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UNLABELLED: On 15 January 2022, Hunga volcano erupted, creating an extensive and high-reaching umbrella cloud over the open ocean, hindering traditional isopach mapping and fallout volume estimation. In MODIS satellite imagery, ocean surface water was discolored around Hunga following the eruption, which we attribute to ash fallout from the umbrella cloud. By relating intensity of ocean discoloration to fall deposit thicknesses in the Kingdom of Tonga, we develop a methodology for estimating airfall volume over the open ocean. Ash thickness measurements from 41 locations are used to fit a linear relationship between ash thickness and ocean reflectance. This produces a minimum airfall volume estimate of 1.8-0.4+0.3 km3. The whole eruption produced > 6.3 km3 of uncompacted pyroclastic material on the seafloor and a caldera volume change of 6 km3 DRE. Our fall estimates are consistent with the interpretation that most of the seafloor deposits were emplaced by gravity currents rather than fall deposits. Our proposed method does not account for the largest grain sizes, so is thus a minimum estimate. However, this new ocean-discoloration method provides an airfall volume estimate consistent with other independent measures of the plume and is thus effective for rapidly estimating fallout volumes in future volcanic eruptions over oceans. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00445-024-01744-6.

On 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami. Plain Language Summary: On 8 October 2023 (UTC), earthquakes were followed by tsunamis in the Izu‐Ogasawara Arc of Japan that were higher than that estimated from the magnitude of the earthquake. The tsunamis were observed on islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japanese islands. Strong T‐phases that are underwater acoustic waves were observed accompanying that event. We estimated that the T‐phases originated from the area around the Sofu Seamount, which has not been recognized as an active submarine volcano until now. One month after the event, we conducted a bathymetric survey of the seamount and revealed the characteristics of a caldera and a central cone within the caldera. Compared to the bathymetric data obtained in 1987, the caldera and central cone topography have changed significantly, such as a crater forming in the central cone, indicating that this seamount is likely to have been an active submarine volcano. The topographic changes that occurred at the depth of the caldera (∼1,000 m) and its scale (diameter ∼5 km) can be associated with the source of the tsunami on 8 October 2023. Key Points: A bathymetric survey of the Sofu Seamount was conducted after the unique T‐phase dominant earthquakes and tsunamis on 8 October 2023Compared to data in 1987, the caldera and central cone topography have changed, indicating that this seamount is an active submarine volcanoThe observed seafloor displacement at the caldera scale may have contributed to the source of the tsunami on 8 October 2023 [ABSTRACT FROM AUTHOR]

An unexpected major tsunami from the region near Sofu Seamount was observed on 8 October 2023. Sofu Seamount is located approximately 600 km from the coast of Japan. Due to far epicentral distances and the successive occurrence of seismic events, the conventional seismic analysis to reveal the accompanying seismic sequence cannot work well. We investigated high‐frequency teleseismic P and regional T waves from the accompanying seismic sequence during the tsunamigenic events near Sofu Seamount. Envelope shapes of teleseismic P and regional T waves were similar, indicating that T‐wave envelopes also reflected source properties of seismic sequence. During seismic events near Sofu Seamount, observed regional envelopes were characterized by weak body waves and large amplitude T waves with durations of 39–68 s. According to numerical simulations of seismic wave propagation using a realistic topography model, characteristics of T waves exhibit weak slope‐angle and strong source‐depth dependencies. Strong T waves with durations less than 60 s only appeared in results with sources at depths ≤0.5 km below the seafloor. We concluded that high‐frequency radiation of the accompanying seismic sequence during the tsunamigenic events near Sofu Seamount possibly occurred at shallower depths just below the seafloor. If seismic and tsunami sources coincide, shallower source depths might cause tsunamigenic uplifts. The observed peak seafloor uplifts and T‐wave amplitudes during tsunamigenic events were scaled. This result suggests the possibility of tsunami forecasting based on T‐wave amplitudes from submarine volcanoes. Plain Language Summary: On 8 October 2023, an earthquake sequence, which included 15 moderate‐size (magnitudes of 4.3–5) earthquakes, was observed in the region near Sofu Seamount. When a moderate‐size (M < 6) earthquake occurs in offshore regions, we generally observe no tsunami signals. However, during this earthquake sequence, an unexpected major tsunami was observed along the coast of Japan. To investigate tsunami sources near Sofu Seamount, we analyzed observed T waves at ocean‐bottom seismometers near the coast of southwest Japan. T waves are ocean acoustic waves and can effectively propagate at distances larger than several thousand kilometers. Observed T waves were significant compared with P waves, propagating through the subsurface. We also analyzed simulation results of high‐frequency seismic wave propagation from Sofu Seamount to the Japanese coast. Comparing simulation results with observed T waves allows us to constrain the depth of moderate‐size earthquakes. Because efficient T‐wave generation only appears in shallower sources, moderate‐size earthquakes should be located at depths ≤0.5 km below the seafloor, and such shallower seismic/deformation phenomena might cause observable tsunamis even for magnitudes ≤5. Key Points: We studied high‐frequency teleseismic P and regional T waves during tsunamigenic events near Sofu SeamountAccording to envelope shapes, accompanying seismic events during tsunamigenic events should be at depths ≤0.5 km below the seafloorShallow sources might cause tsunamigenic seafloor uplifts even for smaller seismic magnitudes [ABSTRACT FROM AUTHOR]

Abstract On 8 October 2023 (UTC), unique earthquakes occurred in the Izu‐Ogasawara Arc, Japan, in which the P‐ and S‐phases were barely visible and only the T‐phases were evident, followed by tsunamis that reached islands in the Izu‐Ogasawara Arc and a wide area of the Pacific coast of southwest Japan. Our estimated T‐phase source area coincides with the Sofu Seamount, which was previously unrecognized as an active submarine volcano. A bathymetric survey of the seamount conducted 1 month after the event revealed characteristics of the seamount with a caldera and a central cone. Compared to the bathymetry in 1987, the topography in the caldera had changed significantly such as a crater forming in the central cone. This seamount is likely to be an active volcano. The topographic changes on the caldera‐sized scale that occurred at the caldera can be explained as a source of the October tsunami.

