Central Caldera Research Articles

Geological history of the Atira Mons large shield volcano, Beta Regio, Venus.

Atira Mons is a large (∼300,000 km2) low-relief (1.8 km) shield volcano, with individual flows extending up to ∼700 km from the central caldera. It is located about 3000 km NW from the major plume center Beta Regio. Detailed mapping of the flows (at 1:500,000 scale, 10x more detailed than previous mapping) has identified fifty-three flow units which are grouped into eleven packages. Flow units are distinguished based on radar brightness, topography, morphology, continuity, structural modification, and sources, while flow packages group flows with clear stratigraphic relationship affinity.Cross-cutting relationships indicate a complex and multi-episodic eruption history with provisional identification of six mapped stages. The most voluminous flows are concentrated in the early stages, while the younger pulses, with a few exceptions, are shorter and less voluminous. A central caldera hosts the youngest volcanism with flows breaching its eastern side. Multiple stages of caldera collapse are indicated.The volume of the volcano is estimated using various methods and yields values range from ∼47,000 to ∼270,000 km3. The larger estimates are consistent with that of the magma volume of Large Igneous Provinces (LIP) on Earth. An appropriate terrestrial analogue is the Benham Rise Oceanic LIP in the western margin of the Philippine Sea, and particularly the Apolaki Caldera, which is the world's largest known basaltic caldera with a diameter of ∼150 km.

Impact assessment caused by bradyseism phenomena in the Campi Flegrei area

Campi Flegrei (Italy) is among the areas with the greatest volcanic explosive risk in the world due to the dangerousness of the expected hazards, the high exposed value (about 500,000 people will be evacuated during the “alarm phase”), and the vulnerability of the urban settlements under the effect of the volcanic phenomena. The last two dramatic bradyseism phases occurred in 1969–1972 and 1982–1984 when Pozzuoli town was affected by rapid ground inflation, which brought an overall higher level of about 3.5 m and caused numerous earthquakes (M ≤ 4.2), with severe damage to buildings. During 1984, the seismicity was intense, with 33 events with 0.5 < M ≤ 3 and six with 3 < M ≤ 3.8. Subsequently, the Campi Flegrei caldera was characterized by general subsidence for about 20 years until 2005, when a new inflation period started and is still ongoing (∼1 m). The areal distribution of the recent uplift is characterized by the maximum vertical displacement in the town of Pozzuoli, with a radial decrease from the caldera center outwards. The need to better understand Campi Flegrei volcanic activity is fundamental to protecting the population from hazards linked to explosive volcanic eruptions and understanding the role of seismicity as a possible precursor of a potential eruption. In this perspective, as part of the activities of the PLINIVS Study Centre (Centre of Competence of Italian Civil Protection Department for Volcanic Risk), the authors developed a procedure, implemented in a web application, that relates the monitoring of the ground deformation with the behavior of buildings to evaluate the level of progressive damage to the ordinary Phlegraean buildings due to bradyseism in near real time. This study describes the models adopted for the three impact/risk factors (hazard, exposure, and vulnerability) used to estimate building damage.

Geosite determination based on geodiversity assessment utilizing the volcanic history of a near-sea-level explosive eruption-dominated volcanic island: Tūhua/Mayor Island, New Zealand

