New perspectives of volcanism at the rift-hosted Santorini-Kolumbo system (South Aegean Volcanic Arc), from IODP deep-drilling

Santorini-Kolumbo is one of the most hazardous volcanic centres in Europe, as highlighted by its VEI-5 explosive eruptions of 726 CE and 1650 CE, and its bradyseismic crises of 2011-12 and 2024-2025. IODP Expedition 398 deep-drilled the volcano-sedimentary infills of marine rift basins at eight sites around Santorini to depths of up to 900 m below the sea floor, and integrated the core stratigraphies with a dense array of seismic profiles from eight expeditions to construct a high-resolution timeline of volcanic activity and to relate it to the basin-fill architecture and tectonic history. In this overview we show that the four drill sites analyzed to date reveal >200 Santorini and 19 Kolumbo tephra layers intercalated in marine sediments. The tephras were correlated chemically between sites, either as the products of individual eruptions or as packages of layers, with the onset of explosive activity at ~1 Ma. The rift basins contain several submarine volcaniclastic megabeds from the caldera-forming eruptions of Santorini and one from the Kos caldera. The megabeds formed when pyroclastic flows poured into the sea and transformed into subaqueous gravity flows. The thickest megabed succession is < 250 ky old and lies on a seismic reflection onlap surface that records a phase of rapid rifting. Sedimentation lagged behind subsidence during rapid rifting, creating bathymetric troughs that served as depocenters for the megabeds. Reconstruction of the basin subsidence history shows that the rift extension rate accelerated markedly about 350 ky ago. This increase in rifting rate preceded, and may have driven, the transition of Santorini from a prolonged state of effusive and moderate explosive activity (~550 – 250 ka) typical of arc stratovolcanoes to one of repeated caldera-forming eruptions (<250 ka). The earliest explosive activity at Kolumbo Volcano is recorded at 265 ka and coincides broadly with the explosive transition at Santorini, suggesting that activity at the volcanic systems is synchronized by tectonic stresses. The main stages of construction of the Kolumbo edifice broadly coincided with periods of caldera-forming silicic volcanism at Santorini, reflecting additional interactions and feedbacks on shorter timescales. The existence of connections between tectonic stresses, fluid pressures, and magma reservoirs of the two neighboring magmatic systems is consistent with concurrent ground movements, seismic swarms and dyke injection at Santorini-Kolumbo in 2024/25.