Volcanic Edifice Research Articles

Geochemistry and 40Ar/39Ar dating of the Sierra de Santa Marta in the tectonically controversial Los Tuxtlas Volcanic Complex (Veracruz, Mexico)

One of the most debated volcanic areas along the eastern coast of Mexico is Los Tuxtlas Volcanic Complex located in the south of the Veracruz state. This volcanic province stands out as an isolated massif between the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Belt to the SE. Los Tuxtlas complex is surrounded by two large alluvial plains formed by the Papaloapan and Coatzacoalcos rivers, and include the active volcano San Martín Tuxtla, the last eruption of which occurred in 1793. The ages, distribution, morphology and geochemistry of the volcanic vents and lavas, particularly the co-existence of the alkaline and subalkaline suites, offer an excellent opportunity to understand, in space and time, the tectono-magmatic history of Los Tuxtlas in comparison with other similar volcanic complexes in the eastern Trans-Mexican Volcanic Belt. Most of the volcanic centers of Los Tuxtlas show NW-SE alignments. This vent arrangement is associated with a left-lateral fault system known as the Veracruz fault, the northwestern expression of which are the submarine volcanoes that form the Anegada High. This fault might be a reactivation of the ancient Tamaulipas-Oaxaca fault related to the opening of the Gulf of Mexico. Based on geomorphological, geochemical, petrographical, and 40Ar/39Ar data, we propose three different stages of volcanic activity for Los Tuxtlas: The older Montepío-El Vigía (7–1.4 Ma), the intermediate Santa Marta (1–0.5 Ma), and the younger San Martín Tuxtla (50 ka to present). The main focus region of this paper is the stratovolcanoes of the Sierra de Santa Marta, formed by at least four major NW-SE aligned volcanic edifices, which from NW to SE are Encanto (950 m.a.s.l; identified and named in this work for the first time), Yohualtajapan (1450 m.a.s.l), Santa Marta (1685 m.a.s.l), and San Martín Pajapan (1250 m.a.s.l). The geological map, the 40Ar/39Ar dates, and the geochemistry data indicate a NW-SE migration of the volcanic activity of the four stratovolcanoes that have created the sierra de Santa Marta. Petrography and chemistry of rocks suggest that the subalkaline rocks of the three magmatic stages were fed by subduction activity of the Cocos Plate. The alkaline suite is likely associated with asthenosphere upwelling and slab edge melting, facilitated by a slab tear of the south Cocos plate evolved since the Early Miocene.

Monitoring of Indonesian volcanoes with the IS06 infrasound array

Detecting and notifying ongoing volcanic explosive eruptions is crucial in supporting the Volcanic Ash Advisory Centre (VAAC). However, local monitoring systems are missing at many active volcanoes, but long range infrasound monitoring might provide useful information if able to detect and notify volcanic explosive events. Indeed, many studies have already highlighted the utility and the potential of long-range infrasound monitoring for this aim, but still open questions remain concerning its actual efficiency and reliability. In this study we investigate the potential of the IS06 array (Cocos Island, Australia) of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to remotely detect volcanic explosive eruptions in the Indonesian Arc between 2012 and 2019, when 11 volcanoes, positioned at a distance between 1000 and 2000 km from the array, erupted with an energy spanning from mild explosions to VEI (Volcanic Explosivity Index) 4 eruptions. For each volcano, using infrasonic data recorded at a single array and accounting for realistic infrasound propagation conditions, we calculate a range corrected Infrasound Parameter (IP) and propose two additional empirical thresholds on signal strength and persistency. The IP is used eventually to define an alert whenever an established threshold is exceeded and the corresponding reliability estimated. Results show that the range corrected IP is highly reliable for events VEI = 3 or greater under favorable propagation conditions, but smaller scale short-lasting explosive eruptions still remain usually undetected. Unresolved ambiguity remains due to short spacing among volcanoes with respect to the array. For regional scale monitoring purposes, this can be solved only considering volcanic sectors rather than single volcanic edifices that, despite preventing unambiguous notification of a given volcano, might allow to increase the attention of the VAAC over a specific area.

