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Coastal environments are highly recognized for their spectacular morphological features and economic activities, such as agriculture, maritime traffic, fishing, and tourism. In the context of climate change and the evolution of physical processes, the occurrence of intense natural phenomena adjacent to populated coastal areas may result in natural hazards, causing human and/or structural losses. As an outcome, scientific interest in researching and assessing multi-hazard susceptibility techniques has increased rapidly in an effort to better understand spatial patterns that are threatening coastal exposed elements, with or without temporal coincidence. The islands of Milos and Thira (Santorini Island) in Greece are prone to natural hazards due to their unique volcano-tectonic setting, the high number of tourist visits annually, and the unplanned expansion of urban fabric within the boundaries of the low-lying coastal zone. The main goal of this research is to analyze the onshore coastal terrain’s susceptibility to natural hazards, identifying regions that are vulnerable to soil erosion, torrential flooding, landslides and tsunamis. Therefore, the objective of this work is the development of a multi-hazard approach to the South Aegean Volcanic Arc (SAVA) islands, integrating them into a superimposed susceptibility map utilizing Multi-Criteria Decision-Making (MCDM) analysis. The illustrated geospatial workflow introduces a promising multi-hazard tool that can be implemented in low-lying coastal regions globally, regardless of their morphometric and manmade characteristics. Consequently, findings indicated that more than 30% of built-up areas, 20% of the transportation network, and 50% of seaports are within the high and very high susceptible zones, in terms of the Extended Low Elevation Coastal Zone (ELECZ). Coastal managers and decision-makers must develop a strategic plan in order to minimize potential economic and natural losses, private property damage, and tourism infrastructure degradation from potential inundation and erosion occurrences, which are likely to increase in the foreseeable future.

Coastal environments are globally recognized for their spectacular morphological characteristics as well as economic opportunities, such as fisheries and tourism industries. However, climate change, growth in tourism, and constant coastal urban sprawl in some places result in ever-increasing risk in the islands of the South Aegean Volcanic Arc (SAVA), necessitating thoughtful planning and decision making. GEOspatial INTelligence (GEOINT) can play a crucial role in the depiction and analysis of the natural and human surroundings, offering valuable information regarding the identification of vulnerable areas and the forecasting of urbanization rates. This work focuses on the delineation of the coastal zone boundaries, semi-automatization of Satellite-Derived Bathymetry (SDB), and urban mapping using a machine learning algorithm. The developed methodology has been implemented on the islands of Thira (Santorini island complex) and Milos. This study attempts to identify inaccuracies in existing open-source datasets, such as the European Settlement Map (ESM), as a result of the unique combination of the architectural style and bare-soil characteristics of the study areas. During the period 2016–2021, the average accuracy of the developed methodology for urban mapping in terms of the kappa index was 80.15% on Thira and 88.35% on Milos. The results showed that the average urbanization expansion on specified settlements was greater than 22% for both case studies. Ultimately, the findings of this study could contribute to the effective and holistic management of similar coastal regions in the context of climate change adaptation, mitigation strategies, and multi-hazard assessment.

Located in the southern Aegean Sea, the Christiana-Santorini-Kolumbo volcanic field is one of the most hazardous volcanic regions in the world and lies in an active continental rift zone. Northeast of Santorini lies the Kolumbo Volcanic Chain (KVC), which comprises more than 20 submarine volcanic cones, with the Kolumbo volcano representing the most prominent edifice of this chain. However, due to their inaccessibility, little is known about the spatio-temporal evolution and tectonic control of these submarine volcanoes and their link to the volcanic plumbing system of Santorini. We will present multichannel reflection seismic data that allow us to image the internal architecture of the KVC and study its link to Santorini. Using a seismostratigraphic framework, we are able to show the KVC evolved during two episodes, which initiated at approx. 1 Ma with the formation of mainly effusive volcanic edifices along a NE-SW trending zone. Most of the cones of the second episode represent submarine pumice cones that were formed by submarine explosive eruptions between 0.7 and 0.3 Ma and partly developed on top of volcanic edifices from the first episode. Our data show that two prominent normal faults underlie the KVC, indicating a direct link between tectonics and volcanism. In addition, we are able to reveal several buried volcanic centers and a distinct volcanic ridge connecting the KVC with Santorini, suggesting a connection between the two volcanic centers in the past. We argue that this connection was interrupted by a major tectonic event and, as a result, the two volcanic systems now have separate, largely independent plumbing systems despite their proximity.

The presence of active hydrothermal vent fields near residential areas and their possible link to volcanic activity pose a potential natural hazard to the environment, to society, and to the economy. Despite the importance of risk assessment and mitigation, the monitoring of volcanic activity is hindered by the remoteness and extreme conditions of underwater volcanoes. By developing a mathematical model for geological and physical processes in these environments we shed light on the underlying dynamics of chemical products emitted from the vents and point to the underlying mechanisms that govern potentially hazardous, underwater volcanic environments. Santorini and Nisyros both belong to the Hellenic Volcanic Arc but appear to have different underlying mechanisms. The Generalized Moments Method (GMM) was applied to data gathered from the Northern Caldera of Santorini and the Nisyros caldera, Avyssos, for the purpose of this work, where we focus on the high-frequency recorded CTD data (Conductivity, Temperature, Depth) in the water column over the hydrothermal vents. The data from Santorini were collected in 2017 using an Autonomous Underwater Vehicle (AUV) during the POS510 mission led by GEOMAR, while the data from Nisyros were gathered with the help of a Remotely Operated Vehicle (ROV) in 2010, during the Nautilus expedition.

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.

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.

In this work, we present a methodology for precise 3D modelling and multi-source geospatial data blending for the purposes of Virtual Reality immersive and interactive experiences. We evaluate it on the volcanic island of Santorini due to its formidable geological terrain and the interest it poses for scientific and touristic purposes. The methodology developed here consists of three main steps. Initially, bathymetric and SRTM data are scaled down to match the smallest resolution of our dataset (LIDAR). Afterwards, the resulted elevations are combined based on the slope of the relief, while considering a buffer area to enforce a smoother terrain. As a final step, the orthophotos are combined with the estimated Digital Terrain Model, via applying a nearest neighbour matching schema leading to the final terrain background. In addition to this, both onshore and offshore points-of-interest were modelled via image-based 3D reconstruction and added to the virtual scene. The overall geospatial data that need to be visualized in applications demanding photo-textured hyper-realistic models pose a significant challenge. The 3D models are treated via a mesh optimization workflow, suitable for efficient and fast visualization in virtual reality engines, through mesh simplification, physically based rendering texture maps baking, and level-of-details.

