It is generally assumed that volatile-rich magmas erupt predominantly effusively at water depths of several hundred meters, as the high hydrostatic pressure suppresses explosive activity. This view is supported by numerous multibeam surveys showing that most submarine volcanoes lack explosive craters. Here, we present high-resolution reflection seismic and bathymetric data from submarine volcanoes located at more than 2km water depth along the southern transtensional diffuse plate boundary of the Azores Plateau. The seismic reflection characteristics of the investigated edifice flanks resemble those of volcanoes elsewhere that have been verified by drilling to consist of highly fragmented volcaniclastic deposits, including ash, lapilli or pumice. Additional evidence for explosive activity comes from a crater that was subsequently filled with mounded coarser volcanic deposits, or even effusive lava, during the waning stages of eruption. Consequently, earlier multibeam studies were unable to detect indicators for explosive eruptions. Our results demonstrate that submarine volcanic morphology alone is insufficient to rule out explosive eruptions and highlight the value of seismic imagery to systematically identify explosive submarine volcanism. Deep volcanic explosions seem more common than generally admitted, even on oceanic plateaus. This has implications for magmatic volatile outputs and their regional to global environmental impacts.

Summary The Red Sea is one of the youngest ultraslow-spreading ridges on our planet and an ideal place to investigate the transition from continental rifting to oceanic spreading. Within this context, the Zabargad Fracture Zone (ZFZ) stands out as a particularly intriguing region. The ZFZ hosts notable features, including an offset in the Red Sea spreading axis, the Mabahiss Mons submarine volcano, and the Kebrit Deep brine pool. Additionally, this region is seismically active, posing a hazard to nearby coastal communities. Despite previous geophysical studies, few seismic velocity models image shallow subsurface structures in the ZFZ. In this work, we use approximately one year of seismic ambient noise recorded by five broadband stations and 14 ocean-bottom seismometers to estimate the shear-wave velocity structure of the ZFZ. For this, we compute vertical-vertical, radial-radial, and transverse-transverse noise correlations, obtain group- and phase-velocity dispersion curves of Rayleigh and Love waves in the 3 to 12 s period band, and employ transdimensional tomography to estimate 1-D and 3-D isotropic shear-wave velocity models of the ZFZ. Our 1-D velocity model suggests that, on average, the ZFZ crustal structure comprises a 1.5 km thick layer including hemipelagic sediments and evaporites, a 2.8 km thick oceanic basement, and a crust-mantle transition extending from 6.5 to 8 km below sea level. Meanwhile, our 3-D model agrees with previous geological and geophysical observations and reveals new subsurface structures. In particular, it shows low velocity areas to the east and south of Mabahiss Deep that correlate with known sedimentary basins. Moreover, our 3-D model contains a low-velocity area near Kebrit Deep that correlates with a region of low seismic activity and a recently inferred spreading-axis segment. Based on previous evidence of inactive hydrothermal vents, we infer that this low-velocity area indicates higher basement temperatures near Kebrit Deep compared to other areas. Lastly, our 3-D model displays a velocity contrast in the southern ZFZ that correlates with a contrast in free-air gravity anomalies and a gradient in evaporite topography. Based on these observations, we interpret this velocity contrast as a lineament related to folded evaporites. Our findings present new constraints on the crustal structure of the ZFZ and serve as a reference for other young ultraslow-spreading ridges.

Shallow eruptions of submarine volcanoes can hamper navigation of ships and alter the biological response of marine ecosystems. Satellite remote sensing can provide timely and continuous information about volcanic activity around dangerous sites contributing to the assessment of pre-, syn- and post eruptive phenomena. Among these, sea-water discoloration is one of the most significant indicators of underwater volcanic activity as its accurate and timely detection may help in revealing possible precursor processes of submarine volcanic eruptions. In this framework, we proposed a novel spectrally-derived method to detect and map discolored plumes around submarine volcanoes in oligotrophic oceans by integrating Sentinel 2A/B-MSI and Landsat 8/9-OLI satellite data. The developed method, combining two discoloration algorithms, was tested around a representative test case, namely the Kavachi Volcano (Solomon Islands, Southwest Pacific Ocean), by using a yearly (2022) MSI-OLI integrated dataset. It exhibited satisfactory validation metrics thus recording overall accuracies (OAs) close to 90% for both the single and integrated (multi-sensor) configuration. Despite the omission errors ranging (OEs) from 18 to 20%, the very low (around 2%) commission (CEs) demonstrated its high level of reliability in mapping discolored waters of volcanic origin. Furthermore, the proven exportability of this method to the Kaitoku Volcano (Japan, Western Pacific Ocean) confirms its capability in detecting underwater volcanic activities regardless of different features of sea-water discoloration (e.g., chemical composition). This method could represent an automated early warning tool to support the operational monitoring of submarine volcanoes arranged by maritime surveillance systems.

