Caldera Rim Research Articles

The pre-Campi Flegrei caldera (>40 ka) explosive volcanic record in the Neapolitan Volcanic Area: New insights from a scientific drilling north of Naples, southern Italy

The oldest volcanism documented in near-vent sections around the Campi Flegrei (CF, southern Italy) caldera does not exceed ~78 ka, even though the mid- to ultra-distal tephrostratigraphic record would suggest that activity in this area started well before that. Reconstructing the activity preceding the large caldera-forming Campanian Ignimbrite (CI) eruption of ~40 ka, via surface geological surveys in proximal areas, is challenging because of the poor accessibility and paucity of sections recording the older chronostratigraphic interval. In order to fill the gap in knowledge of the activity preceding the CI eruption, a 113.2 m deep scientific drillhole was emplaced in the Ponti Rossi area, in the northern part of the city of Naples. The Ponti Rossi area was selected as representative of the stratigraphic setting prior to the CF caldera formation because it is close, although external, to any proposed caldera rim or downthrown area. The cored succession, consisting of pyroclastic deposits separated by paleosols, reworked humified deposits or subaerial erosional surfaces, has been logged and sampled for sedimentological, mineralogical, and geochronological analyses. Thirty-one Pyroclastic Units (PU) were identified. Based on the structural/textural features of the recovered sediments, the first relevant result is the possible absence of the CI, while the deposits of the ~15 ka Neapolitan Yellow Tuff eruption, the second largest caldera-forming event of CF, represent the shallowest sediments. 40Ar/39Ar age determinations on alkali feldspars, extracted from juvenile fragments collected at 45.8–45.9 (PU-29) and 99.5–99.6 (PU-1) metres of depth, yielded ages of 59.03 ± 0.50 ka and 110.00 ± 0.35 ka, respectively. The age obtained for the deepest cored unit, having sedimentological characteristics compatible with proximal deposition, represents the oldest age obtained for a pyroclastic deposit in the sequences near the CF caldera boundaries and extends by 30 ky the explosive history of this area. Furthermore, based on 40Ar/39Ar age constraints, at least 29 eruptions, spanning the ~59–110 ka interval, can be added to the volcanic history of the Neapolitan Volcanic Area. These eruptions can be largely attributed to the CF area, prior to the CI caldera formation, and testify to hitherto unknown, intense explosive activity.

Revisiting the Tianwen Yellow Pumice (TYP) Eruption of Changbaishan Volcano: Tephra Correlation, Eruption Timing and Its Climatostratigraphical Context

AbstractChangbaishan volcano (China/North Korea) is one of the most active and hazardous volcanic centers in Northeast Asia. Despite decades of intensive research, the eruption history of this stratovolcano remains poorly constrained. One of the major puzzles is the timing of the eruption that produced the Tianwen Yellow Pumice (TYP) deposit at the caldera rim. Here we identify a new cryptotephra layer in sediment core 13PT‐P4 from the East Sea. Grain‐specific major, minor, and trace element analyses of glass shards allow a clear correlation of this distal tephra to the proximal TYP deposit of Changbaishan. Age‐depth modeling using radiocarbon (14C) dates of sediment bulk organic fractions and other tephrochronological markers from the sediment sequence constraints the age of the cryptotephra and thus the TYP eruption to 29,948–29,625 cal yr BP (95.4% confidence interval). Our findings lead to a revision of the history of Changbaishan explosive activity, and show that the volcano has been particularly active during ca. 51–24 ka BP in the last 100 ka. Using high resolution palaeo‐proxy records, we find the TYP tephra almost coeval with regional to hemispheric‐scale climatic changes known as Heinrich Event 3 (H3). With its precise age determination and wide geographic dispersion, the tephra offers a key isochron for dating records of past climatic changes and addressing the phasing relationships in environmental response to H3 across East Asia.

