Shallow Hydrothermal System Research Articles

Caldera resurgence and the case-history of Campi Flegrei

Large calderas formed by explosive eruption are often characterized by a structural uplift of the caldera floor that has been named resurgent dome or block. The analysis of recent unrest at several calderas suggests that the resurgent dome is likely formed by the intrusion of magma at shallow depth below the light caldera infill. Campi Flegrei is an active volcano considered the highest risk volcano in Italy and Europe and has been in a state of unrest in the last 70 years. The analysis of the past volcanological history point to an unrest localized in a central resurgent block. Different or paired interpretations on the current unrest suggest either an intrusion of magma at shallow depth (3-5 km) or a deformation governed by the poro-elastic response of a shallow hydrothermal system to changes in fluid pressure and temperature. The invariance of the shape of the deformation, as well as the diffuse degassing of the Solfatara area, hint that the unrest is related with the uplift of the resurgent block driven by magma intrusion at shallow depth.

Electrical Resistivity Imaging of the Northeast Carpathian Volcanic Arc With 3‐D Magnetotellurics Reveals Shallow Hydrothermal System

Electrical Resistivity Imaging of the Northeast Carpathian Volcanic Arc With 3‐D Magnetotellurics Reveals Shallow Hydrothermal System

Functional diversity and metabolic response in benthic communities along an ocean acidification gradient

Altered ocean chemistry caused by ocean acidification (OA) is expected to have negative repercussions at different levels of the ecological hierarchy, starting from the individual and scaling up to the community and ultimately to the ecosystem level. Understanding the effects of OA on benthic organisms is of primary importance given their relevant ecological role in maintaining marine ecosystem functioning. The use of functional traits represents an effective technique to investigate how species adapt to altered environmental conditions and can be used to predict changes in the resilience of communities faced with stresses associated with climate change. Artificial supports were deployed for 1-y along a natural pH gradient in the shallow hydrothermal systems of the Bottaro crater near Panarea (Aeolian Archipelago, southern Tyrrhenian Sea), to explore changes in functional traits and metabolic rates of benthic communities and the repercussions in terms of functional diversity. Changes in community composition due to OA were accompanied by modifications in functional diversity. Altered conditions led to higher oxygen consumption in the acidified site and the selection of species with the functional traits needed to withstand OA. Calcification rate and reproduction were found to be the traits most affected by pH variations. A reduction in a community's functional evenness could potentially reduce its resilience to further environmental or anthropogenic stressors. These findings highlight the ability of the ecosystem to respond to climate change and provide insights into the modifications that can be expected given the predicted future pCO2 scenarios. Understanding the impact of climate change on functional diversity and thus on community functioning and stability is crucial if we are to predict changes in ecosystem vulnerability, especially in a context where OA occurs in combination with other environmental changes and anthropogenic stressors.

Magmatic priming of a phreatic eruption sequence: the 2012 Te Maari eruptions at Mt Tongariro (New Zealand) imaged by magnetotellurics and seismicity

SUMMARY Magnetotelluric data from Mount Tongariro have been analysed using an unstructured tetrahedral finite-element inversion code that incorporates topography, which was not included in previous analysis of these data. Incorporating topography adds information, which stabilizes the resistivity inversion modelling, and for the first time allows details of the shallow hydrothermal system and its relationship with the underlying magmatic system to be resolved. Specifically, an electrically conductive zone between 4 and 12.5 km depth marks the underlying magmatic system, which is shown to directly connect via conductive pathways to the area where the most recent phreatic eruptions at Tongariro occurred in 2012. The resultant phreatic eruptions in 2012 August and November showed no new magmatic component to the eruption deposits. Nevertheless, by combining the magnetotelluric resistivity image with relocated seismicity, we can see that seismicity (a proxy for magma ascent) migrated from the top of the magmatic system into the hydrothermal system in the months preceding these eruptions. Magmatic interaction with the extant hydrothermal system likely caused the over pressurization for the phreatic eruption. This work highlights the utility of combining geophysical methods and petrological data to constrain phreatic eruption processes.

An upper-crust lid over the Long Valley magma chamber.