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita‐Ioto Island, south of Japan, unusual M ∼ 5 non‐double‐couple volcanic earthquakes exhibited quasi‐regular recurrence near a submarine caldera. Following the earthquakes in 2008 and 2015, a distant ocean bottom pressure sensor recorded distinct tsunami signals. In this study, we aim to find a source model of the tsunami‐generating earthquake and quantify the pre‐seismic magma overpressure within the caldera's magma reservoir. Based on the earthquake's characteristic focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish mechanical earthquake models that link pre‐seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring‐fault system and a reservoir of the caldera. The trapdoor faulting with large fault slip due to magma‐induced shear stress in the submarine caldera reproduces well the observed tsunami waveform. Due to limited data, uncertainties in the fault geometry persist, leading to variations of magma overpressure estimation: the pre‐seismic magma overpressure ranging approximately from 5 to 20 MPa, and the co‐seismic pressure drop ratio from 10% to 40%. Although better constraints on the fault geometry are required for robust magma pressure quantification, this study shows that magmatic systems beneath calderas are influenced significantly by intra‐caldera fault systems and that tsunamigenic trapdoor faulting provides rare opportunities to obtain quantitative insights into remote submarine volcanism hidden under the ocean. Plain Language Summary: Monitoring submarine volcanoes is essential to understand and prepare for potential volcanic hazards in/around oceans, but it's challenging because these volcanoes are located in inaccessible environments. In a submarine volcano with a caldera structure in the south of Japan, unusual volcanic earthquakes took place every several years. After one of these earthquakes in 2008, a pressure sensor deployed on the sea bottom recorded a clear signal of tsunami waves. By utilizing the tsunami signal from the earthquake, we attempt to measure how much magma pressure was building up beneath the volcano before the earthquake. Assuming that the earthquake happened with sudden rupture on an intra‐caldera fault system due to highly pressurized magma beneath the volcano, we develop a method to assess the built‐up magma pressure through quantification of the earthquake and tsunami sizes. By applying the method, we estimate that the volcanic edifice was under a highly stressed condition before the earthquake, suggesting the active magma accumulation process that has continued beneath the volcano. Signals emitted from volcanic earthquakes under oceans shed light on the activity of poorly monitored submarine volcanoes. Key Points: Non‐double‐couple earthquakes with seismic magnitudes of 5.2–5.3 recurred in the vicinity of a submarine caldera near Kita‐Ioto IslandA mechanical model of trapdoor faulting based on tsunami data of the 2008 earthquake infers pre‐seismic overpressure in a magma reservoirUncertainty in fault geometry varies our estimate of pre‐seismic overpressure (5–20 MPa) and co‐seismic pressure drop ratio (10%–40%) [ABSTRACT FROM AUTHOR]

Since 2018 an unexpected number of earthquakes have been occurring offshore Mayotte, in the Mozambique Channel. They are linked to the eruption of the Fani Maoré submarine volcano. Using a recently-developed comprehensive automatic catalog, we explore two years of volcano-tectonic (VT) seismicity between March 2019 and March 2021 and analyse in detail the active structures of the magmatic plumbing system using ~33,000 events. The VT earthquakes highlight three magma storage zones and two aseismic conduits that had never been observed before. The temporal evolution of the seismicity reveals a probable regime change in March 2020. While before, the plumbing system reacted to the drainage of magma from a deep reservoir and to the migration of magma towards the seafloor, it is now responding to new migrations of fluids and to the redistribution of the stress-load across the system's pre-existing faults. This analysis is key to better understand long-term volcanism worldwide.

Abstract Voluminous pumice rafts produced by the 2021 phreatomagmatic eruption of Fukutoku-Oka-no-Ba, a submarine volcano located in the Izu-Bonin Islands, reached many Japanese ports and islands, damaging fisheries and hindering marine traffic and trade. This event emphasized the necessity for governments and stakeholders to develop plans in advance and prepare disaster mitigation measures before an eruption. To investigate when and to what extent pumice might intersect trade routes and arrive at islands after future eruptions, we conducted particle tracking simulations of eruptions at major volcanic islands and submarine volcanoes near Japan using the velocity field from the ocean reanalysis dataset. Seven major volcanoes that have produced pumice rafts in the past century were selected: the submarine volcano NNE of Iriomote Island, Izu-Tobu Volcanoes, Miyakejima, Bayonnaise Rocks, Nishinoshima, Kaitoku Seamount, and Fukutoku-Oka-no-Ba. We partly reproduced the distribution of pumice arrivals recorded after the 1986 Fukutoku-Oka-no-Ba eruption, demonstrating the potential effectiveness of the simulations. We report likely pumice raft arrivals and drifting durations for the investigated eruptive scenarios, which may aid future risk assessments for pumice arrivals.

ABSTRACTWe present the bathy-morphological map at a scale of 1: 50,000 of the area around the submerged Ferdinandea Island, the ‘Neverland’ of the Sicily Channel (central Mediterranean Sea). We investigate an area of 100 km2, between 10 and 350 m, which is part of a triangular morphological high, 360 km2 wide, representing the SE-wards prolongation of the Adventure Bank. The study is based on the morphometric analysis based on high resolution multibeam, and sub-bottom CHIRP profiles collected in 2015. The area around the remains of Ferdinandea Island is morphologically shaped by the interplay between volcanic, tectonic, fluid seepage, and oceanographic processes. Since the study area is considered a hot spot of biodiversity affected by maritime traffic (especially in Ferdinandea Channel) and hosting communication pipelines, this map provides insights both for habitat mapping purposes and preliminary marine geohazard assessment due to the occurrence of historically active submarine volcanoes, pockmarks, and mass transport deposits.

The presence of active hydrothermal vent fields near residential areas and their possible link to volcanic activity poses a potential hazard to the environment, society, and the economy. By capitalizing on Autonomous Underwater Vehicle sampling methodologies and applying the Generalized Moments Method model for geological and physical processes in these environments, we shed light on the underlying dynamics shaping the physicochemical characteristics of the vents. In this study, we focus on the Northern Caldera of Santorini and, more specifically, on the recorded CTD data (Conductivity, Temperature, Depth). The data sets were collected in 2017 in Santorini using an Autonomous Underwater Vehicle during the GEOMAR POS510 mission. Our research shows that the active vent field within the caldera probably follows a multifractal behavior and exhibits a weak memory effect. Depth Profiles and Time Series show similar behavior among conductivity and temperature. The variance and moments of both parameters underline the existence of two different mechanisms governing the behavior of the vent field. Finally, the structure function shows that changes in the time series are described by a Cauchy–Lorentz distribution. [ABSTRACT FROM AUTHOR]