Abstract Tūhua/Mayor Island is located approximately 45 km off the NE coast of the North Island of New Zealand. This island was formed by various explosive and effusive volcanic eruptions commonly influenced by magma–water interaction eruption events occurring since the Pleistocene. The wider area of the SW Pacific contains numerous volcanic islands with a similar type of volcanic evolution. Tūhua/Mayor Island should be studied in more detail to understand the underlying volcanic mechanisms and apply this research to other volcanic islands in the SW Pacific. Mayor Island, also known by its indigenous Māori name Tūhua (obsidian in Māori), provides an ideal site for studying current volcanism. The present day island was formed around 150 ka ago and contains several rhyolitic lava-flows from different time periods, pyroclastic-flow deposits generated by small-volume localized eruptions and ignimbrite deposited by large explosive eruptions. Our research utilized a qualitative–quantitative assessment of geodiversity estimates to highlight possible geosites for the collection of precise information about the geological evolution of this area, demonstrating the potential of geoeducational sites. The term geodiversity recognizes geological and geomorphological elements, which have shaped the Earth's surface and underly our abiotic environment. Additionally, volcanic heritage was included in our equation, specifically tailored for Tūhua/Mayor Island and based on expert views (qualitative model). This model allows for a wider diversity for the area of research compared with the original method, which utilized only geological elements. The results show that areas with pyroclastic deposits exposed on the cliffs and in the centre of the collapse caldera should be considered for the further study for geosite planning.

The Ulandryk and related iron oxide-Cu-REE(-Au-U) prospects in the Russian Altai: A large emerging IOCG-type system in a Phanerozoic continental setting

The Ulandryk and related prospects in the Altaid orogenic belt (near the Russian-Mongolian border) comprise intense iron oxide (hematite), Cu, REE, and locally U mineralization and are estimated to contain in excess of 240 Mt iron oxide ores and 3.7 Mt copper. These prospects bear many signatures of the “classic” IOCG (-REE, U) deposits typical of Precambrian terranes but, in contrast, are situated in a Phanerozoic (Paleozoic) setting. They are related to the Early Devonian subvolcanic potassic granite stocks with transitional shoshonitic to A-type granite signatures suggesting a post-orogenic (post-collisional) setting, with some anorogenic (intracontinental to within-plate) affinity. The stocks have been intruded into a coeval local volcanic structure (a volcanic dome complicated by the central caldera) composed of Devonian trachyandesite, trachydacite, quartz latite, rhyolite, rhyolite-dacite, and dominant trachyrhyolite lavas, tuffs and subovolcanic porphyry intrusions.The granitic stocks are accompanied by magmatic and multi-staged hydrothermal breccias, zones of sodic (albite-quartz), potassic (quartz-K-feldspar), propylitic (quartz-albite-chlorite-amphibole) and dominant phyllic to carbonate-phyllic (quartz-sericite to quartz-sericite-carbonate) to possibly argillic (quartz-kaolinite) alteration, with intense development of hematite. Fluorite is also ubiquitous and locally abundant. The mineralization occurs in several wide (typically 60–100 m, up to 200 m) and extended (1.4–6 km) linear to arc-like paralleling steeply dipping zones of breccias surrounding the granite stocks, with the host rock and quartz fragments cemented by fine-grained quartz-hematite and coarser hematite (specularite) material. Copper-gold, REE, and locally U mineralization overprints hematite-rich zones. Wider quartz-sulfide stockworks with Cu-Au mineralization zones containing minor to trace hematite are also present. Quartz is generally abundant forming quartz-hematite veins, stockworks, and breccias (locally with crustiform texture), and zones of quartzite-like pervasive silicification (typically with sericite and clay minerals).Fluid inclusions in quartz contain daughter halite crystals and homogenize by halite dissolution after vapor bubble disappearance indicating a homogenous, high-salinity (35–45 wt%), Na-Ca-K-chloride, aqueous mineralizing fluid. The relatively low homogenization temperatures (∼200–260 °C) and generally subvolcanic-level pressure (∼0.2–0.5 kbar) conditions suggest a shallow level of mineralization possibly corresponding to an upper (to the uppermost) part of a vertically extended magmatic-hydrothermal system.