Geophysical characterization of calderas is fundamental in assessing their potential for future catastrophic volcanic eruptions. The mechanism behind the unrest of Long Valley Caldera in California remains highly debated, with recent periods of uplift and seismicity driven either by the release of aqueous fluids from the magma chamber or by the intrusion of magma into the upper crust. We use distributed acoustic sensing data recorded along a 100-kilometer fiber-optic cable traversing the caldera to image its subsurface structure. Our images highlight a definite separation between the shallow hydrothermal system and the large magma chamber located at ~12-kilometer depth. The combination of the geological evidence with our results shows how fluids exsolved through second boiling provide the source of the observed uplift and seismicity.

Three-dimensional magnetotelluric modeling of Vulcano Island (Eolie, Italy) and its implications for understanding recent volcanic unrest

This paper presents the results of an original short-period magnetotelluric survey performed on Vulcano Island (Italy). The obtained three-dimensional resistivity model details structures up to 2.5 km depth, hitherto unexplored. The La Fossa caldera area corresponds to a moderate resistive anomaly, which extends down to the resolved depth and likely represents a “conduit-like” structure along which magmatic fluids stall and ascend. Other resistive anomalies characterize volcanic edifices, craters, volcanic conduits, and/or eruptive fissures. In addition, the shallower hydrothermal system is detected as a conductive anomaly. Sharp resistivity contrasts generally characterize caldera faults. A main N‒S alignment characterizes the island sector, where considerable amounts of deep subsurface fluids accumulate and mix with the ascending magmas related to the most recent volcanic dynamics. The volcanological interpretation of such findings significantly contributes to understanding the geophysical and geochemical anomalies detected in the last year, which involved the Vulcano shallow hydrothermal system, highlighting the potential for possible hydrothermal/phreatic eruptive events.

Hydrothermal signature on episodic deflation/inflation ground tilt at Aso Volcano

Ground deformation in volcanic areas induced by geothermal fluid circulation can reveal useful information about the dynamical processes occurring in the subsurface hydrothermal system. In the present work, we investigate tiltmeter time-series recorded at Aso Volcano during 2011–2016, a time interval during which different phases of volcanic activity occurred. We performed polarization analysis of the data and identified peculiar long-lasting (hours) transients, defined as Very-Long-period Tilt Pulses. The transients were further characterized in terms of waveform cross-correlation, particle tilt pattern, energy, and time distributions. The analyses indicate that such signals, which appear like deflation–inflation (DI) events, are associated with a Poissonian process whose underlying dynamics evolves over time always driven by a Poissonian mechanism. The obtained results have been interpreted in light of the available geophysical, geochemical and volcanological information. In this framework, the Very-Long-period Tilt Pulses may be ascribed to the depressurization/pressurization of the shallow hydrothermal system according to a fault-valve mechanism, which was active with different efficiency throughout eruptive and inter-eruptive phases.Graphical

Ancient atmospheric noble gases preserved in post-impact hydrothermal minerals of the 200 Ma-old Rochechouart impact structure, France

The evolution of Earth's atmosphere over billions of years provides important constraints on the geological history of our planet. However, major challenges in studying this have been finding samples in which ancient atmospheric signals are reliably preserved and precisely determining the age of the trapped atmosphere. Hydrothermal minerals can preserve fluid inclusions that formed from air-saturated waters. However, the use of hydrothermal minerals as paleo-atmospheric proxies is often limited by contamination by deeply-sourced crustal gases (e.g., radiogenic noble gases) and the difficulty of providing precise age constraints on the crystallization of the hydrothermal minerals.In this study, we tested the hypothesis that meteorite impact craters could provide an untapped archive of atmospheric evolution by investigating noble gases (Ne, Ar, Kr and Xe) contained in fluid inclusions within post-impact hydrothermal minerals from the Rochechouart impact structure (France). Impact craters can be dated using a variety of methods and because meteorite impacts cause localized fracturing of country rocks and the development of minor thermal anomalies, they are often associated with short-lived (up to a few Ma, depending on the size of the crater) and shallow hydrothermal systems dominated by surface-derived hydrothermal fluids.We found that the elemental ratios of noble gases in quartz from Rochechouart are close to the elemental ratios of modern Air. Furthermore, most noble gas isotope ratios are close to modern atmosphere, except for rare deviations caused by radiogenic ingrowth, nucleogenic effects and/or minor mass fractionation. After a minor correction for post-entrapment decay of 40K, an upper limit for the 40Ar/36Ar ratio of the ∼200 Ma atmospheric component trapped in Rochechouart quartz is determined as 292.7 ± 3.6 (2σ s.d.), which is significantly lower than the modern value of 298.6 ± 0.3. A Mesozoic 40Ar/36Ar ratio lower than 292.7 ± 3.6 is in line with the existing paleo-atmospheric dataset and existing models of the temporal evolution of the atmospheric 40Ar/36Ar ratio and conclusively proves the preservation of an ancient atmospheric component in rocks from Rochechouart. Given that meteorite impacts have occurred throughout the entire Earth's history they might collectively provide a unique, yet unexplored, repository of the atmospheric evolution. The same approach might also be applicable to investigating the atmospheric evolution of other terrestrial planets.