The purpose of this particular study is the assessment of landslide susceptibility of Heraklion prefecture area of Crete Island through geospatial analysis in order to present the general susceptibility zonation, so that it may be used as a basis in future extensive and detailed landslide susceptibility research, which will lead to the prevention of possible future landslides. In order to accomplish this task, it is essential to study the geological, geomorphological, geotectonic, hydrological and hydrogeological characteristics of the area, as well as the land use and compare this information with real landslide occurrence data from the study area. Based on this study it became clear that the area is defined by medium to very steep slopes and a geotechnical and geological setting that are extremely favorable for landslides. These reasons, as well as the fact that this prefecture is known to be the most densely populated prefecture of Crete Island created great interest in this area for research of this kind to prevent the loss of human life and the structural damage of properties. In order to study the landslide susceptibility of Heraklion prefecture, firstly, a geodatabase, which includes the landslide inventory, was created. The landslide inventory is consisted of previous landslides, which were collected from related geotechnical reports with all the essential spatial and descriptive information (such as locations and causes of the landslides). Furthermore, the necessary raster and vector data for the geospatial analysis of the area were obtained. Because the landslide susceptibility is a multifactorial problem it was decided to use a relating method to the problem’s nature, which was the methodology of the Analytical Hierarchy Process (AHP), as it is a simple and widely used method for similar studies, which makes it a great tool for this introductory study. In addition, this method was implemented in a GIS environment. For the purpose of the study, ten (10) triggering factors that cause landslides were defined, followed by the creation of a landslide susceptibility assessment map of the study area, depending on the influence of each factor. In accordance with this map, the highest susceptibility mostly corresponds to the mountainous regions of the prefecture. Finally, the success evaluation of the landslide susceptibility model was determined based on the frequency of landslide occurrences of the medium, high, and very high susceptibility zones. Consequently, the model was characterized as 90.48% successful, according to bibliography and the landslide inventory.

Seafloor mapping is closely related to studying and understanding the ocean, which has increasingly raised interest in the past years. Coastal management, habitat loss, underwater cultural heritage, natural disasters, marine resources and offshore installations have underlined the need of charting the seabed. This upturn has been encouraged by many national and international initiatives and culminated in the declaration of the Decade of Ocean Science for Sustainable Development (2021-2030) by the United Nations, 2017. Novel Underwater cloud services offered through the EC H2020 NEANIAS project support this joint quest by implementing Open Science procedures through the European Open Science Cloud (EOSC). The services produce user-friendly, cloud-based solutions addressing bathymetry processing, seafloor mosaicking and classification. Hence, NEANIAS Underwater services target various end-users, representing different scientific and professional communities by offering three applications.The Bathymetry Mapping from Acoustic Data (UW-BAT) service provides a user-friendly and cloud-based edition of the well known open-source MB-System, via Jupyter notebooks. This service produces bathymetric grids and maps after processing the data throughοut a flexible and fit-for-purpose workflow by implementing sound speed corrections, applying tides, filters and choosing the required spatial resolution.The Seafloor Mosaicking from Optical Data (UW-MOS) service provides a solution for representing a large area of the seafloor, in the order of tens of thousands of images, and tackling visibility limitations from the water column. The service performs several steps like camera calibration, image undistortion, enhancement, and quality control. The final product could be a 2D image Mosaic or a 3D model.The Seabed Classification from Multispectral, Multibeam Data (UW-MM) service focuses on seabed classification by implementing cutting-edge machine learning techniques and at the same time providing a user-friendly framework. The service unfolds within four steps: uploading the data, selecting the desired seabed classes, producing the classification map, and downloading the results.Therefore, NEANIAS Underwater services exploit cutting-edge technologies providing highly accurate results, regardless of the level of expertise of the end-user, and reducing the time and cost of the processing. Moreover, the accessibility to sophisticated services can simplify and promote the correlation of interdisciplinary data towards the comprehension of the ocean, and the contribution of these innovative services is expected to be of high value to the marine community.

Morphotectonic analysis of the offshore margins of the Aegean Islands in combination with onshore structures offers a rather complete image of the ongoing deformation within the Aegean micro-plate and especially along its eastern border zone with the Anatolian micro-plate. The swath data, off Lesvos and Samos islands, have been acquired by the hull-mounted RESON SeaBat 7160 on the oceanographic vessel NAFTILOS of the Hellenic Navy Hydrographic Service and gridded at 15m spatial resolution. Active tectonics affect both areas, as recorded by the intense seismic activity along with pronounced erosional and mass wasting processes.The southern margin of Lesvos Island is divided into three sub-basins. The main feature is the central elongated sub-basin extending nearly parallel to the coast, reaching 700m water depth. Its northern margin is bounded by an abrupt WNW-ESE normal fault with morphological slopes up to 41o, whereas its southern one is smoother with 5o of slope and the overall structure corresponds to a half-graben. At its eastern edge, the basin is interrupted by a narrow steep channel, trending NW-SE, and progressively becomes shallower. At the western part of the Lesvos margin, a shallow basin forms an assymetric tectonic graben. Along the northwestern margin, three E-W basins lying approximately at 300-400 m water depth, constitute pull-apart basins within the complex ENE-WSW shear zone of the southern strand of the North Anatolian Fault, bounded by the sub-parallel Skyros and Adramytion Faults. Seismic activity in 2017 comprised a 6.3 magnitude earthquake on the WSW-ESE normal fault of the Lesvos Basin and two major aftershocks of magnitude 5.2 and 5.0 at the NW-SE strike-slip faults of the channel. During 2020 and 2021 normal WNW-ESE faulting with magnitude 5.1 and ENE-WSW dextral strike-slip faulting with magnitudes 4.8, and 5.0 occurred at the western and northwestern basins. However, a magnitude 7.0 earthquake had occurred onshore at the NE-SW Kalloni-Aghia Paraskevi strike-slip fault in 1867.The northern margin of Samos Island is bounded by a normal north dipping E-W fault that generated the strong earthquake of magnitude 7.0 on 30October 2020.The Samos Basin forms a half-graben of 690m water depth with morphological slopes of 31o along the fault zone. Several canyons trending N-S, carve the northern margin ending up between 100m and 600m water depth, and several mass wasting events can be identified alongside the Samos coastline. Westwards, the Ikaria Basin is significantly deeper, reaching 1100m water depth and is delineated by an abrupt zone of nearly 51⁰ slope values, corresponding to the NE-SW Samos active western margin, probably related to strike-slip faulting. Additionally, an impressive retrogressive erosional structure occupies the area between Samos and Ikaria islands, with two prominent meandering narrow canyons debouching at the Ikaria Basin.The combination of E-W to WNW-ESE normal faulting and NE-SW to ENE-WSW dextral strike-slip faulting with minor NW-SE sinistral strike-slip faulting is observed all over the North Aegean Sea, acommodating the southwestward motion of the Aegean micro-plate, relative to the Eurasian plate in the north and the Anatolian micro-plate in the East.

Arc volcanoes are underlain by complex systems of molten-rock reservoirs ranging from melt-poor mush zones to melt-rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, no such a chamber has ever been imaged unambiguously, suggesting that large chambers responsible for caldera-forming eruptions are too short-lived to capture. Here we use a high-resolution imaging method based on finite-length seismic waveforms to detect a small, high-melt-fraction magma chamber embedded in a melt reservoir extending from ~2 to at least 4 km b.s.l. beneath Kolumbo – a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms, and may be a missing link between a deeper melt reservoir and the high-temperature hydrothermal system venting at the crater floor. Though too small to be detected by standard seismic tomography, the chamber is large enough to threaten the nearby islands with tsunamigenic eruptions. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected, and are yet to be discovered, at other active volcanoes.

