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We report on integrated geomorphological, mineralogical, geochemical and biological investigations of the hydrothermal vent field located on the floor of the density-stratified acidic (pH ~ 5) crater of the Kolumbo shallow-submarine arc-volcano, near Santorini. Kolumbo features rare geodynamic setting at convergent boundaries, where arc-volcanism and seafloor hydrothermal activity are occurring in thinned continental crust. Special focus is given to unique enrichments of polymetallic spires in Sb and Tl (±Hg, As, Au, Ag, Zn) indicating a new hybrid seafloor analogue of epithermal-to-volcanic-hosted-massive-sulphide deposits. Iron microbial-mat analyses reveal dominating ferrihydrite-type phases, and high-proportion of microbial sequences akin to "Nitrosopumilus maritimus", a mesophilic Thaumarchaeota strain capable of chemoautotrophic growth on hydrothermal ammonia and CO2. Our findings highlight that acidic shallow-submarine hydrothermal vents nourish marine ecosystems in which nitrifying Archaea are important and suggest ferrihydrite-type Fe(3+)-(hydrated)-oxyhydroxides in associated low-temperature iron mats are formed by anaerobic Fe(2+)-oxidation, dependent on microbially produced nitrate.

Volcanic eruptions stand as destructive threats to adjacent communities, unleashing multiple hazards such as earthquakes, tsunamis, pyroclastic flows, and toxic gases. The imperative for proactive management of volcanic risks and communities' adaptation cannot be overstated, particularly in densely populated areas where the potential for widespread devastation looms large. Kolumbo, an active submarine volcano located approximately 7 km northeast of Santorini Island in Greece, serves as a pertinent case. Its historical record is characterised by an eruption in 1650 CE that produced a catastrophic tsunami. The aftermath witnessed havoc on neighbouring islands, coupled with casualties stemming from noxious gases in Santorini. Eyewitness accounts mention maximum water run-up heights of 20 m on the southern coast of Ios, inundation of an area of 240 m inland on Sikinos, and a flooding of up to 2 km2 inland on the eastern coast of Santorini. Recent studies suggest that a potential future eruption of Kolumbo poses a substantial hazard to the northern and eastern coasts of Santorini. Unfortunately, the absence of a concrete management protocol leaves these areas vulnerable to an impending threat that demands immediate attention. Therefore, it is recommended that a comprehensive approach be adopted, involving scientific research (active monitoring, hazard maps), community engagement, preparedness planning with government agencies, and the development of timely response strategies to reduce the associated risks, prevent casualties, and mitigate the potential consequences on the region's economy and infrastructure. [ABSTRACT FROM AUTHOR]

Volcanic eruptions are driven by magma rising through Earth's crust. The style of an eruption depends on intrinsic and extrinsic parameters and is commonly a dynamic process. Thorough and holistic investigation of the related products is key to understanding eruptive phenomena and assessment of volcano-specific hazards. Models of such phenomena are constrained by quantification of the dispersal, the grain size distribution, and pyroclast textures. Pyroclast texture may be described in part by measurements of density and porosity, which depend on pyroclast volume determination. Yet volume determination of irregularly shaped pyroclasts cannot be achieved with geometrical laws, instead necessitating the use of alternative methodologies. Here, we test three methodologies to quantify pyroclast volume on a set of clasts collected from the Minoan eruption deposits from Santorini, Greece. We compare (1) a manual method for obtaining the lengths of three orthogonal axes of the pyroclast with a caliper, (2) an optical method to measure the longest and shortest axes of the pyroclast via multiple photographs, and (3) an Archimedean buoyancy-based method. While the optical and manual methods provide almost identical values of pyroclast volume when tested under laboratory conditions, there is a discrepancy between these two methods and the Archimedean method, which produces an overestimation of ca. 13% in volume. This discrepancy has little impact on the subsequent assessment of porosity and density for which the natural variability of values is observed to be broader. We therefore propose using the manual approach in the field as a simple and fast, yet reliable, method to obtain large volumes of quantitative data on the texture of erupted products, and we also provide a correction factor for in-field volume assessment of rhyodacitic pumices. [ABSTRACT FROM AUTHOR]