The role of humic-type ligands in the bioavailability and stabilization of dissolved iron in the Western Tropical South Pacific Ocean

The high N2 fixation rate observed in the Lau Basin of the western tropical South Pacific Ocean (WTSP) is fueled by iron (Fe) released from shallow hydrothermal systems. Understanding Fe bioavailability is crucial but the controls on the stability and bioavailability of hydrothermal Fe inputs are still poorly understood. Here, we provide new data on the spatial and vertical distribution of the soluble ubiquitous humic-like ligands (LFeHS) and their associated dissolved Fe (DFe) in the WTSP, including in samples near hydrothermal vents. Our data show that LFeHS are heterogenous ligands with binding sites of both strong and intermediate strengths. These ligands are primarily produced in surface waters and partially mineralized in mesopelagic waters. A substantial fraction of DFe was complexed by LFeHS (mean ~30%). The DFe complexed by LFeHS is likely bioavailable to phytoplankton and LFeHS stabilized Fe released by the mineralization of sinking biomass. However, unsaturation of LFeHS by Fe suggest that part of DFe is not available for complexation with LFeHS. Possible reasons are competition between DFe and other metals, such as dissolved copper, or the inability of LFeHS to access colloidal DFe. The study of two volcanic sites indicates that LFeHS were not produced in these hydrothermal systems. At the active site (DFe ~50 nmol L-1), LFeHS can only partially solubilize the hydrothermal DFe released in this area (1~5.5% of the total DFe). We performed controlled laboratory experiments which show that the observed low solubilization yield result from the inability of LFeHS to solubilize aged Fe oxyhydroxides (FeOx - a kinetically mediated process) and to form stable complexes with Fe(II) species. Our study provides new understanding of the role of LFeHS on the bioavailability and stabilization of hydrothermal DFe.

Influence of shallow hydrothermal fluid release on the functioning of phytoplankton communities

In the Western Tropical South Pacific (WTSP) Ocean, a hotspot of dinitrogen fixation has been identified. The survival of diazotrophs depends, among others, on the availability of dissolved iron (DFe) largely originating, as recently revealed, from shallow hydrothermal sources located along the Tonga-Kermadec arc that fertilize the Lau Basin with this element. On the opposite, these fluids, released directly close to the photic layer, can introduce numerous trace metals at concentrations that can be toxic to surface communities. Here, we performed an innovative 9-day experiment in 300 L reactors onboard the TONGA expedition, to examine the effects of hydrothermal fluids on natural plankton communities in the WTSP Ocean. Different volumes of fluids were mixed with non-hydrothermally influenced surface waters (mixing ratio from 0 to 14.5%) and the response of the communities was studied by monitoring numerous stocks and fluxes (phytoplankton biomass, community composition, net community production, N2fixation, thiol production, organic carbon and metal concentrations in exported material). Despite an initial toxic effect of hydrothermal fluids on phytoplankton communities, these inputs led to higher net community production and N2fixation rates, as well as elevated export of organic matter relative to control. This fertilizing effect was achieved through detoxification of the environment, rich in potentially toxic elements (e.g., Cu, Cd, Hg), likely by resistantSynechococcusecotypes able to produce strong binding ligands, especially thiols (thioacetamide-like and glutathione-like compounds). The striking increase of thiols quickly after fluid addition likely detoxified the environment, rendering it more favorable for phytoplankton growth. Indeed, phytoplankton groups stressed by the addition of fluids were then able to recover important growth rates, probably favored by the supply of numerous fertilizing trace metals (notably Fe) from hydrothermal fluids and new nitrogen provided by N2fixation. These experimental results are in good agreement within-situobservations, proving the causal link between the supply of hydrothermal fluids emitted at shallow depth into the surface layer and the intense biological productivity largely supported by diazotrophs in the WTSP Ocean. This study highlights the importance of considering shallow hydrothermal systems for a better understanding of the biological carbon pump.