Introduction: Active hydrothermal vents of volcanic origin provide a remarkable manifestation of life on Earth under extreme conditions, which may have consequences for our understanding of habitability on other terrestrial bodies as well. Methods: Here, we performed for the first time Illumina sequencing of bacterial and archaeal communities on sub-seafloor samples collected from the Santorini-Kolumbo volcanic field. A total of 19 (3-m long) gravity corers were collected and processed for microbial community analysis. Results: From a total of 6,46,671 produced V4 sequences for all samples, a total of 10,496 different Operational Taxonomic Units (OTUs) were identified that were assigned to 40 bacterial and 9 archaeal phyla and 14 candidate divisions. On average, the most abundant phyla in all samples were Chloroflexi (Chloroflexota) (24.62%), followed by Proteobacteria (Pseudomonadota) (11.29%), Firmicutes (Bacillota) (10.73%), Crenarchaeota (Thermoproteota) (8.55%), and Acidobacteria (Acidobacteriota) (8.07%). At the genus level, a total of 286 known genera and candidate genera were mostly dominated by members of Bacillus, Thermoflexus, Desulfatiglans, Pseudoalteromonas, and Pseudomonas. Discussion: In most of the stations, the Chao1 values at the deeper layers were comparable to the surface sediment samples denoting the high diversity in the subsurface of these ecosystems. Heatmap analysis based on the 100 most abundant OTUs, grouped the sampling stations according to their geographical location, placing together the two hottest stations (up to 99°C). This result indicates that this specific area within the active Kolumbo crater create a distinct niche, where microorganisms with adaptation strategies to withstand heat stresses can thrive, such as the endospore-forming Firmicutes. [ABSTRACT FROM AUTHOR]

Two submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra‐caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault‐reservoir interaction in calderas. Plain Language Summary: Most tsunamis are generated by large earthquakes with seismic magnitudes M > ∼7, but two moderate‐sized earthquakes with only M 5.8 near volcanic islands, north of New Zealand, caused unusual tsunamis with a maximum wave height of about a meter. In this study, we examine the unusual source mechanism of the volcanic earthquakes that excited unexpected tsunamis. By analyzing the records of tsunamis and seismic waves from the earthquakes, we suggest that the inside of a curved fault system beneath a submerged volcano with a caldera structure suddenly moved upward, together with a large intra‐caldera fault slip and a volume increase of a shallow magma reservoir. Overpressure created by magma accumulation beneath the submarine caldera recurrently induces meter‐scale tsunamis without significant ground motions, calling for attention to tsunami hazards from submarine calderas. Key Points: Unusual tsunamis were caused by two Mw 5.8 volcanic earthquakes in 2009 and 2017 near Curtis and Cheeseman Islands in the Kermadec ArcBy using tsunami and seismic data of the 2017 event, we suggest that a trapdoor faulting event occurred at a submerged resurgent calderaTrapdoor faulting events at calderas had abnormally large slips, possibly due to the fault‐reservoir interaction in calderas [ABSTRACT FROM AUTHOR]

Voluminous pumice rafts produced by the 2021 phreatomagmatic eruption of Fukutoku-Oka-no-Ba, a submarine volcano located in the Izu-Bonin Islands, reached many Japanese ports and islands, damaging fisheries and hindering marine traffic and trade. This event emphasized the necessity for governments and stakeholders to develop plans in advance and prepare disaster mitigation measures before an eruption. To investigate when and to what extent pumice might intersect trade routes and arrive at islands after future eruptions, we conducted particle tracking simulations of eruptions at major volcanic islands and submarine volcanoes near Japan using the velocity field from the ocean reanalysis dataset. Seven major volcanoes that have produced pumice rafts in the past century were selected: the submarine volcano NNE of Iriomote Island, Izu-Tobu Volcanoes, Miyakejima, Bayonnaise Rocks, Nishinoshima, Kaitoku Seamount, and Fukutoku-Oka-no-Ba. We partly reproduced the distribution of pumice arrivals recorded after the 1986 Fukutoku-Oka-no-Ba eruption, demonstrating the potential effectiveness of the simulations. We report likely pumice raft arrivals and drifting durations for the investigated eruptive scenarios, which may aid future risk assessments for pumice arrivals. [ABSTRACT FROM AUTHOR]

IntroductionActive hydrothermal vents of volcanic origin provide a remarkable manifestation of life on Earth under extreme conditions, which may have consequences for our understanding of habitability on other terrestrial bodies as well.MethodsHere, we performed for the first time Illumina sequencing of bacterial and archaeal communities on sub-seafloor samples collected from the Santorini-Kolumbo volcanic field. A total of 19 (3-m long) gravity corers were collected and processed for microbial community analysis.ResultsFrom a total of 6,46,671 produced V4 sequences for all samples, a total of 10,496 different Operational Taxonomic Units (OTUs) were identified that were assigned to 40 bacterial and 9 archaeal phyla and 14 candidate divisions. On average, the most abundant phyla in all samples were Chloroflexi (Chloroflexota) (24.62%), followed by Proteobacteria (Pseudomonadota) (11.29%), Firmicutes (Bacillota) (10.73%), Crenarchaeota (Thermoproteota) (8.55%), and Acidobacteria (Acidobacteriota) (8.07%). At the genus level, a total of 286 known genera and candidate genera were mostly dominated by members of Bacillus, Thermoflexus, Desulfatiglans, Pseudoalteromonas, and Pseudomonas.DiscussionIn most of the stations, the Chao1 values at the deeper layers were comparable to the surface sediment samples denoting the high diversity in the subsurface of these ecosystems. Heatmap analysis based on the 100 most abundant OTUs, grouped the sampling stations according to their geographical location, placing together the two hottest stations (up to 99°C). This result indicates that this specific area within the active Kolumbo crater create a distinct niche, where microorganisms with adaptation strategies to withstand heat stresses can thrive, such as the endospore-forming Firmicutes.

Abstract Two submarine earthquakes (Mw 5.8) occurred near volcanic islands, Curtis and Cheeseman, in the Kermadec Arc in 2009 and 2017. Following both earthquakes, similar tsunamis with wave heights of about a meter, larger than expected from their moderate seismic magnitudes, were observed by coastal tide gauges. We investigate the source mechanism for both earthquakes by analyzing tsunami and seismic data of the 2017 event. Tsunami waveform analysis indicates that the earthquake uplifted a submerged caldera around the islands. Combined analysis of tsunami and seismic data suggests that trapdoor faulting, or sudden intra‐caldera fault slip interacted with magma reservoir deformation, occurred due to magma overpressure, possibly in association with caldera resurgence. The earthquake scaling relationship for trapdoor faulting events at three calderas deviates from that for regular earthquakes, possibly due to the fault‐reservoir interaction in calderas.