Inter-eruptive spatiotemporal variation of caldera contraction on Miyakejima volcano revealed by PALSAR and PALSAR-2 time-series data

Monitoring caldera deformation helps improve the understanding of pressure conditions within a volcanic conduit beneath the caldera. Miyakejima volcano in Japan experienced a caldera collapse associated with an eruption in 2000, which included dike intrusions. Although a previous InSAR study suggested deceleration of caldera subsidence in 2009, few observations on the caldera deformation have been reported. We applied SAR time-series analysis to Phased Array-type L-band Synthetic Aperture Radar (PALSAR) and PALSAR-2 data to investigate spatiotemporal variations of ground deformations at Miyakejima volcano during 2006–2011 and 2014–2019. The spatial characteristics of the caldera contraction transformed from a concentric pattern into a north-south asymmetric pattern in mid-2009, implying migration of a deformation peak from the central aspect to the southern edge of the caldera floor. These peaks of the caldera contraction are located at the initial position of the caldera collapse on the central caldera floor, and at an active fumarole on its southern edge. The asymmetric pattern of the caldera contraction continued during 2014–2019 based on PALSAR-2 data. Using the results of numerical modeling and earthquake distribution beneath the caldera, we interpreted that the continuous asymmetric caldera contraction was driven by brittle failures induced by continuous hydrothermal alterations. Although a previous study proposed deceleration of caldera subsidence in mid-2009, we believe that their findings revealed only a snapshot of the spatial variation of the caldera contraction detected in this study.

Spatial Variations in Magmatic Volatile Influx and Fluid Boiling in the Submarine Hydrothermal Systems of Niuatahi Caldera, Tonga Rear‐Arc

AbstractSubmarine caldera volcanoes may host several hydrothermal systems along the caldera wall and related to volcanic cones. Fluid boiling and magmatic volatile influx are common processes in shallow (<2,000 mbsl) subduction zone‐related environments causing variations in the mineralogical and chemical composition of seafloor hydrothermal mineralizations that remain poorly constrained. The submarine caldera of Niuatahi volcano, Tonga rear‐arc, hosts four active vent sites discharging high temperature fluids (<334°C) with variable salinities (369–583 mM Cl) that are indicative of fluid boiling, recorded by distinct Te/As and Te/Au in pyrite, sphalerite, and chalcopyrite. High sulfidation mineral assemblages (e.g., enargite), stable S isotope data and similar trace element signatures in sulfides and native S condensates suggest a minor and/or infrequent contribution of magmatic SO2 to the hydrothermal systems located proximal to the caldera center causing a volatile element (e.g., Se, Bi, Te) enrichment. The hydrothermal system at the northern caldera wall is decoupled from the magmatic SO2 source, as revealed by radiogenic Pb isotopes. Instead, S isotope and trace element constraints propose a host rock‐dominated hydrothermal system, lacking a magmatic volatile influx. The observed hydrothermal fractionation processes (fluid boiling) and the distinct metal (loid) sources (magmatic volatiles vs. host rock) represent a continuum from magmatic volatile‐ to host rock‐dominated hydrothermal systems within the Niuatahi caldera. This leads to seafloor mineralizations with spatially selective trace element enrichments, like Te, Se, and Bi (±Au, Ag) in the caldera center compared to Au, Ag, Zn, Cd, and Pb at the northern caldera wall.

Inflation of Okmok Volcano During 2008–2020 From PS Analyses and Source Inversion With Finite Element Models

AbstractOkmok volcano, located on northeastern Umnak Island along the eastern end of the Aleutian island arc, is one of the most active volcanoes in Alaska, producing multiple eruptions in the past century. The most recent eruption, which occurred during July–August of 2008, was the most explosive since the early nineteenth century. In the years following the 2008 eruption, Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) observations indicate that Okmok has inflated at a variable rate of 40–195 mm/yr. In this study, we investigate the post‐eruptive deformation of Okmok (2008–2020) using InSAR and GNSS. L‐band ALOS‐2, C‐band Sentinel‐1/Envisat and X‐band TerraSAR‐X data are analyzed with Persistent Scatterer (PS) InSAR method. The deformation time series calculated from InSAR and GNSS are assimilated into finite element models using the Ensemble Kalman Filter to track the evolution of the magma system through time. The results indicate that the InSAR‐derived deformation history can be well explained by a spatially stable magmatic source located in the central caldera, at about 3 km beneath the sea level, which is also believed to be the same source that produced the 1997 and 2008 eruptions. Magma accumulation in the reservoir is about from 2008 to 2020, which is about 160% and about 60% of the total reservoir volume changes during the 1997 and the 2008 eruptions, respectively.