Volcanic eruptions can trigger tsunamis, which may cause significant damage to coastal communities and infrastructure. Tsunami generation during volcanic eruptions is complex and often due to a combination of processes. The 1650 eruption of the Kolumbo submarine volcano triggered a tsunami causing major destruction on surrounding islands in the Aegean Sea. However, the source mechanisms behind the tsunami have been disputed due to difficulties in sampling and imaging submarine volcanoes. Here we show, based on three-dimensional seismic data, that ~1.2 km³ of Kolumbo's northwestern flank moved 500–1000 m downslope along a basal detachment surface. This movement is consistent with depressurization of the magma feeding system, causing a catastrophic explosion. Numerical tsunami simulations indicate that only the combination of flank movement followed by an explosive eruption can explain historical eyewitness accounts. This cascading sequence of natural hazards suggests that assessing submarine flank movements is critical for early warning of volcanogenic tsunamis. Three-dimensional seismic data is used to reconstruct the flank collapse of Kolombo volcano in 1650, which led to a catastrophic tsunami event. [ABSTRACT FROM AUTHOR]

Despite their global societal importance, the volumes of large-scale volcanic eruptions remain poorly constrained. Here, we integrate seismic reflection and P-wave tomography datasets with computed tomography-derived sedimentological analyses to estimate the volume of the iconic Minoan eruption. Our results reveal a total dense-rock equivalent eruption volume of 34.5 ± 6.8 km³, which encompasses 21.4 ± 3.6 km³ of tephra fall deposits, 6.9 ± 2 km³ of ignimbrites, and 6.1 ± 1.2 km³ of intra-caldera deposits. 2.8 ± 1.5 km³ of the total material consists of lithics. These volume estimates are in agreement with an independent caldera collapse reconstruction (33.1 ± 1.2 km³). Our results show that the Plinian phase contributed most to the distal tephra fall, and that the pyroclastic flow volume is significantly smaller than previously assumed. This benchmark reconstruction demonstrates that complementary geophysical and sedimentological datasets are required for reliable eruption volume estimates, which are necessary for regional and global volcanic hazard assessments. The authors use seismic and sedimentology data to estimate the volume of the Minoan eruption. The results show that the Plinian phase contributed most to the distal tephra fall, and that the pyroclastic flow volume is significantly smaller than previously assumed. [ABSTRACT FROM AUTHOR]

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 behaviour 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. [ABSTRACT FROM AUTHOR]

