Journal of Volcanology and Geothermal Research最新文献

{"title":"Welding degree assessment of the Escalera Ignimbrite in Central Mexico: Insights from physical and mechanical properties","authors":"Amador Castro-Colín ,&nbsp;Antonio Pola ,&nbsp;Hugo Sereno ,&nbsp;Juan Daniel Pérez-Orozco ,&nbsp;Martha Gabriela Gómez-Vasconcelos ,&nbsp;Elia Mercedes Alonso-Guzmán","doi":"10.1016/j.jvolgeores.2025.108473","DOIUrl":"10.1016/j.jvolgeores.2025.108473","url":null,"abstract":"<div><div>Ignimbrite welding encompasses processes such as glass sintering, deposit compaction, and pumice fragment flattening, influenced by emplacement temperature, cooling rate, and viscosity. These processes significantly impact the physical properties, mechanical behavior, and textural characteristics of ignimbrites. This study examines the Escalera Ignimbrite in Central Mexico to evaluate the welding degree using physical and mechanical properties alongside ultrasonic wave velocities and dynamic elastic constants. The analysis reveals stratigraphic variations in bulk density, porosity, and oblateness (OB) of pumice fragments, with increased compaction and welding intensity observed at greater depths. Lower stratigraphic sections exhibit higher bulk density (2.26 g/cm³) and reduced porosity (16.08 %), correlating with increased uniaxial compressive strength (UCS) values ranging from 60 to 65 MPa. Similarly, splitting tensile strength (<em>σ</em>_<em>t</em>) increases with depth, reaching values of 13.3 to 16.3 MPa in highly welded sections. Dynamic elastic modulus (<em>e.g.</em>, <em>E</em>_<em>d</em>) varies from 3.97 in upper sections to 8.91 GPa in lower (denser layers), reflecting enhanced compaction and reduced porosity. Additionally, P-wave (<em>Vp</em>) and S-wave (<em>Vs</em>) velocities exhibit trends consistent with welding intensity, with values ranging from 1673 to 2497 m/s for <em>Vp</em> and from 887 to 1216 m/s for <em>Vs</em>, suggesting their reliability as welding indicators compared to density and porosity alone. The results highlight a robust correlation between acoustic and mechanical properties with welding degree, emphasizing the role of post-depositional processes in modifying rock properties. This study contributes to the understanding of the mechanical evolution of ignimbrites and the implications of welding on their behavior.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108473"},"PeriodicalIF":2.3,"publicationDate":"2025-10-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333242","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"First year of magnetotelluric observations at Villarrica volcano, Southern Andes","authors":"Daniel Díaz ,&nbsp;Maximiliano Pavez ,&nbsp;Álvaro Amigo","doi":"10.1016/j.jvolgeores.2025.108474","DOIUrl":"10.1016/j.jvolgeores.2025.108474","url":null,"abstract":"<div><div>Villarrica volcano is located in the Southern Andes, one of the world's most productive volcanic arcs of the last decades and stands as the most active volcano in South America, in terms of the number of registered eruptive episodes. Besides its large eruptive record, Villarrica has other particularities, such as being currently an open conduit volcano providing a pathway through which volatiles and magma rise to the surface forming a persistent lava lake at its crater.</div><div>These features make Villarrica one of the most relevant volcanoes to study along the Andes, and efforts to characterize its internal magmatic structure have arisen during recent years. Along with geochemical and geophysical measurements normally used to image the magmatic system of this volcano, the use of monitoring methods to analyze different changes in time are needed to improve the understanding of active volcanic systems such this one.</div><div>Through a research project to test the capabilities of magnetotellurics as a volcano monitoring tool, a first permanent broad-band magnetotelluric station was installed at Villarrica volcano during January 2023, and has been measuring since then. Even considering the limitations of a single station experiment, interesting changes in magnetotelluric parameters measured during 2023 and beginning of 2024 have been estimated, and compared to other geophysical and geological parameters measured during this time, aiming to improve our monitoring capabilities and provide new insights into the magmatic processes at one of the most active volcanoes in the Andes.