Magma chamber decompression during explosive caldera-forming eruption of Aira caldera

Decompression of a magma chamber is a fundamental condition of caldera collapse. Although theoretical models have predicted the decompression of magma chambers before caldera collapse, few previous studies have demonstrated the amount of magma chamber decompression. Here, we determine water content in quartz glass embayments and inclusions from pyroclastic deposits of a caldera-forming eruption at Aira volcano approximately 30,000 years ago and apply this data to calculate decompression inside the magma chamber. We identify a pressure drop from 140–260 MPa to 20–90 MPa during the extraction of around 50 km3 of magma prior to the caldera collapse. The magma extraction may have caused down-sag subsidence at the caldera center before the onset of catastrophic caldera collapse. We propose that this deformation resulted in the fracturing and collapse of the roof rock into the magma chamber, leading to the eruption of massive ignimbrite.

The 2014–2015 complex collapse of the Bárðarbunga caldera, Iceland, revealed by seismic moment tensors

Between August 2014 and February 2015, a caldera collapse process in Bárðarbunga volcano accompanied a fissure eruption at the Holuhraun lava field after magma had propagated laterally 45 km from the Bárðarbunga magma chamber to the lava field. The Icelandic Meteorological Office (IMO) reported around 30,000 earthquakes at the caldera, including more than 70 earthquakes with moment magnitude MW ≥ 5.0. We built a moment tensor catalog of around 230 seismic events at the caldera with MW between 3.7 and 5.5 using a waveform inversion method. We identified five event families based on focal mechanisms, double couple (DC) and compensated linear vector dipole (CLVD) components, epicentral distribution relative to the caldera center, and time of occurrence. The main characteristics of these families are (1) DC to CLVD normal faulting events with steep (50°–70°) to near-vertical (>70°) dip angles on the northern side of the caldera; (2) DC to CLVD normal events with predominant steep (50°–70°) dip angles on the southern side of the caldera; (3) intra-caldera oblique strike-slip events; and (4) thrust events at the eastern side of the caldera with a horizontal tension axis compensated linear vector dipole (T-CLVD) component. Each family provided new insights about the Bárðarbunga collapse mechanism such as (1) a systematic increase in the CLVD component with magnitude, corroborating that the curvature in the ring fault is the likely cause for the vertical CLVD moment tensors observed for the largest events; (2) seismic evidence of near-vertical dip angles in agreement with the vertical faults observed in many calderas worldwide, and numerical and analog models of caldera collapse structures; (3) intra-caldera oblique and strike-slip events on the western side of the caldera occurred by asymmetric collapse; (4) thrust horizontal T-CLVD events observed at the last stage of the eruption, possibly related to a viscoelastic response.

Nisyros Volcanic Island: A Geosite through a Tailored GIS Story

The volcanic island of Nisyros (Greece) is here presented as it presents unique characteristics being a Quaternary volcano in the eastern Aegean Sea, composed of volcanic rocks, and featuring a central caldera that is surrounded by volcanic domes and thick lavas. Its history is strongly connected to the volcano impressing the visitors with steaming hydrothermal craters, intensive smell of sulfur and fumarolic gases, and hot springs. Due to its morphology and geographical position, its cultural and historical heritage has been unchanged in time, bequeath to the island plenty of churches, monasteries with hagiographic frescoes, castles, caves, and spas. To present the geodiversity, the cultural environment, and the biodiversity of this geosite, a relatively new geographic approach was used, ESRI Story Maps. Being web-based applications, they are widespread as an interactive responsive tool used for spatial data communication and dissemination, by combining thematic 2D and 3D webmaps, narrative text, and multimedia content. Such applications can be an ideal way for presenting the available information of places characterized as geosites or protected areas worldwide, providing quick access to the available information to a broader, non-technical audience, developing the interest, and possibly motivating the public to learn more or visit them.