Viscosity of crystal-free silicate melts from the active submarine volcanic chain of Mayotte

Following an unprecedented seismic activity that started in May 2018, a new volcanic edifice, now called Fani Maoré, was constructed on the ocean floor 50 km east of the island of Mayotte (Indian Ocean). This volcano is the latest addition to a volcanic chain characterized by an alkaline basanite-to-phonolite magmatic differentiation trend. Here, we performed viscosity measurements on five silicate melts representative of the East-Mayotte Volcanic Chain compositional trend: two basanites from Fani Maoré, one tephriphonolite and two phonolites from different parts of the volcanic chain. A concentric cylinder viscometer was employed at super-liquidus conditions between 1500 K and 1855 K and a creep apparatus was used for measuring the viscosity of the undercooled melts close to the glass transition temperature in the air. At super-liquidus temperatures, basanites have the lowest viscosity (0.11–0.34 to 0.99–1.16 log10 Pa⸱s), phonolites the highest (1.75–1.91 to 3.10–3.89 log10 Pa⸱s), while the viscosity of the tephriphonolite falls in between (0.89–1.97 log10 Pa⸱s). Near the glass transition, viscosity measurements were performed for one phonolite melt because obtaining pure glass samples for the basanite and tephriphonolite compositions was unsuccessful. This is due to the formation of nanolites upon quench as evidenced by Raman spectroscopy. The phonolite viscosity ranges from of 10.19 log10 Pa⸱s at 1058 K to 12.30 log10 Pa⸱s at 986 K. Comparison with existing empirical models reveals an underestimation of 1.2 to 2.0 log units at super-liquidus and undercooled temperatures, respectively, for the phonolite. This emphasizes (i) the lack of data falling along the alkaline basanite-to-phonolite magmatic differentiation trend to calibrate empirical models, and (ii) the complexity of modeling viscosity variations as a function of temperature and chemical composition for alkaline compositions. The new measurements indicate that, at eruptive temperatures between 1050 °C and 1150 °C (1323–1423 K), the oxidized, anhydrous, crystal-free and bubble-free basanite melt have a viscosity around 2.6 log10 Pa⸱s. In contrast, the anhydrous phonolite crystal- and bubble-free melt would have a viscosity around 6–10 log10 Pa⸱s at expected eruptive temperatures, from 800 to 1000 °C (1073–1273 K). Considering that both basanite and phonolite lavas from the Mayotte submarine volcanic chain contain <6% crystals and a significant amount of water (1-2.3 wt% and 0.8-1.2 wt%, respectively), such viscosity values are probably upper limits. The new viscosity measurements are essential to define eruptive models and to better understand the storage and transport dynamics of Comoros Archipelago magmas, and of alkaline magmas in general, from the source to the surface.

Imaging the lithospheric structure and plumbing system below the Mayotte volcanic zone

Teleseismic receiver-functions and Rayleigh-wave dispersion curves are jointly inverted for quantifying S-wave velocity profiles beneath the active volcanic zone off Mayotte. We show that the lithosphere in the east-northeast quadrant is composed of four main layers, interpreted as the volcanic edifice, the crust with underplating, the lithospheric mantle, and the asthenosphere, the latter two presenting a main low-velocity zone. The depths of the old (10–11 km) and new Moho (28–31 km) coincide with the two magma reservoirs evidenced by recent seismological and petrological methods. We propose that the main magma reservoir composed of mush with an increasing amount of liquid extends down to 54 km depth. This magma storage develops from a rheological contrast between the ductile lower and brittle upper lithospheric mantle and accounts for most of the volcanic eruption-related seismicity. Finally, the abnormally small thickness of the lithospheric mantle (33 km) is likely a result of a thermal thinning since the onset of Cenozoic magmatism.

The growth of polygenetic volcanoes in the laboratory: Control of eruptive volume and magma viscosity on edifice development and morphology

Morphometric parameters have been used to describe and classify the shape of volcanic edifices, but the processes that control these parameters are not fully understood. Here we investigate the influence of the changes in eruptive volume and magma viscosity on the edifice morphology of a simple polygenetic volcano with a fixed stable vent using analogue experiments in the laboratory. We ran the experiments by ejecting vegetable oil repeatedly at a static location in a temperature-controlled room with an adequate interval between ejections to allow this material to solidify. The experiments comprised four different scenarios: constant ejection volume and viscosity, linearly decreasing ejection volume with constant viscosity, exponentially decreasing ejection volume with constant viscosity, and increasing viscosity with constant ejection volume. We built digital elevation models from images captured by digital cameras after each ejection using photogrammetry method. We describe the shapes of the edifices using morphometric parameters, such as height, basal width, H/W ratio, volume, slope, circularity, and regularity. The experiments with decreasing ejection volume produced taller edifices with steeper slopes, especially near the summit, compared to the edifice produced with constant ejection volume. A similar finding was also observed for the edifice resulting from the experiment with increasing viscosity. The circularity and regularity indexes were insignificantly influenced by ejection volume and viscosity changes. Instead, these parameters vary with the height fraction of the edifice where the lower part is more circular, but irregular compared to the upper part. Based on the changes of morphometric variables throughout the experiments, we propose three development stages of volcanic edifice growth: basal foundation, flank construction, and vertical build-up. The development stage in which the edifice currently grows can be inferred from the relative changes in the eruptive volume and the magma viscosity. Therefore, the quantitative characterization of a natural volcanic edifice's morphology can be interpreted to explain the processes that influenced it and its stage of growth.