Magnetic multi-fabrics as tools for understanding ignimbrite emplacement processes: An example from late-Variscan Tharandter Wald Caldera, Bohemian Massif

Felsic collapse calderas pose a hazard to our society, thus detailed knowledge of collapse mechanisms and behavior of eruptive products could help us to better understand the active calderas. We present a model of the eruption and collapse of Late Carboniferous Tharandter Wald Caldera (Variscan Bohemian Massif) based on field mapping and a magnetic multi-fabric approach. We investigate intra-caldera rhyolitic ignimbrites using a combination of conventional in-phase and relatively new out-of-phase anisotropy of magnetic susceptibility, along with the anisotropy of anhysteretic remanent magnetization. A two-stage caldera evolution model is proposed. (1) The high-energetic and high-temperature ignimbrites filled the paleo-valleys to form a thick ignimbrite sheet with welded to rheomorphosed central portion. Concurrently, eruptions emptied the source magma chamber triggering the piston caldera collapse. (2) The subsidence caused a monoclinal ductile bending of the still-hot and ignimbrite sheet around the caldera rim towards its center. Caldera activity was terminated by the emplacement of subvolcanic ring and radial dikes associated with hydrothermal activity. The latter likely induced abundant magneto-mineralogical alteration and formed composite fabrics. Hence, the application of the multi-fabric approach may serve as a useful tool to extrapolate reliable geodynamic proxies from the magnetic fabric of old, eroded, or altered caldera ignimbrites.

Historical vertical ground movements in the Campi Flegrei volcano: A new transect across the caldera rim

This research focuses on a geoarchaeological study of the coastal sector extending from Fusaro coastal lake to Punta Pennata Islet, located in the western peripheral area of Campi Flegrei Caldera (Gulf of Pozzuoli, Italy), to reconstruct relative sea-level (RSL) oscillations and related vertical ground movements which have occurred since the Roman period. Campi Flegrei Caldera is characterized by sudden vertical ground movements of volcano-tectonic origin (i.e. bradyseismic crisis) which strongly modified the coastal landscape during the Holocene. Despite this, the area has been densely inhabited since the Greek-Roman times, preserving numerous traces of its ancient splendor. Part of these findings can be interpreted as high- or low- precision sea-level markers, helping in the reconstruction of the RSL change history. By using a multi-technique approach made of both direct and indirect surveys, the latter performed at the key-site of Punta Pennata through the use of an unmanned surface vessel prototype (ARGO), we investigated different archaeological sites with a twofold purpose. We reconstructed the RSL change of the study area during the last 2.1 ka and, by comparing the RSL data with site-specific geophysical models, we demonstrated that during the considered period the area went through five different evolutive phases characterized by an alternation of prevailing uplift, stability, and subsidence of volcano-tectonic origin, bringing the RSL from −4 and +5 m MSL, up to its current elevation. Moreover, by comparing the RSL data with the present-day sea level, we observed a differential overall subsidence of almost 1-m between the inner and outer sectors of the caldera at Miseno Cape. This validated the presence of a fault, crossing the promontory in an NNW-SSE direction, previously mentioned by other authors. Moreover, the integration of our data and historic sources enabled reconstructing the palaeo-environmental scenario of this area during the 1st century BC.

Seafloor Investigations of the Kemp Caldera, the Southernmost Arc Caldera Volcano From the South Sandwich Island Arc

AbstractKemp Caldera, situated in the south of the intra‐oceanic South Sandwich arc, is one of the least explored submarine calderas that hosts hydrothermally active vent sites. The caldera was discovered in 2009. Since then, the focus has been primarily on biological studies. During the R/V Polarstern cruise PS119 in 2019, we gained new insights into the morphology, petrology and the formation of the Kemp Caldera. The ship's multibeam data provide an overview of the caldera bathymetry and backscatter characteristics. The new data revealed that the caldera is nested with two or possibly three concentric calderas. TV‐sled and remotely operated vehicle (ROV) observations provide detailed visual data for the hydrothermally active sites of the vent field at the central resurgent cone and flare site at the NNW caldera rim. The central vent field is dominated by white smokers, where clams, sponges and other fauna thrive, while at the flare site inactive as well as actively venting chimneys have been found. The latter are characterized by metal‐enriched fluids of temperatures ≥200°C. During ROV dives, rock samples were collected from the cone, providing the first information about the Kemp Caldera rock composition. The caldera rocks are dacitic, in contrast to the basalts and andesites of the neighboring Kemp Seamount. This suggests that the dacitic cone was formed by one or more later eruptions of differentiated magma, probably stored in shallow intrusions which are driving hydrothermal activity.