Arc volcanoes are underlain by complex systems of molten-rock reservoirs ranging from melt-poor mush zones to melt-rich magma chambers. Petrological and satellite data indicate that eruptible magma chambers form in the topmost few kilometres of the crust. However, very few chambers have ever been definitively located, suggesting that most are too short-lived or too small to be imaged, which has direct implications for hazard assessment and modelling of magma differentiation. Here we use a high-resolution technology based on inverting full seismic waveforms to image a small, high-melt-fraction magma chamber that was not detected with standard seismic tomography. The melt reservoir extends from ~2 to at least 4 km below sea level (b.s.l.) at Kolumbo – a submarine volcano near Santorini, Greece. The chamber coincides with the termination point of the recent earthquake swarms and may be a missing link between a deeper melt reservoir and the high-temperature hydrothermal system venting at the crater floor. The chamber poses a serious hazard as it could produce a highly explosive, tsunamigenic eruption in the near future. Our results suggest that similar reservoirs (relatively small but high melt-fraction) may have gone undetected at other active volcanoes, challenging the existing eruption forecasts and reactive-flow models of magma differentiation.

Hydrographic survey of the southeastern coastal margin of Lesvos Island (Greece) was conducted in January 2018 by R/V METEOR. The main morphological characteristic of the southern margin of Lesvos Island is a sub-parallel to the coast elongated basin, 30 km long, 5–10 km wide, and 700 m deep. It is interrupted by an oblique NW–SE narrow channel of 650 m depth, which separates the adjacent platform areas surrounding the longitudinal basin. The northern margin of the basin is abrupt with morphological slopes of more than 10°, following the major WNW-ESE normal fault surface, running along the coastal zone, with an overall throw of more than 800 m. In contrast, the southern margin shows a gradual slope increase from 1° to 5°, where several shallow sub-bottom profiles across the basin have indicated the existence of some minor WNW-ESE normal faults and one reverse/strike-slip fault zone, with throws of a few up to 13 m. However, the NW–SE narrow channel intersecting the basin is bounded by two sub-vertical fault zones with sinistral strike-slip motion and throws of more than 150 m. The above structure was reflected in the mainshock of magnitude 6.3 (12/6/2017), corresponding to a WNW-ESE normal fault dipping to the SSW and the two major aftershocks of magnitude 5.2 and 5.0 corresponding to the two sub-parallel NW–SE sinistral strike-slip faults of the channel. This active deformation comprising WNW-ESE normal faults and NW–SE or NE-SW strike-slip faults is compatible with the area’s overall tectonic frame at the eastern margin of the Aegean microplate.

Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.

At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short‐period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper‐crustal structure (

The Christiana–Santorini–Kolumbo volcanic field in the South Aegean Sea (Greece) is one of the most important in Europe, having produced more than 100 explosive eruptions in the last 400,000 years. Its volcanic centers include the extinct Christiana Volcano and associated seamounts, Santorini caldera with its intracaldera Kameni Volcano, Kolumbo Volcano, and 24 other submarine cones of the Kolumbo chain. Earthquakes, volcanic eruptions, submarine mass wasting, neotectonics and gas releases from these centers pose significant geohazards to human populations and infrastructures of the Eastern Mediterranean region. Defining the geological processes and structures that contribute to these geohazards will provide an important framework to guide future monitoring and research activities aimed at hazard mitigation.

As ocean acidification intensifies, there is growing global concern about the impacts that future pH levels are likely to have on marine life and ecosystems. By analogy, a steep decrease of seawater pH with depth is encountered inside the Kolumbo submarine volcano (northeast Santorini) as a result of natural CO 2 venting, making this system ideal for ocean acidification research. Here, we investigated whether the increase of acidity towards deeper layers of Kolumbo crater had any effect on relevant phenotypic traits of bacterial isolates. A total of 31 Pseudomonas strains were isolated from both surface- (SSL) and deep-seawater layers (DSL), with the latter presenting a significantly higher acid tolerance. In particular, the DSL strains were able to cope with H + levels that were 18 times higher. Similarly, the DSL isolates exhibited a significantly higher tolerance than SSL strains against six commonly used antibiotics and As(III). More importantly, a significant positive correlation was revealed between antibiotics and acid tolerance across the entire set of SSL and DSL isolates. Our findings imply that Pseudomonas species with higher resilience to antibiotics could be favored by the prospect of acidifying oceans. Further studies are required to determine if this feature is universal across marine bacteria and to assess potential ecological impacts.

The lack of direct seismological evidence for large molten magma chambers is considered to be one of the most important arguments in support of the mush paradigm. However, most published melt fraction estimates based on interpretation of seismological data are associated with large uncertainties because of two limitations: i) inherent limits to resolution of seismic tomography and, ii) trade-offs in the constitutive relationships that tie seismic properties to melt fraction. Low-velocity volumes associated with magma storage are particularly difficult to image with conventional travel-time tomography due to limited resolution and wavefront healing, resulting in blurred images and a high velocity bias. We tackle these limitation by applying full waveform inversion to active source seismic data collected over the Kolumbo submarine volcano (Greece). We recover a previously undetected Vp anomaly of –50% beneath the volcano and interpret this as a shallow magmatic intrusion. Extension of this approach to the wider Santorini volcanic system is ongoing. Concurrently, we are tackling the second limitation, which is the result of the dependence of elastic properties on the microgeometry of the melt. Seismological melt estimates rely on the assumption that the melt pore space can be represented by simple geometrical shapes, usually ellipsoids, with a given aspect ratio. Since the aspect ratio is poorly constrained, this results in a trade-off between melt fraction and melt geometry. We have adapted a method for the homogenisation of the elastic properties of multi-phase composites and applied it to calculating the elastic properties of partially molten rocks starting from the melt microstructure determined by X-ray CT scanning. The microgeometry of the mush can be inferred from the study of glomerocrysts: crystal mush inclusions with quenched interstitial melt that are carried to the surface by erupted lava. After the sample is digitized and segmented into its constitutive phases (crystals, melt, vesicles), the average elastic properties are determined by numerical homogenisation which consists of numerically simulating the deformation of the sample under load and predicting its elastic response. The results are compared to a semi analytical solution for ellipsoidal inclusions. We apply this approach to a plutonic nodule from St Kitts and show that the melt microstructure leads to an elastic response equivalent to that of ellipsoidal melt inclusions with an aspect ratio of 0.1 (oblate spheroids). This equivalent aspect ratio is used to refine melt estimates for Montserrat, Santorini and Kolumbo volcano.

