Scientific ocean drilling in the Santorini caldera reveals a surprisingly large eruption in historic times

Santorini is one of the best-studied volcanic archipelagos in the world. IODP Expedition 398 has now made a surprising discovery that solves a historical enigma. We found the products of an explosive eruption from 726 CE, the extent of which was previously only mentioned in historical writings.
Scientific ocean drilling in the Santorini caldera reveals a surprisingly large eruption in historic times
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Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera - Nature Geoscience

Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Kameni’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km3 of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas.

With its white houses with blue roofs and breathtaking views, Santorini is one of Europe's top tourist destinations. Two million tourists visit every year. At the center of the archipelago lies the active Kameni volcano, whose two islands Palea Kameni (Old Burnt Island) and Nea Kameni (New Burnt Island) are popular day-trip destinations. The story of these volcanic islands began after the devastating Minoan eruption some 3600 years ago, when the previous volcano erupted vast amounts of ash and pumice and eventually collapsed, creating the iconic caldera walls of Santorini.

Santorini caldera at sunset. The iconic white houses with blue roofs overlooking of the caldera are characteristic of Santorini. Not far from the buzzing tourist hubs lies the active Kameni Volcano, the peaks of which form the two islands of Nea Kameni and Palea Kameni that can be seen on the left in this photo. Photo: Jonas Preine.

This was not Santorini's first caldera collapse, as we now know of at least five such events within the last half million years. Like other large volcanic systems (e.g. Campi Flegrei in Italy or Taupo in New Zealand), Santorini goes through caldera cycles. After a very large caldera-forming eruption such as the Minoan eruption, the new cycle is thought to begin with small but frequent eruptions as the volcanic system is recharging. Subsequently, the volcanic system continues to mature and eruptions become larger but less frequent before the system is ready to produce another caldera-forming eruption. This typically occurs over time scales of a few tens of thousands of years.

Santorini is currently in a recharging phase. We know from historical records of nine eruptions that created the two Kameni islands. The last eruption took place in 1950. Eyewitness accounts tell us that these eruptions were spectacular to watch, but generally effusive or only mildly explosive. However, one event stands out. Historical writers noted this:

‘In the early summer of 726 [CE], the sea was seen to boil as if it had been heated by an incandescent furnace. Heavy steam rose from it, and the smoke became denser and denser until it was joined by incandescent pyroclastic ejecta. The jet was so hot that it seemed to be completely on fire. Large blocks of pumice were ejected in such quantity that they covered the surface of the sea over an immense area, and wind carried them to the coasts of Asia Minor and Macedonia.’

Fouque, 1879.

It has even been suggested that the sudden appearance of floating rocks on the surface of the Aegean Sea may have prompted Emperor Leo III of Constantinople, fearing divine displeasure, to impose iconoclasm, a ban on the display of religious symbols, which caused severe socio-economic instability in the Byzantine Empire.

But where are the deposits of this supposedly significant eruption? Onshore Palea Kameni, only a thin mantle of pumice and a small lava flow at the northwestern tip of the island have been associated with the eruption in 726 CE. No other products are observed that would hint at an eruption of the scale of that described. Were the accounts exaggerated? Were the historical writers confusing Santorini with another volcano?

Deciphering the history of the Kameni volcano was one of the many motivations that led some of our team to submit a proposal to the International Ocean Discovery Program (IODP) in 2015 to drill the ocean basins in and around Santorini. Only here could we hope to have a complete archive of the eruptions of Santorini and its neighbouring volcanoes. High-resolution reflection seismic data, recorded on the research vessel RV Poseidon in 2019, indicated that the basins within the Santorini Caldera contain sediments up to 300 m thick, but their origin remained unclear. At the end of 2022, we had the opportunity to sample these deposits by deep-drilling.

Research at sea on board the JOIDES Resolution. In December 2022, a team of international scientists, technicians and crew set sail on board the JOIDES Resolution for more than eight weeks to carry out scientific drilling in and around Santorini. Photos: Eric Bravo (upper left, right and lower left) and Acacia Clark (lower right).

IODP Expedition 398 set sail from Tarragona, Spain, aboard the iconic JOIDES Resolution drillship. On board was an international team of 30 interdisciplinary scientists, supported by 30 technical staff and 60 crew members. Together, we drilled at four locations in the basins of the caldera, the deepest hole reaching 120 m. Having found the uppermost layers in the caldera to be mainly smaller ash and some smaller lapilli intervals from different eruptions, we were surprised to find many meters of grey pumice and ash from what turned out to be a single eruption. Up to 40 meters thick in the cores, we were able to calculate a bulk volume of up to 2.7 km3 of this deposit using our dense grid of seismic data. These profiles showed that the thickness of this deposit increased towards the Kameni Volcano, and that the offshore continuity of the lava flows from the Kameni islands allowed no other conclusion than that what we had recovered were the deposits of the 726 CE eruption - effectively reconciling historical accounts with scientific evidence.

3D view into the Kameni volcano. The combination of ocean drilling, seismic reflection surveying and the digital elevation model of Santorini, allowed the lava flows of Nea Kameni and Palea Kameni to be traced offshore and correlated with the drilled lithologies. See the published paper for more details.

The fact that no deposits have been found on Santorini’s mainland suggests that the eruption was largely confined to the submarine realm, which is in line with historical accounts (“… the sea was seen to boil as if it had been heated by an incandescent furnace. Heavy steam rose from it…’). Towards the end of the eruption, the vent became subaerial, which explains the occurrence of a lava flow and a thin layer mantle on Palea Kameni. Chemical fingerprinting of a young ash layer in sediment cores around Santorini further indicates that the eruption produced an ash plume, increasing the eruption volume to up to 3.7 km3. This estimate does not include any pumice raft deposits, which are described as significant, reaching the coasts of Macedonia and the strait of the Dardanelles, more than 400 km away.

Our study indicates a magnitude of the Volcanic Explosivity Index of 5, of the same order as the 2022 Hunga Tonga–Hunga Ha‘apai eruption, but about 30 times smaller than the Minoan eruption of Santorini. While there is no indication that a similar eruption will occur in the near future, our new estimate of the size of the 726 CE eruption significantly exceeds previously considered worst-case eruption scenarios for the Kameni Volcano and needs to be taken into account in hazard assessments for Santorini.

Furthermore, our study shows that the lava flows of the subsequent eruptions of Nea Kameni lie on top of the unconsolidated deposits of the 726 CE eruption, forming a steep submarine cone around Kameni. From the 2018 sector collapse of Anak Krakatau in Indonesia , we know that intra-caldera volcanoes may become unstable, and strong lithological boundaries can act as potential detachments for flank collapses. This should be carefully monitored on Santorini.

If we were unaware of the deposits of such a large eruption from a volcano as well studied as Santorini, we must assume that our global eruption records have a significant blind spot for submarine explosive eruptions.

https://doi.org/10.1038/s41561-024-01392-7

The JOIDES Resolution in front of the caldera walls of Santorini. Only with the help of scientific ocean drilling can the history of marine volcanoes be fully reconstructed. Photo: Thomas Ronge.

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Volcanology
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geodynamics > Volcanology
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Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geophysics
Geomorphology
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