Volcanoes in marine settings of island arcs – chains of volcanoes situated above subduction zones – are a major source of natural hazards. Pyroclastic currents (fast moving currents of hot volcanic rock and gas) from explosive eruptions pour into the sea, generating tsunamis that impact coastal communities and infrastructures. While some eruptions occur subaerially, others take place from submarine caldera vents, so the eruptive products are largely confined to the sea floor. Consequently, such eruptions are not recognized in studies of the onland eruptive record. An example was the eruption of Hunga Tonga Hunga Ha’apai Volcano in the Pacific in January 2022, which was one of the most powerful eruptions on historical record but which generated deposits confined to the sea bed.
The Christiana-Santorini-Kolumbo Volcanic Field (CSKVF) lies on the Aegean island arc that stretches between Greece and Turkey in the heart of continental Europe. It is one of the most explosive island arc volcanic fields in the world, and was the site of the iconic Minoan eruption of the Late Bronze Age. Over the last three decades, some of our research team have studied the onland eruptive record of the CSKVF in detail, piecing together a high-resolution chronology of eruptions since 0.3 My ago. However, the eruption history older than that remains sketchy, with very little information concerning its record prior to emergence. We know that there was an ancestral submarine volcano called Akrotiri, which later transitioned into the modern volcanic system. But were there explosive eruptions in the distant past? How large were these eruptions, and could they serve an analogues for present-day submarine caldera volcanoes on island arcs elsewhere?
In 2015, some of our team conceived the idea of using the marine rift basins around Santorini as archives of past eruptions. When the volcano erupts, pyroclastic currents flow into the basins where they form a layer in the marine sediments, as does falling pumice and ash. The volcano-sedimentary fills of the basins, in places over a 1 km in thickness, therefore offer a continuous record of eruptions of the whole CSKVF since several million years ago. The proposal was submitted successfully to the International Ocean Discovery Program (IODP) deep-sea drilling program. From 11 December 2022 to 10 February 2023, some 30 international scientists, ably supported by 30 technical personnel and 60 crew, drilled the marine basins on the JOIDES Resolution, the workhorse ship of IODP. As expected, we penetrated the eruptive deposits of the last 0.3 My, as well as the deposits of many unknown smaller eruptions from that period. We also sampled many volcanic layers from deeper than 0.3 My that are enabling us to reconstruct the full history of the CSKVF.
Unexpectedly, we penetrated a layer of deeply buried pumice and ash with a distinctive chemical composition at seven of the eight rift drill sites. The layer in these holes was up to 70 m thick, but extrapolation using our dense array of seismic profiles shows that it is locally as thick as 150 m. Shipboard palaeontologists used microfossil assemblages in the sediments above and below the layer to date the eruption at 0.52 My at a time when Santorini was largely submarine, much earlier than the previously known large explosive eruptions of the volcano. We interpret the layer, which we call the Archaeos Tuff (Gr: ancient) as emplaced by huge turbidity currents and slurries formed when pyroclastic currents were erupted into the water column and, through entrainment of water, transformed into water-supported gravity flows of pumice and ash. The turbidity currents then spread out across the basin, covering the seafloor up to 70 km away from its source and generating a volcaniclastic megaturbidite with a volume we calculate to be about 90 cubic kilometres. This is the largest pyroclastic current deposit of the CSKVF and is possibly as large as the great Kos Plateau Tuff eruption of the eastern Aegean island arc 160,000 years ago. In a subsequent twist we also found that the submarine ash layer correlates chemically with veneers of pyroclastic current deposits just a few metres or less thick on Christiana, Santorini, and Anafi islands. This shows that, although probably largely submarine, the eruption breached the sea surface and sent pyroclastic currents across the sea for as much as 55 kilometres from the vent site. This is a particularly convincing example of the ability of gas-charged pyroclastic currents to travel across open sea.
The Archaeas Tuff eruption shows many chemical similarities to the early Akrotiri volcanics of Santorini. Our interpretation is that the eruption marked the culmination of the largely submarine Akrotiri Volcano half a million years ago before the emergence of modern Santorini.
Our study documents both the offshore and onshore deposits from a large, shallow submarine explosive eruption, well constrained by volume, age, bathymetric, field and geochemical data, the pyroclastic currents from which were more than ten times larger than those of the ~6 km3 of Hunga Tonga–Hunga Ha‘apai eruption in 2022. This changes our current understanding of the Aegean island arc, revealing a greater capacity for highly hazardous submarine volcanism than previously known. It highlights the importance of deep drilling in unravelling the full secrets of island arcs, particularly in densely populated regions like the Mediterranean.