Insights into the explosive eruption history of the Campanian volcanoes prior to the Campanian Ignimbrite eruption

The Campanian Volcanic Zone (CVZ) comprises multiple active volcanoes and includes the highly productive Campi Flegrei and Ischia caldera systems. These caldera volcanoes have produced probably the largest eruptions in Europe in the past 200 ka, such as the Monte Epomeo Green Tuff (MEGT; Ischia) at ca. 56 ka and the Campanian Ignimbrite (CI; Campi Flegrei) at ca. 40 ka, which form widespread isochrons across the Mediterranean region. These closely-spaced volcanic centres erupt phonolitic to trachytic glass compositions that are similar, and thus it can be challenging to correlate tephra deposits to specific volcanic sources. Here we present a detailed tephrostratigraphy for pre-CI eruption activity using the units preserved within a sequence at the coastal Acquamorta outcrop, on the western side of the CI caldera rim. Both the MEGT and CI units are present in the section, and they bracket twelve eruption units that were logged and sampled. New major and trace element glass chemistry data have been acquired for these Acquamorta tephra deposits. Three eruption deposits from Ischia and nine from Campi Flegrei are identified, which helps constrain the tempo of volcanic activity of these centres between the large caldera-forming eruptions. The three Ischia tephra deposits between the MEGT and the CI are indistinguishable based on both major and trace element glass chemistry and cannot be correlated to a specific or known eruption in this interval, such as the Schiappone tephra. The compositional variations between the Campi Flegrei eruptions reveal temporal shifts in the composition of the tephra deposits that reflect changes in the magmatic system prior to the CI eruption. These deposits indicate that there were at least nine eruptions at Campi Flegrei within 16 ka of the enormous CI eruption, and suggest that there was no significant period of repose before the caldera generating eruption.

Genesis and evolution of the post-caldera pyroclastic rhyolites from La Primavera caldera, Jalisco, Mexico: A crystal mush perspective

La Primavera is a Quaternary rhyolitic volcanic complex located in the northwestern part of the Trans-Mexican Volcanic Belt. It consists of lavas, domes, and composite volcanoes emplaced between 143.5 and 25.5 ka, both inside and outside the caldera rim. Post-caldera volcanism occurred between ∼95 and 25.5 ka, erupting 2 lavas, 22 domes, and 5 composite volcanoes. Here, we present new petrographic descriptions, mineral chemistry, and whole-rock chemical compositions of juvenile fragments from the post-caldera pyroclastic units originated from the Nejahuete, San Miguel, Planillas, and Tajo composite volcanoes. Based on their major, trace, LREE (light rare earth elements), and isotopic compositions, we identify four compositional groups: 1) Colli dome (low-LREE and low-Zr), 2) E to N pyroclastic units (low-LREE and intermediate-Zr), 3) B to D pyroclastic units, and 4) GP to A pyroclastic units (high-LREE and high-Zr). Based on their composition, we propose a three-step model to explain the genesis of the post-caldera magmas: 1) injection of a calc-alkaline subduction-related melt modified by an OIB mantle-derived component (e.g., the Gachupín basaltic-andesite) into the magmatic chambers and generation of at least two crystal mushes, one formed from crystallization of a mantle-derived magma (i.e, Gachupín), and the other from a differentiated residual liquid derived from the same mantle-derived magma, 2) formation of the REE-poor and REE-rich compositions of the La Primavera magmas by different degrees of partial melting of the crystal mushes, and 3) generation of the REE-intermediate compositions of the La Primavera magmas by mixing of the REE-poor and REE-rich melts produced from partial melting of the crystal mushes to generate the LREE and Zr variations of the La Primavera magmas.

Complex styles of phreatomagmatic explosions at Kīlauea Volcano, Hawaii, controlled by magma structure

Explosive eruptions at basaltic volcanoes remain poorly understood. Kīlauea Volcano is a type locality for basaltic eruptions and is well-known for effusive activity. However, more than 7 m of phreatomagmatic Keanakākoʻi Tephra unit D deposits from explosive eruptions crown the southern rim of the summit caldera and provide a stark reminder of Kīlauea’s explosive past and future potential. We used detailed field observations as well as granulometric and morphological analysis of 100 samples from two proximal sections to assess the eruption style and fragmentation mechanism. The deposits can be divided into four subunits, six different lithofacies, and contain three juvenile tephra components. Each juvenile component shows distinct shape variability resulting from molten fuel-coolant interaction (MFCI) explosions of magma of variable vesicularity. Fragmentation of dense glass generates olive-green ash, fragmentation of low to moderately vesicular magma generates a dark gray ash-lapilli component, and fragmentation of highly vesicular magma generates light-yellow pumice. Our work shows that magma structure impacts MFCI explosion efficiency. Small-scale planar bedding throughout most of the deposit points to a general eruption style of small, frequent explosions generating low plumes. Thicker beds of accretionary lapilli of fine-extremely fine ash are related to very efficient magma-water mixing. Pyroclastic density current (PDC) deposits in the upper part of the stratigraphy contain at least three flows but show no significant dune or cross-bedding structures. We suggest that this is a function of the vent being situated in a caldera that was then ∼600 m deep, where the caldera wall acted as a barrier and changed the flow dynamics to very dilute overspills and co-PDC plume falls over the wall. Deconvolution modeling of the polymodal grain size distributions is used to assess grain size changes of each juvenile component for this deposit, which greatly improves interpretation of lithofacies generation and eruption dynamics. Size-correlated shape parameters show that shape data across a wide size range are needed to accurately track grain shapes. This study demonstrates how careful examination of grain size and shape of juvenile tephra clasts can help volcanologists understand how effusive basaltic volcanoes can become violently explosive.