Abstract Repurposing a submarine telecommunication cable as a distributed acoustic sensor (DAS) significantly enhances earthquake monitoring in the Canary Archipelago, where much of the seismicity occurs offshore. We interrogated two ∼60 km segments of a dark-fiber cable between Tenerife and Gran Canaria from 26 July to 29 September 2020. P and S waves of local earthquakes were first detected using a deep learning model for phase picking and then merged with arrival-time readings from the Instituto Geográfico Nacional seismic network. To prevent overweighting the dense DAS array relative to the sparser land-based stations, we designed a selection strategy that uses a machine-learning-based channel quality index algorithm to automatically retain a limited number of high-signal-to-noise DAS channels per event. The selected channels are spaced according to the actual interstation distances of the National Seismic Network on the islands. We relocated 31 earthquakes (1.1≤mbLg≤3.2) using a nonlinear, probabilistic earthquake location method. The addition of DAS data narrowed the median azimuthal gap by ∼12° and reduced the median depth uncertainty from over 12 to 3.4 km. Within the Enmedio submarine volcano corridor, the hybrid DAS-land network detected 66% of events with mbLg≥1.5, and all events above mbLg=2.0 within 50 km of the cable. These results demonstrate that even a modest number of well-selected DAS channels can significantly improve the precision of earthquake location in submarine environments, providing a scalable and operationally compatible path for enhancing offshore seismic and volcanic monitoring. The reduced volume of data also ensures that DAS integration does not overwhelm existing processing pipelines, making it feasible for use in real-time alert systems.

Seasonal variations in mercury species and selenium in plankton from Kagoshima Bay, southern Kyushu, Japan: Impact of active submarine volcanoes on ecosystems

Abstract The deep submarine volcano Enmedio is located between Tenerife and Gran Canaria, the two most populated islands of the Canary archipelago. This study integrates multidisciplinary data gathered from 2015 to 2024, enabling a comprehensive analysis of the geomorphological, petrological, geochemical, and oceanographic features of this volcano. The combined findings from these disciplines confirm hydrothermal activity at the submarine volcano. The main volcanic cone is characterized by a fracture zone, which crosses it from NNW to SSE and passes over the summit, and a half-moon-shaped depression in the south-western flank, likely caused by a collapse event. In the surrounding area, at least 20 smaller volcanic reliefs have been identified, five of which present crater-like depressions. Backscatter data and seismic profiles suggest significant rock alteration at the fracture zone of Enmedio volcano. This alteration is corroborated by rock samples exhibiting surface modifications characterized by Fe-oxyhydroxide deposits, alongside microscopic filamentous textures typical of biogenic Fe-rich formations. These features suggest a strong association with hydrothermal environments. Additionally, geochemical analyses point to magmatic differentiation processes. The physical, chemical, and biological oceanographic parameters in the seawater show temperature anomalies, water column instabilities, enhanced turbidity, enrichment in inorganic nutrient concentrations, and an overall increase in prokaryotic abundance. These anomalies were found linked to some relevant geomorphological features, such as the summit fracture zone, the half-moon depression, and the two closest smaller volcanic cones. This integrated perspective highlights Enmedio as a focal point of hydrothermal and volcanic activity, shedding light on the dynamic relationship between geological and oceanographic processes in the Canary archipelago.

