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Volcanic island sector collapses have the potential to trigger devastating tsunamis and volcanic eruptions that threaten coastal communities and infrastructure. Considered one of the most hazardous volcano-tectonic regions in the world, the Christiana-Santorini-Kolumbo Volcanic Field (CSKVF) lies in the South Aegean Sea in an active rift zone. Previous studies identified an enigmatic voluminous mass-transport deposit west and east of Santorini emplaced during the early evolution of the edifice. However, the distribution and volume as well as the nature and emplacement dynamics of this deposit remained unknown up to now. In this study, we use an extensive dataset of high-resolution seismic profiles to unravel the distribution and internal architecture of this deposit. We show that it is located in all basins surrounding Santorini and has a bulk volume of up to 125 km3, thus representing the largest known volcanic island mass-transport deposit in the entire Mediterranean Sea. We propose that the deposit is the result of a complex geohazard cascade that was initiated by an intensive rift pulse. This rifting event triggered a series of smaller precursory mass-transport events before large-scale sector collapses occurred on the northeastern flank of the extinct Christiana Volcano and on the southeastern flank of the nascent Santorini. This was followed by the emplacement of large-scale secondary sediment failures on the slopes of Santorini, which transitioned into debris and turbidity flows that traveled far into the neighboring rift basins. Following this cascade, a distinct change in the volcanic behavior of the CSKVF occurred, suggesting a close relationship between crustal extension, mass transport, and volcanism. Cascading geohazards seem to be more common in the evolution of marine volcanic systems than previously appreciated. Wider awareness and a better understanding of cascading effects are crucial for more holistic hazard assessments.

The Christiana-Santorini-Kolumbo volcanic field (CSKVF) in the Aegean Sea is one of the most active volcano-tectonic lineaments in Europe. Santorini has been an iconic site in volcanology and archaeology since the 19th century, and the onshore volcanic products of Santorini are one of the best-studied volcanic sequences worldwide. However, little is known about the chronology of volcanic activity of the adjacent submarine Kolumbo volcano, and even less is known about the Christiana volcanic island. In this study, we exploit a dense array of high-resolution marine seismic reflection profiles to link the marine stratigraphy to onshore volcanic sequences and present the first consistent chronological framework for the CSKVF, enabling a detailed reconstruction of the evolution of the volcanic rift system in time and space. We identify four main phases of volcanic activity, which initiated in the Pliocene with the formation of the Christiana volcano (phase 1). The formation of the current southwest-northeast–trending rift system (phase 2) was associated with the evolution of two distinct volcanic centers, the newly discovered Poseidon center and the early Kolumbo volcano. Phase 3 saw a period of widespread volcanic activity throughout the entire rift. The ongoing phase 4 is confined to the Santorini caldera and Kolumbo volcano. Our study highlights the fundamental tectonic control on magma emplacement and shows that the CSKVF evolved from a volcanic field with local centers that matured only recently to form the vast Santorini edifice.

Located on the Hellenic Arc, the Christiana-Santorini-Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano-tectonic systems in the Mediterranean. Although this rift zone has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high-resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre-volcanic NNE-SSW-oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE-WNW-oriented basins, which was overprinted by a NE-SW-oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small-scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep-seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large-scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos-Nisyros-Yali volcanic field zone and Rhodos highlights concurrent regional volcano-tectonic changes, suggesting a potential arc-wide scale of the observed volcano-tectonic interplay. Key Points We reconstruct the volcano-tectonic evolution of the Christiana-Santorini-Kolumbo rift zone using multichannel seismic data The overprint of a Pleistocene NE-SW striking fault system on a Pliocene E-W oriented system initiated the emergence of

With a maximum volume estimate of up to 86 km³ dense-rock equivalent (DRE), the Minoan Eruption is considered one of the largest Holocene eruptions. However, as most eruption products were deposited in the Mediterranean, previous volume estimates based on a limited database. We present new marine geophysical and sedimentological datasets allowing us to reassess the volume of the Minoan eruption in unprecedented detail. We combine high- resolution reflection seismic and P-wave tomography datasets with more than 40 marine sediment cores, constrained by X-ray computer-tomography (CT) scanning. The reflection seismic profiles indicate an ignimbrite volume of ~5.7 km³ DRE deposited on Santorini’s flanks, which is seven times smaller than previous estimates, while the P-wave tomography indicates ~5.5 km³ DRE of Minoan intra-caldera deposits, which is four times smaller than previous estimates. CT-guided analysis of the sediment cores allows us to differentiate two ash layers, which are separately integrated into ash deposit isopach maps and allows determine deposit porosities with high accurary. The combined ash volume of ~19.5 km³ DRE is in the same order as previous estimates. This yields a total eruption volume of ~31 km³ DRE. In addition, we use the new datasets to constrain the post-eruptive topography of the caldera and estimate the caldera collapse volume to be ~31.5 km³. The internal consistency of both independent approaches implies high confidence in our estimates, likely representing the most precise volume reconstruction of any major (M6.5+) volcanic eruption in the Holocene. Our analysis implies that the Minoan Eruption was smaller and produced significantly less ignimbrites than previous reconstructions indicated, while still causing a devastating tsunami. This highlights the significant tsunamigenic potential of submarine- emplaced ignimbrites