Caldera-Like Delineation and Geological Resources Identification of Majenang Area, Indonesia by Gravity and Magnetic Data Inversions

Indonesia’s archipelago formed by the tectonic evolution proceeded with subduction, which was accompanied by volcanism. The systematic subduction zones produce the magmatic arcs with different periods since the Permian up to the Tertiary. However, only the recent Quaternary volcanic arc is recognized and lacked of information about the ancient volcanic environment. Calderas are a crucial feature in any volcanic environment due to the prospect site of geological resources. The Gravity and Magnetic methods are commonly used for preliminary study in almost any cases due to their light-weight, low-cost, and ability to map a wide area rapidly. The inverse-modeling scheme was invoked to estimate the sub-surface situation during the interpretation processes. The study intended to show the ability of both gravity and magnetic method for delineating of caldera-like environment including geological resources prospective site identification. According to the research, a northwest-southeast dextral strike-slip fault found in the area and belongs to the Pamanukan-Cilacap Fault Zone (PCFZ). A circular caldera-like anomaly delineated and interpreted as the ring fault of an ancient volcanic caldera in the study area. Several high gravity anomalies found within the caldera rims are interpreted as lava domes or intrusion rocks, while the high-density found following the outer part of the ring-fault terrain is interpreted as the buried lava. The ancient eruption point inferred around Majenang city, thus the study area proposed as the Majenang Caldera. A mineralization zone identified around the study area, which is comparable with the Cihonje people’s gold mining site as the proven prospective area.

Formation of hydrothermal ferromanganese oxides from the Daigo-Kume Knoll in the middle Okinawa Trough, Japan

Many active hydrothermal sites resulting from active volcanism have been recognized in the Okinawa Trough, a nascent back-arc basin. Despite its importance for understanding whole hydrothermal circulation, low-temperature (LT) hydrothermal activity, which precipitates ferromanganese (Fe–Mn) oxides, has not been actively studied compared to high-temperature (HT) hydrothermal activity, which forms sulfide deposits. Therefore, in this study, we investigated the origin and formation of Fe–Mn oxides obtained from the outer rim of the Daigo-Kume Knoll caldera in the middle Okinawa Trough based on their chemical and mineral compositions. Composed of birnessite and todorokite, the Fe–Mn oxide samples contain low levels of trace elements, indicating their precipitation from LT hydrothermal fluids. This is the first report of hydrothermal activity at the Daigo-Kume Knoll. Based on an independent component analysis of elemental mapping data, it was determined that the hydrothermal activity consisted of two stages: a primary hotter LT hydrothermal fluid (roughly 30 °C–150 °C) that altered silicate debris and precipitated mainly Fe oxides, and a secondary colder LT hydrothermal fluid (roughly 30 °C) that precipitated mainly Mn oxides without alteration. The growth rate, estimated from the chemical composition, and the thickness of the Fe–Mn oxide samples indicated that the duration of the hydrothermal activity was about 17.7–29.5 kyr. In contrast to their LT hydrothermal origin, the high Mo/Mn ratio of the Fe–Mn oxide samples suggested that the LT hydrothermal fluids that precipitated the Fe–Mn oxide samples might be originally produced by HT water–rock interactions.