A submarine volcanic eruption has the potential to generate a dangerous local tsunami. To better understand the free surface disturbance generated by an underwater volcanic eruption, which will form the initial condition for any subsequent wave generation, we conducted a series of laboratory experiments. In these experiments, compressed air was injected into a tank filled with water to simulate an underwater eruption. The experiments were repeated over a range of different pressures and water depths. Each eruption can be divided into three phases: A momentum‐driven jet, a buoyancy‐driven plume, and a fountain‐generation regime. Our experiments exhibit two fountain regimes (a dome regime and a finger regime), with a transition between them. These fountain regimes have been observed in several real submarine volcanic eruptions. This paper proposes a Froude number criterion to combine the water depths and source conditions together with the aspect ratios of fountains to quantify different fountain regimes. This quantitative relationship holds for two real subaqueous volcanic eruption cases (Myojin‐Sho eruption in 1952 and 1996 eruption in Karymskoye Lake). The fountain of the Myojin‐Sho shallow submarine eruption on September 23,1952 appears to have been in the dome regime, which means it was a relatively weak eruption. Unlike other eruptions from this volcano, which did generate tsunamis, no tsunami waves were detected on September 23. This study contributes to an enhanced understanding of the usually unseen mechanism of free surface disturbances by volcanic gas injection during submarine eruptions. Plain Language Summary: Underwater volcanic eruptions inject hot magma into cold ambient water. The interaction between magma and water creates steam explosions which can displace a large amount of water. An eruption occurring in shallow enough water and with sufficiently strong energy will be able to disturb the water surface and even initiate a tsunami. As these eruptions are hidden beneath water, their direct observation near their vents is difficult and dangerous, while the water surface disturbances are much easier and safer to measure. In order to understand the relationship between different kinds of eruptions and their disturbance of the water surface, we model underwater volcanic eruptions in the laboratory by injecting compressed air into water. We observe three main shapes of fountains on the water surface in our experiments: a) finger‐like; b) dome‐like; c) shapes intermediate between them. A finger‐shaped fountain accompanied by a large number of splashes normally occurs at shallow‐water depths and/or in an eruption with intense source strength, while a dome‐shaped fountain occurs under the opposite conditions. We find a good consistency between our experimental results and field observations, which means that we are able to estimate the unseen source conditions from the observed free surface disturbances. Key Points: We undertake novel experiments to quantify the evolution of an underwater gas eruption and its free surface responseThree types of fountain regimes observed in our experiments are consistent with field observations of underwater volcanic eruptionsFinger regimes occur at shallow depths or in the eruptions with strong source energy, while dome regimes occur under the opposite conditions [ABSTRACT FROM AUTHOR]

Submarine volcanic eruptions have the potential to generate tsunamis, which can cause destruction well beyond the range of the eruption itself. Here, we present a series of underwater eruption experiments in which a non‐condensing gas was injected into a water tank with a range of water depths and applied pressures. This study proposes an effective scaled water depth and categorizes underwater eruptions into three types: deep‐water eruptions, intermediate‐water eruptions, and shallow‐water eruptions. In deep‐water eruptions, most of the energy is dissipated within the water column before the plume reaches the surface, and negligible waves are generated. In intermediate‐water eruptions, reductions in water depth reduce the loss of energy to the water column, leaving more energy available for wave generation. This causes an increase in wave heights as water shallows, up to a point. In sufficiently shallow‐water cases, the water depth is so small that almost all of the energy from the eruptive jet or plume passes through the water and is dissipated into the air, so there is only small wave‐making potential, even with relatively intense source strength. Therefore, there exists a critical water depth at which an eruption with a given source intensity will generate the largest waves. That depth lies at the boundary between the intermediate‐ and shallow‐depth regimes, where the energy available for wave generation is at a maximum. This research reveals fundamental wave generation mechanisms related to underwater gas eruptions, thereby extending our understanding of submarine volcanic tsunami generation and providing a foundation for future hazard assessment. Plain Language Summary: Tsunamis are most often generated by seafloor motion due to earthquakes. However, tsunamis generated by underwater volcanic eruptions are less common and are often underestimated by researchers. In order to study the waves generated by short‐lived underwater volcanic eruptions, we injected compressed air from a submerged vent into a tank filled with water in the laboratory. We find that for an eruption of a given source intensity, the maximum wave heights first increase and then decrease as water depths increase from shallow to deep. This is because the ejected jet passes through the free surface and its energy is lost in air in shallow‐water depths, while most of the energy is lost underwater during the plume motion in deep‐water depths. Therefore, there is an optimal depth where an eruption with a given source intensity can generate the largest waves. These results are significant because they show the most dangerous conditions in tsunamis generated by submarine volcanic eruptions. The riskiest conditions need to be better accounted for in the future tsunami risk assessments. Key Points: We present new physical experiments to model waves generated by underwater gas eruptionsBased on an effective scaled depth, underwater eruptions are divided into three types: deep‐, intermediate‐, and shallow‐water eruptionsWe hypothesize that there is a critical water depth at which a subaqueous eruption of a given intensity will generate the largest waves [ABSTRACT FROM AUTHOR]