Large amounts of pumice stone generated by the submarine volcanic eruption at Fukutoku Okanoba on 13 August 2021 drifted ashore, affecting ship navigation and fishery operations and posing challenges for disposal and a risk to benthic sea-life. As a new approach to managing ejected pumice from submarine eruptions, we investigated the possibility of using pumice as a sand-capping material for eutrophic sediments through laboratory experiments. Crushed pumice as a sand cover material effectively reduced the sedimentary oxygen consumption rate. Nutrient release from sediment showed a similar trend, with ~25% and 82% reductions in NH4-N and PO4-P release rates, respectively. Furthermore, bivalve exposure experiments using crushed pumice suspended in seawater showed no adverse effects specific to pumice and lowered bivalve mortality to a greater extent than did using kaolin at the same concentration. This could be related to the differences in gill accumulation and blockage owing to the particle size variation of suspended particles. These results suggest that crushed pumice is effective for sand-capping and suitably suppresses oxygen consumption and nutrient release from sediments. [ABSTRACT FROM AUTHOR]

Fukutoku‐Oka‐no‐Ba is a submarine volcano located at 24°17.1′ N/141°28.9′ E in the Izu–Bonin arc, and is one of the most active volcanoes in Japan. This volcano produced an explosive eruption in August 2021 that generated a large amount of volcaniclastic material, some of which drifted westward to Japan and the coastal area of East Asia as a pumice raft. The pumice clasts that drifted for >1000 km were mostly homogeneous and identical to those produced by past historical eruptions. The clasts have trachytic compositions (SiO2 = 61–63 mass% and Na2O + K2O = 8.6–10.0 mass%) and contain augite, plagioclase, olivine (Mg# ~65), and magnetite, along with a small number of mafic enclaves containing diopside and high‐Mg olivine (Mg# ~ 92). We undertook a research cruise to investigate the proximal volcaniclastic materials by dredging. The proximal materials include pumice, weakly vesiculated lapilli, and volcanic blocks, which have trachytic composition (SiO2 contents up to 64.5 mass%). The main minerals in the proximal material are similar to those in the drift pumice, although remnants of mafic magma do not occur in the SiO2‐rich samples. The petrographic and geochemical characteristics of the proximal and drift ejecta from Fukutoku‐Oka‐no‐Ba suggest the magma reservoir was stratified into two parts. The major part experienced magma mixing with a limited volume of mafic magma, whereas the other part was more differentiated. The differentiated high‐SiO2 magma accumulated in the upper part of the magma reservoir and avoided the mixing with and feed of volatile from the mafic magma, then were pushed out from the volcanic vent without extensive bubbling to sunk in the proximal area. [ABSTRACT FROM AUTHOR]

We present the bathy-morphological map at a scale of 1: 50,000 of the area around the submerged Ferdinandea Island, the ‘Neverland’ of the Sicily Channel (central Mediterranean Sea). We investigate an area of 100 km², between 10 and 350 m, which is part of a triangular morphological high, 360 km² wide, representing the SE-wards prolongation of the Adventure Bank. The study is based on the morphometric analysis based on high resolution multibeam, and sub-bottom CHIRP profiles collected in 2015. The area around the remains of Ferdinandea Island is morphologically shaped by the interplay between volcanic, tectonic, fluid seepage, and oceanographic processes. Since the study area is considered a hot spot of biodiversity affected by maritime traffic (especially in Ferdinandea Channel) and hosting communication pipelines, this map provides insights both for habitat mapping purposes and preliminary marine geohazard assessment due to the occurrence of historically active submarine volcanoes, pockmarks, and mass transport deposits. [ABSTRACT FROM AUTHOR]

The D. João de Castro is a submarine volcano with known hydrothermal activity located in the Terceira ultra-slow spreading rift within the Azores triple junction (ATJ). Several well-known mafic and ultramafic rock-hosted seafloor hydrothermal systems lay along the Mid-Atlantic Ridge, to the north and to the south of the Azores platform, yet little is known about seafloor hydrothermal activity, ore-metal availability, and magmatic–hydrothermal interactions within the ATJ. Here, we investigate multi-phase melt inclusions hosted in early formed phenocrysts (olivine, clinopyroxene and plagioclase), and metallic precipitates found in groundmass vesicles. Combining detailed petrographic observations with geochemical data and thermobarometry calculations, we assess P–T conditions of early formed phenocrysts, melt pathways towards surface, timing of sulfide saturation and composition of immiscible sulfide melts. Results show that D. João de Castro is characterized by a multi-level magmatic system where primary melt segregated from the upper mantle and moved up through the oceanic crust with little residence time. Sulfide saturation with the formation of immiscible magmatic sulfide liquid (Fe–Ni–Cu) occurred early in primitive magmas with clinopyroxene and olivine crystallization and continued during plagioclase crystallization. At shallower levels, the magmatic degassing of volatiles carrying base metals (Cu–Zn–Pb–Co) and Ba have contributed to the element budget of the D. João de Castro hydrothermal system. The study of multi-phase melt inclusions and vesicles at D. João de Castro submarine volcano contributes to the understanding of source to surface magmatic processes at the Terceira Rift and underline the importance of magmatic degassing into seafloor hydrothermal systems. [ABSTRACT FROM AUTHOR]

Submarine volcano 7.10, which is part of the North Iturup group of submarine volcanoes of the Kuril island arc, was studied in 5 voyages of the research vessel Vulkanolog in 1982–1989. Comprehensive studies have shown that the edifice of the 7.10 submarine volcano is composed of rocks of a range from dacites to basalts. The summit and slopes of the volcano are devoid of sediments, and the base is overlain by a sedimentary stratum, thickness of which reaches 800 m. The minimum depth recorded above the top of the volcano is 210 m. In the volcanic edifice, subvertical, northeastern and northwestern feeder channels, as well as peripheral magma chambers at depths of 2.5–3.0 km, were identified. It is assumed that during the terminal eruption, small lava flows poured out in the northeast and southwest directions, while the main lava flow poured out in the southeast direction and reached the base of the volcanic edifice.

