Caldera-forming Eruption Research Articles

Evolution of eruption rate between two caldera-forming eruptions in the Jemez Mountains volcanic field, New Mexico, USA

Rhyolites that erupted between the Otowi and Tshirege members of the Bandelier Tuff, known as the Valle Toledo Member, were investigated in the Jemez Mountains volcanic field to infer changes in eruptive rate and flux between two caldera-forming eruptions. Our analysis combines high-precision 40Ar/39Ar single-crystal laser-fusion dating of sanidine, matrix glass compositions and mineralogy of pumice, depositional textures, and volumetric estimates of erupted rhyolites to present a revised Valle Toledo Member eruptive chronology that integrates our observations with those of previous studies. With the improved temporal resolution of our high-precision eruption ages (median 2σ uncertainty of ±2.9 ka), the updated Valle Toledo Member eruption chronology consists of at least 42 temporally or mineralogically distinct eruptions that we group into four periods of time based on eruption rate. Within the first 8.1 ± 5.1 kyr (~1605–1597 ka) that followed the Otowi caldera-forming eruption at 1605.4 ± 2.3 ka, six eruptions are recognized. This suggests an average recurrence interval on the order of 1.4 ± 0.9 kyr. Twenty-seven eruptions occurred during the next 194.4 ± 5.1 kyr (from ~1597–1403 ka) and the average recurrence interval increased to 7.2 ± 0.2 kyr. Following this second period of slower eruption rate, a previously unrecognized eruption hiatus of up to 162.4 ± 3.1 kyr occurred from 1402.9 ± 2.3 ka to 1240.5 ± 2.1 ka. Resumption of volcanic activity is characterized by a series of at least nine volcanic eruptions during the next 8.6 ± 2.5 kyr, culminating in the eruption of the 400 km3 Tshirege Member of the Bandelier Tuff and formation of Valles caldera at 1231.9 ± 1.3 ka. This last phase of pre-caldera activity has the shortest observed average recurrence interval (1.0 ± 0.3 kyr) and greatest eruptive flux (≥ 0.4 ± 0.2 km3/kyr) that occurred between the two caldera-forming eruptions. We interpret the shifts in eruption rate and flux following the 162.4 ± 3.1 kyr eruption hiatus, in addition to mineralogical changes present in post-hiatus rhyolites, as indicators that the Bandelier system was trending towards another major eruption. The magmatic system may have grown gradually throughout the ca. 162 kyr period of volcanic repose; however, the heightened eruption rate in the ca. 10 kyr before caldera collapse is consistent with relatively rapid growth of the magmatic system before the Tshirege event as previously proposed by other studies. Furthermore, the new dataset is consistent with prior geochronology studies of the region that show the four largest explosive eruptions in the Jemez field were all proceeded by very slow eruptive rates or hiatuses. This demonstrates that sequences of heightened volcanic activity can initiate on rapid geological timescales from states of volcanic quiescence in the Bandelier system.

Melt embayments record multi-stage magma decompression histories

Over the last decade, the melt embayment has proven its merit as a robust petrological tool capable of recording magma decompression rates for explosive eruptions. However, the models developed and applied to extract this information from embayments have not accounted for the complexity and nonlinearity of magma flow in the conduit. We present Embayment Decompression in Two Stages (EDiTS): a numerical model for extracting magma decompression rates from measured volatile diffusion profiles preserved in crystal-hosted embayments, approximating magma acceleration using two constant-rate decompression paths. This model solves for three unknown parameters: initial (deeper) and final (shallower) decompression rates, as well as the pressure where a transition occurs. We successfully benchmark EDiTS against existing numerical diffusion models, and use controlled multi-stage decompression experiments on natural quartz-hosted embayments to test the ability of our model to recover known decompression paths. We find that EDiTS is able to closely approximate the known two-stage path in the mixed-volatile (H2O + CO2) experiment, while a constant-rate modeling approach is unable to simultaneously fit H2O and CO2 gradients. However, in the H2O-saturated experiment, there is no unique solution to the resulting gradient, with both constant-rate and two-stage models reproducing the measured profile, and EDiTS notably overestimating the known total ascent time by several hours. Using decompression experiments, we show that constant-rate models can provide misleadingly good fits to embayment H2O gradients produced by more complex decompression histories, and thus the measurement and modeling of multiple diffusing species, when available, can provide crucial constraints. We then apply EDiTS to re-evaluate mixed-volatile embayment datasets from explosive silicic arc and caldera-forming eruptions from five volcanic centers (Yellowstone, WY, USA; Bandelier, NM, USA; Long Valley, CA, USA; Taupo, NZ; Mount St. Helens, WA, USA), where all systems contain initial CO2. In contrast to the minutes to hours of total ascent time extracted from embayment volatile profiles using constant-rate models, our two-stage model resolves slower initial ascent times that span several to >10 h. Final ascent rates are 1–2 orders of magnitude faster than the initial extracted rates, in agreement with theoretical conduit flow model predictions. Application of EDiTS to embayments from the May 18th, 1980 eruption of Mount St. Helens results in an initial stage of ascent on the order of hours consistent with the timing of magma arrival at the surface from the seismically-inferred storage region (7–9 km) ~3.5 h after the initial blast, and a final stage of ascent (<1–5 min) in close agreement with time-integrated bubble number densities. Our combined numerical, experimental, and natural results suggest that, with the application of more advanced models, the melt embayment can provide a complete picture of magma decompression timescales from the deep conduit to the surface.