Abstract: Kolumbo submarine volcano, located 7km northeast of Santorini, Greece in the Aegean Sea, last erupted in 1650AD. Submarine and subaerial explosive activity lasted for a period of about four months and led to the formation of thick (~250m) highly stratified pumice deposits on the upper crater walls as well as extensive pumice rafts that were dispersed throughout the southern Aegean Sea. Subaerial tephra fallout from eruption columns that breached the surface occurred as far east as Turkey. Remotely operated vehicle (ROV) video observations from transects of the northern and southwestern crater walls have been used to create stratigraphic sequences of the 1650AD deposits. Petrographic and geochemical analyses reveal that the bulk of the deposits consists of white, highly vesicular, sparsely porphyritic, biotite-bearing rhyolite pumice. Pre-eruption volatile contents determined from plagioclase-hosted melt inclusions yield a median value of 6.5wt.%. This corresponds to a pre-eruption storage pressure of ~200MPa, or a depth of ~6km assuming a H2O-saturated magma. Comparison of the natural glass compositions and mineral assemblage of the Kolumbo samples with experimental results on other rhyolite magmas of similar composition in the modified haplogranite system supports the pressure and total volatile estimates. Pre-eruption temperature was calculated as 750°C based on plagioclase–melt geothermometry. The high volatile content of the Kolumbo magma and historical accounts of substantial subaerial eruption plumes suggest that the explosive eruption was driven by primary volatile degassing. Three phases of activity can be inferred from historical accounts and deposit stratigraphy; an initial submarine eruption during which time the vent became shallower as pyroclastic material accumulated, a transitional phase with substantial fallout from a hybrid submarine/subaerial eruption plume, and a final subaqueous phase as the eruption waned. Depositional processes included a complex interplay of submarine fallout, pyroclastic density currents, and clast segregation associated with the transition from submarine to subaerial plumes, producing a highly stratified pumiceous sequence. This is the first study of the in situ pyroclastic deposits of a well-documented silicic submarine explosive eruption in the relatively shallow (<500m below sea level) marine environment. [Copyright &y& Elsevier]

The Minoan eruption of Santorini is one of the largest Holocene volcanic events and produced several cubic kilometers of pyroclastic flows emplaced on the submerged flanks of the volcano. Marine geophysical surveys reveal a multitude of undulating seafloor bedforms (USBs) around Santorini. While similar structures are known from other volcanoes worldwide, Santorini offers the unique opportunity to relate USB formation with volcanic processes during one of the best-studied volcanic eruptions worldwide. In this study, we combine high-resolution seismic reflection data with multibeam echosounder bathymetry to reveal the internal architecture of USBs around Santorini and to relate their morphological characteristics to formational processes. The USBs around Santorini were formed during the Minoan eruption and represent the seafloor expression of mass transport deposits. Three types of deposits differ in composition or origin. (1) Depositional USBs, which can only be found to the north of the island, where Minoan eruption ignimbrites reach their maximum thickness and the undulating topography is the result of thrusting within the deposit. (2) USBs related to slope failures of volcaniclastics from the entire Thera Pyroclastic Formation, which can be found east, south, and west of the island. (3) USBs associated with deep-seated deformation, which occurs on the southwestern flank along an area affected by rift tectonics and extends to a depth of more than 200 m below the seafloor. In cases (2) and (3), the USBs are formed upslope by block rotation and downslope by thrusting. Our study indicates that these processes may have contributed to the generation of the devastating Minoan tsunami. Since Santorini is located in one of the most tectonically active regions in the Mediterranean, capable of producing earthquakes with magnitude M7+, our study has important implications for hazard assessment. A strong earthquake located close to the island may have the potential to reactivate slope instabilities posing a previously undetected but potentially significant tsunami hazard. • The Minoan eruption formed undulating seafloor bedforms (USBs) around Santorini. • USBs are the result of pyroclastic flows, slope failure and deep-seated deformation. • Deep-seated deformation affects the southwestern flank of Santorini. [ABSTRACT FROM AUTHOR]

