Active Caldera Research Articles

Newly identified small vulcanian eruptions during the caldera-forming stage of Towada Volcano, Northeast Japan

Understanding the detailed eruptive history and conditions during the buildup to catastrophic caldera-forming eruptions is essential for understanding the evolution of caldera volcanoes and predicting eruptive hazards. Towada Volcano is an active caldera volcano in the northern part of the Northeast Japan Arc. At least two catastrophic caldera-forming eruptions (eruptive episodes N and L) occurred during its caldera-forming stage (61–15.7 ka). A detailed geological survey identified three small vulcanian tephra layers that were erupted during the caldera-forming stage. The tephra layers are blue–gray ash fall deposits that consist mainly of fresh blocky dacite–rhyolite fragments. In ascending order, these deposits are assigned to eruptive episodes O′, N′, and M′. Each eruption had a volume of <0.11 km3, which is much smaller than that of other eruptions during the caldera-forming stage. The eruptive ages of episodes O′, N′, and M′ are estimated to be ca. 40, 23.1, and 17.2 ka, respectively, based on new 14C ages and paleosol thicknesses data. At least three cycles of a medium- to large-scale (3–20 km3) explosive eruption, preceded by a small vulcanian eruption (<0.11 km3), occurred during the latter half of the caldera-forming stage. The small vulcanian eruptions (episodes O′, N′, and M′) occurred after relatively long periods of quiescence (4000–14,000 years) and were followed by medium- to large-scale explosive eruptions (episodes N, M, and L) 1500–4000 years later. This cyclic activity probably reflects changes in overpressure in the magma reservoir. As the overpressure increased, episodes O′, N′, and M′ probably occurred when the overpressure had not increased sufficiently to trigger a medium- to large-scale explosive eruption, and episodes N, M, and L occurred at higher overpressures. These cycles characterize the fermentation phase of Towada Volcano, and terminated with the formation of the Towada Caldera during episode L at 15.7 ka. The magma composition and eruption frequency changed abruptly after episode L, and a small stratovolcano was formed through intermittent eruptions of mafic magma. This change suggests that the caldera collapse during episode L changed the entire shallow magmatic system that existed during the fermentation phase, and the system shifted to a recovery (post-caldera) phase. Based on eruptive volumes and frequencies, the present Towada Volcano has not yet reached the conditions that existed during the late caldera-forming stage and is therefore unlikely to produce a catastrophic caldera-forming eruption in the near future.

Improving the Accuracy of Digital Terrain Models Using Drone-Based LiDAR for the Morpho-Structural Analysis of Active Calderas: The Case of Ischia Island, Italy

Over the past two decades, the airborne Light Detection and Ranging (LiDAR) system has become a useful tool for acquiring high-resolution topographic data, especially in active tectonics studies. Analyzing Digital Terrain Models (DTMs) from LiDAR exposes morpho-structural elements, aiding in the understanding of fault zones, among other applications. Despite its effectiveness, challenges persist in regions with rapid deformation, dense vegetation, and human impact. We propose an adapted workflow transitioning from the conventional airborne LiDAR system to the usage of drone-based LiDAR technology for higher-resolution data acquisition. Additionally, drones offer a more cost-effective solution, both in an initial investment and ongoing operational expenses. Our goal is to demonstrate how drone-based LiDAR enhances the identification of active deformation features, particularly for earthquake-induced surface faulting. To evaluate the potential of our technique, we conducted a drone-based LiDAR survey in the Casamicciola Terme area, north of Ischia Island, Italy, known for the occurrence of destructive shallow earthquakes, including the 2017 Md = 4 event. We assessed the quality of our acquired DTM by comparing it with existing elevation datasets for the same area. We discuss the advantages and limitations of each DTM product in relation to our results, particularly when applied to fault mapping. By analyzing derivative DTM products, we identified the fault scarps within the Casamicciola Holocene Graben (CHG) and mapped its structural geometry in detail. The analysis of both linear and areal geomorphic features allowed us to identify the primary factors influencing the current morphological arrangement of the CHG area. Our detailed map depicts a nested graben formed by two main structures (the Maio and Sentinella faults) and minor internal faults (the Purgatorio and Nizzola faults). High-resolution DEMs acquired by drone-based LiDAR facilitated detailed studies of the geomorphology and fault activity. A similar approach can be applied in regions where the evidence of high slip-rate faults is difficult to identify due to vegetation cover and inaccessibility.