Hydrological and volcano-related gravity signals at Mt. Somma–Vesuvius from ∼20 yr of time-lapse gravity monitoring: implications for volcano quiescence

SUMMARY We report on about 20 yr of relative gravity measurements, acquired on Mt. Somma–Vesuvius volcano in order to investigate the hydrological and volcano-tectonic processes controlling the present-day activity of the volcano. The retrieved long-term field of time gravity change (2003–2022) shows a pattern essentially related to the subsidence, which have affected the central part of the volcano, as detected by the permanent GNSS network and InSAR data. After reducing the observations for the effect of vertical deformation, no significant residuals are found, indicating no significant mass accumulation or loss within the volcanic system. In the north-western sector of the study area, at the border of the volcano edifice, however, significant residual positive gravity changes are detected which are associated to ground-water rebound after years of intense exploitation of the aquifers. On the seasonal timescale, we find that stations within the caldera rim are affected by the seasonal hydrological effects, while the gravity stations at the base of the Vesuvius show a less clear correlation. Furthermore, within the caldera rim a multiyear gravity transient is detected with an increase phase lasting about 4 yr followed by a slower decrease phase. Analysis of rain data seem to exclude a hydrological origin, hence, we hypothesize a deeper source related to the geothermal activity, which can be present even if the volcano is in a quiescent state. We infer the depth and volume of the source by inverting the spatial pattern of the gravity field at the peak of the transient. A volume of fluids of 9.5 × 107 m3 with density of 1000 kg m−3 at 2.3 km depth is capable to fit reasonably well the observations. To explain the gravity transient, simple synthetic models are produced, that simulate the ascent of fluids from a deep reservoir up to the depth of 2.3 km and a successive diffusion within the carbonate aquifer hosting the geothermal system. The whole process appears to not significantly affect the seismicity rate and the deformation of the volcano. This study demonstrates the importance of a 4-D gravity monitoring of a volcano to understand its complex gravity signals that cover different spatial and temporal scales. Discriminating the different contributions that mix up in the observed gravity changes, in particular those due to hydrologic/anthropogenic activities form those due to the geothermal dynamics, is fundamental for a complete and reliable evaluation of the volcano state.

DInSAR Data Reveal an Intriguing Contemporaneous Onset of Deep Deflation below Vesuvio and the Ongoing Campi Flegrei Uplift

Campi Flegrei and Vesuvio volcanoes are only about 25 km apart, located on opposite sides of the densely inhabited area of Naples (Italy). Since neighbouring volcanoes may influence each other’s activity, it is of great interest to identify signs of any mutual interaction between Campi Flegrei and Vesuvio, or at least note coincidences in their recent deformation dynamics. After a large uplift, Campi Flegrei was generally subsiding from 1985 to 2001, while it has been uplifting—probably driven by deep magma inflation—at an accelerating rate since then. Here, we analysed the ground displacement in the whole Vesuvian area and its surroundings around the early 2000s using 1993–2010 ERS/ENVISAT ascending- and descending-orbit line-of-sight displacements obtained through the Small BAseline Subset Differential Synthetic Aperture Radar Interferometry technique. Although ground deformation is slow—a few millimetres per year—Empirical Orthogonal Function analysis shows a sudden trend change around 2001. Pre-2001 velocity maps confirm previously published results: subsidence mainly occurred inside the caldera rim—probably because of the sliding and compaction of young incoherent materials—and in a few spots around 10 km from the summital crater; eastward displacement occurred in a lobe east of Vesuvio, and westward displacement occurred in a lobe west of Vesuvio, as in the case of the spreading of the volcanic edifice and/or extensional tectonics. We attribute the subsidence spots to the previous high local number of new buildings per year. Post-2002 velocity maps provide evidence of a very different scenario: general subsidence in the whole Vesuvian area, westward displacement in a lobe east of Vesuvio, and eastward displacement in a lobe west of Vesuvio. This last arrangement of the ground displacement field is made even clearer by subtracting the post-2002 velocity from the pre-2001 value. The results of our analyses are consistent with the deflation of a deep pressurised source. Additionally, Vesuvio’s deep seismicity decreased at the beginning of 2002. The coincidence between the transition from deflation to inflation at Campi Flegrei and the onset of deflation below Vesuvio may suggest the possible transfer of magma and/or magmatic fluids between the two plumbing systems.

New insights into geological setting of the summit area of mount Etna volcano (Italy) inferred from 2D gravity data modelling

Gravimetric observations were carried out in 2015, to image the uppermost portion of the volcanic plumbing system of Mt. Etna (Italy). Gravity measurements were performed using two relative gravimeters, along a profile that crosses the summit craters area (elevations between 2,820 and 3,280 m a.s.l.). Accurate positioning of the gravity observation points was determined through GPS measurements. After applying elevation and terrain corrections, the reduced gravity data were used to build a 2D density model of the uppermost part of the volcano edifice. This model was constrained using to-date knowledge of the structural setting of the area and the available volcanological data. We highlighted the presence of low-density material below the summit craters, down to the depth of about 2.1 km, interpreted as highly altered, fumarolized and structurally weakened material. It is also likely that the close presence of the conduits feeding the summit craters of the volcano contributes to the gravity low in the SW half of the measurement profile. Conversely, the gravity low observed at the northern edge of the profile could reflect the high concentration of faults and eruptive fissures in the Pizzi Deneri area, in correspondence of the Ellittico caldera rim.