The application of photogrammetry to volcanic areas is usually made using UAVs for collecting pictures aimed at producing high-resolution orthomosaic and digital surface models. In the present work, instead, we use a boat-camera-based photogrammetry approach, as a tool for orthomosaic, digital surface modelling and virtual outcrop production at an almost vertical 300-m-high geological feature: the northern caldera wall of Santorini. This is a geological structure of great interest, where many tens of dykes crop out within a heterogeneous host rock made of sequences of effusive and explosive volcanic deposits. Some active and inactive faults also dissect the caldera wall. Thus, the study area is almost inaccessible for classic field surveys due to challenging logistic conditions and landslide hazard.We used a 20 MPX camera run by an operator who collected a total of 887 pictures almost continuously, orthogonal to the ground and opposite to the target, during a 5.5-km-long boat survey. We performed the study along the northern caldera wall, at a constant boat velocity and at a distance from the coast/caldera wall that varied between 35.8 m and 296.5 m. The outcomes of the photogrammetry application include: 1) a high-resolution 3D model of the study area, 2) a high-resolution virtual outcrop for two selected parts of the caldera, 3) qualitative and quantitative structural data (dyke attitude, thickness, cross-cutting relationships, host rock lithology) along the vertical caldera cliff. Our method represents a new approach for 3D outcrop building for research under extreme logistic conditions.

The Milos hydrothermal field is one of the largest known shallow water hydrothermal systems, and shows both fluid and gas outflow through the seafloor. Recent studies based on imagery acquired by both aerial and submarine drones (Puzenat et al., submitted) reveal several types of fluid outflow associated with bacterial mats along the SE coast of the island (Paleochori, Spathi, and Agia Kyriaki bays). From these observations? include: a) zones of polygonal hydrothermal outflow and associated bacterial mats, b) extended white (bacterial) patches, and c) isolated ones. Subseafloor hydrothermal circulation is hosted in sediments with subseafloor temperatures >50°C, and there is a clear association between hydrothermal circulation and active degassing.To understand the controls on and relationships between fluid and gas outflow in the area, we need to characterise: a) the nature of the subseafloor (sediment thickness, composition & permeability); b) the distribution of gas and subseafloor fluids, and c) the distribution of gas flares emanating from the seafloor. In November 2020, we conducted a short pilot geophysical study at Paleochori Bay, deploying a towed catamaran with a multibeam echo sounder (iXblue Seapix) to obtain seafloor bathymetry, acoustic backscatter and water column detection of gas flares. We also deployed a sub-bottom profiler (iXblue Echoes 3500 T1) to image sediment architecture and gas/fluid diffusion within the sediment. Our survey focused on Paleochori Bay, surveing areas from ~5 m (nearshore) to ~100 m waterdepth (offshore).Preliminary results of this geophysical survey suggest that subseafloor gas accumulations play a major role on the nature and structure of hydrothermal activity at Milos. These gas accumulations within the sediments develop along an onshore/offshore fault system, and likely control the shallow subseafloor thermal structure, establishing a thin thermal conductive layer between the roof of gas pockets and the seafloor.[GJ1] [je2] We will report on the link between the distribution and geometry (extent, depth, acoustic nature of the accumulations) of gas pockets, fluid outflows, and gas outflows, all of which will be characterised from both seafloor imagery and subsurface geophysical surveys. We will also discuss how gas pocket geometry may be linked to both fluid flow and subseafloor temperature structure. [HA3]

IODP proposal VolTecArc aims at deep-sea drilling in and around the Christiana-Santorini-Kolumbo (CSK) marine volcanic field to investigate interactions and feedbacks between tectonics and volcanism and how volcanoes interact with their marine environments. The volcanic field lies in a rift system 100 km long and 45 km wide, oblique to the South Aegean volcanic arc, that is one of the most volcanically and seismically active regions of Europe. The volcanoes include three polygenetic and over 20 monogenetic centers that have jointly produced over a hundred explosive eruptions over the last few hundred thousand years. The volcanoes pose important hazards to the Eastern Mediterranean region. Unrest at Santorini caldera in 2011-12 raised awareness of eruption threat at an island archipelago visited by 1.5 million tourists per year.The results of onland volcanological research, eruption dating, multi-beam sea floor mapping, shallow sediment coring and dredge sampling, combined with a high-quality site-survey database of multichannel seismic profiles and a recent seismic tomography experiment, make deep drilling at the CSK volcanic field very timely. Deep drilling will enable characterization and interpretation of depositional packages visible on seismic images, chemical correlation of Santorini-derived volcanic layers in the rift fills with the dated onshore stratigraphy, and provide a tight chronostratigraphic framework for marine successions. Some objectives of drilling are to: (1) document the history of tectonics, subsidence, sedimentation and volcanism in an arc-rift environment, and how volcanism has evolved spatially and temporally since rift initiation; (2) determine how the genesis and compositions of magmas and their associated volatiles have evolved in time and space over the lifetime of the rift; (3) document the dynamics and environmental impacts of arc eruptions and calderas, including eruption frequencies, magnitudes and rates, the mechanisms of caldera collapse, and the origin of caldera unrest events.

The Hellenic arc hosts several active volcanic centers, of which the Milos, Santorini-Kolumbo and Kos-Yali-Nisyros volcanic fields present particularly high threats due to recent unrest (2011-2012 and 1996-1997 at Santorini and Nisyros, respectively). These volcanic centers have repeatedly produced highly explosive eruptions (VEI 4 to 7) from ~360 ka into historic times. The marine tephra record provides information not only on the number of events, but also on their magnitudes and intensities inferred from tephra dispersal characteristics, and is thus essential to quantitatively assess future volcanic hazards and risks.Here we complement earlier work on distal to ultra-distal east-Mediterranean sediment cores, which captured the largest eruptions. We present results from a grid of medial to distal sediment cores collected in 2017 during RV Poseidon cruise POS513 with core positions both comparatively close to and between the three volcanic fields, in order to record medium- to large-scale eruptions.During this cruise, 47 gravity cores up to 7.4 m long, and 3 box cores of the uppermost 0.5 m sediment were recovered, which contain more than 220 primary ash layers. The compositions of glass shards from all layers were characterized by major (EMP) and trace-element (LA-ICPMS) analyses.Geochemical fingerprinting supports correlations with 20 eruptions from all three volcanic fields as well as with the 39 ka Campanian ignimbrite eruption from the Campi Flegrei, Italy. Correlations with eleven eruptions from Santorini-Kolumbo (Kameni, Kolumbo 1650, Minoan, Cape Riva, Cape Tripiti, Upper Scoria 1 and 2, Middle Pumice, Cape Thera, Lower Pumice, Cape Therma 3) are established, and we newly identify two widespread tephras from eruptions on Milos (Lower and Upper Firiplaka). We have probably been able to solve some previous chronostratigraphic problems at Kos-Yali-Nisyros by correlating marine tephras with the Kos Plateau Tuff, and with the Yali 2 tephra, whereby we identify a second, less evolved facies produced by that eruption that has not yet been recognized on land. We also find tephras from four eruptions on Nisyros (Nisyros 1 to 4) including the previously established Lower (Nisyros 4) and Upper (Nisyros1) Nisyros Pumice eruptions.These correlations also provide new age constraints for hitherto poorly or non-dated Aegean tephras based on sedimentation rates derived between multiple anchor points of dated terrestrial tephra ages. We deduce ages of ~22 ka and ~36 ka for Upper and Lower Firiplaka tephras from Milos (the latter overlying the Campanian ash) which are significantly younger than other eruption ages known from Milos, ~54 ka, ~62 ka, ~69 ka, and ~76 ka for the Nisyros 1 to 4 tephras, and ~52 ka for the Yali 1 tephra as well as a verified age of 33 ka for the Yali 2 tephra with its two contemporaneous facies.These new tephrostratigraphic results help to improve quantifications of distribution and eruption characteristics for all these eruptions, and provide important pre-site survey data for the Santorini IODP proposal VolTecArc.