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108474"},"PeriodicalIF":2.3,"publicationDate":"2025-10-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333269","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Structural parameters of the late Ediacaran dyke swarm in the Saghro Massif (Eastern Anti-Atlas, Morocco): Implications for the depth of magma origin, paleo stress reconstruction and emplacement models","authors":"Ahmed Baamar ,&nbsp;Ezzoura Errami ,&nbsp;Brahim Karaoui ,&nbsp;Zakarya Yajioui ,&nbsp;John Browning","doi":"10.1016/j.jvolgeores.2025.108470","DOIUrl":"10.1016/j.jvolgeores.2025.108470","url":null,"abstract":"<div><div>The study of dyke swarms is crucial for understanding the processes of magma intrusion and the tectonic environments that influence volcanic activity. Dyke swarms are essential records of tectonic and magmatic events, offering insights into magma ascent, chamber dynamics, and stress conditions during emplacement. In this context, the dyke swarm of the central part of the Saghro Massif (Eastern Anti-Atlas) is investigated for the first time, using structural aspects of the dykes and paleo stress inversion to constrain magma overpressure, the depth of magma reservoir, the state of stress during emplacement, and the tectonic regime responsible for emplacement. We selected seventy-eight mafic and intermediate dykes in the swarm of the Central Part of the Saghro Massif (CPSM), characterized by different orientations N-S to NN<em>E</em>-SSW, NE-SW, E-W and NW-SE. In addition, the size distribution of thicknesses and lengths follows a power-law and a log-normal distribution respectively. We calculate magmatic overpressure using selected dyke aspect ratios to estimate the depth of the magma reservoir. This study suggests that the CPSM dykes form mainly due to the injection of magma from a deep magma reservoir at depths &gt;31 km, hence close to the Moho crust-mantle boundary (31–33 km). Paleo stress reconstruction shows that the CPSM dyke swarm was emplaced when the minimum principal compressive stress (σ₃) was oriented WNW-ESE, and the maximum principal compressive stress (σ₁) was vertical. The reconstruction also shows that the emplacement of the CPSM swarm occurred during a transtensional tectonic regime, associated with the WACadomian orogeny.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108470"},"PeriodicalIF":2.3,"publicationDate":"2025-10-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333268","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Vent-specific unrest at Pavlof Volcano, Alaska: Insights from multidisciplinary data","authors":"Darren Tan ,&nbsp;David Fee ,&nbsp;Pavel Izbekov ,&nbsp;Taryn Lopez ,&nbsp;Társilo Girona ,&nbsp;Vanesa Burgos ,&nbsp;Stephen McNutt ,&nbsp;Matthew Haney ,&nbsp;Valerie Wasser ,&nbsp;Jessica Larsen ,&nbsp;Ronni Grapenthin ,&nbsp;Mario Angarita ,&nbsp;Pablo Saunders-Shultz ,&nbsp;Tara Shreve ,&nbsp;Jamshid Moshrefzadeh","doi":"10.1016/j.jvolgeores.2025.108465","DOIUrl":"10.1016/j.jvolgeores.2025.108465","url":null,"abstract":"<div><div>Pavlof Volcano, a frequently active volcano in the Aleutian arc, has erupted six times between 2007 and 2022. Many Pavlof Volcano eruptions lack detectable precursory seismicity or ground deformation, making them challenging to forecast. Since 2007, these eruptions have ranged from Strombolian to Subplinian (VEI 2–3), with their characteristics seemingly related to their active vent location. Summit vent eruptions tend to be abrupt and explosive, while southeast flank vent eruptions tend to be more effusive and longer-lived. To better understand Pavlof Volcano’s plumbing system and improve eruption forecasts, we integrate geophysical, petrologic, and satellite-based thermal infrared and gas data to conduct a multidisciplinary data synthesis for eruptions between 2007 and 2022. Notably, a recently developed volcano seismicity detection model reveals unique pre- and syn-eruptive seismic tremor regimes associated with each vent system. The summit vent eruptions show greater tremor diversity and resurgent eruptive behavior, whereas the southeast flank vent eruptions show a steady tremor transition over a single eruptive phase. We use local infrasound data and air-to-ground coupled waves to build explosion catalogs, revealing gas-rich explosions during the 2021–2022 eruption from the summit vent, which was previously thought to be inactive during that eruption. Whole-rock composition and ash analyses indicate a near-uniform basaltic andesite composition spanning multiple eruptions, and a consistent anomalous presence of altered olivines in erupted material. We use these findings to propose a new conceptual model for Pavlof Volcano’s plumbing system: a shallow T-junction outlet controlling seismic tremor diversity and the partitioning of gas-charged magma; and an elongated, heated conduit transporting magma from depth and storing partly molten, residual magma between eruptions. We suggest that magma ascent rate and vent sealing modulate eruption style at Pavlof Volcano, where the T-junction directs rapidly ascending, gas-rich magma to the summit vent, and diverts slower rising, gas-poor magma towards the southeast flank vent.