Abstract Back‐arc basins provide insights into the processes governing the evolution of continental rifting to seafloor spreading. The Bransfield basin hosts a back arc rift that is hypothesized to be in the late stages of this transition. Orca volcano is a submarine volcano that lies on the most evolved portion of the rift. It coincides with a transition in the rifting style from a clearly delineated magmatic rift to the southwest to a wide and deeper faulted graben with thicker sediment to the northeast. We conducted an active‐source tomography experiment to image the three‐dimensional isotropic and anisotropic P‐wave velocity structures of the upper‐crust of Orca Volcano. We developed a method to incorporate secondary arrivals to improve the imaging of the volcano's magma chamber. The magma chamber extends from ∼1 km beneath the seafloor to 3–4 km depth with a maximum melt fraction of 16%–41% and a melt volume of 0.8–2.3 km 3 . The southwest rift of Orca volcano is characterized by a narrow zone of low velocities that connects to the magma chamber and by strong rift parallel anisotropy at shallow depths. Its structure has similarities to a slow spreading ridge consistent with a rift that is transitioning to seafloor spreading. Extension to the northeast of Orca is more distributed and not clearly linked to the volcano. We infer that the change in extensional style results from a more hydrated and weaker mantle to the northeast that may be caused by a tear in the Phoenix slab and a consequent change in slab depth.

The paper presents data on the mineralogy and petrochemistry of basaltoids, dacite, and gabbro dredged from Esmeralda submarine volcano, southern Mariana island arc, during Cruises 4 and 5 of the R/V Vulkanolog. The basaltoids and gabbros of the tholeiitic series are highly fractionated rocks, in contrast to most of the studied rocks from submarine volcanoes and basins in the southern Mariana island-arc system. The rocks contain elevated iron concentrations. The elevated Fe# is consistent with the high Fe content of the pyroxenes and plagioclase, which classes most of the dredged rocks with the association of arc ferrotholeiites. Similar to the basaltoids, the dacite sample contains both an elevated Fe content and a high Fe2O3 concentration in plagioclase, suggesting that the dacite may be genetically related to the dredged basaltoids. The lower concentrations of high-field strength (HFSE) and heavy rare earth (LREE) elements in the rocks than in N-MORB suggest that their parental melts were derived from a depleted mantle source at a spinel-facies depth level. The high FeOtotal/MnO values and discriminant petrochemical characteristics suggest that the deep source contained pyroxenite. The enrichment of the basaltoids and gabbro in large-ion lithophile elements (LILE) and the correlations between some elemental ratios (Th/Nb, Ba/Nd, Ba/Yb, Th/Yb, etc.) indicate that the magma was generated in the presence not only of a low-temperature fluid component but also of a high-temperature magmatic melt, produced by the melting of sedimentary rocks.

First data on the family Dulichiidae (Amphipoda: Senticaudata) from communities of the Piip submarine volcano, the Bering Sea, with the description of a new species of Dulichiopsis

The Antimilos volcanic field of the Aegean Volcanic Arc is studied, based on an oceanographic survey off the northwestern margin of Milos Island, conducted onboard R/V Meteor, comprising high resolution bathymetric data, AUV operations and seismic parasound profiles. The scope of the paper is to describe the three submarine volcanic cones adjacent to Antimilos Island and the interaction of volcanism and tectonics in the Milos-Antimilos region. The results comprise a bathymetric map and a morphological slope map, utilizing a 5-meter grid interval. A morphotectonic map off Antimilos Island was compiled, comprising the three submarine volcanic domes and a hummocky relief extending to the Myrtoon Basin in the NW up to 800 m depth. NW-SE fault zones delimit the tectonic graben between the Cycladic plateau and the western Ridge, hosting the volcanic features and a 72 km 2 hummocky area with VDA deposits representing some recent sliding hazard events triggered by earthquakes. A prominent ENE-WSW fault delimits the Antimilos volcanic field from the NW Milos structure with disruption of the shelf breakThe shelf area at 130–150 m depth is well developed around Antimilos Island and on top of the two shallow submarine volcanic cones at least during the last three low stand periods, indicating their onshore/offshore evolution. The geological hazard of the studied area comprises earthquakes of magnitude Mw 6.0-6.6, submarine landslides with extended VDA deposits and volcanic activity related to the ongoing intense submarine hydrothermal vents.

Submarine volcanoes and faults pose hazards to nearby populated islands, yet their inaccessibility limits monitoring efforts. The Christiana-Santorini-Kolumbo volcanic field is capable of generating devastating eruptions, earthquakes and tsunamis. The 2025 earthquake swarm near Kolumbo, causing the evacuation of thousands from their homes, underlines the need for accurate and real-time monitoring. We interrogate a 45 km dark fibre that extended from Santorini past the submarine volcano Kolumbo for two months in 2021, comparing the performance of the fibre with the existing monitoring network for earthquake detection and location. The detected quakes originated all over Greece, coming from any azimuth. We can reliably identify events, doubling the number of detections in the vicinity of the fibre and Kolumbo. For event location, the azimuthal coverage of the existing seismometer network outperforms the fibre, emphasising the importance of a nonlinear fibre layout. Our findings suggest that while the higher detection sensitivity of DAS leads to an information gain, the data analysis remains challenging. The data quality may be insufficient for automated workflows. The need for human input limits the potential of DAS for real-time monitoring, although the enhanced detection sensitivity in remote areas justifies the continued research of DAS for submarine volcano monitoring.