Comparison of empirically-based and physically-based analyses of coseismic landslides: A case study of the 2016 Kumamoto earthquake

This study compares the performance of empirically- and physically-based methods for predicting coseismic landslides around Aso caldera during the 2016 Mw 7.0 Kumamoto earthquake. The physically-based method couples a regional-scale wave propagation simulation with a site-scale Newmark-type sliding analysis, while the empirically-based method predicts sliding displacements based on seismic recordings and/or ground motion prediction equations. By using the well-documented landslide inventory, the predictive capacity of each method is quantitatively assessed.The case study demonstrates that the physically-based method has the advantage of simulating complicated near-fault and topographic effects. It captures around 44% of observed landslides at the caldera rim, which is significantly better than the 20% captured from the empirically-based prediction. Both methods have similar prediction performance, capturing around 56% actual landslides at the central cone region. The empirically-based method is simple to use and provides fast landslide prediction. It can be further improved by incorporating a simple prediction model for the topographic amplification effect, which increases the percentage of captured landslides by around 8% across the caldera rim.

An integrated geochemical spatial and temporal survey of thermal springs to characterize the geothermal resource of a volcano (Piton des Neiges, Réunion Island)

Geothermal exploration can be challenging in the absence of obvious surface indicators of a high enthalpy geothermal resource such as geysers, fumaroles or thermal springs with temperatures near 100 °C. This is the case at the Piton des Neiges volcano (Réunion Island), which displays evidence of hydrothermal activity mostly as thermal springs with temperatures not exceeding 50 °C, affected by mixing and reequilibration, and emerging in a complex geological framework, resulting from the polygenic history of this dormant volcano. Yet, these thermal springs keep a partial record of the reservoir geochemistry. Here, we propose an exploration method to set this thermal signature apart from the contribution of other processes and trace back the equilibrium conditions, enabling us to locate thermal fluid circulations, and evaluate their temperature and availability. We report its application at Piton des Neiges volcano. We led an extensive geochemical survey which nearly doubled the number of thermal springs reported on the volcano and dramatically extended the existing geochemical database in space and time. In tandem, we also monitored δ18O and δD values in rainwaters and thermal springs over two years to compare their temporal variations.Our surveys show that the repartition of thermal springs is controlled by only one of the volcanic rift zones (the N30°E axis), which is thus identified as the heat source of hydrothermal circulations. This N30°E rift zone is also a preferential pathway for the degassing of mantellic CO2. In addition, the composition of thermal waters is influenced by syenite intrusions along the rim of the caldera, acting as drains for the uprise of hydrothermal waters. O, H and Sr isotopic signatures of springs show that the geothermal resource of Piton des Neiges is in fact made of two independent and unconnected hydrothermal systems on the northern and southern circular depressions of the volcano (the cirques of Salazie and Cilaos, respectively). Their recharge is meteoric, multi-year, and mostly achieved by cyclonic events. Thermal waters experience reequilibration during their ascent and various degrees of CO2 input or degassing. Last equilibrium temperatures of water/rock interaction, estimated by multicomponent geothermometry, reach up to 125 ± 6 °C in the cirque of Salazie, and up to 141 ± 15 °C in the cirque of Cilaos, with possibility for hotter waters before reequilibration. Taken together, our results suggest that the most promising target for future exploration is located in the cirque of Cilaos, along the caldera rim, and in proximity of syenite intrusions.With a thorough prospecting of thermal springs, a comprehensive geochemical study and a geochemical monitoring of the different water reservoirs, we improved our understanding of the geothermal resource and identified future exploration targets at Piton des Neiges volcano.A separation scheme for strontium and light rare earth elements and its application to the isotopic analysis of strontium and neodymium in silicate rocks are presented. This method benefits from the selectivity and high capacity of two newly introduced extraction Chromatographic materials, referred to as Sr.Spec and TRU.Spec, respectively. These afford a straightforward separation of Sr and Sm + Nd with high yield, good purity and satisfactory blank levels, on very small (0.25 ml) columns using small volumes of solutions of a single mineral acid, HNO3. The validity of the method is illustrated by the measurement of 87Sr/ 86Sr and 143Nd/144Nd ratios in twelve international standard reference materials.