High‐Resolution InSAR Reveals Localized Pre‐Eruptive Deformation Inside the Crater of Agung Volcano, Indonesia

AbstractDuring a volcanic crisis, high‐rate, localized deformation can indicate magma close to the surface, with important implications for eruption forecasting. However, only a few such examples have been reported, because frequent, dense monitoring is needed. High‐resolution Synthetic Aperture Radar (SAR) is capable of achieving <1 m spatial resolution and sub‐weekly revisit times, but is under‐used. Here we use high‐resolution satellite SAR imagery from COSMO‐SkyMed, TerraSAR‐X, and Sentinel‐1 to detect intra‐crater uplift preceding the November 2017 onset of eruptive activity at Agung, Indonesia. Processing the SAR imagery with an up‐to‐date, accurate, high‐resolution digital elevation model was crucial for preventing aliasing of the deformation signal and for accurate georeferencing. We show that >15 cm of line‐of‐sight shortening occurred over a 400‐by‐400 m area on the crater floor in September‐October 2017, accompanying a deep seismic swarm and flank dyke intrusion. We attribute the deformation to the pressurization of a shallow (<200 m deep) hydrothermal system by the injection of magmatic gases and fluids. We also observe a second pulse of intra‐crater deformation of 3–5 cm within 4 days to 11 hr prior to the first phreatomagmatic eruption, which is consistent with interaction between the hydrothermal system and the ascending magma. This phreatomagmatic eruption created the central pathway used during the final stages of magma ascent. Our observations have important implications for understanding unrest and eruption forecasting, and demonstrate the potential of monitoring with high‐resolution SAR.

Shallow resistivity structure around the 2018 craters of Mt. Motoshirane of Kusatsu-Shirane Volcano, Japan, revealed by audio-frequency magnetotellurics

In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement.Graphical

Identification of Seismo‐Volcanic Regimes at Whakaari/White Island (New Zealand) Via Systematic Tuning of an Unsupervised Classifier

AbstractWe present an algorithm based on Self‐Organizing Maps (SOM) and k‐means clustering to recognize patterns in a continuous 12.5‐year tremor time series recorded at Whakaari/White Island volcano, New Zealand (hereafter referred to as Whakaari). The approach is extendable to a variety of volcanic settings through systematic tuning of the classifier. Hyperparameters are evaluated by statistical means, yielding a combination of “ideal” SOM parameters for the given data set. Extending from this, we applied a Kernel Density Estimation approach to automatically detect changes within the observed seismicity. We categorize the Whakaari seismic time series into regimes representing distinct volcano‐seismic states during recent unrest episodes at Whakaari (2012/2013, 2016, and 2019). There is a clear separation in classification results between background regimes and those representing elevated levels of unrest. Onset of unrest is detected by the classifier 6 weeks before the August 2012 eruption, and ca. 3.5 months before the December 2019 eruption, respectively. Regime changes are corroborated by changes in commonly monitored tremor proxies as well as with reported volcanic activity. The regimes are hypothesized to represent diverse mechanisms including: system pressurization and depressurization, degassing, and elevated surface activity. Labeling these regimes improves visualization of the 2012/2013 and 2019 unrest and eruptive episodes. The pre‐eruptive 2016 unrest showed a contrasting shape and nature of seismic regimes, suggesting differing onset and driving processes. The 2016 episode is proposed to result from rapid destabilization of the shallow hydrothermal system, while rising magmatic gases from new injections of magma better explain the 2012/2013 and 2019 episodes.

Multiple spatial and temporal scales of deformation from geodetic monitoring point to active transcrustal magma system at Uturuncu volcano, Bolivia

Abstract Uturuncu volcano in southern Bolivia last erupted around 250 ka but is exhibiting signs of recent activity, including over 50 yr of surface uplift, elevated seismic activity, and fumarolic activity. We studied the spatial and temporal scales of surface deformation from 1992 to 2021 to better understand subsurface activity. We tracked Uturuncu’s recent deformation using interferometric synthetic aperture radar (InSAR) data and the global navigation satellite system (GNSS) station UTUR, located near Uturuncu’s summit. We observed a spatially coherent signal of uplift from 2014 to 2021 from Sentinel-1 A/B satellites that indicates the Altiplano-Puna magma body, located 19–24 km below ground level, and previously noted as the source of the large region of deformation, is still active. The ground is now uplifting at a rate of ~3 mm/yr compared to prior rates of ~10 mm/yr. We corroborated this waning uplift with in situ data from station UTUR. We combined the Sentinel-1 data with TerraSAR-X interferograms to constrain an ~25 km2 region of subsidence located 11 km SSW of Uturuncu, with a source depth of 2.1 km below ground level to an active period of ~2.5 yr with ~5 mm/yr subsidence. We developed a conceptual model that relates these varying depths and time scales of activity in a transcrustal magmatic system. We associate the surface uplift with pressurization from ascending gases and brines from magmatic reservoirs in the midcrust. We infer the existence of brine lenses in the shallow hydrothermal system based on low subsurface resistivity correlated with surface subsidence.

