Santorini caldera

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Santorini Caldera
Santorini island group from space
Highest point
Coordinates36°23′44″N 25°27′33″E / 36.39556°N 25.45917°E / 36.39556; 25.45917
Geography
Map
LocationAegean Sea, Greece
Geology
Mountain typeCaldera (active)
Last eruptionJanuary to February 1950

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 (known as Thera in antiquity), the main island, Therasia and Aspronisi at the periphery, and the Kameni islands at the center. It has been designated a Decade Volcano.

Geography

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Photograph of Santorini caldera from the air.

The caldera measures about 12 by 7 km (7.5 by 4.3 mi), with 300 m (980 ft) high steep cliffs on three sides, and a maximum depth of 385 m (1,263 ft).[1]

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 75.8 km2 (29.3 sq mi), Therasia 9.3 km2 (3.6 sq mi), and the uninhabited islands of Nea Kameni 3.4 km2 (1.3 sq mi), Palea Kameni 0.5 km2 (0.19 sq mi) and Aspronisi 0.1 km2 (0.039 sq mi).

Santorini's high walls, draped by whitewashed villages, combined with a sunny climate and good observation conditions, have made it a magnet for volcanologists,[2] as well as a highlight of tourism in the Aegean.

Geology

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The South Aegean Volcanic Arc includes the volcanoes of Methana, Milos, Santorini and Nisyros.

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.[3]

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.[4]

The Kameni islands at the center of the caldera are made of lava rocks.

Volcanology

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The caldera wall of Santorini island.

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.[5] 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.[6]

Palea Kameni and Nea Kameni were formed as a result of multiple, initially submarine, smaller eruptions at the center of the caldera.[7]

Eruptive history

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Following is a list of the major eruptive events of Santorini beginning with the catastrophic Minoan eruption, as noted by the Smithsonian National Museum of Natural History's Global Volcanism Program:[6]

Start date Stop date Characteristics of eruption
1610 BC ± 14 years
("Minoan eruption") 		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 (excentric) 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.

The huge Minoan eruption of Santorini in the 17th century BC may have inspired the legend of Atlantis.[8] It was rated 7 in the Smithsonian Global Volcanism Program's Volcanic Explosivity Index.[9]

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.[2] GPS instruments registered renewed deformation around the caldera in 2011 and 2012.[10]

Current unrest

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Displacement time series of GNSS station; Santorini 2024-2025

In July 2024, Santorini entered a new phase of volcanic unrest. Starting in mid-summer of 2024, seismic activity increased significantly. The most intense volcano-tectonic seismic sequence occurred in the broader Anydros islet area, ∼30 km to the northeast of Santorini since late January 2025. However, low-seismicity intra-caldera unrest since early July 2024 also occurred.[11]

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.[12] 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.[13][14][15][16]

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 (b-value), and spatiotemporal clustering. They identify four stages:

  • (1) an initial, volcano-driven phase beginning in summer 2024 with slightly accelerating moment release and growing focus toward the Amorgos area
  • (2) A progressive onset in January 2025 marked by stronger clustering, a nearly steady b-value, and rapidly increasing magnitude variability was captured.
  • (3) A short, intense and chaotic episode in early February 2025, when those metrics signal peak irregularity
  • (4) a post-crisis interval in which the statistics reorganize toward a more regular, tectonic-like pattern with aftershock-style behavior.

The study argues that these quantitative precursory and evolutionary patterns are operationally useful for observatories: tracking shifts in clustering, b-value stability (consistency of the b-value over time and space, which is not always stable and is influenced by factors like stress, fluids, and geological structure.), and variability/entropy can help flag when a Santorini–Amorgos-type crisis is tipping from a fluids-affected swarm into a fault-controlled one, with implications for short-term hazard communication during future unrest in the area.[17]

On June 15, 2025, a new seismic swarm was detected under Santorini. These earthquakes where 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.[18]

IUGS geological heritage site

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In respect of it being 'one of the largest calderas in the Mediterranean Sea formed by Plinian eruptions in a volcanic arc tectonic framework', the International Union of Geological Sciences (IUGS) included 'The Quaternary Santorini Caldera' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.'[19]