Abstract: Between January 2011 and April 2012, Santorini volcano (Greece) experienced a period of unrest characterised by the onset of detectable seismicity and caldera-wide uplift. This episode of inflation represented the first sizeable intrusion of magma beneath Santorini in the past 50 years. We employ a new approach using 222Rn– systematics to identify and quantify the source of diffuse degassing at Santorini during the period of renewed activity. Soil CO2 flux measurements were made across a network of sites on Nea Kameni between September 2010 and January 2012. Gas samples were collected in April and September 2011 for isotopic analysis of CO2 ( ), and radon detectors were deployed during September 2011 to measure (222Rn). Our results reveal a change in the pattern of degassing from the summit of the volcano (Nea Kameni) and suggest an increase in diffuse CO2 emissions between September 2010 and January 2012. High-CO2-flux soil gas samples have . Using this value and other evidence from the literature we conclude that these CO2 emissions from Santorini were a mixture between CO2 sourced from magma, and CO2 released by the thermal or metamorphic breakdown of crustal limestone. We suggest that this mixing of magmatic and crustal carbonate sources may account more broadly for the typical range of values of CO2 (from to ) in diffuse volcanic and fumarole gas emissions around the Mediterranean, without the need to invoke unusual mantle source compositions. At Santorini a mixing model involving magmatic CO2 (with of and elevated (222Rn)/CO2 ratios ) and CO2 released from decarbonation of crustal limestone (with (222Rn)/CO2 ∼ 30–300 Bq kg−1, and of ) can account for the and (222Rn)/CO2 characteristics of the ‘high flux’ gas source. This model suggests of the carbon in the high flux deep CO2 end member is of magmatic origin. This combination of and (222Rn) measurements has potential to quantify magmatic and crustal contributions to the diffuse outgassing of CO2 in volcanic areas, especially those where breakdown of crustal limestone is likely to contribute significantly to the CO2 flux. [Copyright &y& Elsevier]

Santorini is the most active arc volcano of the Hellenic subduction zone and has a history ofalternating effusive, shield-forming eruptions and explosive caldera collapse. The most recentcaldera-forming event is the well-studied 3.6 kyr Late Bronze Age (LBA/Minoan) eruption.In 2011-2012, the volcano experienced inflation and seismicity attributed to magmaintrusion between 3.3 and 5.9 km depth. To investigate processes that control calderaformation and the accumulation of magma at arc volcanoes, we use a high-resolution,P-wave, seismic velocity model together with existing geological and geochemicalconstrains. The PROTEUS experiment collected dense, 3D, active-source marine-landseismic data using 90 ocean bottom seismometers, 65 seismic land stations, and∼14,300 controlled-sound marine sources from the U.S. R/V Marcus Langseth.We use seismic tomography to invert &#x003E;220,000 Pg travel times to image of theupper crustal structure to 7 km depth. Regional geology and tectonic features arereflected in the near-surface velocity structure. Magmatism has localized along NE-SWbasin-like structures that connect the Christiana, Santorini, and Kolumbo volcaniccenters, indicating a strong interaction between magmatism and tectonism. In theupper 3 km beneath the north-central caldera, we find a ∼3-km-wide, cylindricallow-velocity anomaly (-2 km/s) that lies directly above the pressure source of 2011-2012inflation and between two NE-SW-striking tectonic lineaments. Vents that formedduring the first three phases of the LBA eruption locate near the edges of the imagedstructure. We infer that collapse of a limited area of the caldera floor resulted ina high-porosity, low-density cylindrical volume, which formed by either chaoticcollapse along reverse faults, wholesale subsidence and infilling with tuffs andignimbrites, phreatomagmatic fracturing, or a combination of these processes. LBAphase 4 eruptive vents are located along the margins of the topographic calderaand the velocity structure indicates that the wider topographic caldera formed bycoherent down-drop following the more limited collapse in the northern caldera. Thisprogressive collapse sequence is consistent with models for multi-stage formationof nested calderas along conjugate reverse and normal faults. The upper-crustaldensity differences inferred from the seismic velocity model predict differences insubsurface gravitational loading that correlate with 2011-2012 edifice inflationindicating that sub-surface density anomalies may influence present-day magmaaccumulation. Directly beneath this structure, we image a second low-velocityanomaly that is 3-6 km below the northern caldera basin, 3-5 km wide, and elongated∼15 km NE-SW, parallel to tectonic structures. The largest-amplitude recoveredvelocity anomaly (-1 km/s) is consistent with 4-10% melt and coincides with the2011-2012 Mogi pressure source. Significant ray bending indicates that the recoveredsize and amplitude of the low velocity volume are minimum bounds. We infer anupper-crustal intrusion rate (∼2.6 km3/ky) consistent with the inferred long-termgrowth of the shallow magma system at Santorini. Future analysis of waveforms,reflectors, and shear waves, and full waveform inversion, will better constrain themelt content and distribution of crustal magma accumulation beneath Santorini. [ABSTRACT FROM AUTHOR]