Controlling influence of water and ice on eruptive style and edifice construction in the Mount Melbourne Volcanic Field (northern Victoria Land, Antarctica)

The Mount Melbourne Volcanic Field (MMVF) is part of the West Antarctic Rift System, one of Earth’s largest intra-continental rift zones. It contains numerous small, compositionally diverse (alkali basalt–benmoreite) flank and satellite vents of Late Miocene–Pliocene age (≤12.50 Ma; mainly less than 2.5 Ma). They demonstrate a wide range of morphologies and eruptive mechanisms despite overlapping compositions and elevations, and they occur in a relatively small area surrounding the active Mount Melbourne stratovolcano. The volcanic outcrops fall into several main categories based on eruptive style: scoria cones, tuff cones, megapillow complexes, and shield volcanoes. Using the analysis of lithofacies and appraisal of the internal architectures of the outcrops, we have interpreted the likely eruptive setting for each center and examined the links between the environmental conditions and the resulting volcanic edifice types. Previous investigations assumed a glacial setting for most of the centers but without giving supporting evidence. We demonstrate that the local contemporary environmental conditions exerted a dominant control on the resulting volcanic edifices (i.e., the presence or absence of water, including ice or snow). The scoria cones erupted under dry subaerial conditions. Products of highly explosive hydrovolcanic eruptions are represented by tuff cones. The water involved was mainly glacial (meltwater) but may have been marine in a few examples, based on a comparison of the contrasting internal architectures of tuff cones erupted in confined (glacial) and unconfined (marine, lacustrine) settings. One of the glaciovolcanic tuff cones ceased activity shortly after it began transitioning to a tuya. The megapillow complexes are highly distinctive and have not been previously recognized in glaciovolcanic successions. They are subglacial effusive sequences emplaced as interconnected megapillows, lobes, and thick simple sheet lavas. They are believed to have erupted at moderately high discharge and reduced cooling rates in partially drained englacial vaults under ice, probably several hundred meters in thickness. Finally, several overlapping small shield volcanoes crop out mainly in the Cape Washington peninsula area. They are constructed of previously unrecognized multiple ‘a‘ā lava-fed deltas, erupted in association with a thin draping ice cover c. 50–145 m thick. Our study highlights how effectively water in all its forms (e.g., snow, ice, and any meltwater) or its absence exerts a fundamental control on eruption dynamics and volcano construction. When linked to published ages and40Ar/39Ar dates produced by this study, the new environmental information indicates that the Late Pliocene–Pleistocene landscape was mainly an icefield rather than a persistent topography-drowning ice sheet. Ice thicknesses also generally increased toward the present.

From ‘source to sink’ to ‘sink to source’: a review of volcanic fluvial and lacustrine successions in Japan

Abstract Volcaniclastic sedimentological studies in Japan have addressed Miocene–Holocene volcaniclastic fluvial and lacustrine successions. These research approaches can be categorized into: (1) the ‘from volcanic source to sink’ concept focusing on hydrological volcaniclastic sedimentation (including lahars), environmental impacts and cascading processes by explosive volcanism; and (2) the ‘from sink to volcanic source’ view, with a focus on elucidating volcanic and eruptive information such as the location of the eruption centre, eruption style and eruption age from analyses of distally deposited volcaniclastic sediments other than tephra fall deposits, despite the occasional absence of correlative eruption records in proximal areas and/or on the volcanic edifice itself. Geological records, which have a longer time span, can reveal decadal- to millennial-scale hydrological and cascading volcaniclastic sedimentation that cannot be assessed currently by direct observation. In addition, distal volcaniclastic successions in sinks have a continuous depositional record that can provide precise ages and the recurrence intervals of past eruptions and eruption-associated hydrological events. Both perspectives, beyond the borders of sedimentology and volcanology, are necessary to capture the geological record fully and to understand past volcanisms and their impacts, which will eventually lead to better prediction and preparation for future volcanic events.