The Christiana-Santorini-Kolumbo (CSK) volcanic field in the South Aegean Sea is one of the most active volcanic-tectonic lineaments in Europe, having produced numerous explosive eruptions, catastrophic earthquakes, and disastrous tsunamis in the past 350,000 years. The present-day Santorini caldera is located in the centre of the NE-SW trending CSK field, which extends for more than 60 km from the extinct Christiana volcano in the southwest to the active submarine Kolumbo volcano northeast of Santorini. While the onshore architecture of Santorini has been well studied, little is known about its offshore architecture. Further, the past volcanism of Kolumbo is only known for its last eruption in 1650 AD and that of Christiana is completely unknown. Based on the available onshore datings of the volcanic formations, it has been proposed that volcanism in the CSK field initiated at Christiana, then migrated northeast towards Santorini and later to Kolumbo. This, however, has yet to be confirmed by offshore investigations. To fully constrain and understand the initiation and evolution of volcanism in the CSK field, we combine an extensive collection of high-resolution multichannel and vintage seismic data covering the entire zone.With these seismic profiles, we are able to (1) correlate the seismo-stratigraphy of the Christiana basin west of Santorini with that of the Anhydros Basin east of Santorini, (2) identify major phases of extrusive and intrusive activity of individual volcanic vents, and (3) establish a regional chrono-stratigraphic framework in which we chronologically integrate these phases. We conclude that volcanism occurred repetitively during distinct phases of activity, which are separated from each other by periods with little or no volcanic activity. The onset of volcanism occurred without the generation of significant pyroclastic flows and was mainly characterized by shallow intrusions, clearly visible at Christiana, at its neighbouring volcanic cones, and at the southwestern flank of Akrotiri. The next phase saw the formation of the Kolumbo Volcanic Chain in the Anhydros Basin and the formation of sill intrusions in the Christiana Basin, which we find below a chaotic, presumably pyroclastic unit. This was followed by a major regional event on Santorini, during which a thick transparent subunit was deposited in all surrounding basins. The most recent phase was dominated by the volcanoclastic deposits from Santorini’s eruptive cycles and the recent eruption of Kolumbo. Using estimates of sedimentation rates, we convert this chrono-stratigraphic scheme into an approximate timeline, which implies that the initiation of volcanism occurred during Late Pliocene to Early Quaternary - much earlier than revealed by onshore dating.

Marine microbial communities are an untapped reservoir of genetic and metabolic diversity and a valuable source for the discovery of new natural products of biotechnological interest. The newly discovered hydrothermal vent field of Santorini volcanic complex located in the Aegean Sea is gaining increasing interest for potential biotechnological exploitation. The conditions in these environments, i.e., high temperatures, low pH values and high concentration of heavy metals, often resemble harsh industrial settings. Thus, these environments may serve as pools of enzymes of enhanced catalytic properties that may provide benefits to biotechnology. Here, we screened 11 metagenomic libraries previously constructed from microbial mat samples covering the seafloor and the polymetallic chimneys of Kolumbo volcano as well as mat samples from Santorini caldera, to mine, in silico, genes associated with bioenergy applications. We particularly focused on genes encoding biomass hydrolysis enzymes such as cellulases, hemicellulases and lignin-degrading enzymes. A total of 10,417 genes were found for three specific groups of enzymes—i.e., the endoglucanases, the three different beta-glucosidases BGL, bglX and bglB, and the alpha-galactosidases melA, and rafA. Overall, we concluded that the Santorini–Kolumbo volcanic ecosystems constitute a significant resource of novel genes with potential applications in bioenergy that deserve further investigation.

Dacite lavas erupted from Kameni Islands volcanic centre (Greece) during the last 2000 years have a limited range in chemical composition (SiO2 = 64.0–68.5%) which contrasts with their wide range in plagioclase abundance (3–22%) and crystal size distributions. Most plagioclase crystals have simple zoning and occur independently or in loose clusters with finer-grained cores. We propose that magmatic diversity was produced by the interaction between crystals that formed at the base of a magma reservoir and bubbles produced by injection and vesiculation of more mafic magma. Two end-member situations can be identified: in juvenile systems, the basal crystal mass is loosely connected and readily disrupted by bubble formation. The crystal–bubble couples accumulate at the top of the reservoir, from where they can enter the sub-volcanic plumbing system to produce high-crystal content, chemically unevolved magmas. In the mature system, the crystal mass is well connected so bubbles displace the evolved, interstitial magma and liberate only a smaller number of crystals from the crystal mass. This process produces chemically evolved magmas, with lower crystal contents. The oldest lavas seem to have been produced from mature systems, whereas the youngest eruptions were of lavas produced from juvenile systems. This progression may reflect an overall reduction in repose times during the last 2000 years.

Although the rise of antibiotic and multidrug resistant bacteria is one of the biggest current threats to human health, our understanding of the mechanisms involved in antibiotic resistance selection remains scarce. We performed whole genome sequencing of 21 Pseudomonas strains, previously isolated from an active submarine volcano of Greece, the Kolumbo volcano. Our goal was to identify the genetic basis of the enhanced co-tolerance to antibiotics and acidity of these Pseudomonas strains. Pangenome analysis identified 10,908 Gene Clusters (GCs). It revealed that the numbers of phage-related GCs and sigma factors, which both provide the mechanisms of adaptation to environmental stressors, were much higher in the high tolerant Pseudomonas strains compared to the rest ones. All identified GCs of these strains were associated with antimicrobial and multidrug resistance. The present study provides strong evidence that the CO2-rich seawater of the volcano associated with low pH might be a reservoir of microorganisms carrying multidrug efflux-mediated systems and pumps. We, therefore, suggest further studies of other extreme environments (or ecosystems) and their associated physicochemical parameters (or factors) in the rise of antibiotic resistance.