Constraints on ground deformation processes at the Tulu Moye volcanic complex, Main Ethiopian Rift

Tulu Moye is an actively deforming volcanic complex with a geothermal field in the Main Ethiopian Rift. We use InSAR between 2014 and 2022, integrated with other geophysical data, to investigate the temporal and spatial characteristics of the deformation signal in the area, and to model its source. Velocity maps and time series analysis show a deformation signal consistent with uplift at a velocity of up to 50 mm/yr in the satellite Line-of-Sight (LOS) in 2014–2017, then decreasing to 12 mm/yr until 2022. The centre of deformation is located about 10 km west of a main geothermal drilling site at Tulu Moye, between the Bora, Berecha, and Tulu Moye volcanoes, with a NW-SE elongation direction. Our best-fit model suggests that the deformation is caused by an 8.7 km by 1.2 km sill situated ∼7.7 km below the surface (∼5.9 km below sea-level), elongate in the N54°W direction and dipping S11°W, and experienced an average velocity of volume change of ∼8.9 × 106 m3/yr in 2014–2017. The surface projection of the sill overlaps with local transverse faults and hydrothermal manifestations. The sill is ∼1–2 km below clusters of microseismic swarms and a region of high resistivity, both indicating hydrothermal fluid flow. The location and geometry of the sill correlates with the upper edge of high conductivity interpreted as a zone of partial melt, and we therefore attribute the uplift at the Tulu Moye volcanic complex to inflow of magma in the sill. We also suggest that the transverse caldera rims faults may restrict magma flow, and also facilitate both vertical and lateral hydrothermal fluid flow.

Two Decades of Monitoring Hydrothermal Plumes at the Brothers Submarine Volcano, Kermadec Arc, New Zealand

Abstract Brothers volcano is arguably the most well-studied submarine arc volcano on Earth. Between 1996, when massive sulfides were first recovered by dredging, and 2018, when International Ocean Discovery Program (IODP) Expedition 376 recovered cores from as deep as 453 m below the sea floor at two chemically distinct hydrothermal upflow zones, over 60 conductivity-temperature-depth (CTD) vertical casts and tow-yo operations mapped hydrothermal plumes over and around the edifice by employing hydrothermal tracer-specific sensors. These surveys started in 1999 and were completed during nine separate expeditions at one- to three-year intervals, except for a six-year gap between 2011 and 2017. Hydrothermal plume distributions over this two-decade period show variability in the intensity and vertical rise height of plumes from the four main vent fields (Upper Cone, Lower Cone, NW Caldera, and Upper Caldera, with the latter not discovered until 2017). Upper Cone plumes were more intense than all other sites in 1999, 2002, 2007, and 2009, then significantly diminished from 2011 to 2018. The Lower Cone plume was the most intense in 2004, then the NW Caldera site became the dominant source of hydrothermal particles from 2011 to 2018. Despite the gap of six years between 2011 and 2017, hydrothermal output appears to have increased within the caldera sometime after the 2009 survey while simultaneously decreasing in intensity at the cone sites. This supports other evidence of linkages between the cone and caldera sites in the deep hydrothermal circulation system, and may be related to the predicted deepening of hydrothermal circulation, infiltration of seawater to facilitate “mining” of magmatic brines, and modulation of subseafloor mineralization processes associated with a modeled, pulsed injection of magmatic gasses. The surveys also revealed ways in which the highly variable regional hydrographic environment impacts the flux of hydrothermal products to the surrounding ocean. Plumes from sources located above the caldera rim disperse hydrothermal components without hindrance, but particles and heat from sources within the caldera become trapped and are dispersed episodically by caldera-flushing events. While on site for 18 days in 2018, repeat CTD casts into the deepest part of the caldera, which was isolated from the surrounding ocean, showed a progressive increase in temperature, representing a net heat flux of 79 MW from conductive and advective sources deeper than 1,570 m.