The Helium and Carbon Isotope Characteristics of the Andean Convergent Margin

Subduction zones represent the interface between Earth’s interior (crust and mantle) and exterior (atmosphere and oceans), where carbon and other volatile elements are actively cycled between Earth reservoirs by plate tectonics. Helium is a sensitive tracer of volatile sources and can be used to deconvolute mantle and crustal sources in arcs; however it is not thought to be recycled into the mantle by subduction processes. In contrast, carbon is readily recycled, mostly in the form of carbon-rich sediments, and can thus be used to understand volatile delivery via subduction. Further, carbon is chemically-reactive and isotope fractionation can be used to determine the main processes controlling volatile movements within arc systems. Here, we report helium isotope and abundance data for 42 deeply-sourced fluid and gas samples from the Central Volcanic Zone (CVZ) and Southern Volcanic Zone (SVZ) of the Andean Convergent Margin (ACM). Data are used to assess the influence of subduction parameters (e.g., crustal thickness, subduction inputs, and convergence rate) on the composition of volatiles in surface volcanic fluid and gas emissions. He isotopes from the CVZ backarc range from 0.1 to 2.6 RA (n = 23), with the highest values in the Puna and the lowest in the Sub-Andean foreland fold-and-thrust belt. Atmosphere-corrected He isotopes from the SVZ range from 0.7 to 5.0 RA (n = 19). Taken together, these data reveal a clear southeastward increase in 3He/4He, with the highest values (in the SVZ) falling below the nominal range associated with pure upper mantle helium (8 ± 1 RA), approaching the mean He isotope value for arc gases of (5.4 ± 1.9 RA). Notably, the lowest values are found in the CVZ, suggesting more significant crustal inputs (i.e., assimilation of 4He) to the helium budget. The crustal thickness in the CVZ (up to 70 km) is significantly larger than in the SVZ, where it is just ∼40 km. We suggest that crustal thickness exerts a primary control on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the ACM. We also report carbon isotopes from (n = 11) sites in the CVZ, where δ13C varies between −15.3‰ and −1.2‰ [vs. Vienna Pee Dee Belemnite (VPDB)] and CO2/3He values that vary by over two orders of magnitude (6.9 × 108–1.7 × 1011). In the SVZ, carbon isotope ratios are also reported from (n = 13) sites and vary between −17.2‰ and −4.1‰. CO2/3He values vary by over four orders of magnitude (4.7 × 107–1.7 × 1012). Low δ13C and CO2/3He values are consistent with CO2 removal (e.g., calcite precipitation and gas dissolution) in shallow hydrothermal systems. Carbon isotope fractionation modeling suggests that calcite precipitation occurs at temperatures coincident with the upper temperature limit for life (122°C), suggesting that biology may play a role in C-He systematics of arc-related volcanic fluid and gas emissions.

Hydrothermally induced edifice destabilisation: The mechanical behaviour of rock mass surrounding a shallow intrusion in andesitic lavas, Pinnacle Ridge, Ruapehu, New Zealand