The Christiana-Santorini-Kolumbo volcanic field (CSKVF) in the Aegean Sea is one of the most active volcano-tectonic lineaments in Europe. Santorini has been an iconic site in volcanology and archaeology since the 19th century, and the onshore volcanic products of Santorini are one of the best-studied volcanic sequences worldwide. However, little is known about the chronology of volcanic activity of the adjacent submarine Kolumbo volcano, and even less is known about the Christiana volcanic island. In this study, we exploit a dense array of high-resolution marine seismic reflection profiles to link the marine stratigraphy to onshore volcanic sequences and present the first consistent chronological framework for the CSKVF, enabling a detailed reconstruction of the evolution of the volcanic rift system in time and space. We identify four main phases of volcanic activity, which initiated in the Pliocene with the formation of the Christiana volcano (phase 1). The formation of the current southwest-northeast–trending rift system (phase 2) was associated with the evolution of two distinct volcanic centers, the newly discovered Poseidon center and the early Kolumbo volcano. Phase 3 saw a period of widespread volcanic activity throughout the entire rift. The ongoing phase 4 is confined to the Santorini caldera and Kolumbo volcano. Our study highlights the fundamental tectonic control on magma emplacement and shows that the CSKVF evolved from a volcanic field with local centers that matured only recently to form the vast Santorini edifice.

Highlights • Overview on geochemical composition of pore water and solid phase of sediments on the Azores Plateau. • Evidence for deep marine hydrothermal activity on the Azores Plateau. • Pore water data suggest ongoing anaerobic oxidation of methane and carbonate recrystallization. The Azores Plateau is an active magmatic region in the Central North Atlantic Ocean. In this study, we present a comprehensive data set of major element compositions and 87Sr/86Sr ratios of pore waters from surface sediments (0–9 mbsf) of the Azores Plateau. Based on distinct geochemical signatures we can separate normal marine from hydrothermally affected sediments. Normal marine sediments can further be differentiated by their ash content. Pore waters of ash rich gravity cores (GCs) do not show any deviations from seawater values except of a minor increase in Sr. In contrast, ash poor GCs generally show a trend for decreasing Ca with increasing depth, accompanied by a minor SO4 decrease and a more pronounced Sr increase. We suggest that these deviations are caused by processes such as anaerobic oxidation of methane and carbonate recrystallization. At four additional sample locations we observed a decrease in Mg and SO4 accompanied by a Ca increase in the pore waters, a pattern typical for hydrothermal fluids. The existence of hydrothermal systems in this region are corroborated by multi-channel seismic data, suggesting that sill or dyke intrusions are present in the subsurface close to the core locations. Overall, our observations offer preliminary indications for the existence of submarine hydrothermal systems on the Azores Plateau away from the Mid- Atlantic Ridge.

The E-W trending∼1 600 km long Azores-Gibraltar Fracture Zone (AGFZ) establishes the Africa-Eurasia plate boundary in the Atlantic Ocean. The AGFZ is intersected by the Madeira-Tore Rise (MTR), a N-S trending 1 100 km complex volcanic ridge. MTR magmatism in the Eurasia plate is mainly of Early Cretaceous age while in the Africa plate it varies from Neogene to Present, mainly. At the AGFZ-MTR intersection magmatism is 0.5 My. The tectonic style along the AGFZ varies dramatically across the MTR. West of the MTR the trace of the plate boundary is discrete and defined by the strike slip Gloria Fault, while east of the MTR the plate boundary becomes diffuse, and deformation is partitioned along strike slip and thrust faults. However, instrumental earthquakes from M=7 to M=8.4 have occurred in the last 80 years, not mentioning the historical Lisbon earthquake with estimated magnitude M=8.1 to 8.9. The PROPEL (PROPagation of the Eurasia-Africa pLate boundary East of the GLoria Fault), a multichannel seismics marine campaign funded by EUROFLEETS-2 aimed at imaging for the first time the crustal through shallow mantle structure of the AGFZ across the MTR. It was run on board the R/V OGS Explora, using a 1500 m long streamer with 120 channels, an array of airguns (16.4 L or 8.2 L) and a magnetometer. A total of 722 km along 11 profiles were acquired. Processing of the seismic data allowed imaging the transform plate boundary showing a series of various geological phenomena, such as: i) successive events of transtensional and transpressive tectonics interfering with magmatism, ii) the structure and multiple tectono-magmatic evolution of the MTR, iii) the transition from the Horseshoe and Tagus Abyssal Plains towards the MTR and iiv) large scale landslides (>50 km of length). PROPEL seismic data also allow for better understanding the nature of magnetic anomalies in this region of the Atlantic, as well as a variety of submarine morphological features of both tectonic and ocean