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Explosive eruptions of island volcanoes such at Tambora (1815 CE), Krakatau (1883 CE) and Santorini (1600 BCE) commonly pour pyroclastic currents (high-temperature mixtures of volcanic particles and gases) into the sea, but the ultimate fate of these flows and their impacts on the submarine realm are poorly documented. Recent studies of submarine products from the 2022 eruption of Hunga-Tonga Volcano in the Pacific Ocean have shown that the submarine continuations of such flows are able to travel under water at high speeds, eroding the seabed, impacting seafloor cable networks, and laying down submarine ash deposits. The large scale at which these processes can operate was not, however, fully comprehended.
The eruption of Kos Volcano on the eastern Hellenic Volcanic Arc 161,000 years ago laid down a thick layer of ash on Kos islands (the Kos Plateau Tuff), and its pyroclastic currents crossed the shoreline. The total volume of magma emitted was ten times larger than that at Hunga-Tonga. However, until now the impact of its pyroclastic flows on the submarine realm was completely unknown. Recent drilling of the seafloor sediments NE of Santorini by IODP Expedition 398 have now revealed that the pyroclastic currents from the Kos eruption had an unexpectedly large impact. Upon entering the sea, they entrained water and transformed into cold suspensions of ash and water. These suspensions then flowed down low gradients of the sea floor for over 140 km until they met a series of deep, fault-defined submarine basins north-east of Santorini, where they ponded and deposited their loads of suspended ash to thicknesses of 200 to 300 meters in some places. Since particles in the ash bed are largest at the base and smaller at the top, the 'megabed' is interpreted as having been deposited by a single event: the Kos volcanic eruption followed by perhaps several weeks to months of upstream remobilization of the primary deposit. Scouring of the sea bed by the flows during their emplacement is recorded by large quantities of rock fragments and fragments of marine organisms such as corals at the base of the ash bed. 
Ash beds hundreds of meters thick have been described previously from ancient submarine sediment successions that have been uplifted above sea level. However, the present discovery is the first example of a thick submarine ash deposit from the pyroclastic currents of a young (Quaternary) eruption that has been well characterized through traditional onland studies.
The submarine ash bed from the Kos Plateau Tuff eruption was discovered by deep-drilling during IODP Expedition 398 in the Aegean Sea, and is interbedded within the submarine products of nearby Santorini Volcano. Despite the much greater distance to Kos, the Kos Plateau submarine ash is much thicker than the ash beds from nearby Santorini, showing how large the Kos eruption was, even significantly larger than previously known: more than 200 cubic kilometers of erupted ash if the new submarine ash bed is added to the volume of the previously known ash deposits on Kos and its neighbouring islands.
The study highlights the efficient transport of large volumes of volcanic ash across the sea floor around explosive island volcanoes. It changes our perspective of the potentially large, hidden volumes of volcanic arc eruptions, and the ability of submarine flows to disperse volcanic ash far from the eruption source. The large volume and great extent of Kos Plateau Tuff submarine ash suggest that the volumes of some other explosive eruptions from island volcanoes may have been underestimated. While emphasis is commonly placed on the onland and atmospheric impacts of large explosive eruptions, the present study serves to illustrate the huge but largely invisible effects of large explosive volcanic eruptions on the submarine realm.

 
Scientific Operations:

The deep-drilling expedition was conducted by the JOIDES Resolution Science Operator (JRSO) as part of the IODP. The IODP was a multidecadal, international research program supported by 22 nations, with the goal of exploring Earth's history and structure recorded in seafloor sediments and rocks and monitoring sub-seafloor environments. Expedition 398 sailed with 32 scientists from 9 countries, with expertise in a range of geoscience disciplines. Associated professor Dr. Steffen Kutterolf, volcanologist at ºÚÁÏÊÓƵ Helmholtz Centre for Ocean Research Kiel, took on one of the two scientific leadership positions.