Evidence From Intermediate‐Depth Earthquakes of Slab‐Derived Fluids Beneath the Taupō Volcanic Zone

AbstractAhi Tupua, the central section of the Taupō Volcanic Zone in the central North Island of New Zealand, encompasses Taupō and Ōkataina calderas and has been the most frequently active and productive silicic magma system worldwide during the Quaternary. The entire Taupō Volcanic Zone is underlain by the Hikurangi Subduction Zone where a Large Igneous Province, the Hikurangi Plateau, is being subducted, but Ahi Tupua exhibits much higher rates of magma output than either the northern or the southern sections. Trace element signatures of Ahi Tupua eruptive products suggest that both decompression melting and flux melting play a role in this rifted arc setting, with fluid flux signatures being more prevalent in regions of active caldera volcanism. Intermediate‐depth (50–300 km) earthquakes provide a means of studying faulting and fluid flow processes within and around the subducting slab beneath Ahi Tupua. Using data from the national seismic network and a 13‐station temporary network (“ECLIPSE”), we repick and relocate 397 intermediate‐depth earthquakes of magnitudes M2.5+ that occurred in a 29‐month interval, and compute focal mechanisms for a subset of 47 earthquakes. We observe some seismicity that may be within the mantle wedge but most of the relocated earthquakes occur within the crust and uppermost mantle of the subducted slab and exhibit a weak transition from predominantly normal‐faulting in the slab crust to predominantly reverse‐faulting in the slab mantle. No double seismic zone is observed beneath Ahi Tupua. These observations are consistent with dehydration of the slab and flow of slab‐derived fluids along existing faults into the mantle wedge, that drives flux melting and accounts for the distinctive geochemical signatures and voluminous output of Ahi Tupua calderas.

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.

Principal component analysis on twenty years (2000–2020) of geochemical and geophysical observations at Campi Flegrei active caldera

Campi Flegrei (CF) is an active and densely populated caldera in Southern Italy, which has manifested signs of significant unrest in the last 50 years. Due to the high volcanic risk, monitoring networks of the most sensitive unrest indicators have been implemented and improved over time. Precious database constituted by geophysical and geochemical data allowed the study of the caldera unrest phases. In this paper we retrace the caldera history in the time span 2000–2020 by analyzing displacement, seismicity and geochemical time series in a unified framework. To this end, Principal Component Analysis (PCA) was firstly applied only on geochemical data because of their compositional nature. The retrieved first three components were successively analyzed via PCA together with the geophysical and thermodynamical variables. Our results suggest that three independent processes relay on geochemical observations: a heating/pressurizing of the hydrothermal system, a process related to magmatic fluids injection at the hydrothermal system roots, and third process probably connected with a deeper magmatic dynamic. The actual volcano alert state seems mainly linked to the variation of the hydrothermal system activity. Our approach made it possible to explore the interrelation among observations of different nature highlighting the importance of the relative driving processes over time.

A shifty Toba magma reservoir: Improved eruption chronology and petrochronological evidence for lateral growth of a giant magma body

Polycyclic caldera complexes hold clues to understanding why some magmatic systems develop into supersized magma bodies and how they can recover to produce several caldera-forming eruptions. However, the geologic records of the transitions between successive caldera events are very often inaccessible due to limited preservation of eruptive products of inter-caldera activity, prompting the search for alternative archives of magma evolution such as accessory minerals. Here we applied multiple geochemical tools to study one of the most active caldera centres of the Quaternary, the Toba caldera complex in Sumatra (Indonesia), which produced at least four caldera-forming eruptions in the last 1.6 My, including the iconic Youngest Toba Tuff at 74 ka. We combined feldspar 40Ar/39Ar and zircon U–Pb geochronology of proximal pyroclastic deposits with glass and mineral chemistry of both the tuffs and distal marine tephra to revise the eruption chronology of Toba, obtaining new eruption ages of 1417 -31/+14 ka (zircon) or 1339 ± 39/39 ka (plagioclase, internal/full external 2σ uncertainty) for the Haranggaol Dacite Tuff, 783.81 ± 0.85/1.32 ka (sanidine) for Oldest Toba Tuff, and 503.61 ± 1.36/1.50 ka (sanidine) for Middle Toba Tuff. Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb crystallisation ages, trace element contents and Hf isotopic ratios of zircons illuminate changes in the shallow magma reservoir which saw near-continuous zircon crystallisation over 1.6 My. Prolonged build-ups to each eruption with highly scattered zircon trace element compositions reflect a complex, heterogeneous character of the shallow reservoir, without a clear temporal trend or indications of the eruption trigger. In contrast, hafnium isotopes in zircon display a pronounced shift towards unradiogenic values immediately after the OTT caldera collapse, followed by a gradual recovery to a baseline value of εHf = -7 at the time of YTT eruption, interpreted as a reflection of the shift in magma reservoir position corresponding to change in the character of assimilated crust. We can show in unprecedented detail how a large caldera collapse affects magma geochemistry; however, identification of patterns in the behaviour of the Toba system and making geochemistry-based predictions about its future development remain a challenge.