Tephra layers in marine sediments are widely used to correlate and date paleoclimate and paleoceanography records, and to determine spatiotemporal changes in magmatic evolution and eruption frequency. Dissolved matrix glass H2O contents of marine tephra could potentially inform understanding of eruption processes but are rarely used due to the issue of secondary hydration after deposition. Recent advancements in Fourier transform infrared spectroscopy (FTIR) volatile analysis have enabled reconstruction of original water contents of hydrated volcanic glasses. These new Fourier transform infrared spectroscopy analysis methods offer a new way to investigate tephra stored in marine sedimentary archives. We present a case study of the Od-1 tephra layer in marine sedimentary core C9010E, drilled ∼40 km south of the Boso peninsula in Japan. This tephra was erupted by the shallow silicic submarine Oomurodashi volcano in the northern Izu-Bonin arc at ∼13.5 ka. Our Fourier transform infrared spectroscopy volatile data show it has been affected by secondary hydration, with the extent of hydration controlled by grain size and porosity characteristics. Numerical modelling of low temperature hydration suggests Fourier transform infrared spectroscopy data offer an additional method for estimating eruption ages of marine tephra. OH contents, unaltered by low temperature secondary hydration, record low ambient eruptive pressures for all grain sizes and tephra types i.e., blocky and dense or pumiceous. Consideration of hydrostatic pressure gradients and past sea level at Oomurodashi shows that the majority of tephra volatile data cannot be explained by quench within a submarine eruption plume. Instead, OH contents record quench fragmentation within the shallow submarine edifice. Physical characteristics of the tephra are consistent with the formation of these tephra by explosive phreatomagmatic eruption processes. Together these OH data and tephra characteristics support the interpretation that the Od-1 tephra layer was formed by the same shallow phreatomagmatic eruption that formed the existing Oomuro Hole crater and that produced subaerial tephra deposits on nearby Izu-Oshima and Toshima islands. This study demonstrates the crucial contribution that imaging Fourier transform infrared spectroscopy analysis can make to the interpretation of degassing and eruption processes of volcanic glasses, particularly vesicular pyroclasts and/or glasses affected by secondary hydration, adding an important new dimension to marine tephra research.

The presence of active hydrothermal vent fields near residential areas and their possible link to volcanic activity poses a potential hazard to the environment, society, and the economy. By capitalizing on Autonomous Underwater Vehicle sampling methodologies and applying the Generalized Moments Method model for geological and physical processes in these environments, we shed light on the underlying dynamics shaping the physicochemical characteristics of the vents. In this study, we focus on the Northern Caldera of Santorini and, more specifically, on the recorded CTD data (Conductivity, Temperature, Depth). The data sets were collected in 2017 in Santorini using an Autonomous Underwater Vehicle during the GEOMAR POS510 mission. Our research shows that the active vent field within the caldera probably follows a multifractal behavior and exhibits a weak memory effect. Depth Profiles and Time Series show similar behavior among conductivity and temperature. The variance and moments of both parameters underline the existence of two different mechanisms governing the behavior of the vent field. Finally, the structure function shows that changes in the time series are described by a Cauchy–Lorentz distribution.

The main cause of tsunamis is large subduction zone earthquakes with seismic magnitudes Mw > 7, but submarine volcanic processes can also generate tsunamis. At the submarine Sumisu caldera in the Izu–Bonin arc, moderate‐sized earthquakes with Mw < 6 occur almost once a decade and cause meter‐scale tsunamis. The source mechanism of the volcanic earthquakes is poorly understood. Here we use tsunami and seismic data from the recent 2015 event to show that abrupt uplift of the submarine caldera, with a large brittle rupture of the ring fault system due to overpressure in its magma reservoir, caused the earthquake and tsunami. This submarine trapdoor faulting mechanism can efficiently generate tsunamis due to large vertical seafloor displacements, but it inefficiently radiates long‐period seismic waves. Similar seismic radiation patterns and tsunami waveforms due to repeated earthquakes indicate that continuous magma supply into the caldera induces quasi‐regular trapdoor faulting. This mechanism of tsunami generation by submarine trapdoor faulting underscores the need to monitor submarine calderas for robust assessment of tsunami hazards. Plain Language Summary: Tsunamis are mainly caused by large submarine earthquakes, but submarine volcanic processes can also trigger tsunamis. Disproportionately large tsunami waves have been generated every decade by moderate‐sized volcanic earthquakes at a submarine volcano with a caldera structure, called Sumisu caldera, in the Izu–Bonin arc, south of Japan. Despite the moderate earthquake size, the maximum wave heights of the tsunamis were about a meter, and their source mechanism has been controversial. In this study, we used tsunami and seismic data from a recent earthquake to show that the submarine caldera abruptly uplifts due to brittle rupture of its intra‐caldera fault system driven by overpressure of magma accumulating in its underlying magma reservoir and generates large tsunamis almost once a decade. The atypical source mechanism for tsunami generation suggests that it is important to monitor active submarine calderas for assessing tsunami hazards. Key Points: Large tsunamis are generated by moderate‐sized volcanic earthquakes at a submarine calderaTsunami and seismic data indicate that abrupt uplift of the submarine caldera by trapdoor faulting causes large tsunamisContinuous magma supply into the submarine caldera induces submarine trapdoor faulting on a decadal timescale [ABSTRACT FROM AUTHOR]

Abstract The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region.

Submarine hydrothermal systems along active volcanic ridges and arcs are highly dynamic, responding to both oceanographic (e.g., currents, tides) and deep-seated geological forcing (e.g., magma eruption, seismicity, hydrothermalism, and crustal deformation, etc.). In particular, volcanic and hydrothermal activity may also pose profoundly negative societal impacts (tsunamis, the release of climate-relevant gases and toxic metal(loid)s). These risks are particularly significant in shallow (

Submarine volcanic eruptions can perturb the water surface and subsequently generate localized tsunamis. Herein waves are produced in analogous laboratory experiments by injections of non‐condensable expanding air into fresh water. By varying source pressures, jet durations and water depths, we obtain two key results. There exists a saturation duration above which the maximum wave height will not increase as the eruption continues. Apart from in extremely shallow or deep water depths, this saturation duration is positively correlated with the source intensity and negatively with the water depth. There also exists a critical water depth where an eruption with a given source intensity can generate the largest waves. When an eruption lasts less than the saturation duration, increasing the eruption duration will increase the critical water depth and the corresponding maximum wave height. The saturation duration and critical water depth are the primary controls on tsunami size and hazard. Plain Language Summary: When volcanoes erupt underwater, they can generate waves that are dangerous even at coastlines far away from the eruption itself. In this study, we investigated whether the wave heights changed when the underwater eruption lasted a different amount of time. Our experiments modeled an underwater eruption by releasing compressed air into a tank of water. We carefully controlled the duration of the eruption and measured the size of the waves generated in the tank. Although at first longer eruptions generated larger wave heights, eventually the height reached a maximum value. Further increases in the duration did not lead to larger wave heights. Changing the water depth also affected the wave heights; there is a water depth where a given eruption can generate the largest wave heights. Our experiments show how this "critical" water depth changes depending on the eruption duration. Because both the depth and duration of eruption are important in determining the wave height, they should be considered in future hazard assessments of tsunamis generated by underwater volcanic eruptions. Key Points: We investigate the effect of underwater volcanic eruption duration on the properties of generated waves, using analogous gas jet experimentsThere is a saturation duration above which the maximum wave amplitude does not increase with increasing eruption durationFor durations shorter than the saturation duration, longer eruptions increase the critical water depths associated with the largest waves [ABSTRACT FROM AUTHOR]