Kolumbo represents one of the most hazardous, currently active, volcanoes in the eastern Mediterranean. Its last eruption in 1650 AD was associated with a vast explosion, causing a tsunami of regionally devastating impact. The eruption was also associated with the voluminous and rapid release of toxic gases asphyxiating humans and animals on the nearby Islands. Earthquake records from the recent decades document on-going unrest beneath the volcano. Remotely operated vehicle dives revealed several hydrothermal vent sites and bacterial mats at the crater floor, which are concentrated near the northern crater wall. The vents emit mainly CO2, leading to the accumulation of acidic waters in the crater. Accordingly, one of the main volcanic hazards associated with Kolumbo is that rapid overturning of water in the crater may release harmful amounts of toxic gases. Monitoring the hydrothermal processes inside the Kolumbo crater will provide an important contribution to the understanding and evaluation of this and other volcanic hazards.In October 2019, we deployed an ocean bottom seismometer and hydrophone (OBS/H) inside the Kolumbo crater. During the four days of passive recording we identified about 100 so-called short duration seismic events, which were only present on the seismometer channels, while generally being absent on the hydrophone channels. The events have durations of less than one second with dominant frequencies between 5 to 30 Hz. Most of the events represent well-polarized seismic phases, which enables us to determine their azimuth angle (with a 180-degree bias) and angle of incidence at the OBS/H. We cross-correlated all polarized seismic waveforms and subsequently used the cross-correlation coefficients for a hierarchical cluster analysis to elaborate whether the events have a random origin or originated from a common origin. Our analysis revealed that the majority of events is associated with two clusters. The azimuth angles of all events in the largest cluster coincide with the azimuth angle between station and the field of hydrothermal vents and bacterial mats inside the crater. This coincidence suggests that the origin of the short duration events is associated with the sub-seafloor migration of fluids or the fluid discharge process at the crater floor. In fact, short-duration events of similar characteristics, recorded by OBS/H, were previously attributed to sub-seafloor fluid migration and the discharge of fluids at the seafloor. Our analyses indicate that seismic monitoring of submarine volcanoes should include the detection and analysis of short duration events, which may act as a novel tool in the characterization of volcanic unrests and volcanogenic geohazard monitoring in general.

In the present work, we test the application of boat-camera-based photogrammetry as a tool for Virtual Outcrops (VOs) building on geological mapping and data collection. We used a 20 MPX camera run by an operator who collected pictures almost continuously, keeping the camera parallel to the ground and opposite to the target during a boat survey. Our selected target was the northern part of Santorini's caldera wall, a structure of great geological interest. A total of 887 pictures were collected along a 5.5-km-long section along an almost vertical caldera outcrop. The survey was performed at a constant boat speed of about 4 m/s and a coastal approaching range of 35.8 to 296.5m. Using the Structure from Motion technique we: i) produced a successful and high-resolution 3D model of the studied area, ii) designed high-resolution VOs for two selected caldera sections, iii) investigated the regional geology, iv) collected qualitative and quantitative structural data along the vertical caldera cliff, and v) provided a new VO building approach in extreme logistic conditions.

Generally, key geological outcrops are inaccessible for classical mapping due to the hard-logistic conditions of their location in remote or dangerous areas like active volcanoes or fault zones. The UAV-based photogrammetry is a helpful technique to overcome such difficulties in site investigation. It allows a very high-detailed 3D model reconstruction of relevant outcrops, providing also the possibility to cover wider areas.In this study, we tested the use of a “Selfie drone” aimed at outcrops reconstruction for 3D mapping of volcano-tectonic features. Two different sites in Santorini volcanic complex with different characteristics have been chosen: i) the Vlychada Beach, located in the southern part of the island, characterized by vertical cliffs that offer great exposure of the pumice layers from the well-known Late Bronze Age (LBA) (Minoan) eruption and ii) a historical volcanic crater located in the northern part of Nea Kameni island, related to the 1570 A.D. eruption, with a diameter of about 85 m, which is mostly inaccessible within its internal part and cannot be studied by classical field methods. The “Selfie drone” which was used for the photo collection, is a 0.300-kg quadcopter equipped with a 12 Megapixel camera, EXIF information (Exchangeable Image File Format) and GPS coordinates. This drone has a flight time of approximately 16 minutes. A total of about 1900 photos has been collected, considering both sites, that have been reconstructed using photogrammetry techniques.The resulting 3D models are characterized by a sub-centimetric texture resolution, allowing detailed mapping of the Minoan pumice layers, fractures, crater geometry, and related volcanic deposits, proving the usefulness of “Selfie drones” for geological – tectonic mapping.

Field studies are vital for mapping and understanding active geological processes on Earth. Such studies commonly inform analogue and numerical modelling setups and provide insights over a variety of scales. However, geological field studies have several limitations as they are sensitive both to field-based conditions (e.g. weather conditions, geomorphology, weathering, erosion and access) and the experience of the researchers conducting the work. All of these limitations can add significant error or uncertainty to geological measurements. At the same time, new geological measurement techniques (e.g. photogrammetry) are easy to access, fast and friendly to use, but also often depend on ground truthing parameters.In this study, we compared two different methods for mapping and surveying volcanotectonic processes related to dyking events: classical field analysis and boat-based photogrammetry. We tested the two approaches on dykes located within a section of a steep cliff face that makes up part of the Santorini caldera. The caldera wall is accessible by land only in the upper most parts and so most measurements require access by boat or by abseiling down the cliff faces. The latter is very dangerous and not recommended.The core of the work is to carefully compare field data with the equivalents collected on photogrammetry-derived 3D model, focusing on the sea level area in order to compare reliable dataset. Data comparison is focused on dyke attitudes, thicknesses, petrological descriptions, along the 4-km length profile of the northern caldera wall of Santorini volcano.We collected a series of high-resolution images, around 800 pictures in total, aimed at 3D modelling the dyke swarm using photogrammetry methods. They have been collected using a 20 MPX hand-held camera equipped with commercial GPS from a boat, moving parallel and to a constant distance from to the caldera wall.Comparison of both datasets allowed insights into 1) the completeness and, 2) the limitations of each technique. Here we assess the various advantages to design a novel multidimensional methodology that allows fast, accurate and low-cost data generation in difficult working conditions, such as at steep cliff faces and flooded terrains.