Geochemical Features of Volcanic Rocks from the Shaerbuti Mountain Complex, West Junggar, Xinjiang, China: Implications for Recycling of Materials

In this paper, we focus on the geological features of volcanic edifices and the geochemistry of intermediate–basic volcanic rocks of Shaerbuti Mountain, which offer a new perspective on recycled materials in the study area. The Shaerbuti volcanic rocks consist of calc-alkali basalt and andesite formed in an arc setting. The porphyroclastic texture of basalt, explosive breccia rock, and the distribution of both breccia and agglomerate provide robust evidence that a volcanic edifice exists in Shaerbuti Mountain. Based on geochemical features, the Shaerbuti volcanic rocks have been identified as being of two types. Type I volcanic rocks have light rare earth element (LREE)-enriched patterns, with La/Sm ratios of 2.27–4.03, Th/Yb ratios of 0.50–1.46, and Nb/Yb ratios of 1.11–2.28. Type II volcanic rocks display a flat rare earth element (REE) pattern, with La/Sm ratios ranging from 1.83 to 2.43, Th/Yb ratios ranging from 0.24 to 0.45, and Nb/Yb ratios ranging from 0.87 to 0.93. In the studied rocks, MgO-Cr, MgO-Ni and MgO-CaO present a positive relationship, which indicates clinopyroxenes crystallized. The Sr-Nd-Pb isotopic compositions of these basalts present values of 0.7045 to 0.7063 ((87Sr/86Sr)i), 6.4 to 6.6 (εNd(t)), and 17.1300 to 18.3477 ((206Pb/204Pb)i), respectively. According to Sr-Nd-Pb isotope features, we argue that melts of altered oceanic crust and sediments were incorporated into the source. We also evaluate the water content (0.55%–6.72%) of the studied volcanic rocks.

Разнообразие рельефа подводных вулканических гор Северо-Восточной Атлантики

The structure of the volcanic seamounts of the Atlantis-Great Meteor group, as well as areas of the archipelagos of the Cape Verde Islands, the Canary Islands and the Azores was analyzed. Their simplest type is a cone-like structure with one peak (Mt. Maio). A more complex type is represented by two closely spaced volcanic edifices (Mt. Nola). Seamounts were also formed as a result of the activity of a central-type volcano and a fissure system (Mt. Te-Paps). A more complex morphology is represented by mountains that were formed as a result of the activity of an underwater volcano of the central type, which was combined with the activity of 3–4 radial fissure volcanic systems (Mt.Tropic). Seamounts were also formed only during fissure eruptions (Mts. Kondor, Kadamosto). They are ridges, tens of kilometers long without a clearly defined peak. All of the above types of mountains, with their rise above sea level, subsequent abrasion and subsidence, lead to the creation of flat-topped structures (guyots) (Mt. Sengor).

Elemental and mineralogical characterization of marine sediments and their relationship to sedimentary and oceanographic processes in Central Bransfield Basin.

This work consists of the sedimentological, mineralogical, and geochemical characterization of eight marine sediment cores collected in the Central Bransfield Basin, along a transect between the South Shetland Islands to the Antarctic Peninsula and its correlation to the sedimentary and oceanographic processes of the area. A chemical characterization based on X-ray fluorescence dispersive spectrometry was implemented to obtain geochemical data of the marine sediment while the minerals were identified by X-ray diffraction. The study allowed to classify the cores into three groups according to their sediment source and chemical and mineralogical characteristics. The joint assessment of the geochemical and mineralogical signature of the sediment has confirmed that the elemental ratios Ti/Ca and Fe/Ca can be applied as proxies in the reconstitution of the terrigenous contribution to the Central Bransfield Basin if we consider the sedimentary contribution of the volcanic edifices present in the region. The Fe/K ratio associated with the Chemical Index of Alteration reinforced an increase in the degree of weathering near South Shetland Island, which is also pointed out by other authors in studies on climate change mainly in the subantarctic islands. The trend of temperature increase implies the importance of monitoring the region.