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108465"},"PeriodicalIF":2.3,"publicationDate":"2025-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145363370","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Discrimination between dome-forming and explosive eruptions in the stratigraphic record – field, textural and petrographic evidence from the Taiheizan eruption, Mt. Sanbe, Japan","authors":"Andreas Auer ,&nbsp;Keiko Suzuki-Kamata ,&nbsp;Tetsuya Kogure ,&nbsp;Daisuke Endo ,&nbsp;Hiroshi Kitagawa ,&nbsp;Shun Orui ,&nbsp;Katsura Kobayashi","doi":"10.1016/j.jvolgeores.2025.108462","DOIUrl":"10.1016/j.jvolgeores.2025.108462","url":null,"abstract":"<div><div>Arc volcanoes often show an alternating behaviour between explosive and effusive activity. Although considered an effusive phenomenon, emplacement of highly evolved lava domes is almost always accompanied by explosive activity and processes that initiate the formation of pyroclastic density currents. Lava dome forming eruptions have distinct hazard patterns and whether a specific volcano has produced lava domes during its history is important. While the presence of a lava dome is self-evident for modern (witnessed) eruptions – the question becomes more difficult for deposits in the stratigraphic record. We here review the most recent eruption of Sanbe Volcano, SW Japan, and highlight challenges in the discrimination between explosive and effusive eruption styles. The Taiheizan eruption produced a large number of deposits from pyroclastic density currents (PDC) of the block and ash flow type, leading to the undisputed interpretation that an active lava dome was present during this eruption about 4000 years ago. However, re-evaluation of the deposits suggests they formed by eruption column collapse during a Plinian eruption. The term “block and ash flow deposit” (a descriptive expression for a deposit with a bimodal grainsize distribution), is often tied to an interpretation (formed by lava dome collapse) in the volcanological literature. There are many processes around active volcanoes that can produce deposits with large blocks in a fine-grained matrix and a dominantly bimodal GSD. This makes recognition of dome forming eruptions in the eruptive record challenging but also gives room for misinterpretation. Beside our re-evaluation of the Taiheizan eruption style, we provide a list of features that can help to identify dome forming eruptions in the stratigraphic record.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108462"},"PeriodicalIF":2.3,"publicationDate":"2025-10-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333271","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Temporal evolution of fumarolic gas geochemistry at the Nevados de Chillán Volcanic complex (2013−2023): Signals of volcanic unrest and insights into the hydrothermal system","authors":"Daniele Tardani ,&nbsp;Marco Taussi ,&nbsp;Franco Tassi ,&nbsp;Jorge E. Romero ,&nbsp;Philippe Robidoux ,&nbsp;Camila Poblete-González ,&nbsp;Fernanda Álvarez-Amado ,&nbsp;Lorenzo Manosalva ,&nbsp;Claudia Berrios ,&nbsp;Daniele L. Pinti ,&nbsp;Martin Reich ,&nbsp;Diego Morata ,&nbsp;Valentina Mura ,&nbsp;Gloria Arancibia","doi":"10.1016/j.jvolgeores.2025.108467","DOIUrl":"10.1016/j.jvolgeores.2025.108467","url":null,"abstract":"<div><div>Gas geochemistry is key to understanding volcanic processes, offering insights into subsurface magma dynamics and aiding eruption forecasting. We present a 10-year monitoring case study from Andean Southern Volcanic Zone (SVZ), one of the world's most active regions, demonstrating its value in assessing volcanic unrest. The geochemical composition of fumarolic gas emissions from the Nevados de Chillán Volcanic Complex (Chile) strongly varied across different phases of volcanic activity, permitting the evaluation of the dynamic interplay between magmatic and hydrothermal processes. During volcanic quiescence periods, (2013 and 2023), fumarolic gases were predominantly controlled by shallow meteoric-hydrothermal circulation, as suggested by Ar (⁴⁰Ar/³⁶Ar ∼ 270–290) and water isotopic signatures, low ³He/⁴He (or Rc) ratios (∼3.5 times the atmospheric value Ra), moderate CO₂ levels (between ∼2200 and ∼ 9800 μmol/mol), and the absence of magmatic gaseous species (SO₂, HCl, and HF). In contrast, the unrest phase (2016 and 2017) was marked by a rapid and significant gas character shift. Rc/Ra values increased up to &gt;6, CO₂ concentrations exceeded 12,000 μmol/mol, and magmatic gaseous species became detectable, with SO₂, HCl, and HF reaching 8.5, 4.3, and 0.21 μmol/mol, respectively. Additionally, simultaneous Ar (⁴⁰Ar/³⁶Ar ∼ 370–410) and water isotopic signature shifts from meteoric origin toward a mixing with deep components were also detected. Gas geothermometry, computed through the H₂/Ar* – CH₄/CO₂ and H₂/Ar* - CO/CO₂ equilibria systems, revealed a rise from 290 ± 10 °C in quiescence to 340 ± 10 °C during the unrest phase. The long-term geochemical surveillance helped detect a substantial signature of volatile-rich magmatic fluids influx into the hydrothermal system before the eruption phase (2018–2021), offering information on the early detection of unrest.