References

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  1. ^ Preine, Jonas; Karstens, Jens; Hübscher, Christian; Druitt, Tim; Kutterolf, Steffen; Nomikou, Paraskevi; Manga, Michael; Gertisser, Ralf; Pank, Katharina; Beethe, Sarah; Berthod, Carole; Crutchley, Gareth; McIntosh, Iona; Ronge, Thomas; Tominaga, Masako (2024). "Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera". Nature Geoscience. 17 (4): 324. Bibcode:2024NatGe..17..323P. doi:10.1038/s41561-024-01392-7. ISSN 1752-0894. It separates the caldera into a northern and a southern basin, which have maximum depths of 385 m and 290 m, respectively.
  2. ^ a b "Introduction – Santorini Volcano". Retrieved 19 April 2011.
  3. ^ "Tectonic setting of Santorini". Retrieved 20 April 2011.
  4. ^ "Geology of Santorini – The prevolcanic basement". Retrieved 20 April 2011.
  5. ^ Preine, J.; Karstens, J.; Hübscher, C.; Nomikou, P.; Schmid, F.; Crutchley, G.J.; Druitt, T.H.; Papanikolaou, D. (2022). "Spatio-temporal evolution of the Christiana-Santorini-Kolumbo volcanic field, Aegean Sea". Geology. 50 (1): 96–100. Bibcode:2022Geo....50...96P. doi:10.1130/G49167.1.
  6. ^ a b "Santorini". Global Volcanism Program. Smithsonian Institution. Retrieved 29 June 2021.
  7. ^ "Santorini Volcanic Caldera, Greece". Retrieved 19 April 2011.
  8. ^ Sparks, R. S. J. (2000). "Friedrich, W. L. 2000. Fire in the Sea. The Santorini Volcano: Natural History and the Legend of Atlantis". Geological Magazine. 137 (5). Cambridge University Press: 593. doi:10.1017/s0016756800224618. S2CID 128708746.
  9. ^ "Large Volcano Explocivity Index". Countries of the World. Archived from the original on 30 September 2020. Retrieved 17 April 2021.
  10. ^ NBC News "Volcano on scenic Greek island getting a little restless 3 March 2012[dead link]
  11. ^ Papazachos, C.; Foumelis, M.; Bitharis, S.; Pikridas, Ch.; Trasatti, E.; Papageorgiou, E.; Ventouzi, Ch.; Kkallas, Ch.; Vougioukalakis, G.; Vaselli, O. (16 July 2025). "The Santorini 2024–2025 Volcano-Tectonic Sequence: Constraining the Initial Phase of the Intra-Caldera Unrest". Geophysical Research Letters. 52 (13) e2025GL115856. Earth and Planetary Science Letters. Bibcode:2025GeoRL..5215856P. doi:10.1029/2025GL115856.
  12. ^ Briole, Pierre; Ganas, Athanassios; Serpetsidaki, Anna; Beauducel, François; Sakkas, Vassilis; Tsironi, Varvara; Elias, Panagiotis (11 July 2025). "Volcano-tectonic interaction at Santorini. The crisis of February 2025. Constraints from geodesy". Geophysical Journal International. 242 (3) ggaf262. doi:10.1093/gji/ggaf262. Retrieved 10 November 2025.
  13. ^ Papanikolaou, Nikos; Stallard, Esme (5 February 2025). "Thousands evacuate Santorini after earthquakes shake Greek island". www.bbc.com. BBC. Retrieved 10 November 2025.
  14. ^ "Evacuation of Greece's Santorini island continues as tremors persist". NBC News. Reuters. 6 February 2025. Retrieved 10 November 2025.
  15. ^ Smith, Helena (6 February 2025). "State of emergency declared on Santorini after earthquakes shake island". The Guardian. The Guardian. Retrieved 10 November 2025.
  16. ^ BECATOROS, ELENA (11 February 2025). "Scientists say several thousand earthquakes detected near Greece's island of Santorini". AP News. Associated Press. Retrieved 10 November 2025.
  17. ^ Zaccagnino, Davide; Michas, Georgios; Telesca, Luciano; Vallianatos, Filippos (2025). "Precursory patterns, evolution and physical interpretation of the 2025 Santorini–Amorgos seismic sequence". Earth and Planetary Science Letters. 671 119656. Bibcode:2025E&PSL.67119656Z. doi:10.1016/j.epsl.2025.119656. Retrieved 9 November 2025.
  18. ^ "Santorini volcano (Greece) - seismic crisis Feb 2025 updates and information". Volcano Discovery. 15 June 2025. Retrieved 10 November 2025.
  19. ^ "The First 100 IUGS Geological Heritage Sites" (PDF). IUGS International Commission on Geoheritage. IUGS. Retrieved 13 November 2022.