КАЛЬДЕРА КРАШЕНИННИКОВА (ВОСТОЧНАЯ КАМЧАТКА): ВОЗРАСТ И МАГНИТУДА ИЗВЕРЖЕНИЯ

New data on the composition of the pyroclastic deposits of the Krasheninnikov caldera allowed us to correlate it to the previously studied tephra horizon Geys30, which until recently was erroneously attributed to an eruption within the Geysernaya caldera. Pyroclastic flow deposits from the eruption that led to the formation of the Krasheninnikov caldera were found on the southern shore of Kronotsky Lake, and its distal tephra was found in the sediments of the Central Kamchatka Depression (CKD) at a distance of up to 200 km from the source (from Milkovo to Kliuchi villages). The age of this tephra was previously estimated at ~30 ka, which now allows us to accept this estimate for the Krasheninnikov caldera. The identification of the tephra of the caldera-forming eruption in distal sections allows a minimum estimate of the erupted pyroclastic volume of ~13 km3 and the eruption magnitude of 6.1.

Characteristics of the Density and Magnetic Susceptibility of Pumice from the Maninjau Caldera-Forming Eruption, Indonesia

Abtract- Lake Maninjau is a volcanic caldera formed by a volcanic eruption located in West Sumatra. This volcanic eruption threw pyroclastic materials, one of which is pumice, as far as ± 75 km from the center of the eruption. This study aims to determine density and magnetic susceptibility characteristics of pumice from maninjau caldera-forming eruptions, indonesia. The method used is the rock-magnetic method. It is used to measure magnetic susceptibility based on the distribution of volcanic material. The samples were selected from 3 different locations around Mount Maninjau with distances varying between ±12-32 km. A Bartington Magnetic Susceptibility Metre Type B (MS2B) instrument was utilised to determine the susceptibility of rocks and X-ray fluorescence (XRF) to identify rock element levels. The results of the data analysis obtained have magnetic density and susceptibility values in the NGS (Ngarai Sianok) area, which ranged from 0.67-1.77×103 kg/m3 and 4.4-826×10-8m3/kg, the LBS (Lubuk Basung) area which ranged of 0.74-1.34×103 kg/m3 and 6.6-625.4×10-8m3/kg, and the PRM (Pariaman) area, which ranged from 0.68-3.63×103 kg/m3 and 8-359×10-8m3/kg. The results showed that when sampling closer to Mount Maninjau, the pumice is characterized by dense, fresh, but very small pore size and dominated by slightly greyish white pumice with few white crystals on its surface. Therefore, the location of the pumice from Mount Maninjau has a significant influence on its characteristics, including density, magnetic susceptibility, and iron (Fe) and titanium (Ti) content. Keywords: Pumice, Density, Magnetic Susceptibility, Mount Maninjau, XRF

Submarine cores record magma evolution toward a catastrophic eruption at Kikai Caldera

Magma evolution toward a caldera-forming eruption remains uncertain in many cases owing to the lack of successive volcanic records before catastrophic eruptions. Here we take an approach to this issue by analyzing a submarine core sampled near Kikai Caldera, southern Japan, which has recorded two caldera-forming eruptions at 95 and 7.3 ka and small eruptions between them. Discovery of mafic glass fragments in the submarine deposits of the 95-ka eruption, which had not been recognized in subaerial outcrops, implies the involvement of mafic magma in felsic magma-driven caldera-forming eruption. Inter-caldera volcanic activity resumed with binary mafic and felsic magma extrusions but then shifted to eruptions predominated by felsic magmas. In the final stage preceding the 7.3-ka caldera-forming eruption, the most felsic composition did not appear in glass fragments. We suggest that this period was the phase of felsic melt accumulation to grow a magma reservoir toward the next catastrophic eruption.