New Insights on Ground Deformation at Campi Flegrei Caldera Inferred From Kinematics and Dynamics Investigation of Borehole Tilt

AbstractThe study of the ground deformation in active calderas provides valuable indications of the ongoing dynamical processes. In this framework, we analyze the borehole tiltmeter data recorded at Campi Flegrei (Southern Italy) from their first installation (1 April 2015), in order to retrieve the kinematics of the ground deformation and its main anomalies. Our approach involves the nonlinear dynamics analysis aimed at the identification of the degrees of freedom of the system and thus its complexity. Starting from the second part of the year 2020, the behavior of the dynamical system becomes collective, and a locally significant deviation of the deformative pattern from the background trend is observed. In particular, a series of 15 slow tilt variations (like jumps lasting a few days) appears in the ground deformation. They are associated with a very low‐dimensional system likely generated by a local second‐order source. The latter is related to fluid migration and it is superimposed on the primary one located in Pozzuoli town and driving the current uplift of Campi Flegrei. The effects of such a local, second‐order stress field are, indeed, evident in the area of the volcanic dome of Mt. Olibano, where they are measured as tilt steps. The superposition of primary and second‐order sources of deformation acting contemporaneously can justify the recent variations in geophysical and geochemical parameters. Our approach based on the joint dynamical and kinematical analyses of the tiltmeter data can be applied to other volcanic/tectonic areas or extended to other geophysical and geochemical variables.

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.

Understanding caldera degassing from a detailed investigation at Lake Rotoiti, Okataina Volcanic Centre, New Zealand

Volcanic lakes in large silicic caldera volcanoes are an important source of CO2 emissions. However, quantifying CO2 output is challenging due to the lack of observed historical CO2 flux records and the large size of the volcanic and hydrothermal systems. Twenty percent of the surface area of Okataina Caldera, Taupo Volcanic Zone, New Zealand is covered with lakes. Geothermal expressions, predominantly on the Okataina caldera margin, occur in at least six different locations, with surface expressions both on land and under water. Lake Rotoiti is located at the northwest edge of the Okataina Caldera, spans across the Tikitere and Taheke geothermal fields, and has inputs from on-land thermal springs, lake floor hydrothermal vents, and from Lake Rotorua. CO2 flux from Lake Rotoiti was assessed using the accumulation chamber method and three techniques are used to process the data for total CO2 emission of the lake: (1) sequential Gaussian simulation (sGs) method allows quantification of CO2 emission with spatial control, (2) graphical statistical approach (GSA) allows the quantification of CO2 emission from different degassing regimes, and (3) a method based on water chemistry of the lake. We find CO2 is mostly emitting at Tumoana Bay and Central Basin, and the emission rate is 271 ± 38 t d−1 of CO2 (based on sGs). We then added this estimated emission data to the existing CO2 data for individual geothermal systems distributed around the active Okataina volcanic centre and calculate that the entire caldera is emitting at least 1856 t d−1 of CO2. The total emission is dominated by the lakes, where the topography is lower. We discuss the implications of this study in terms of preferential degassing locations and the amount of degassing, in particular for CO2, in an active caldera setting.