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108467"},"PeriodicalIF":2.3,"publicationDate":"2025-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145363369","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Benchmarking and testing poroelastic dynamic magma mush volcano deformation models","authors":"Rami Alshembari,&nbsp;James Hickey,&nbsp;Lorenzo Mantiloni","doi":"10.1016/j.jvolgeores.2025.108468","DOIUrl":"10.1016/j.jvolgeores.2025.108468","url":null,"abstract":"<div><div>Volcano deformation studies provide crucial insights into magmatic processes, offering a key perspective on the dynamics of magma reservoirs and their eruption potential. Here, we benchmark a Finite Element (FE) poroelastic magma mush model with an analytical model of a spherical melt core and mush shell undergoing melt resupply. In our comparisons, we investigate key reservoir parameters, such as melt core pressure, deformation at the core-shell and shell-rock boundaries, and the evolution of tensile stress. The benchmarking provides excellent agreement between the FE results and the analytical solutions. We extend the analysis by incorporating a previously neglected free surface effect in the numerical models, allowing for the examination of surface deformation beyond the original analytical solution. Comparison of the surface deformation resulting from a core-shell magma mush model to that from a uniform poroelastic magma mush model highlights differences in the predicted surface displacement patterns, emphasizing the importance of understanding the ways in which melt can be stored in the crust. Additionally, we show that melt compressibility plays a more complex role in the core-shell model than in the uniform poroelastic model. A compressible melt in the core stores more pressure during melt resupply, which, upon release, drives post-injection inflation. In contrast, an incompressible melt in the core with a compressible interstitial melt in the shell shows post-injection deflation, as the shell absorbs residual pressure when injection ceases. Our results demonstrate how different reservoir configuration and melt distribution could impact the response of the Earth's crust to magma injections.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108468"},"PeriodicalIF":2.3,"publicationDate":"2025-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333272","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Transport and sedimentation of Pyroclastic Density Currents across topographic obstacles","authors":"Lucas Corna ,&nbsp;Gert Lube ,&nbsp;Daniel H. Uhle ,&nbsp;Ermanno Brosch ,&nbsp;Jim R. Jones ,&nbsp;Michael Manga ,&nbsp;Benjamin Andrews","doi":"10.1016/j.jvolgeores.2025.108469","DOIUrl":"10.1016/j.jvolgeores.2025.108469","url":null,"abstract":"<div><div>Pyroclastic density currents (PDCs) can cross significant topographic obstacles. The processes that govern the interaction of PDCs with obstacles remain poorly understood leaving uncertainty in hazard planning and mitigation. Here, we report the results of large-scale experimental PDCs comprising hot volcanic particles and gas propagating across ridge-shaped obstacles. Observations from high-speed video and measurements of the velocity, density and temperature structure of the flows are used to identify the flow processes that occur when PDCs propagate across and become partially blocked by hill-shaped topographic obstacles; and how these characteristics are recorded in PDC deposits. The experiments show that the interaction of PDCs with ridges generate strong local perturbations to the internal flow velocity, density and temperature structure. These flow changes are linked to three main processes: the blocking of the lower, concentrated flow region in front of the obstacle; the compression and acceleration of the non-blocked flow regions on the stoss side; and the flow detachment behind the crest and formation of a turbulent wake before flow re-attachment downstream. Flow-topography interactions result in deposition and erosion rates that vary by three and two orders of magnitude, respectively, which explain the strong asymmetry of PDC deposits across topographic obstacles. The facies architecture of experimental deposits across ridges resembles those of natural PDC deposits from Te Maari and Taupō volcanoes (New Zealand). The findings of this study can guide the interpretation of PDC deposits or be taken into consideration in numerical models simulating the propagation of PDCs across complex topography for hazard forecast.