Volcanic edifices are composed of an assortment of materials with varying mechanical behaviours. The range in mechanical behaviours of these rock materials presents a critical unknown element to volcanic stability assessments. In this study, we explore the deformation behaviour (e.g. uniaxial, triaxial, and tensile deformation behaviour) of volcanic materials from seven geotechnical units associated with a small, shallow intrusion at Pinnacle Ridge, Ruapehu, New Zealand. First, we provide a complete characterisation of the rock masses using field (e.g. rock mass characterisation) and lab (compressive and tensile strength analyses) data for each geotechnical unit. The new data show a range of tensile and triaxial compressive strengths dependent largely on porosity, which is further modified by hydrothermal alteration. More altered rocks are generally weaker (with the exception of the brecciated lava margins) under triaxial conditions. Altered rocks generally transition from brittle to ductile behaviour at lower confining pressure, although this depends on the porosity.Using the stratigraphy and geomorphology of Pinnacle Ridge in conjunction with the rock property data we collect, we then use finite element modelling to depict the deformation behaviour of these units under different pore pressure and seismic conditions. The modelling shows that a relatively minor increase in pore pressure (e.g. < 3 MPa above hydrostatic conditions) is sufficient to change the slope failure type from deep-seated rotational sliding to extensional rupture. The presence of a low-permeability, low-rock mass strength material, even if very localised like a hydrothermal vein, can reduce slope stability by locally increasing pore pressure and providing a weakness plane that facilitates failure. The modelling also shows that a relatively minor increase in pore pressure (e.g. < 6 MPa above hydrostatic conditions) in a shallow hydrothermal system is sufficient to cause localised failure in the material, especially in the hydrothermal vein material. Additionally, the low-permeability hydrothermal vein material can cause pore pressure compartmentalisation, decreasing the strength factor in all materials in the compartment. While Pinnacle Ridge and its associated hydrothermal veins are >1 km from the currently active hydrothermal system, similar veins may be present near active parts of the hydrothermal system at Ruapehu and are an important consideration for future stability models at Ruapehu and other volcanoes.

Decoding water-rock interaction and volatile input at La Soufriere volcano (Guadeloupe) using time-series major and trace element analyses in gas condensates

The geochemical behavior of major and trace elements in fumarolic gas condensate samples collected between April 2017 and January 2021 was investigated at La Soufriere hydrothermal volcano. Samples collected from Cratère Sud and Napoleon Nord fumaroles (temperature varying from 93.9 to 108.4 °C) offer a unique opportunity to reveal the physicochemical processes that affect the deeper magmatic and shallow hydrothermal systems feeding fumaroles. Gas condensate samples exhibit abundant solid particles that suggest strong water-rock interaction and erosive processes in fumarolic conduits at La Soufriere. These particles were incorporated during sampling, except for rare Si-rich spherules that appeared to have precipitated during storage time. Gas condensate samples displayed abundant concentrations of S, halogens (Cl, Br, I), metals/metalloids (B, Ni, Cu, Zn, As, Mo, Ag, Cd, Sb, Pb, and Bi), as well as enhanced concentrations of rock-related lithophile and siderophile elements. The relative abundance of major and trace elements in gas condensates suggests the permanent contribution of volatile-rich magmatic fluids and strong water-rock interaction processes. Based on elemental correlations and the physical properties of chemical elements, it is postulated that most of them are discharged in the aerosol phase, except for the halogens and S, which can be found in the gaseous phase in low-temperature fumarolic emissions. Metals and metalloids (Sb, Bi, Zn, Mo, Ni, Ag, Cd, As, and Pb) enriched in gas condensates were transported within tiny aqueous droplets, while rock-related elements (including REEs) were transported in the aerosol phase. Trace element enrichments in La Soufriere's condensates are comparable with those found in worldwide magmatic-hydrothermal systems such as Kawaj Ijen (Indonesia), Lastarria (Chile), and White Island (New Zealand) volcanoes. The size of the hydrothermal system and the degree of water-rock interaction are thought to be important parameters in controlling the abundance of major and trace elements in low-temperature fumarolic discharge.

Intracaldera explosions and lava emissions during the 2007 caldera collapse of Piton de la Fournaise, La R\xe9union Island

The March–April 2007 Piton de la Fournaise basaltic eruption was the most significant eruption on La Réunion Island in historical times. On 2 April, a fissure opened on the southeastern flank of the volcano. Vigorous fountains fed lavas that rapidly reached the coast. Three days later, on the 5–6 April, major caldera collapse occurred at the summit, affecting the floor and walls of Dolomieu caldera. Monitoring records, primarily webcam images, have been analysed and integrated with geophysical data to reconstruct the chronology of events at the summit during caldera collapse. Those events included progressive subsidence of the former caldera floor, landslides, explosions, lava emissions and steam fumaroles, and lasted until 19 April though diminished greatly in frequency after 7 April. For two days after the main caldera collapse on 5 April, subsidence increments, intracaldera lava emission and explosions were closely associated in time and in location. Abundant steam and wet talus on the caldera walls imply that the shallow hydrothermal system and/or groundwater were exposed by subsidence. The presence of juvenile components in ash deposited at the summit during caldera collapse and the close link between intracaldera lavas and explosions suggest that many explosions were phreatomagmatic. Although caldera collapse was related to magma withdrawal from beneath the summit via the flank vent activity, numerous intracaldera lava emissions indicated that magma was nevertheless present at the summit during caldera collapse. We infer that the lava emissions were fed by an intrusion emplaced at the end of March into the eastern summit region and that the intrusion was being actively recharged during caldera collapse. Caldera collapse involved a complex combination of magma withdrawal and magma replenishment at separate summit reservoirs.