Storage, evolution, and mixing in basaltic eruptions from around the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand

The Okataina Volcanic Centre (OVC) is the most recently active caldera system in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are several examples of basaltic to basaltic-andesite contributions to OVC eruptions. These range from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions from within and around the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that eruptions sample multiple distinct reservoirs during ascent to the surface. The most abundant basaltic component is generated by cooling-induced crystallisation of a common, oxidised, volatile-rich basaltic melt at various depths within the crust that mixes upon ascent. Despite similar bulk compositions, these four eruptions are texturally distinct from each other as a result of their wide variation in eruption style.

Catastrophic caldera-forming eruptions and climate perturbations: the result of tectonic and magmatic controls on the Paleocene-Eocene Kilchrist Caldera, Isle of Skye, NW Scotland

Caldera-forming eruptions are amongst the most hazardous events in the Earth’s history, and understanding their formation is essential to forecasting activity at active calderas worldwide. In this study we present new field and geochronological evidence for a Paleocene-Eocene caldera from Skye, NW Scotland. Magma exploited a regional thrust fault as a conduit, and then ponded against intrusive igneous rocks emplaced against a regional extensional fault. Replenishment of silicic magma reservoirs with basaltic magma triggered eruptions. The eruptions typically deposited extremely coarse ignimbrites, demonstrating catastrophic collapse of the caldera, which occurred via an inner ring fault and a complexly faulted marginal zone. Collapse was followed by remobilisation of silicic magma and caldera resurgence. The magma consumed dolostone country rocks, causing significant release of CO2 and contributed to the Paleocene-Eocene Thermal Maximum. Our results demonstrate how tectonics localise magma and caldera development, and how this can cause cataclysmic volcanic and climatic hazards.

Influences on geothermal circulation in the Okataina Volcanic Centre, New Zealand

The Okataina Volcanic Centre (OVC) in the Taupo Volcanic Zone (TVZ) is New Zealand's most recently active caldera and lava dome complex, erupting 85 km3 of magma equivalent in the past 21 ka. Successive caldera collapses over the past ∼550 kyr have created a complex crustal structure through which fluids now migrate, evidenced by hot springs and several geothermal fields located predominantly around the caldera margins. While recent magnetotelluric (MT) modelling suggests that high-temperature upwelling fluids originate from mid-crustal locations at the margins of an inferred magmatic complex, it is still uncertain to what degree various factors influence these fluid pathways from source to surface. Here, we use heat and fluid flow models to explore the roles that simplified geological structures, topography, and deep, localised heat sources play in determining the locations of upflow at the OVC.To isolate and understand individual processes that modulate fluid flow, we created TOUGH2 heat and fluid flow models that encompass a 35 by 39 km2 area extending beyond the topographic boundaries of the OVC calderas. An average heat input of 700 mW/m2 was set at the base of the models at 5 km depth, either as a uniform hot plate or as discrete zones of higher heat input with locations derived from MT models. The upper boundary of the models followed water table elevation, which is a slightly muted reflection of topography. We constructed a suite of models, ranging from uniform rock properties to more complex permeability variations that correspond to basement/volcanic rocks and large-scale faults inferred from gravity data and surface mapping. Models ran for 50 kyr, the approximate time since the most recent caldera-forming event. Elevated model temperatures were compared with low-resistivity zones from shallow (<500 m) Direct Current (DC) apparent resistivity values and deeper sensing MT resistivity models.Our models suggest that topography and localised basal heat sources are the largest influences on the locations of geothermal upflows. In the western half of the study area, observed upflows beneath Waimangu and Tikitere can be modelled with localised heat sources and topography alone. Upflow under Rotoma to the north-east is not replicated by any of our suite of models, suggesting that unmapped heat sources are important in this area. In the east of the study area, modelled upflow is dominated by present-day topography which is strongly influenced by <25ky Tarawera and Haroharo Volcanic Complexes and < 1ky Tarawera River valley. Our models suggest that strong permeability contrasts associated with large-scale faults would have effects on regional-scale fluid circulation that are not observed in real life, and that the coincidence of OVC topographic boundaries and geothermal systems is due to the boundaries' also being topographic lows rather than fluid conduits.