Volcano‐Magnetic Signal Reveals Rapid Evolution of the Inner Structure of Piton de la Fournaise

AbstractNear‐real time analysis of magnetization can provide important information for the imaging of volcano systems and their spatiotemporal evolution. This study focuses on the contribution of volcano‐magnetic signals from reiterations of ground magnetic measurements to investigate the evolution of active structures at the Piton de la Fournaise volcano from 2017 to 2020. Changes are demonstrated by magnetic anomalies along a reference profile by means of the reiteration periods. These variations are first modeled qualitatively in 2D using electrical resistivity constraints in order to investigate the evolution of magnetization at depth through time, and the model is subsequently compared with the 3D intrusive activity from depth up to the surface from Interferometric Synthetic Aperture Radar (InSAR) inverse modeling. The shallow areas of demagnetization modeled from one reiteration to another are consistent with the geometry and location of the underlying intrusions revealed by the 3D InSAR models, suggesting strong thermal, stress, and electrokinetic effects due to magmatic activity not only at the surface but also at depth, along the main magmatic paths. It also raises a question as to the extent of the associated thermal diffusion processes at the scale of individual magma injections. This study confirms that detecting resistivity and magnetization anomalies, and quantifying their spatiotemporal evolution, can provide powerful tools for imaging volcanic systems at various scales and for providing warning of associated hazards. It also highlights the necessity for 4D monitoring of volcanic edifices using this method to provide greater precision, an important issue that is now made possible by the use of Unmanned Aerial Vehicle measurements.

Natural Semantic Networks in the study of risk perception in the face of volcanic ash fall in Tonila, Jalisco, México.

The Colima volcano is one of the most active in Mexico. Near the foothills of this there are several communities from the north of the state of Colima and the south of Jalisco, Mexico that are settled there, which in addition to experiencing volcanic hazards, in particular experience the fall of ash, this not only because of the proximity to the volcanic building, but also by the direction and speed of the winds that occur at the time the volcanic activity. The fall of volcanic ash becomes a social problem, since not only can it cause damage to people's health, but also the effects that they could have on their agricultural activities as well as the damage to animals generate an economic problem. for families. In this context, studying the perception of risk in the face of volcanic ash fall is essential, since we must not forget that human beings react to the risks they perceive, and the type of response they give depends on the level of knowledge they have regarding the situation that arises. is being presented to them, as in this case is the fall of volcanic ash. In this sense, the way in which the experts will manage the risk will depend on how the population acts. This article, in addition to the contributions it has in understanding the perception of risk that the population of Tonila, Jalisco, Mexico has in the face of volcanic ash fall, reflects on the methodological usefulness of natural semantic networks on this area.

Shallow recycling of lower continental crust: The Mahoney Seamount at the Southwest Indian Ridge

The Mahoney Seamount is a recently discovered volcanic edifice located 4 km north of the ultra-slow spreading Southwest Indian Ridge (SWIR). The SWIR is one of the slowest spreading ridges worldwide with a full spreading rate of ∼14 mm/year and low magmatic productivity. We report that highly vesicular basalts from the Mahoney Seamount have unradiogenic Nd-Hf together with radiogenic Sr isotopic compositions. Their distinct low 206Pb/204Pb isotope signature combined with high 207Pb/204Pb and 208Pb/204Pb is best explained by melting of a mantle that has been strongly influenced by stranded lower continental crust. The geographic distribution of the isotopic variability favors the idea of shallow recycling of lower continental crust isolated for a longer period contributing to melts forming Mahoney Seamount through off-axis fault systems. The isotopic composition of Mahoney Seamount lavas shares many characteristics with EM-1 sources and the DUPAL signature. Previous isotopic studies of the SWIR basalts proposed recycling of ancient subcontinental lithospheric mantle (SCLM) or pelagic sediments with oceanic crust to be responsible for this enriched isotopic signature. Lu/Hf and Sm/Nd ratios of pelagic sediments would result in decoupled 143Nd/144Nd and 176Hf/177Hf ratios. This decoupling is also observed in Ejeda-Bekily dikes from Madagascar, but those are believed to sample the SCLM dispersed in the Indian Ocean. However, Mahoney Seamount shows no decoupling in those isotopic systems and the restricted occurrence of the extreme lower continental crustal signature at Mahoney Seamount implies that the enriched isotopic signature has a different origin.