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region. [ABSTRACT FROM AUTHOR]

Phytoplankton community was investigated during two contrasting periods using offshore plankton samples in the volcanic area of Methana peninsula (Saronikos Gulf): the first at early autumn (warm period, September 2016) and the second one at early spring (cold period, March 2017). In order to investigate the phytoplankton community structure in the complex geo-biochemical conditions of the area, samples were collected from stations near the CO2 hydrothermal vents, at the hydrothermal sulfur and radioactive springs and at a fishery nearby Methana town. Three major phytoplankton groups, Bacillariophyceae, Dinophyceae, and Prymnesiophyceae, were studied, using inverted microscopy. In early autumn, Dinophyceae were dominant in the majority of the stations with cell concentrations of Prorocentrum spp. up to ~ 35.5 × 103 cells l−1. In early spring, the dominant class was Bacillariophyceae with dominant genus Nitzschia/Pseudo-nitzschia presenting cell concentrations up to ~ 33.9 × 103 cells l−1. Furthermore, Prymnesiophyceae appeared in both spring and autumn samples with small fluctuations. Total phytoplankton cell concentrations followed a seasonal trend, presenting slightly lower values in the hydrothermal-effected area in comparison with the broader Saronikos Gulf, confirming the prevalence of oligotrophic conditions. Seasonal variation was very strong, revealing an association with water temperature and nutrient content. Those environmental variables proved to have a strong effect that was reflected in the phytoplankton community structure. [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]

Volcanic activity is one of the primary sources of arsenic in nature, and chemical forms of arsenic discharged by volcanic activity are inorganic. Kagoshima Bay has active submarine volcano located in the 200 m deep basin of its northern part. Since dissolved inorganic arsenic is more toxic than organic arsenic it is important to understand the dispersion of discharged arsenic in seawater. To estimate the influence of arsenic emitted from the bottom vents seasonal changes in As(III) and As(V) concentrations in seawater were studied. There are few studies on the impact of arsenic discharged by submarine volcanos in the inner bay. The total concentrations of As(III) and As(V) (T-As) in seawater at the background site in Kagoshima Bay was 1.71 ± 0.35 μg L−1 (av. ± s.d.), similar to the world average value of 1.7 μg L−1. The highest T-As value at the nearest point to the submarine volcano, 3.48 μg L−1, was observed in summer. Since the vertical mixing of seawater cannot occur in summer due to the formation of the thermocline, the deep water is strongly affected by volcanic activity. The increase in T-As was caused by the increase in As(III) concentration suggesting the discharge of As(III) from the vents. [ABSTRACT FROM AUTHOR]

Many of the world's volcanoes are hidden beneath the ocean's surface where eruptions are difficult to observe. However, seismo‐acoustic signals produced by these eruptions provide a useful means of identifying active submarine volcanism. A literature survey revealed reports of 119 seismo‐acoustically recorded submarine eruptions since 1939. Submarine eruptions have been recorded in all major tectonic settings, with a range of geochemistries, and at a variety of water depths, but the reports are dominated by eruptions in the Pacific and at only a few locations. Many of the reports offer little detail, with over half of the observations made from distances >500 km, and only about half were confirmed as eruptions by non‐seismo‐acoustic evidence. The reported seismo‐acoustic signals cover a wide variety of processes, including earthquakes, explosions, various types of tremor, signals related to lava extrusion, and landslides. Recorded signals can sometimes be difficult to classify or confidently associate with an eruption, although there has been progress in this regard. Real‐time monitoring of submarine eruptions has been on‐going for several decades on regional and global scales with growing interest and effort in local networks. Real‐time networks are complemented by short‐term instrument deployments that often give more detailed insights into the dynamics and processes of submarine eruptions. Thorough seismo‐acoustic monitoring and study has increased our understanding of submarine eruptions, especially of deep‐sea volcanoes and spreading centers. Despite this, there are still many outstanding questions that need to be addressed for submarine volcanoes to be as well understood and monitored as their terrestrial counterparts. Key Points: A literature survey found reports of 119 submarine volcanic eruptions recorded by seismic, hydroacoustic, or acoustic instruments since 1939This review summarizes the current state of submarine eruption seismo‐acoustics based on the literature survey resultsSeismo‐acoustic monitoring and study has increased our understanding of submarine eruptions but many scientific opportunities remain [ABSTRACT FROM AUTHOR]

Offshore northern Ishigaki‐Jima Island, in the southern Okinawa Trough, offers outstanding opportunities to explore the rifting stage of a backarc system. We report the results of integrated marine geological and geophysical surveys with high‐density survey lines in this area. We identify a graben bounded by normal faults and extending approximately 59 km in an ENE‐WSW direction off‐axis of the southern Okinawa Trough. Submarine volcanoes with active hydrothermalism and associated intrusive structures lie in the graben. Magnetic anomaly and seismicity data in and around the graben suggest the presence of relatively shallow magma acting as a heat source. All features identified in and around the graben suggest active rifting in the southern Okinawa Trough. Plain Language Summary: The Okinawa Trough, adjacent to the southwesternmost Japanese islands, has experienced rifting of its continental crust since 2 Ma. The southern part of the Trough includes a rift valley, the Yaeyama Rift, where rifting is active. Herein, using marine geological and geophysical data, we report the discovery of a graben bounded by normal faults on the southern margin of the southern Okinawa Trough. The graben hosts several submarine volcanoes, under which magnetic anomalies suggest relatively shallow magma. Because these geologic features resemble graben in the axial trough, we propose that the newly identified graben represents a rifting episode previously undocumented in the geological evolution of the Okinawa Trough. Key Points: A graben controlled by a normal fault was discovered in the southern Okinawa Trough offshore northern Ishigaki‐Jima IslandMagnetic anomaly and seismicity data indicate relatively shallow magma acting as a heat sourceThe graben likely manifests embryonic rifting on the southern margin of the southern Okinawa Trough [ABSTRACT FROM AUTHOR]