A decade of earthquake activity at Taupō Volcano, New Zealand

Taupō, New Zealand, is an active caldera volcano that in recent times has erupted on average every ~500 years, with the latest explosive eruption in 232±10 CE. Monitoring at Taupō is challenging as there has been no eruptive activity in documented history; however, Taupō does undergo periods of unrest on roughly a decadal timescale, such as in 2019. Key to identifying these unrest periods is understanding what represents 'normal' inter-unrest activity. In this study, we generate an earthquake catalogue for Taupō for 2010–2019 inclusive, consisting of 46,481 earthquakes. This shows that the Taupō region has background earthquake rates of 50–200 earthquakes per month and the 2019 unrest episode was preceded by an exponential increase in earthquake rate. We also show that when attenuation is accounted for there is no evidence for low-frequency earthquakes at Taupō, and that this is an important consideration for volcano monitoring and determining the presence of significant magma movement.

Assembly and development of large active calderas hosting geothermal systems: Insights from Los Humeros volcanic complex (Mexico)

Caldera volcanoes are complex geological systems that show, during and after their formation, a wide variability in terms of eruptive styles, magmatic and geochemical evolution, and volcanic structures. Los Humeros Volcanic Complex (LHVC) is a key area where to study these factors. It is located in the easternmost sector of the Trans-Mexican Volcanic Belt and hosts an active geothermal system. LHVC exemplifies the complex nature of calderas, recording periods of alternated effusive and explosive eruptive phases, a heterogenous magmatic source, diverse basaltic to rhyolitic products, and the overlapping of caldera collapse events. This study provides an up-to-date interpretation of the caldera framework. Our work is based on a detailed analysis of literature and novel data aimed at the morpho-structural caldera configuration, and supported by geophysical modeling, and petrology of the volcanic products. The new results confirm a more complex evolution of LHVC involving geometric configurations related to multiple caldera collapse events producing both asymmetric trapdoor, and piecemeal styles. The present configuration of the caldera framework involved the initial formation (at 164 ka) of a large caldera (Los Humeros) of ca. 15–17 km diam (shorter than previously reported), which was overlapped asymmetrically by the younger (69 ka) and smaller (ca. 8.5–10 km diam) Los Potreros caldera over the west side, as a result of multiple sequential collapsing events. The final configuration of the caldera rims (scarps) was promoted by incremental growth. A post-caldera resurgence phase was induced by the injection of multiple shallow intrusions of silicic bodies, causing a more localized deformation of the caldera floor. The Holocene volcanism records the recurrent injection of compositionally distinct magma batches uprising from different depths of the LHVC transcrustal magmatic plumbing system. This latter indicates the transition from the caldera stage magmatic system dominated by a single, large, and shallow magmatic body to a more complex, polybaric post-caldera stage plumbing system, made up of a lower crust mafic reservoir feeding smaller magma batches vertically distributed in the whole crust. The integration of all the existing data constitutes the archive to better understand how large active calderas hosting exploitable geothermal systems assemble through time.

Imaging the Absorbing Feeding and Eruptive Pathways of Deception Island, Antarctica

AbstractDeception Island is one of the most active and best‐documented volcanoes in Antarctica. Since its last eruption in 1970, several geophysical surveys have targeted reconstructing its magmatic systems. However, geophysics fails to reconstruct the pathways magma and fluids follow from depth to erupt at the surface. Here, novel data selection strategies and multi‐frequency absorption inversions have been framed in a Geographical Information System, using all available geological (vents and faults distribution), geochemical and geophysical knowledge of the volcano. The result is the detection of these eruptive pathways. The model offers the first image of the magma and associated fluids pathways feed the 1967, 1969, and 1970 eruptions. Results suggest that future ascending paths might lead to active research bases and zones of planned helicopter rescue. The connection between seismic absorption, temperature, and fluid content makes it a promising attribute for detecting and monitoring eruptions at active calderas.