SUMMARY The seismo-volcanic crisis near Mayotte (Comoros archipelago, Indian Ocean)—marked by the discovery of a new submarine volcano east of the island—is characterized by persistent very-low-frequency (VLF) seismic activity. These events feature long-lasting oscillations exceeding 25 min, with dominant spectral peaks at 67 and 167 mHz. We analyse more than 780 repetitive VLF events recorded between June 2018 and January 2022 using a dense local seismic network, including broad-band ocean-bottom hydrophones. To investigate these events, we developed a spectral-based detection algorithm, a 3-D back-projection location method and performed full-waveform modelling. These techniques constrain the temporal evolution, spectral characteristics, and first-order source properties, including source mechanism and geometry. The VLF sources are located at depths of 20–30 km, near the top of the proximal volcano-tectonic swarm, approximately 12 km east of Petite-Terre. The signals likely originate from the resonance of a magma-filled sill, triggered by transient increases in magma pressure and/or aseismic fault slip acting on a piston-like structure within a complex transcrustal magmatic system. Our findings highlight the diagnostic power of advanced seismological methods for imaging deep submarine volcanic processes and provide new insights into magma-tectonic coupling during the Mayotte seismo-volcanic crisis.

Abstract The Kolumbo submarine volcano, located NE of Santorini (Greece), represents the major source of volcanic hazard in the region, with a historical explosive eruption in 1650 CE that caused extensive damage and multiple deaths. To understand its behaviour and related hazard, we conducted a micro-analytical petrological study on the 1650 CE juvenile products to reconstruct the feeding system architecture and magma dynamics. The samples collected are heterogenous and consist of variably vesiculated white pumices , grey banded/convoluted pumices and dense juveniles , with prevalent homogeneous rhyolitic composition. Mafic enclaves are also present, mainly in the grey and denser lithotypes, ranging from basalts to dacite and showing different Sr-Nd-Pb isotopic compositions than rhyolites. Distinct mineral parageneses characterise the enclaves and host rhyolites with mainly plagioclase (An15-25), biotite and orthopyroxene in the latter and plagioclase (mostly An80-95), amphibole, clinopyroxene and olivine in the former. Among rhyolites two distinct groups (Group-A and Group-B), representative of two spatially close magma reservoirs feeding the eruption, are recognised. They differ for crystal contents and compositions and for enclave type and abundance. Group-A rhyolites show evidence of late mingling with more mafic magmas, whereas Group-B magmas were more significantly affected by early mixing with rhyodacitic melts. A deeper (~ 7–13 km), mafic system of multiple magma bodies sequentially fed the shallower (~ 3–4.5 km) rhyolitic reservoirs prior to (and during) the 1650 CE eruption and, in turn, repeatedly received new inputs of more primitive magmas, with enclave formation already in the deeper system. Our proposed dynamics and architecture of the 1650 CE plumbing system well reflect those hypothesized by geophysical imaging for the present-day volcano, also providing further, more direct evidence, with significant implications for volcanic risk assessment.

Abstract Due to the properties of sound propagation underwater, including the existence of the SOFAR channel, sounds in the ocean can propagate over large distances with little attenuation. This makes passive acoustics a relevant method for monitoring natural events such as ice calving, earthquakes, and underwater volcanic eruptions. However, the lack of automated techniques (e.g., such as beamforming for small‐aperture array) for large (50 km) hydrophone arrays makes the data analysis very time‐consuming; indeed, analysts must recognize signals such as earthquake T‐phases on several hydrophones, manually pick and associate their arrival times to build event catalogs. This lengthy iterative process by trial and error often leads to underutilization of the recorded data and exposes the resulting catalogs to the analyst's subjectivity. Here we propose a pipeline that fully automates the creation of event catalogs from continuous hydroacoustic data in two steps: (a) detection of events at each recording station and (b) association of detections at multiple stations to trilaterate the source. In order to evaluate this approach, we tested it on real data recorded by the Mayotte Hydrophone Network (MAHY), which monitors the activity of the new underwater volcano off Mayotte Island in the Mozambique Channel. Its seismicity and volcanic activity were analyzed with the catalogs obtained.