A machine learning approach to joint gravity and cosmic-ray muon inversion at Mt Usu, Japan

SUMMARY The ability to accurately and reliably obtain images of shallow subsurface anomalies within the Earth is important for hazard monitoring and a fundamental understanding of many geologic structures, such as volcanic edifices. In recent years, machine learning (ML) has gained increasing attention as a novel approach for addressing complex problems in the geosciences. Here we present an ML-based inversion method to integrate cosmic-ray muon and gravity data sets for shallow subsurface density imaging at a volcano. Starting with an ensemble of random density anomalies, we use physics-based forward calculations to find the corresponding set of expected gravity and muon attenuation observations. Given a large enough ensemble of synthetic density patterns and observations, the ML algorithm is trained to recognize the expected spatial relations within the synthetic input–output pairs, learning the inherent physical relationships between them. Once trained, the ML algorithm can then interpolate the best-fitting anomalous pattern given data that were not used in training, such as those obtained from field measurements. We test the validity of our ML algorithm using field data from the Showa-Shinzan lava dome (Mt Usu, Japan) and show that our model produces results consistent with those obtained using a more traditional Bayesian joint inversion. Our results are similar to the previously published inversion, and suggest that the Showa-Shinzan lava dome consists of a relatively high-density (2200–2400 km m–3) cylindrical anomaly, about 300 m in diameter. Adding noise to synthetic training and testing data sets shows that, as expected, the ML algorithm is most robust in areas of high sensitivity, as determined by the forward kernels. Overall, we discover that ML offers a viable alternate method to a Bayesian joint inversion when used with gravity and muon data sets for subsurface density imaging.

Evolution of magma supply system beneath a submarine lava dome after the 7.3-ka caldera-forming Kikai-Akahoya eruption

Kikai caldera in SW Japan, which is a submarine volcano, has undergone repeated caldera-forming eruptions. After the 7.3-ka caldera-forming Kika-Akahoya eruption, basaltic and rhyolitic subaerial eruptions occurred at Inamura-dake, Iwo-dake and Showa-Iwojima volcanic centers on the caldera rim. Fumarole activity still continues at Iwo-dake. Recent submarine surveys have revealed that a 600 m high central lava dome grew inside the Kikai caldera, with a volume of 32 km3. In addition, several satellite volcanic cones formed to the east and the west of this central lava dome. These features inside the submarine caldera would postdate the 7.3-ka Akahoya eruption. However, the formation mechanism of these submarine volcanic edifices is yet to be well understood. Here we present the whole-rock and mineral chemistry of dredged rocks from the central lava dome and satellite volcanic cones inside the submarine caldera. The results are used to constrain magmatic conditions that formed these submarine volcanic edifices. Most rhyolites from the central lava dome and the western satellite volcanic cones are compositionally distinct from those of the Akahoya eruption and the old Iwo-dake volcanic stage (5.2–3.9 ka) rhyolites. Instead, the chemical compositions of these submarine rhyolites overlap with those of the volcanic products of the young Iwo-dake, which is active since 2.2 ka. The compositional similarity suggests that the central lava dome tapped the same magma supplying the young Iwo-dake volcanism. By contrast, the dredge samples from the eastern satellite volcanic cones exhibit binary chemical trends: one group having chemical composition that overlaps with those of the young Iwo-dake rhyolites; the other group being chemically similar to the rhyolites from the old Iwo-dake and the Akahoya eruption. The magma temperature estimated from Fe-Ti oxide thermometry (922 °C on average) is almost equal to those estimated for subaerially erupted rhyolites at Iwo-dake during the young Iwo-dake volcanic stage and the Showa-Iwojima eruption in 1934–1935. The oxygen fugacity estimated from Fe-Ti oxygen barometry is 0.8 log unit higher than Ni-NiO buffer (NNO + 0.8), on average, which is lower than that estimated for the subaerially erupted rhyolites. A magmatic hygrometer estimated H2O content of the melt ranging 2.2–3.5 wt%. Polybaric crystallization from H2O-saturated melts may have occurred beneath the submarine lava dome at depths of 2–4 km. These results suggests that a separate magma supply system with basaltic and rhyolitic magmas may exist beneath the submarine central lava dome and that underlying basaltic magma, such as that beneath Iwo-dake and Showa-Iwojima, may have heated rhyolitic magmas stored at depths of 2–4 km. Additionally, the lack of mafic enclaves, which are by contrast commonly observed in the subaerially erupted rhyolites at Iwo-dake and Showa-Iwojima on the caldera rim, implies that direct interaction between basaltic and rhyolitic magmas is limited beneath the center of the submarine caldera.