Submarine hydrothermal activity is common at the flanks of volcanic islands, and in some cases, occurring at very shallow water (0-100 meter depth). These sites are a key target for systematic seafloor mapping to understand the location, geometry and nature of hydrothermal discharge. These data are also critical for monitoring the temporal variability of these dynamic systems, while providing a context for instrumental measurements, sampling and other observations (e.g., temperature of outflow, chemistry, etc.). Here we present a systematic mapping of the Milos hydrothermal system in the Hellenic volcanic Arc, characterized by submarine gas emissions, high-temperature outflow, bacterial mats, precipitation of hydrothermal minerals, and small hydrothermal constructs and edifices. We have mapped this site at regional scales using satellite imagery (World-View2 images from the DigitalGlobe foundation), complemented with aerial photography acquired with drones, and high-resolution seafloor photomosaics (Our drone and AUV mapping ground truths the correlation between patterns in satellite imagery and hydrothermal outflow, associated to mineral precipitates and/or bacterial mats at the seafloor. This mapping also reveals a clear organization of the hydrothermal outflow in sandy areas. In particular, polygonal patterns are common and often associated with inactive or actively bubbling pockmarks. These areas, showing white bacterial mats and hydrothermal precipitates, are rippled, suggesting that the hydrothermal precipitates do not consolidate the sediment. White precipitates display subseafloor temperatures >50°C at depths of 10 to 50 cm. The white areas are bound by bands of seafloor with a hummocky structure due to intense bioturbation, that obliterates the ripples, with widths of up to a few meters. This area shows subseafloor temperatures of 20-40°C, and corresponds to a transition from the high-temperature white zones and the seafloor with ripples and no hydrothermal precipitates. This area exhibits subseafloor temperatures similar to those of seawater, and can be associated with seagrass. These patterns reveal a clear organization of a narrowly focused hydrothermal outflow that controls the biological communities at the seafloor and subseafloor. We will discuss the implications of these observations to quantify hydrothermal fluxes in the study area.

Detailed knowledge about geometry and physical properties of magmatic systems at arc volcanoes promises to better constrain models of magma differentiation, transit and storage in the crust, and to help assess volcanic hazard.Unfortunately, low-velocity zones associated with melt accumulation are particularly difficult to image by conventional travel-time tomography due to its limited resolving power, resulting in blurred boundaries and underestimated velocity contrasts.Here we alleviate these issues by applying full-waveform inversion (FWI) to study a magmatic system of Santorini - an active, semi-submerged volcano with a known record of large, caldera-forming eruptions.We use a 3D wide-angle, multi-azimuth seismic dataset from the recent PROTEUS experiment acquired with ca. 150 ocean-bottom/land seismic stations and ca. 14,000 air-gun shots. We implement a finite-difference immersed boundary method to simulate reflections off the caldera’s irregular topography, and pressure-velocity conversion to take full advantage of the multi-component data. We perform inversion with careful data-selection, increasing frequency up to 6 Hz, and extensive quality-control based on a phase spatial-continuity criterion.A final P-wave velocity model of the upper crust offers a high-resolution image of Santorini magmatic and hydrothermal systems with pronounced low-velocity zones due to a high melt and water content respectively. The features are better resolved and the velocity contrasts distinctly sharper than in the starting model obtained with travel-time tomography. We also recover a previously undetected low velocity anomaly of >40% beneath Kolumbo - a submarine volcanic cone to the NE of Santorini caldera. We interpret this anomaly as a magmatic sill.

New bathymetric and seismic reflection data from the Santorini–Amorgos Tectonic Zone in the southern Cyclades have been analysed and a description of the morphology and tectonic structure of the area has been presented. The basins of Anhydros, Amorgos and Santorini–Anafi have been distinguished together with the intermediate Anhydros Horst within the NE-SW oriented Santorini–Amorgos Tectonic Zone which has a length of 60–70 km and a width of 20–25 km. The basins represent tectonic grabens or semi-grabens bordered by the active marginal normal faults of Santorini–Anafi, Amorgos, Ios, Anhydros and Astypalaea. The Santorini–Anafi, Amorgos and Ios marginal faults have their footwall towards the NW where Alpine basement occurs in the submarine scarps and their hangingwall towards the southeast, where the Quaternary sediments have been deposited with maximum thickness of 700 m. Six sedimentary Units 1–6 have been distinguished in the stratigraphic successions of the Santorini–Anafi and the western Anhydros Basin whereas in the rest area only the upper four Units 3–6 have been deposited. This shows the expansion of the basin with subsidence during the Quaternary due to ongoing extension in a northwest-southeast direction. Growth structures are characterized by different periods of maximum deformation as this is indicated by the different sedimentary units with maximum thickness next to each fault. Transverse structures of northwest-southeast direction have been identified along the Santorini–Amorgos Tectonic Zone with distinction of the blocks/segments of Santorini, Anhydros/Kolumbo, Anhydros islet and Amorgos. Recent escarpments with 7–9 m offset observed along the Amorgos Fault indicate that this was activated during the first earthquake of the 7.5 magnitude 1956 events whereas no recent landslide was found in the area that could be related to the 1956 tsunami.

Submarine hydrothermal activity is responsible for heat and chemical exchanges through the seafloor. Shallow-water hydrothermal systems (SWHS), while identified around the globe, are often studied in a way that is less comprehensive than their deep-ocean counterparts (e.g., along ridges), where systematic optical and acoustic mapping is more prevalent and coupled to in situ observations and sampling. Using aerial drones, an AUV, and temperature measurements at 10–40 cm subseafloor, we investigated in 2019 one of the most extensive SWHS known to date, in Paleochori and nearby Spathi and Agia Kyriaki Bays (south of Milos, Greece). Hydrothermal venting, found from the shore to water depths of almost 500 m, shows emissions of gases and high-temperature fluids, often associated with bacterial mats and/or hydrothermal mineral precipitates. This study provides extensive drone mapping coupled with local AUV surveys for seafloor characterization and ground-truthing from the shore to ~20 m water depth. Seafloor photomosaics also provide a detailed context to samples, measurements and observations carried in situ. We interpret the photomosaics to define distinct seafloor types, linked to this hydrothermal activity. White hydrothermal patches (WHPs) often show a clear polygonal organization, together with outflow areas that are both more dispersed and distributed. Polygonal patterns likely result from fluid convection in a sandy porous medium heated from below. These WHPs display elevated subseafloor temperatures, typically >50°C, with maximum values of ~75°C. Photomosaics also display textures of biological origin, including seagrass and bioturbation patterns. Widespread bioturbation by burrowing shrimps is often associated with WHPs, bounding them, but also occurs on sandy seafloor away from hydrothermal patterns. Subseafloor temperatures at these bioturbated areas are of ~30–40°C, and are thus transitional between hot WHPs and sedimented seafloor unaffected by hydrothermal activity (~24°C). In addition to linking subseafloor temperature data and interpreted seafloor photomosaics, our results provide a comprehensive general overview of this SWHS, of the organization of its hydrothermal outflow through the seafloor, and of the underlying subseafloor fluid circulation. This paper also gives the first perspectives on the heat fluxes of the system, and constitutes a background for other studies on the nature and distribution of microbial communities, controlled by this hydrothermal activity.

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920 mg/kg) and Nb (245 mg/kg), in the coastal sand. The low REE concentration (ΣREE + Y = 240 mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE. The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions.

Microbathymetry data, in situ observations, and sampling along the 13°20′N and 13°20′N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along-extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20′N OCC, and gabbro and peridotite at 13°30′N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30′N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20′N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.

