward a more regular, tectonic-like pattern with aftershock-style behavior.
On June 15, 2025, a new seismic swarm was detected under Santorini. These earthquakes were small and a continuation of the seismic crisis that began in the summer of 2024. The earthquakes were mostly smaller than magnitude 3, and clustered at depths between 5-20 km along the SW-NE oriented tectonic Kameni line, which has been the main source of magma ascent at the volcano over the past few hundred thousand years. The earthquake swarm was triggered by the movement of magma through a fault system to the northeast of Santorini. However, the earthquakes ended about three months after they began, and the magma remained more than underground.