Structure and Present State of the Astroni Volcano in the Campi Flegrei Caldera in Italy Based on Multidisciplinary Investigations

AbstractDespite its known reconstructed volcanic history, the structural setting and present state of the Astroni Volcano of the Campi Flegrei caldera in Italy are still poorly defined. Through structural, geophysical, and geochemical investigations, we elucidate the structure and present volcanic activity of the Astroni Volcano, which hosts tuff cones, scoriae cones, lava domes, and lakes on the crater floor. A volcano‐tectonic analysis focused on the entire volcano edifice, coupled with electrical resistivity tomography of the shallower part of the Astroni crater, revealed the main rock formations, faults, and possible fluid patterns within the first 150 m depth. Two main NE–SW and NW–SE trending fault sets were imaged using electrical resistivity modeling and measurements along the wall of the volcanic edifice; they likely delimit a maar‐like structure resulting from the highest energetic subplinian Astroni 6 eruption event and acted as magma pathways during the late eruptive activity stage. A 3D view of the reconstructed resistivity model revealed both deep root‐conduit‐like structures and shallower dome‐like shapes for volcanic edifices on the crater floor. Gas and carbon compositions in the NNE sectors of the Astroni Lago Grande are similar to those of the Solfatara fumarole fluids, suggesting common hydrothermal origin and a possible link with a deep hydrothermal reservoir. This fluid‐emission area along the border of the younger volcanic structure exhibits a +40°C maximum soil‐temperature anomaly. The proposed volcano‐tectonic architecture should improve the unrest scenarios in case of reactivation in this Campi Flegrei caldera sector and the monitoring strategies for the Astroni Volcano.

Long-term change of the eruption activities of Sakurajima volcano, Japan, inferred from the fallout tephra deposits

Stratigraphic analysis of fallout tephra deposits in and around a volcano provides a framework for understanding the long-term temporal change in the volcano’s activities. Here, we reconstruct the evolution of the volcanic activities of Sakurajima volcano based on reconstructed tephra stratigraphy by original field surveys, compilations of geological and archeological data, and new 14C dating. We define three eruption stages of Stage 1 (30–24 ka), Stage 2 (12.8–4.8 ka), and Stage 3 (4.5 ka–present), based on a major hiatus and shifting of the volcanic centers. Stages 2 and 3 are further subdivided according to the predominant mode of volcanic activity. Revised distribution of tephra deposits indicates that a total of 14.5 km3 (5.8 km3 dense rock equivalent) of fallout tephra has erupted from Sakurajima. Among them, Stage 2a (13–8 ka) produced the largest volume of tephra fallout deposits, suggesting that Sakurajima peaked in magma discharge during Stage 2a (2.9 km3/kyr) and then decreased rapidly toward Stage 2b (8–4.8 ka; 0.07 km3/kyr). The eruption of large-volume tephra deposits in Stage 2a, followed by the development of a thick volcanic ash layer in Stage 2b, indicates the eruption style shifted from explosive pumice eruptions in Stage 2a to ash-producing Vulcanian activity in Stage 2b, with decreasing magma discharge rate. Thick volcanic ash deposits covered by several fallout tephra deposits during Stage 3 also reflect a shift in activity from repeated Vulcanian explosions with lava effusions in Stage 3a (4.5–1.6 ka) to predominantly pumice eruptions during the historical period (Stage 3b) with an increasing magma discharge rate. The case study of Sakurajima presented here demonstrates that the combined analysis of the distribution, stratigraphy, and age of pumice fall layers with the lava and pyroclastic cone deposits on the volcanic edifice is a powerful tool for deciphering the growth history of complex stratovolcanoes.

Insights into the plumbing system of Colima Volcanic Complex from geophysical evidence

The Colima Volcanic Complex (CVC) consists of three stratovolcanoes: El Cántaro, Nevado de Colima and Volcán de Colima, aligned from north to south, with the same chronological order. It has been assumed that the magmatic system also has migrated in the same direction, with the surface trend of the volcanic edifices following the same pattern at depth. However, 3D inversion of gravimetric and aeromagnetic data, combined with spectral analysis (Werner deconvolution) give a detailed analysis of the CVC magmatic system, resulting in a new model of its geometry and migration, which takes into account the rifting process of the Colima Graben, and the separation of the Jalisco and Michoacán Blocks. The resulting fractures favor magma ascent through the western Michoacán Block boundary, which follows a NE-SW direction and gradually changing its angle, becoming more vertical close to Volcán de Colima. This research shows that the CVC magmatic system is best described as a Trans-crustal mush system, characterized by a zone of crystal rich magma storage located within the structural system. The direction of the magmatic migration at depth does not correspond to vertical ascent, illustrated by the lack of correlation with the volcanic edifices on the surface. Additionally, in the results it is possible to identify the separation between distinct phases of the magma, whereby the lower viscosity mobile phase is located in distinct zones within the upper crust, feeding the activity of Volcán de Colima.