Assembly and Development of Large Active Calderas Hosting Geothermal Systems: Insights from Los Humeros Volcanic Complex (Mexico)

Assembly and Development of Large Active Calderas Hosting Geothermal Systems: Insights from Los Humeros Volcanic Complex (Mexico)

The Quaternary Kurobegawa Granite: an example of a deeply dissected resurgent pluton

The Quaternary Kurobegawa Granite, central Japan, is not only the youngest known granitic pluton exposed on the Earth’s surface, it is one of few localities where both Quaternary volcanics and related plutons are well exposed. Here, we present new zircon U–Pb ages together with whole rock and mineral geochemical data, revealing that the Kurobegawa Granite is a resurgent pluton that was emplaced following the caldera-forming eruption of the Jiigatake Volcanics at 1.55 ± 0.09 Ma. Following the eruption, the remnant magma chamber progressively cooled forming the voluminous Kurobegawa pluton in the upper crust (~ 6 km depth) until ~ 0.7 Ma when resurgence caused rapid uplift and erosion in the region. This is the first study to document the detailed spatiotemporal evolution of resurgent pluton for a Quaternary caldera system. Our new findings may contribute significantly to understanding the fate of active caldera systems that can produce supereruptions.

Volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, Mexico

ABSTRACT This work presents the volcanic geology of the easternmost sector of the Trans-Mexican Volcanic Belt, including the Serdán-Oriental basin and Cofre de Perote-Citlaltépetl volcanic range, two contrasting Quaternary volcanic fields. The first comprises dominant monogenetic volcanism of bimodal composition including isolated rhyolitic domes and tuff rings, basaltic andesite maar volcanoes, cinder and lava cones, and an active caldera complex (Los Humeros). The second is dominated by large composite polygenetic volcanoes of andesitic-dacitic composition, including the shield-like compound Cofre de Perote volcano, La Gloria and Las Cumbres complexes, and the active Citlaltépetl stratovolcano. Mapping units include a pre-volcanic basement made of metamorphic (Paleozoic), and sedimentary (Jurassic-Cretaceous); intrusive (Miocene) rocks; multiple basaltic-andesitic lava flows and rhyolitic domes; volcaniclastic sequences (debris avalanches and lahars; and pyroclastic deposits (block-and-ash flows, ignimbrites, fallouts, and surges) (Miocene-Holocene). This map provides a comprehensive view of the highly diverse volcanism, which may reference for future research work.

Reply to Norini and Groppelli's comment on “Estimating the depth and evolution of intrusions at resurgent calderas: Los Humeros (Mexico)” by Urbani et al. (2020)

Abstract. Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

Modeling the multi-level plumbing system of the Changbaishan caldera from geochemical, mineralogical, Sr-Nd isotopic and integrated geophysical data

Changbaishan, an intraplate volcano, is characterized by an approximately 6 km wide summit caldera and last erupted in 1903. Changbaishan experienced a period of unrest between 2002 and 2006. The activity developed in three main stages, including shield volcano (basalts), cone-construction (trachyandesites to trachytes with minor basalts), and caldera-forming stages (trachytes to comendites). This last stage is associated with one of the more energetic eruptions of the last millennium on Earth, the 946 CE, VEI 7 Millennium Eruption (ME), which emitted over 100 km3 of pyroclastics. Compared to other active calderas, the plumbing system of Changbaishan and its evolution mechanisms remain poorly constrained. Here, we merge new whole-rock, glass, mineral, isotopic, and geobarometry data with geophysical data and present a model of the plumbing system. The results show that the volcano is characterized by at least three main magma reservoirs at different depths: a basaltic reservoir at the Moho/lower crust depth, an intermediate reservoir at 10–15 km depth, and a shallower reservoir at 0.5–3 km depth. The shallower reservoir was involved in the ME eruption, which was triggered by a fresh trachytic melt entering a shallower reservoir where a comenditic magma was stored. The trachytes and comendites originate from fractional crystallization processes and minor assimilation of upper crust material, while the less evolved melts assimilate lower crust material. Syn-eruptive magma mingling occurred during the ME eruption phase. The magma reservoirs of the caldera-forming stage partly reactivate those of the cone-construction stage. The depth of the magma storage zones is controlled by the layering of the crust. The plumbing system of Changbaishan is vertically extensive, with crystal mush reservoirs renewed by the replenishment of new trachytic to trachyandesitic magma from depth. Unlike other volcanoes, evidence of a basaltic recharge is lacking. The interpretation of the signals preceding possible future eruptions should consider the multi-level nature of the Changbaishan plumbing system. A new arrival of magma may destabilize a part of or the entire system, thus triggering eruptions of different sizes and styles. The reference model proposed here for Changbaishan represents a prerequisite to properly understand periods of unrest to potentially anticipate future volcanic eruptions and to identify the mechanisms controlling the evolution of the crust below volcanoes.