Low-Temperature Hydrothermal Systems Response to Rainfall Forcing: An Example From Temperature Time Series of Fumaroles at La Soufrière de Guadeloupe Volcano

Volcanoes with highly-developed and shallow hydrothermal systems may be subject to sudden increases of their surface steam emission at vents in response to either deep forcing (e.g. increase of heat flux coming from the magma chamber) or external forcing (e.g. sudden decrease of atmospheric pressure or variation of meteoric water input). Because the vent plumbing has a limited heat and mass transfer capacity, the rise of steam pressure accompanying the increase of flux may destabilize the system in order to augment its net transfer capacity. This reorganization may, for instance, take the form of an enlargement of existing conduits and vents or to the creation of new ones. In such a case, local and extremely dangerous blast phenomena are likely to occur with devastating consequences several hundreds of meters around. Even volcanoes with a moderate activity and considered safe by the local population are exposed to such abrupt and dangerous events. The detection of early warning signals through temperature monitoring in the vents is of a primary importance and a main difficulty is to correctly interpret temperature jumps in order to reduce false alarms. We analyze time series of the temperature measured in three fumaroles located at the top of La Soufrière volcano in Guadeloupe, which are characterized by their relatively low temperature around 99°C, slightly above the boiling temperature of water at this altitude. Thanks to the long duration of the records from January to August 2017 and to their short 1-s sampling interval, a multiscale analysis can be performed over several orders of magnitude. We show that, despite their complex and sometimes erratic appearance, the temperature variations observed in the vents contain components highly correlated with rain input variations. Some remarkable patterns recurrently appear at different periods and we show that the main temperature variations of more than 10°C are related to the rainfall intensity. Our results illustrate the importance of external forcing on the otherwise complex and possibly chaotic dynamics of the shallow hydrothermal system of La Soufrière. They also reveal that a careful analysis of rainfall forcing must be done to be able to draw any conclusion concerning changes caused by the underlying hydrothermal system.

Utilizing Thermal Demagnetization Events to Evaluate Volcanic Unrest and the Prospects for Eruption Forecasting

Volcanoes with shallow hydrothermal systems are often accompanied by background volcanic activity such as fumarolic activity, microseismicity, and ground deformation even in the non-eruptive phase. When elevated, they are said to be in a state of “unrest.” It is not difficult to imagine that such events of unrest reflect changes in the state of the shallow hydrothermal system beneath a volcano. However, there is currently no method by which these events can be used to quantitatively evaluate eruption imminency or predict eruption intensity based on physical and/or chemical models. A potentially useful application of such unrest events for probabilistically forecasting eruptions is discussed. First, the method proposed by Hashimoto et al. (2019) for compiling and evaluating the sources of unrest events, such as thermal demagnetization, is described. Then, the volcanic unrest index (VUI) of Potter et al. (2015a) is proposed as another key tool. Finally, a concept is proposed for integrating the VUI and the unrest data to make probabilistically forecasting eruptions feasible. Also described is a recent attempt to introduce the VUI for evaluating a volcano in Japan. Information on sources of unrest in the form of the scatter plot of Hashimoto et al. (2019) can be used as one of the rating criteria on the VUI worksheet. The key idea is to divide the source diagram into regions based on the probability of posterior eruptions given unrest events and to assign VUI scores to these regions. Such a procedure may augment the VUI's function, partially enabling probability-based eruption forecasting. Irrespective of whether the VUI is applied or not, it is essential to obtain temporally homogeneous monitoring data during both eruptive and non-eruptive periods for a quantitative evaluation of unrest events. Surveys and analyses carried out regularly over long time periods also play an equally important role. Therefore, to realize of probabilistic eruption forecasting, it is fundamentally important that monitoring networks are run properly and the data are shared appropriately.