Elastic interaction between Mauna Loa and Kīlauea evidenced by independent component analysis

The contrasting dynamics between Mauna Loa and Kīlauea have been studied over the last 100 years from multiple viewpoints. The fact that dynamic changes of one volcano trigger a dynamic response of the other volcano indicates a connection may exist. Petrological works show a direct relationship between the magmatic systems of these two volcanoes is not possible. We analysed DInSAR data and GPS measurements of ground deformation patterns associated with the activity of Mauna Loa and Kīlauea volcanoes. The DInSAR SBAS dataset spans the interval between 2003 and 2010, and was acquired along ascending and descending orbits of the ENVISAT (ESA) satellite under different look angles. Of the 10 tracks that cover the Big Island (Hawai‘i), 4 cover both volcanic edifices. Using GPS measurements, we computed the areal strain on 15 triplets of stations for Kīlauea volcano and 11 for Mauna Loa volcano. DInSAR data was analysed by applying Independent Component Analysis (ICA) to decompose the time-varying ground deformation pattern of both volcanoes. The results revealed anticorrelated ground deformation behaviour of the main calderas of Mauna Loa and Kīlauea, meaning that the opposite response is seen in the ground deformation of one volcano with respect to the other. At the same time, Kīlauea exhibits a more complex pattern, with an additional component, which appears not to be correlated with the dynamics of Mauna Loa. The GPS areal strain time series support these findings. To corroborate and help interpret the results, we performed inverse modelling of the observed ground deformation pattern using analytical source models. The results indicate that the ground deformation of Mauna Loa is associated with a dike-shaped source located at 6.2 km depth. In comparison, the anticorrelated ground deformation of Kīlauea is associated with a volumetric source at 1.2 km depth. This excludes a hydraulic connection as a possible mechanism to explain the anticorrelated behaviour; instead, we postulate a stress-transfer mechanism. To support this hypothesis, we performed a 3D numerical modelling of stress and strain fields in the study area, determining the elastic interaction of each source over the others. The most relevant finding is that the Mauna Loa shallow plumbing system can affect the shallowest magmatic reservoir of Kīlauea, while the opposite scenario is unlikely. Conversely, the second independent component observed at Kīlauea is associated to a sill-shaped source located at a depth of 3.5 km, which is less affected by this interaction process.

Key factors controlling deep Carboniferous volcanic reservoirs in the east slope of Mahu Sag, Junggar Basin, NW China

Considering the complex factors controlling volcanic reservoirs, the Carboniferous strata in the eastern slope area of the Mahu Sag (ESMS) in the northwestern Junggar Basin (NJB) were investigated using rock cores, thin sections, scanning electron microscope (SEM), physical properties, major elements, X-ray fluorescence (XRF), well logging, and seismic data. The volcanic rocks revealed by drilling are mostly weathering crust reservoirs (WCRs), the formation of which in and around the study area is significantly controlled by weathering and leaching (WL). Most types of volcanic rock can be improved by long-term weathering. Favorable reservoirs in the ESMS are often developed within 150 m below the tectonic unconformity boundary at the top of the Carboniferous. The longer the weathering duration, the better are the overall quality of the WCRs. Weathering duration of about 40 Ma is probably an important threshold in the NJB. Ultra-long leaching of atmospheric water and strong late dissolution of acidic fluids before oil and gas accumulations are important for reservoir development and petroleum accumulation in volcanic strata filled with authigenic minerals, especially calcite. The early regional tectonic movement affected the volcanic eruption and controlled the lithofacies distribution. The linear density of fractures was negatively correlated with the distance from the main controlling fault. Owing to the relatively weak filling, high-angle fractures contribute significantly to the reservoir permeability. The physical properties of volcanic breccia are better than those of tuff, and the porosity, permeability, and fracture density of andesite are higher than those of basalt. The physical properties of near-source facies belts of a volcanic edifice are better than those of far-source facies ones. Favorable exploration areas are the structural highs and fault zones where the duration of WL is more than 40 Ma, explosive facies and effusive facies near the crater are developed, or the inherited ancient buried hills transformed by faults and fractures near excellent source rocks, where the dissolution of atmospheric water and organic acidic fluids are strong.