Through a glass darkly: In-situ x-ray computed tomography imaging of feed melting in continuously fed laboratory-scale glass melter

This study describes the first direct in-situ 3-D observation of a steady-state melting process by imaging a laboratory-scale slurry-fed glass melter in operation by x-ray computed tomography. Features of the reacting glass-feed, the foam layer underneath, and cavities in the glass melt pool are reconstructed in three-dimensional images. A slurry pool formed in a deep central caldera of dense dried feed, which penetrated into the glass melt. Slurry overflow from the caldera led to fast-dried and highly porous feed structure. A thin layer of foam separated the caldera from the melt. Bubbles ~5–15 mm in diameter were seen to grow beneath the reacting feed and move through the melt to escape at the edge. Pore morphology is benchmarked against computed tomography scans of a pellet of reacting simulated waste glass feed, and evolved gas analysis describes the gases generated as a function of temperature. Cooling artifacts are imaged and compared to previous studies of quenched cold caps. Detailed understanding of processes occurring during the conversion process in and below the reacting feed layer is necessary for the development of representative models of the melting process.

Continuous evolution of oceanic crustal structure following an eruption at Axial Seamount, Juan de Fuca Ridge

Abstract We present the first continuous observations of the temporal evolution of oceanic crustal shear velocity beneath Axial Seamount, a submarine volcano on the Juan de Fuca Ridge (offshore northwestern North America). Weekly values of seafloor compliance, the periodic deformation of the seafloor under ocean waves, were estimated over the time period between December 2014 and May 2018 using data from two cabled broadband ocean-bottom seismometers with collocated absolute pressure sensors. We inverted these measurements for shear-wave velocity within the volcano beneath the two stations as a function of depth and time. Our results, combined with estimates of seismic compressional wave velocity, suggest that the shallow melt reservoir and the lower crust beneath the central caldera contain melt fractions of 14% and at least 4%, respectively. The eruption of April 2015 induced a dramatic drop in shear velocities beneath the central station, primarily in the lower crust, which could have been caused by an increase in melt fraction, a change in small-scale melt geometry, or both. The absence of such a change beneath the eastern flank of the caldera indicates that there is a lower-crustal conduit beneath the caldera center, which is much narrower in cross section (<1 km2) than the overlying melt reservoir (≥42 km2). Our study demonstrates the promise of using continuous data to understand submarine volcanism and crustal accretionary processes.

Reconstruction of the fossil hydrothermal system at Lake City caldera, Colorado, U.S.A.: Constraints for caldera-hosted geothermal systems

Reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings. The 22.9 Ma Lake City caldera in Colorado, U.S.A., is well-exposed and contains an exhumed fossil shallow hydrothermal system. In this study, alteration mineralogy, vein textures and fluid inclusions are used to characterise the temperature and composition, spatiotemporal variability, and structural controls of the hydrothermal system. At paleo-depth equivalent between 1 and 2 km, the hydrothermal system was dominantly moderate temperature (up to ~290 °C), low salinity (<3% NaCl equivalent) and neutral to weakly acidic pH. There is evidence for boiling in veins throughout the exposed depth range; however, boiling textures are most common at the highest elevations of the system (~1150 m paleo-depth), and in structurally controlled fluid conduits. Quartz-illite alteration assemblages in the centre of the caldera indicate slightly more fluid-dominated conditions compared to those that formed propylitic alteration on the caldera margin. These alteration types reflect contrasting fluid pathways; a more pervasively fractured and faulted resurgent dome in the caldera centre, compared to fewer, larger conduits in the basement granite at the caldera margin. Based on the lack of high-temperature (<350 °C), hypersaline (>20 wt% NaCleq) fluid inclusions within the caldera centre, we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established. In contrast, in the eastern portion of the caldera, late distinct magma batches provided high temperature (up to ~540 °C) and hypersaline (up to ~65% NaCl eq.) magmatic fluid input above intrusions. Our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings. We also recognize the contrasting hydrothermal manifestations of a waning, degassed magma batch “left-over” from a caldera forming eruption, compared to fresh, volatile-rich magma.