The Aegean Sea, as part of the Mediterranean, is an ecologically and geologically diverse marine system, with a wide range of habitats including coastal and deep-sea oxic sediments, and submarine volcanoes with seafloor hydrothermal vents and CO2-venting active, and inactive, polymetallic (Au, Ag, Sb, Cu, Pb, Zn) hydrothermal chimneys. Generally, our knowledge regarding microbial community composition and function in the benthic layers such as the sediments and hydrothermal vents of the Aegean Sea is limited. Here, we summarize our current knowledge on benthic microbial communities in diverse marine habitats of the Aegean Sea. Data based on sequencing technologies, such as Sanger-based clone libraries, pyrosequencing, and Illumina sequencing are presented.

International audience; The Pico-Faial ridge is a steep WNW-ESE volcanic ridge that has developed within the Nubia-Eurasia diffuse plate boundary, close to the Azores Triple Junction. The ridge comprises two islands, Pico and Faial, separated by a shallow (< 100 m depth) and narrow (< 8 km) channel. Despite some similarities, the two islands show contrasting features still deserving explanation: (1) meaning of GPS data; (2) meaning of palaeomagnetic data; (3) the island-scale Faial Graben does not have a counterpart in Pico; (4) both islands comprise a main central volcano each, but the one in Faial is symmetrical and the one in Pico is asymmetrical; (5) Pico shows evidence of at least two large-scale flank collapses, but none has been recognized so far in Faial. The new data reported here lead to the following probable answers: (1) the GPS data show much larger vertical and horizontal ground motions in Faial, which could be the result of recent volcanism and tectonics; (2) the cyclicity inferred from the paleomagnetic data in Pico could be related to inflation/deflation cycles that could have triggered the inferred flank collapses; (3) the Faial Graben ends abruptly at the eastern edge of the island, which we interpret as inhibited propagation to the east because of the load and stresses imposed by the nearby large Pico Volcano; (4) we attribute the asymmetry of the Pico Volcano to partial flank collapse recognizable in new offshore seismic profiles; (5) either the Faial island is actually gravitationally more stable, or there have been flank collapses not yet recognized. All these processes have concurred to shape the two islands and the ridge to their current morphology, but the current morphologies of Pico and Faial result mainly from three of these processes: volcanism, tectonics and large-scale landslides.

This study reports the first detailed geochemical characterization of Kolumbo submarine volcano in order to investigate the role of source heterogeneity in controlling geochemical variability within the Santorini volcanic field in the central Aegean arc. Kolumbo, situated 15 km to the northeast of Santorini, last erupted in 1650 AD and is thus closely associated with the Santorini volcanic system in space and time. Samples taken by remotely-operated vehicle that were analyzed for major element, trace element and Sr-Nd-Hf-Pb isotope composition include the 1650 AD and underlying K2 rhyolitic, enclave-bearing pumices that are nearly identical in composition (73 wt.% SiO2, 4.2 wt.% K2O). Lava bodies exposed in the crater and enclaves are basalts to andesites (52-60 wt.% SiO2). Biotite and amphibole are common phenocryst phases, in contrast with the typically anhydrous mineral assemblages of Santorini. The strong geochemical signature of amphibole fractionation and the assimilation of lower crustal basement in the petrogenesis of the Kolumbo magmas indicates that Kolumbo and Santorini underwent different crustal differentiation histories and that their crustal magmatic systems are unrelated. Moreover, the Kolumbo samples are derived from a distinct, more enriched mantle source that is characterized by high Nb/Yb (>3) and low 206Pb/204Pb (

The structural evolution of the South Aegean Sea is little explored due to the lack of marine seismic data. Our present day understanding is mainly based on some island outcrops and GPS measurements. In this study we discuss the rather incremental opening of the Anydros Basin in the Pliocene during six major tectonic pulses and the subsequent basin fill processes by interpreting seismic data and derived time isochore maps. Between the active pulses basin floor tilting persisted on a much lower rate. Seismic data illustrate the depositional processes in the emerging Anydros Basin. The observation of onlap fill strata, divergent reflection pattern, moat channels and contourite drifts imply that deposition was controlled by turbidity and contour currents as well as the tilting basin floor. The metamorphic Attico–Cycladic basement shows a rise that aligns along an NW–SE directed axis crossing Anydros island. This axis marks a structural change of the Santorini–Amorgos Ridge and thus represents a major structural boundary. Dip angles of NE–SW trending major faults, like the Santorini–Amorgos Fault, indicate normal faulting to be the superior mechanism forming the present horst and graben environment. Hence, the area is likely to be in a state of NW–SE directed extensional stresses forming the asymmetric graben structure of Anydros. Secondary fault clusters strike the same direction but show much steeper dip angles, possibly indicating strike-slip movement or resulting from deformational stresses along the hinge zones of the normal faults. The majority of the faults we discovered are located in the area of earthquake clusters, which is another indication of recent faulting. Ring faults around Kolumbo submarine volcano, result from caldera collapse and mark the diameter of the magma chamber approximately to 20 km.

Co-auteur étranger; International audience; One of the best known places on Earth where volcanology meets archaeology and history is the volcanic island of Santorini (Thira), Greece. It is famous for the cataclysmic Late Bronze Age (Minoan) Plinian eruption which destroyed the Minoan culture that flourished on the island. Hosting a central, flooded caldera bay and, within that the active islands of Palaea and Nea Kameni, Santorini volcano has been the focus of international research efforts for over one and a half centuries. In this paper, we summarize recent findings and related ideas about the Minoan physiography of the island, also known as Strongyli, from a volcanological, geomorphological and archaeological point of view. As proposed as early as the 1980s, a central caldera bay existed prior to the Late Bronze Age. Probably characterised by a smaller size and located in the northern part of the present-day caldera, this earlier caldera bay was formed during the previous Plinian eruption - called Cape Riva eruption - c. 22,000 years ago. Within the caldera bay, a central island, Pre-Kameni, existed, named after the present-day Kameni Islands. High-precision radioisotopic dating revealed that Pre-Kameni started to grow c. 20,000 years ago. Whereas volcanologists have accepted and refined the caldera concept, archaeologists have generally favoured the theory of an exploded central cone instead of a pre-existing central caldera. However, analysis of the Flotilla Fresco, one of the wall paintings found in the Bronze Age settlement of Akrotiri, reveals the interior of a Late Bronze Age caldera that may be interpreted as a realistic landscape. Approximately 3600 years ago, the island of Strongyli was destroyed during the explosive VEI = 7 Minoan eruption. Pre-Kameni was lost by this eruption, but its scattered fragments, together with other parts of Strongyli, can be recovered as lithic clasts from the Minoan tuffs. On the basis of photo-statistics and granulometry of the lithic clasts contained in the Minoan tuffs, complemented by volumetric assessment of the erupted tephra and digital elevation model (DEM) analysis of alternative models for the pre-eruptive topography, the volume of Pre-Kameni can be constrained between 1.6 and 3.0 km(3), whereas the volume of the destroyed portion of the ring island of Strongyli between 9.1 and 17.1 km(3). Of these, the larger values are considered more realistic, and imply that most of the destroyed part of Strongyli was incorporated as lithic components in the Minoan tuffs, whereas up to 3 km(3) of Strongyli might have been downfaulted and sunken during caldera formation and is not accounted for in the Ethics.