Variations in stress drops of earthquakes associated with the April and May 2015 eruption of Axial Seamount, on the Juan de Fuca Ridge, suggest a reduction in crustal strength as a result of the eruption. Seismicity during the inflation and deflation periods was well recorded by ocean bottom seismometers located within and along the caldera. We use these nearby recordings and an empirical Green's function spectral ratio method to obtain corner frequencies for stress drops of earthquakes on caldera ring faults. We find stress drops from 0.6 to 43 MPa for 423 ring fault earthquakes (1.6 ≤ MW ≤ 3.6) and an average stress drop two times higher during the inflation period (6.4 MPa) prior to the eruption, than during the subsequent deflation (3.2 MPa). Stress drops also correlate with spatially varying shear wave speed, possibly reflecting a region of pervasive cracking in the northern caldera. Plain Language Summary: How do earthquakes reflect changes in crustal strength at a volcano? Seismometers on the seafloor at Axial Seamount (an underwater volcano off the coast of the U.S. Pacific Northwest) recorded thousands of earthquakes associated with Axial's 2015 eruption. We investigate changes in earthquake behavior that reveal a reduction in the strength of volcanic ring faults as a result of this eruption. We find that earthquakes release more stress when the volcano inflates before the eruption, than as the volcano deflates during and after the eruption. This suggests that increased crustal damage resulting from the eruption limits stress accumulation. Key Points: Ocean bottom seismic data are used to obtain stress drops of 423 ring fault earthquakes associated with the eruption of a submarine volcanoHigher stress drops during inflation and lower stress drops during deflation suggest decreased fault strength as a result of the eruptionStress drop spatial heterogeneity correlates with changes in shear wave speed, suggesting increased damage in Axial's northern caldera [ABSTRACT FROM AUTHOR]

Monowai is an active submarine volcanic centre in the Kermadec Arc, Southwest Pacific Ocean. Multi-beam data acquired during expedition SO225 aboard R/V SONNE in December 2012 indicates that the topography of the main stratocone has evolved significantly since the last survey in June 2011. Bathymetric measurements of the edifice reveal differences of up to 42 m in seafloor depth and indicate a net volume increase of ∼0.037 km3 across the summit area. Explosive volcanism observed onsite during the SO225 mapping campaign could be linked to a 20h-long swarm of unusually coherent T phase arrivals, suggesting that Monowai is a prime source of broadband seismic noise in the Southwest Pacific region during times of activity. Our findings further document the dynamic nature of volcanic processes at Monowai and have implications for future expedition planning. [ABSTRACT FROM AUTHOR]

The aim of the study is to understand the characteristics of a volcano occurred in marine environment, as Weh Island where Sabang City located is still demonstrated its volcanic cone morphology either through satellite imagery or bathymetric map. Methods used were marine geology, marine geophysics and oceanography. Results show that surface volcanism (sea depth less than 50 m) take place as fumaroles, solfataras, hot ground, hot spring, hot mud pool and alteration in the vicinities of seafloor and coastal area vents. Seismic records also showed acoustic turbidity in the sea water column due to gas bubblings produced by seafloor fumaroles. Geochemical analyses show that seafloor samples in the vicinities of active and non-active fumarole vent are abundances with rare earth elements (REE). These were interpreted that the fumarole bring along REE through its gases and deposited on the surrounding seafloor surface. Co-existence between active fault of Sumatra and current volcanism produce hydrothermal mineralization in fault zone as observed in Serui and Pria Laot-middle of Weh Island which both are controlled by normal faults and graben.

Extensive deep (>1500 m) non-hydrothermal ash-rich plumes were present in the water column surrounding West Mata submarine volcano (NE Lau basin) during eruptive phases, but were absent when West Mata was quiescent. Deep ash plumes were chemically distinct from hydrothermal plumes originating at West Mata and other hydrothermal sources in the region, and were dominated by very fine shards compositionally similar to the lava erupting at the ∼1200 m summit with morphologies indicative of magma degassing and shattering. Deep ash plume layers occurred at variable water depths (and distances above the seafloor) and were transported by local currents to 10s of km from West Mata. We interpret these deep ash plumes to be syn-eruptively generated ash transported downslope via sediment gravity flows of varying magnitudes, runout distances, and liftoff densities. Final deposition of individual particles will be by settling from suspension (i.e., fallout deposits). The mechanism for initiating gravity flows is uncertain, but their spatial and temporal distribution suggests that eruption column collapse and/or mobilization of unstable volcaniclasts accumulating on steep slopes during the eruption are likely sources. Turbidity within the deep ash plumes decreased by 80% over a 3 week period in 2017, suggesting the eruption that created a new deposit, constrained between March 2016 and November 2017, was likely active as recently as a few weeks to months prior to the FK171110 expedition. Similar deep ash plumes have been observed surrounding other erupting submarine volcanoes. This pattern of syn-eruptive dispersal suggests that fine ash does not have to be lofted high into the water column in event plumes, or enter the ocean from subaerial sources, to be dispersed by local currents and contribute to fallout deposits in regional and distal sediments, which may appear as cryptotephra or thin ash layers if the ash supply exceeds pelagic sedimentation rates; deposits that are often attributed to subaerial eruptions alone. These results also suggest that the presence of deep ash plumes surrounding a submarine volcanic edifice, along with a magmatic-hydrothermal plume above the summit, may be a valuable diagnostic exploration tool for identifying active, or very recently active, submarine eruptions.

Oceanic plateaus are large igneous provinces in the oceans, created by massive underwater eruptions, but their late-stage volcanism is poorly understood. With the addition of recent high-quality bathymetric data to existing data, 286 secondary cones were discovered over Shatsky Rise oceanic plateau. These cones with steeper flank slopes (mean 6.1° ± 4.4°) and smaller sizes (102–1923 m in height) are morphologically distinct from the plateau, and they are thought to have formed after the main volcanic episodes. Cone height and characteristic height (420 m) are close to seamounts in the Pacific Ocean, whereas greater than those in the Atlantic Ocean. Mean flatness of Shatsky Rise's cones (0.25 ± 0.20) are similar to that of seamounts in both Pacific and Atlantic Oceans, but notably density in cone distribution (0.56 km−3) and their mean slope are significantly lower than those of seamounts in the two oceans. Lower slopes of secondary cones within Shatsky Rise may be explained by higher effusion rates of remaining magma. Although cone formation was expected to have a link to rifting by seafloor spreading, weak relationship between cone orientation and magnetic anomaly pattern implies that the expectation is negative. Moreover, weak correlation between the cone height and depth indicates it is not true that volcanic cones grow taller when they occur closer to the massif summits with thicker oceanic crust, which was suggested as the increase in hydraulic pressure. Cone height and flatness are also not strongly related, implying that remaining magma supply was too limited to foster the cones to critical height. [ABSTRACT FROM AUTHOR]