High Rates of Inflation During a Noneruptive Episode of Seismic Unrest at Semisopochnoi Volcano, Alaska in 2014–2015

AbstractMagma intrusion rate is a key parameter in eruption triggering but is poorly quantified in existing geodetic studies. Here we examine two episodes of rapid inflation in this context. Two noneruptive microseismic swarms were recorded at Semisopochnoi Volcano, Alaska in 2014–2015. We use differential SAR techniques and TerraSAR‐X images to document surface deformation from 2011 to 2015, which comprises island‐wide radial inflation totaling ~25 cm (+/−1 cm) line of sight displacement in 2014–2015. Multiple source geometries are tested in an inversion of the deformation data, and InSAR data are best fit by a spheroid trending to the northeast and plunging to the southeast, with a major axis of ~4 km and minor axes of ~1 km, directly under the central caldera of Semisopochnoi. In 2014, a modeled influx of 0.043 km3 of magma caused line of sight displacement of ~17 cm. This magma was stored at a depth of ~8 km, until 2015 when 0.029 km3 was added. Along with the definition of inflation source parameters, the recorded seismic events are relocated using differential travel times. These relocated events outline a linear aseismic area within a larger zone of shallow (&lt;10 km) seismicity. This aseismic region aligns with the centroid of the deformation model. Based on these geodetic and seismic models, the plumbing system at Semisopochnoi is interpreted as a spheroidal magma storage zone at a depth of ˜8 km below a linear feature of partial melt. The observed deformation and seismicity appear to result from rapid injection into this main storage region.

Focal mechanisms of recent seismicity at Campi Flegrei, Italy

The present study analysed volcano tectonic earthquakes that occurred in the caldera of Campi Flegrei, Italy from 2000 to 2019 to compute source focal mechanisms. We used data from a large number of seismic stations to estimate reliable fault plane solutions for 71 events of a maximum duration magnitude (MD) ranging between 0.5 ≤ MD ≤ 2.5. We found that most solutions were of a normal type and were rake-angle negative focal mechanisms in 86% of the cases. Only a few earthquakes occurred on normal dip-slip faults; the majority occurred on varying degrees of oblique slip. Only one event had a pure reverse-mechanism, and it was located far from the caldera centre. The spatial distribution of the computed mechanisms exhibited a remarkable mix of fault orientations without any relationship to the location area or source depth. The predominance of normal kinematics on high-angle and nearly vertical faults and the very shallow source depth exhibited good agreement with the fact that seismicity at Campi Flegrei occurred during periods of ground uplift. In fact, ground uplift elongated the shallowest crust, thus reducing the normal stress on existing locked faults; this facilitated earthquakes, particularly those characterised by normal, dip-slip, and strike-slip mechanisms. In contrast, ground subsidence produced a horizontal shortening of the upper crust, which increased the normal stress on high-angle faults and reduced earthquake occurrence. Our results indicate that the driving force of the recent volcano-tectonic seismicity in the study region is the ongoing ground uplift.