Abstract The 1650 CE Kolumbo (Greece) submarine eruption resulted in the reported deaths of 50 people and thousands of animals on Santorini (Thera) due to exposure to a cloud of noxious volcanic gases. Lack of ash in the cloud indicates that the gas release was unrelated to a magmatic explosive eruption. Medical evidence from historic accounts suggests lethal exposure to CO 2 and H 2 S, which were likely the main hazardous gases, whereas strongly acidic gases such as SO 2 , HCl, and HF were less relevant due to their high solubility in seawater. Expert elicitation indicates significant uncertainty, with probabilities of gas releases conditional on the occurrence of an eruption in the next 30 years ranging in 15–58–93% (5th–50th–95th percentiles), and a 2–17–48% likelihood of the gas cloud reaching Thera. A transient 3D multiphase fluid dynamics model (ASHEE) is employed to simulate turbulent gas cloud propagation and dilution under scenarios with source and meteorological conditions informed by expert elicitation and ECMWF-ERA5 2005−2016 data. A novel component of this study is the derivation of analytical predictive relationships through a non-dimensional scaling analysis, based on parameters like the Richardson number. Simple models are presented for weak, intermediate, and strong wind regimes, providing a generalized framework for gas transport and hazard assessment in similar volcanic settings. Results indicate about 50% probability of the gas cloud reaching Thera, even with a relatively modest volumetric flow rate of 10 3 m 3 s −1 and a wind speed half of the average. However, hazardous concentrations (above 200 ppm of H 2 S and 10 vol.% of CO 2 ) along the NE coast of Thera are reached if source gas flux exceeds 10 4 m 3 s −1 . By integrating elicitation outcomes, physical modeling, and probabilistic analysis, this study estimates a 16% and 17% likelihood of hazardous gas exposure for CO 2 and H 2 S, respectively, along the NE coast of Thera in case of formation of a gas density current in the next 30 years.

Abstract Unexpected major tsunamis have occurred at Sumisu Caldera with an interval of about 10 years. Recently, a similar unexpected major tsunami from Sofu Seamount was reported. Sumisu Caldera and Sofu Seamount are submarine volcanoes far south of Japan. Large T waves from the accompanying seismic events were observed during these tsunamigenic events. Characteristics of T waves could be useful as monitoring metrics for seismicity and tsunami excitation at submarine volcanoes. To understand T-wave generation around submarine volcanoes, I conducted numerical simulations of seismic-acoustic wave propagation from regions around Sumisu Caldera and Sofu Seamount and evaluated T-wave characteristics at various source locations. For sources just below the seafloor, efficient T waves are generated in wider areas of the simulation regions. As depth below the seafloor increases, epicenters with large-amplitude and short-duration T waves are concentrated within the submarine volcanoes of Sumisu Caldera, Sofu Seamount, and Sofugan Volcano. Large-amplitude and short-duration T waves at a certain seismic station are generated from the seismic source located near the steep (> 10º) downslopes oriented toward the seismic station. Large-amplitude and short-duration T waves from regions far from steep downslopes oriented toward the seismic station could not be observed at the station. Both slope angle and downslope direction are important for T-wave generation. The heterogeneous variation of downslopes at submarine volcanoes also causes anisotropic azimuthal patterns of T waves at far stations. The heterogeneities of bathymetric features around source regions cause variability of T-wave characteristics, and consequently, horizontal and vertical locations of seismic events could be constrained by not only body and T-wave travel times, but also T-wave characteristics. Graphical Abstract

Abstract On 15 January 2022, the Tonga submarine volcano eruption generated atmospheric shocks, air pressure perturbations, tsunamis, etc., inducing traveling atmospheric disturbances (TADs) and traveling ionospheric disturbances (TIDs). Data from the Himawari-8 satellite, ionosondes, as well as nearby barometers and tide gauges are employed to study the perturbations over Taiwan and Japan. Ionograms reveal that TIDs associated with the thermospheric and tropospheric Lamb wavefronts propagate with the northwestward (upward) speeds of 539.0 ± 22.3 and 338.3 ± 4.0 m/s (21.0 ± 5.0 and 12.4 ± 3.3 m/s), which further uplift the ionosphere by 86–155 and 50–110 km, respectively. The latter TID is followed by distinct TAD signatures, including prominent near-Earth surface air pressure pulses and significant sea level fluctuations, whereas the former one is not. The two TIDs significantly uplift the ionosphere which increase the growth rate of Rayleigh–Taylor instability, and result in the occurrence of ionospheric spread-F layers.