The paper presents results of a comprehensive study of ferromanganese crusts from a submarine volcano on the Sonne Ridge, which was discovered during the 178th cruise of the RV Sonne (2004) investigating the northern slope of the Kuril basin in the framework of the KOMEX program. Results of study of the structures, textures, and mineral and elemental compositions of the crusts suggest their hydrogenous origin. The contents of major, trace, and rare-earth elements and Y (REY) in four mineral phases of ferromanganese crusts of the Sea of Okhotsk have been determined by phase analysis for the first time. The REY patterns of the manganese and ferrous in-situ phases confirms the hydrogenous genesis and indicates no hydrothermal influence. [ABSTRACT FROM AUTHOR]

Marine geoheritage comprises unique geological features of modern and ancient seas and oceans. The Russian Altai (southern Siberia) is a vast and geologically rich area, which was covered by a marginal sea of the Panthalassa Ocean in the Devonian. New geosites representing shallow- and deep-marine depositional environments and palaeoecosystems of submarine volcano slopes are proposed, namely, Melnichnye Sopki and Zavodskie Sopki. They are located near the town of Zmeinogorsk (Altai Region of the Russian Federation). These pieces of marine geoheritage are valuable on an international scale. Special geoconservation procedures are recommended to manage the proposed geosites efficiently. They can be included in a geopark, which is reasonable to create due to the concentration of geological and mining heritage in the study area.

In this paper, a three-dimensional, nonlinear model is proposed to incorporate the effect of submarine volcanic eruption on equatorial oceanic flows. The model inspired by the Constantin and Johnson model, considers an appropriate density function to model the density of the oceanic system with a volcano erupting at a point on the sea-bed, synchronizing with the temperature variations. The formulated model contains term related to Coriolis effect and is consistent with β -plane approximation. Exact results for the model have been obtained using the asymptotic system of equations. [ABSTRACT FROM AUTHOR]

Organic mercury, inorganic mercury and total mercury concentrations in phytoplankton (<0.1 mm) and zooplankton (>0.1 mm) collected in Kagoshima Bay, Japan were measured from 2017 to 2019 to estimate the impact of mercury discharged from submarine volcanoes on ecosystems; submarine volcanic activity continues at a depth of 200 m in the inner part of Kagoshima Bay. The total mercury concentrations in phyto- and zooplankton collected by vertical hauling at 0–200 m at just above the submarine volcano were in the range of 0.11–2.0 mg kg−1 (avg. 0.67 mg kg−1) and 0.090–0.56 mg kg−1 (avg. 0.21 mg kg−1), respectively. These values were one order of magnitude higher than the values in plankton collected in the central part of Kagoshima Bay. Organic mercury concentrations in phyto- and zooplankton were <0.010–0.071 mg kg−1 (avg. 0.028 mg kg−1) and 0.012–0.25 mg kg−1 (avg. 0.10 mg kg−1), respectively, for the inner part, and <0.010–0.040 mg kg−1 (avg. 0.010 mg kg−1) and <0.010–0.025 mg kg−1 (avg. 0.012 mg kg−1), respectively, for the central part. The values obtained in the inner part of the bay increased in summer and decreased in winter, which was consistent with changes in seawater mercury concentrations affected by volcanic activity. The organic mercury concentration in zooplankton collected just above the submarine volcano showed a size dependency, and a higher value was observed in the larger size, which suggested that the discharged mercury from the volcano was absorbed and concentrated through the ecosystem. [Display omitted] • T-Hg in plankton around submarine volcano is several times higher than reference site. • Approximately 40% of T-Hg in zooplankton is organic mercury. • The organic Hg in zooplankton collected around the volcano showed a size dependency. • The discharged Hg from the submarine volcano was concentrated through the ecosystem. [ABSTRACT FROM AUTHOR]

High-resolution bathymetric mapping using an autonomous underwater vehicle and a survey vessel was conducted at Daisan–Miyako Knoll in the southern Ryukyu Arc. This paper presents for the first time the detailed volcanic morphology of a submarine edifice that previously was poorly explored, and discusses its formation process and relation to arc volcanism and tectonics. Bathymetry shows morphological evidence that Daisan–Miyako Knoll is a stratovolcano with three caldron-like depressions (>1 km in diameter), a central cone, and multiple lava flows at the summit. The main edifice is conical with a basal diameter of 8–10 km and relief of 1000–1100 m, with a summit depth of 779 m. The central cone that stands inside the depressions has a circular crater. Numerous blocks up to 10 m wide are distributed around the crater, indicating a previous explosive eruption. Lava flows erupted from the central cone have a maximum thickness of 46 m at the flow margins and a maximum flow length of 1.9 km. The total flow area is 1.5 km 2 and the estimated volume is 0.03 km 3 . The relatively thick lava flows (up to 46 m) with steep-sided margins (30–40°) and curtain-folded surface textures suggest that silicic magmas erupted, and their fresh appearance indicates that lava-flow eruptions (the most recent volcanic activity) occurred during the Quaternary. The volcanic features at Daisan–Miyako Knoll are aligned mainly in the NW–SE direction, suggesting that volcanism here may be controlled primarily by arc-parallel extension. No volcanic edifices have been confirmed on the trench side of Daisan–Miyako Knoll, therefore this edifice defines the location of the arc volcanic front in this region. From the larger rhyolitic edifices (>10 km wide) around Daisan–Miyako Knoll along the front, it is inferred that silicic arc volcanism is extending from the central to southern submarine Ryukyu Arc. [ABSTRACT FROM AUTHOR]

Over the last decades, there has been growing interest about the ecological role of hydrothermal sulfide chimneys, their microbial diversity and associated biotechnological potential. Here, we performed dual-index Illumina sequencing of bacterial and archaeal communities on active and inactive sulfide chimneys collected from the Kolumbo hydrothermal field, situated on a geodynamic convergent setting. A total of 15,701 OTUs (operational taxonomic units) were assigned to 56 bacterial and 3 archaeal phyla, 133 bacterial and 16 archaeal classes. Active chimney communities were dominated by OTUs related to thermophilic members of Epsilonproteobacteria, Aquificae and Deltaproteobacteria. Inactive chimney communities were dominated by an OTU closely related to the archaeon Nitrosopumilus sp., and by members of Gammaproteobacteria, Deltaproteobacteria, Planctomycetes and Bacteroidetes. These lineages are closely related to phylotypes typically involved in iron, sulfur, nitrogen, hydrogen and methane cycling. Overall, the inactive sulfide chimneys presented highly diverse and uniform microbial communities, in contrast to the active chimney communities, which were dominated by chemolithoautotrophic and thermophilic lineages. This study represents one of the most comprehensive investigations of microbial diversity in submarine chimneys and elucidates how the dissipation of hydrothermal activity affects the structure of microbial consortia in these extreme ecological niches. [ABSTRACT FROM AUTHOR]