The Internal Structural Evolution of Calderas: Results from 3D Discrete Element Simulations

The structural evolution of calderas is a key issue in volcanology and has profound implications for hazard analysis and the exploitation of geothermal energy and hydrothermal ores. However, their internal geometry at depth and the detailed fault and fracture distribution are unclear and debated. In order to better constrain the internal structural evolution of calderas, I have developed a 3D discrete element model of a frictional cover undergoing piston-like subsidence at its base, simulating magma chamber deflation and cover collapse. I examine two piston geometries, simulating magma chambers with roofs that are circular and rectangular in plan view, to investigate patterns of faulting and subsidence in three dimensions. In both models a complex arrangement of normal and reverse faults accommodates deeper subsidence at higher structural levels. Bell- to cone-shaped, outward-dipping ring faults are consistently the first structures to develop; these faults propagate upwards from the piston edges towards the surface. Later caldera growth is mainly the result of movement on vertical, or steeply inward-dipping, normal ring faults which enclose the earlier reverse faults. As a result, all calderas widen, in terms of their surface expression, with time. The final stage of caldera development includes significant collapse of the caldera walls and transport of this material towards the caldera center. The results confirm that the evolutionary patterns/stages proposed from 2D numerical and analogue models can be generalized to three dimensions, although significant differences between long- and short-axis geometries do occur when the piston is elongate. Compared to 2D simulations, however, 3D results show the geometric complexity of ring faulting, with variations in strain and fault activity at various stages of development demonstrating that often a simple, continuous ring fault structure is not developed.

Magma interactions, crystal mush formation, timescales, and unrest during caldera collapse and lateral eruption at ocean island basaltic volcanoes (Piton de la Fournaise, La Réunion)

The dynamics of magmatic processes at large mafic ocean island volcanoes control the likely locations (central caldera versus flanks) and timing of their eruptions. Crystals and their melt inclusions are key witnesses of these processes but are rarely studied in detail and in the same samples. Here we report the crystal and melt inclusion compositions of the April 2007 caldera-forming eruption of Piton de la Fournaise volcano and discuss how they relate to geophysical unrest monitoring data. Olivine crystals show mainly normal zoning (decrease in Mg/Fe) towards the rims, and also around some melt inclusions. Many crystals also show fine-scale skeletal structures defined by high phosphorus concentrations. Melt inclusions contain 53–205 ppm CO2 and 0.25–1.1 wt% H2O, and δD (δD values expressed as δDVSMOW) ranges from −135 to 62‰. Monitoring data show that inflation of the edifice started about a month before the first 2007 eruption: magma intrusion occurred at ≈3 km below sea level, and quickly migrated towards shallower depths (about 1 km above sea level). Such a time frame of magma movement is recorded in the chemical zoning of the olivine crystals that massively and quickly crystallized when reaching shallow depth, without significant interactions between resident and intruding magmas. The intrusion was followed by lateral flank eruption and caldera collapse. The chemical zoning of the olivine crystal rims and around the melt inclusions indicates that the newly created crystal-mush moved laterally towards the surface in matter of days to 3 months. Post-caldera samples show significant H+ loss, likely due to the depressurization of the magmatic system stored at shallow level. Our findings are different from other mafic ocean island volcanoes or stratovolcanoes (e.g. Kilauea, Canary Islands, and Etna), where crystals commonly record magma mixing between evolved and shallow melts and intruding mafic melts. We speculate that the difference between our findings and those of similar mafic ocean island volcanoes is due to the variety of magma supply rates from depth.

Multi-temporal airborne structure-from-motion on caldera rim: Hazard, visitor exposure and origins of instabilities at Piton de la Fournaise