Subduction-related volcanism plays an important role in society through its impact on life and the environment. Better understanding of island-arc volcanism and mitigation of associated risk requires study of the processes that drive such magmatism, the interactions and feedbacks between tectonics and volcanism, and how volcanoes interact with their marine environments. The Christiana-Santorini-Kolumbo (CSK) volcanic field on the Hellenic Volcanic arc of Greece is a unique system for addressing these issues. It lies in a rift system 100 km long and 45 km wide, oblique to the island arc, that is one of the most volcanically and seismically active regions of Europe. The volcanoes include three polygenetic and over 20 monogenetic centers that have jointly produced over a hundred explosive eruptions over the last few hundred thousand years. The volcanoes pose an important threat to the Eastern Mediterranean region. The Santorini eruption of the Late Bronze Age (LBA) is an iconic event in volcanology and archaeology. Unrest at Santorini caldera in 2011-12 raised awareness of eruption threat at an island archipelago visited by 1.5 million tourists per year. The results of extensive onland volcanological research, eruption dating, multi-beam sea floor mapping, shallow sediment coring and dredge sampling, combined with a high-quality site-survey database of multichannel seismic profiles and a recent seismic tomography experiment, make deep drilling at the CSK volcanic field very timely. Deep drilling is essential to identify, characterize and interpret depositional packages visible on seismic images, to chemically correlate Santorini-derived volcanic layers in the rift fills with the dated onshore stratigraphy, to provide a tight chronostratigraphic framework for marine successions, and to sample ancient shallow marine hydrothermal systems, pore waters and microbial colonies. Some main objectives of drilling are to: (1) document the history of tectonics, subsidence, sedimentation and volcanism in an arc-rift environment, and how volcanism has evolved spatially and temporally since rift initiation; (2) determine how the genesis and compositions of magmas and their associated volatiles have evolved in time and space over the lifetime of the rift; (3) document the dynamics and environmental impacts of arc eruptions (including the LBA eruption) and calderas, including eruption frequencies, magnitudes and rates, the mechanisms of caldera collapse, and the origin of caldera unrest events; (4) document the occurrence and nature of deep caldera-hosted ecosystems. We will drill into the volcano-sedimentary fills of the rifts (up to several hundred metres thick) to access a near-continuous record of volcanism, sedimentation and basin subsidence since the onset of rifting in the Pliocene, and to seek relationships between volcanism and tectonics. Drilling inside Santorini caldera will address fundamental questions about how arc calderas form, and will investigate the nature of a unique funnel-shaped zone of low seismic velocity identified by seismic tomography beneath the focus of caldera-floor inflation in 2011-12. The proposal arose from a MagellanPlus meeting in Athens. The participants of that meeting are coauthors on this abstract. [ABSTRACT FROM AUTHOR]