The Early Chemical Characteristics of Geothermal Surface Features of Mount Lompobattang-Bawakaraeng, Gowa and Bantaeng Regency, South Sulawesi, Indonesia

The Early study of surface geothermal features characteristics, based on surface chemical characteristics fluids is compulsory for understanding the geothermal system. The geothermal area of Mount Lompobattang-Bawakaraeng is mostly located in Gowa and Bantaeng Regency, South Sulawesi and these data were taken in 2018. The stratigraphy is composed of Tertiary Volcanic Sediments and Lompobattang-Bawakaraeng Quaternary Volcanic. The geological structures consist of caldera rim on the peak of Mount Lompobatang-Bawakaraeng and the fault structure of southwest-northeast and northwest-southeast which controls the appearance of geothermal manifestations on the surface. The active thermal manifestations are only warm springs, i.e., Batu Belerang, Pencong, and Parangloe warm springs. Batu Belerang warm spring is situated at an altitude of around 1400 masl, while Pencong and Parangloe warm springs are about 24 km from Batu Belerang warm springs with a lower altitude of around 168 masl. The temperature of the warm springs is around 31- 49.2 °C. There is one inactive surface geothermal manifestation, i.e., the Cold Acid Crater Lake Blue Luraya (Danau Biru Luraya) at the elevation of 1561 masl, the water temperature of 17 °C. The result of water analysis shows that warm springs Pencong and Parangloe have more dominant HCO3. The Batu Belerang and the Cold Acid Crater Lake Blue Luraya (Danau Biru luraya) are more dominated by SO4. In the triangular diagram that compares the relative concentrations of Na, K, Mg shows that the Pencong group (Pencong and Parangloe warm springs) is plotted in the lower boundary zone of partial equilibrium, while Lompobattang group (Batu Belerang warm springs and Cold Acid Crater Lake Blue Luraya (Danau Biru Luraya) is plotted in the corner of Mg (immature waters zone). The temperature of the system is probably only estimated at the shallow part of the system by SiO2 geothermometer about 125 °C and the Batu Belerang could be more than 125 °C.

Unraveling the Geothermal Potential of Corbetti Caldera Using Integrated Geophysical Methods, Central Main Ethiopian Rift

In this paper integrated geophysical techniques, involving electrical resistivity, gravity and magnetic surveys have been carried out over the Corbetti Caldera in the central Main Ethiopian Rift (CMER) to verify the geothermal potential of the caldera. Corbetti geothermal prospect is located at about 250 km south of Addis Ababa and 20km from Awassa city and the area is bounded by Lake Awassa to the south and Lake Shalla to the north with geographic location between latitude 7.170N-7.250N and longitudes 38.300E-38.470E. The Corbetti Caldera is characterized by Quaternary volcano-tectonic activity which is mainly silicic volcanism and a resurgent caldera structural system. Quaternary volcanism is associated with a wide spread of steaming ground and fumarolic activity which could confirm the existence of a heat source at depth. Fifty six (56) VES points, 200 gravity and about 200 magnetic data have been used and analyzed. The results are presented as magnetic, gravity and electrical counter maps for qualitative interpretation. Constrained gravity and magnetic 2D model and results obtained from the geoelectric section were made for quantitative interpretation. The complete Bouguer gravity anomaly in conjugation with the total magnetic field anomaly map of the area indicates the existence of intrusion beneath the caldera, i.e. the highest Bouguer gravity anomaly resulting from the higher density of the intrusion. Correspondingly the shallower heat source caused by this intrusion is characterized by the lowest magnetic anomaly response. Results from vertical electrical soundings along profile-2 indicate the presence of a middle thicker conductive zone which is associated with the increase in temperature and alteration of rocks. Apparent resistivity map for AB/2=1810m and AB/2=2700m shows low resistivity anomaly follows the eastern and northern caldera rim, stretching north of the caldera towards Lake Shalla.

Bouguer Anomaly Re-Reduction and Interpretative Remarks of the Phlegraean Fields Caldera Structures (Southern Italy)

Phlegraean Fields is a large, active caldera located in the densely populated westernmost sector of Naples’s Bay (Southern Italy). Several Bouguer anomaly surveys are available for this area with different resolution and accuracy; gravity data derive from the integration of stations placed below and above the sea level as the caldera develops both onshore and offshore. The comparison of these maps with the Digital Elevation Model shows a still remaining Terrain Effect hiding the shallower and deep caldera structure’s signal. This effect has an impact on the modelling of the gravity source’s depth and geometry. In this research, we apply a geologically constrained terrain correction method to the higher resolution Free Air dataset available for the study area to enhance the complete Bouguer reduction. The correlation analysis between the residual and the topography allows us to assess the quality of the outcomes. The results represent an improvement in the anomalies’ isolation and clearly show a continuous circular-like clustering of maxima related to the geometry of the caldera rim. The minima are associated with volcano-tectonic depression filled with pyroclastic and sediment. Furthermore, features alignments overlap the fault systems, along which the volcanic activity occurred.