Piton de la Fournaise is one of the world’s most active and visited volcanoes. Its summit crater (Cratère Dolomieu), the main tourist attraction, underwent a major caldera collapse in 2007 and its rim is not yet stabilized. In order to assess the caldera rim instability risk for visitors, we followed its structural evolution from 2007 to 2015. Using aerial photogrammetry campaigns, we mapped the unstable sites very precisely, carried out a quantitative analysis of the temporal evolution of these instabilities, and assessed the risks for visitors. Considering the 2008–2015 period, four sites close to the crater’s edge showed significant horizontal ground motion (0.5–2 m), fracture widening (average of 0.3–0.56 m) and large-scale mass wasting volumes (total of 1.8±0.1 × 106 m3). We infer two different processes at work: (1) to the west and north, toppling of the basalt units occurs after periods of fracture widening due to the combined effect of magmatic intrusions and long-term inflation/deflation cycles; (2) to the south and east, parts of the caldera rim slowly slide towards the caldera centre, with significant accelerations during periods of enhanced volcanic activity (in 2008–2010 and 2014–2015). The official observation platform is the most stable zone to overlook the Cratère Dolomieu. By contrast, the most frequently visited area of the rim (northwest) outside the official platform is also the most unstable.

Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)

The study area is located at the transition between the northern end of the Tonga Trench and the North Fiji fracture zone, where tectonic movements are reputed to be the fastest in the world. To the southeast of Futuna Island, a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a NE-SW oriented volcanic graben and the N-S oriented Alofi ridge. In 2010, the active volcano Kulo Lasi, which represents the most recent volcanic episode in the Futuna area, was discovered in the center of this extensive volcanic zone. Kulo Lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor. It is composed of basaltic to trachy-andesitic lava with no obvious geochemical affinity with the Tonga subduction that occurs 500 km to the east. The central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m. Diving operations with the submersible Nautile and high-resolution AUV mapping, have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera. Four tectono-volcanic stages can be distinguished at Kulo Lasi caldera. In stage 1, the shield volcano is built. Annular reverse faults develop at the summit and control circulation of water/rock-dominated hydrothermal fluids and high-temperature alteration of rocks along the nascent normal faults. Mixing of hydrothermal fluids with seawater is favored along normal superficial faults, leading to the formation of low-temperature Fe/Mn mineralization at the summit of the volcano. During stage 2, the caldera collapse, gradually revealing outcrops of the altered and mineralized zones formed during Stage 1. As the magma chamber cools and collapses, less heat is available. As a result, medium to low-temperature (<100 °C) Fe/Si deposits form on the floor of the caldera. In stage 3, refilling and ascent of the magma chamber at depth promote the uplift of a central resurgent dome devoid of recent lava emission. The depth of the magma chamber under the dome is estimated to be 1.6 km. Annular dykes feeding the new lava flows are controlled by the deep reverse faults at the outer rim of the caldera floor. The eruptive events are accompanied by short-lived emission of magmatic fluids, rich in SO2, materialized by the presence of native sulfur depositions on the surface of the most recent lava flows. During stage 4 (present day), black smoker sulfide chimneys, controlled by the deep reverse faults, form at the surface of the most recent lava flows at the outer part of the caldera floor. The fluids emitted result from the mixing of a deep fluid of the water/rock reaction type and of shallow seawater superheated in contact with the hot dykes. Hydrogen is generated during reaction of seawater with the hot dikes and H2 is enriched in the mixed fluids. Active (43 °C) siliceous chimneys at the base of the caldera walls and low-temperature vents (6 °C) perched on the caldera walls suggests a decrease in the venting temperature at a distance from the floor of the caldera. In chimneys, the sphalerite/pyrrhotite/isocubanite/barite paragenesis, probably linked to the high H2 concentrations in the fluids, indicate unusual reducing conditions for a back-arc. The sulfide mineralization also has unusual trace element concentrations. The concentration in Pb and Ba appears characteristic of back-arc environments while the enrichment in Co, Se, and Sn is more common to mineralization associated with basalt in mature back-arcs and associated with ultramafic rocks on mid ocean ridges.

Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology, oxygen isotope, and hafnium isotope analyses of zircon

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15−10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10−6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ18O (ranges of up to 6‰) and eHf (ranges of up to 24 e units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 104–106 years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ18O and eHf) and a combination of Precambrian basement rock (normal δ18O and eHf down to − 60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ18O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ18O compositions over time.