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108469"},"PeriodicalIF":2.3,"publicationDate":"2025-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145333273","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sources of helium and associated heat in hydrothermal fluids from the central western branch of the East African Rift System (Democratic Republic of Congo and Burundi)","authors":"Wisdom Kambale Kavyavu ,&nbsp;Daniele L. Pinti ,&nbsp;Bienfait Kambale Simisi ,&nbsp;Dario Tedesco","doi":"10.1016/j.jvolgeores.2025.108464","DOIUrl":"10.1016/j.jvolgeores.2025.108464","url":null,"abstract":"<div><div>The area around Lake Kivu, situated in the western branch of the East African Rift System (WBEARS), which spans the Democratic Republic of Congo, Rwanda, and Burundi, is characterized in its southern part by numerous hot springs. Chemistry and isotopic signatures of hydrothermal fluids have been studied in the northern part of Lake Kivu due to the presence of one of the world's active volcanoes, Nyiragongo. However, the areas along the coast of Lake Kivu, extending south to the shores of Lake Tanganyika, where active volcanism is currently absent, have been largely overlooked, despite the presence of several tens of hydrothermal manifestations with temperatures ranging from 25 to 86 °C, located in a highly faulted region characterized by low to moderate seismicity. This study aimed to fill the gap by collecting samples from 15 hot springs, four lake waters, and one cold spring, located in the Democratic Republic of Congo and Burundi. Stable isotopes of water (δ²H and δ ¹⁸O), noble gas isotopes of He, Ne, and Ar, Sr isotopic ratio (⁸⁷Sr/⁸⁶Sr), and major ions and trace elements were measured. Most water samples are classified as Na-K-HCO₃ or Ca-Mg-HCO₃ types. Stable isotopes indicate that the waters are of meteoric origin. The helium isotopic ratios (³He/⁴He or R), normalized to the atmospheric ratio (Ra = 1.384 × 10⁻⁶), range from 0.058 to 1.304, indicating a dominant crustal helium source and a maximum of 13 % of mantle helium. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.70362 in the Lac Vert sample, typical of mafic volcanic products, to 0.77520 in hot springs of the Ruzizi basin, a very radiogenic value possibly indicating water-rock interaction with the Proterozoic silicate basement of Central Africa. The ⁸⁷Sr/⁸⁶Sr shows a rough trend with the ⁴⁰Ar/³⁶Ar ratios, indicating a slightly detectable terrigenic ⁴⁰Ar* excess in hot spring water. These results contrast with those on the northern shore of Lake Kivu, which are characterized by magmatic helium, suggesting that moving further south, the heat is controlled by the local geothermal gradient, with fluids likely circulating deeper into the crust and warming up. This circulation is facilitated by the extensional faults of the African rift, where most of the hot springs are located. Calculated low crustal fluxes of helium in the Lake Kivu region are insufficient to create economically valuable helium reserves, as found in the southern termination of the WBEARS, in the Rukwa Rift Basin.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"468 ","pages":"Article 108464"},"PeriodicalIF":2.3,"publicationDate":"2025-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145363368","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Detailed resistivity structure by high-density AMT surveys in the geothermal area: A case study near Unzen volcanoes, Japan","authors":"Nagi Yamashita ,&nbsp;Tada-nori Goto ,&nbsp;Keiichi Ishizu ,&nbsp;Kodo Umakoshi ,&nbsp;Hiroshi Sasaki","doi":"10.1016/j.jvolgeores.2025.108466","DOIUrl":"10.1016/j.jvolgeores.2025.108466","url":null,"abstract":"<div><div>Hydrothermal fluids are well known to circulate through fracture zones (e.g., faults) in geothermal areas. Impermeable clay layers can play a role of caprock, retaining geothermal reservoirs underneath. Hydrothermal alteration layers are known to include clay minerals and to have lower hydraulic permeability and higher electrical conductivity than the surrounding rocks. The magnetotelluric (MT) method of geophysical exploration can estimate the resistivity structure below the surface. Therefore, MT method can reveal relations among hydrothermal fluid circulation, impermeable clay layers, and faults. Nevertheless, because of sparseness of observation sites and the resulting low spatial resolution, earlier studies using the MT method have rarely revealed such detailed relations. For this study, high-density audio-frequency MT (AMT) soundings were conducted at 83 sites in the Unzen hot spring area, Japan, with site spacing of approximately 50–150 m to estimate details of the three-dimensional (3-D) resistivity structure near the surface (shallower than 1 km). Consequently, the distribution of conductive bodies was clearly identified in the shallow subsurface. Some conductive bodies are exposed on the surface, where hot springs and fumaroles are distributed. The conductive bodies are regarded as representing hydrothermal alteration layers. However, no shallow conductive body was found on the north side of an active fault (Oshidori-no-ike fault; OF). This feature suggests sharp variation of subsurface temperatures at OF, possibly because of recharging of meteoric water along OF. We conclude that high-density 3-D AMT surveys can reveal shallow subsurface geothermal systems.</div></div>","PeriodicalId":54753,"journal":{"name":"Journal of Volcanology and Geothermal Research","volume":"469 ","pages":"Article 108466"},"PeriodicalIF":2.3,"publicationDate":"2025-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145520731","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}