Abstract Submarine volcanic eruptions can produce diverse surface phenomena and hazards that differ from those of eruptions on land, such as phreatomagmatic explosions, tsunamis, and pumice rafts. Understanding the causes and processes of these surface phenomena and hazards posed especially by explosive submarine eruptions is essential for disaster prevention and mitigation for volcanoes near or below sea level. I constructed a global database of all known submarine eruptions and extracted their general characteristics. The detailed surface phenomena and processes of noteworthy explosive submarine eruptions in recent years are also summarized. Such data and analyses are useful to elucidate key phenomena and processes during submarine eruptions and to evaluate the types and effects of hazards. I focus on the relationship between water depth and the occurrence of hazardous surface phenomena, including explosivity, subaerial pyroclastic density currents (PDCs), and tsunamis. The number of explosive phenomena on the sea surface and tsunamis produced decreases dramatically where the vent is at depths ≥ 400 m, and eruption detection methods are almost entirely limited to seismoacoustic signals, pumice rafts, discolored water, and direct observation by on-site expeditions. However, because of the difficulty of detecting deep-sea eruptions without surficial manifestations, they are almost certainly under-reported in the database. Eruptions accompanied by PDCs over the sea surface represent less than 11% of the total, although eruptions in shallow water are more likely to be accompanied by PDCs. The database analysis improves our knowledge of hazardous phenomena caused by submarine eruptions, and also contributes to constraining phenomena during past historical eruptions, such as the 1650 CE eruption of Kolumbo Central Volcano, southern Aegean, and assessing future hazards at Kolumbo and other submarine volcanoes.

In January 2020, we published a paper [1] showing that the Intergovernmental Panel on Climate Change (IPCC) claimed that Earth’s temperature would reach 1.5 °C above pre-industrial days by 2030. At the same time, those who study the Sun, such as Valentina Zharkov, indicated that by 2030, the Earth's temperature would be much lower and close to pre-industrial times. The satellite temperature measurements of the Earth’s temperature are the best because they cover the whole Earth and minimize the urban heat island effect. This record shows that from January 2019 to January 2023, the temperatures dropped by approximately 0.5 °C, which indicates that the Earth is cooling. On January 15, 2022, the Hunga Tonga submarine volcano violently erupted, sending approximately 146 million tonnes of water into the stratosphere. This water reduced the heat flow to space. It increased the Earth’s temperature to a peak in April 2024 of approximately 0.45 °C above the safe upper limit of 1.5 °C above pre-industrial levels, as promoted by the IPCC. There was no disastrous effect. We are halfway to 2030, and there are serious problems with the IPCC estimate. It is recommended that the IPCC consider revising its estimates of the impact of CO2 on warming the Earth’s atmosphere.

The article considers the distribution features of siliceous mineralization in ferromanganese crusts of the Sea of Japan. The crusts are formed near the top parts of submarine volcanos. Both manganese crusts and ferrosiliceous crusts with manganese admixture were found on some of them, among the dredging material. The main rock-forming elements in the crusts are Mn (0.2–63.1 %), Fe (0.01–42.5 %) and Si (0.4–37.9 %). The Fe/Si value varies from 0.01 to 5.5, Mn/Si — from 0.01 to 177.5. The silicon content varies significantly not only in samples collected at different rises, but also in samples raised at the same station. In addition to ferrosiliceous crusts, formed as a result of quartz cementation of grains of presumably glauconite (celadonite), crusts where the same glauconite is cemented by manganese hydroxides have a high silicon content. The samples with amorphous silica areas along with the manganese matrix, or where the sediment is impregnated with manganese hydroxides, have an increased Si content. The general pattern of ore deposition on underwater volcanoes in the Sea of Japan is as follows: Fe-Si  Si  Mn  Ba. This pattern is distorted in some cases, which is most likely conditioned by the pulsating flow of hydrothermal fluids during subsequent volcanic activity.