Geology of the Kebasen Paleovolcano in Banyumas Regency, Central Java Province, Indonesia

Abstract The new interpretation on geology of Kebasen district in Central Java, Indonesia which is located in the middle parts of Sunda Arc indicates the existence of a paleovolcano. This conclusion is achieved through 1:25,000 geological mapping with a focus on volcanic facies, characterization of lithology types, mineralogy, and fossil analysis, to clarify its volcanic history and paleo-environment. Laboratory analysis works on representative rock samples are done using petrography and fossil identification on calcareous sedimentary members. Our research results conclude that Kebasen paleovolcano was a composite volcano with a diameter of about 8 km, elongated in NW-SE orientation, with Kebasen city located at the SW rim of the old volcanic edifice. The volcanic history and resulting volcanic rocks consisted of two contrasting volcanic stages. The first one was explosive caldera-forming eruptions that produced thick and partly welded pyroclastic rocks, dominated by pyroclastic breccia at the lower parts and finer tuff members at the upper parts. After caldera-forming eruptions, volcanic activity was followed by effusive eruptions of basaltic to andesitic lavas that covered the caldera floor. The lavas have abundant pillow structures indicating a submarine environment, intermediate to basaltic composition, and porphyritic texture, with plagioclase and pyroxene as the most abundant phenocryst minerals. Available rock geochemistry data demonstrate that lavas have mafic to intermediate compositions, with magma affinity at the transition of calc-alkaline to tholeiitic series. Results of geomorphological and lithology association analysis conclude that the research area was part of the central to distal facies of a paleovolcano complex, with the paleo depositional environment in the open marine environment. The results of fossil analysis on calcareous sedimentary rocks between volcanic layers indicate that the age of volcanism was Early Pliocene (N18) with the paleobathymetry transitioning from upper-middle bathyal to outer–middle neritic, which were parts of the continental margin. It is concluded that Kebasen paleovolcano is a newly identified submarine caldera-type Pliocene volcano within the middle parts of the Sunda Arc in Central Java, Indonesia.

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.

The Neapolitan Yellow Tuff record in the Gaeta Gulf (Eastern Tyrrhenian margin, Southern Italy)

Abstract. The Neapolitan Yellow Tuff caldera-forming eruption (ca. 15 kyr) is a large event that occurred at Phlegrean Fields during the Late Glacial–Holocene time span. The eruptive products represent a widespread isochronous marker that links the marine and terrestrial stratigraphic record of the central Mediterranean and Northern Europe sequences. In this research, we describe the stratigraphic signature of the NYT deposits in the Late Glacial–Holocene shelf sequence of the Gulf of Gaeta (northern Campania region, southern Tyrrhenian Sea) throughout the seismo-stratigraphic interpretation of a grid of high and very high-resolution Chirp seismic profiles. The seismo-stratigraphic data allowed to individuate in the Upper Pleistocene–Holocene stratigraphic architecture a continental shelf characterized by an offlap prograding succession followed by a very thick transgressive onlapping unit, and by an upper highstand unit mainly characterized by undulations probably linked to gas-prone sediment. The NYT reflector observed was parallel and with a high amplitude. It was continuously detected from the shelf to the slope within transgressive deposits. On the inner shelf, its continuity was interrupted by shallow gas pockets. The NYT depths ranges from 2 m b.s.f. on the slope to 38 m b.s.f. on the continental shelf. The NYT event appeared to have played a key role in controlling the stratigraphic architecture of the studied area during the last transgression, acting as a trigger for the formation of the aggradational sedimentary successions fed by pyroclastic and volcaniclastic deposits.

Evolution of a large-scale phreatoplinian eruption: Constraints from the 40 ka caldera-forming eruption of Kutcharo volcano, eastern Hokkaido, Japan

“Phreatoplinian” is an explosive phreatomagmatic eruption style that is defined by the fragmentation of magma and widespread dispersal of the resulting fine ash and accretionary lapilli. These eruptions pose significant future risks at caldera volcanoes that host lakes and abundant groundwater. There have been no direct observations of a phreatoplinian eruption, therefore, constraining the detailed mechanisms and sequences of such events relies on studying the deposits of previous eruptions. In order to advance our understanding of these hazardous phenomena we conducted a case study of the 40 ka caldera-forming eruption (Kp I) from Kutcharo volcano in eastern Hokkaido, Japan. We subdivided Kp I eruption deposits into 7 units (Units 1 to 7 in ascending order). Units 1 to 6 are air fall deposits consisting of alternating thin pumice and thick silty ash layers with abundant spherical accretionary lapilli. Stratigraphically higher ash fall units are thicker, finer in grain-size, and more widely distributed. The maximum eruption column height and mass-discharge rate were calculated to be 40 km and 1.4 × 109 kg/s, respectively. Unit 7 is a climactic ignimbrite (76 km3), which is distributed widely over the area north of Kutcharo caldera.Unit 6 is the largest air fall unit and can be considered to have been deposited by a phreatoplinian eruption, given its abundant accretionary lapilli, wide dispersion, and high degree of fragmentation. Unit 6 had the highest mass discharge rate (1.4 × 109 kg/s), suggesting the interaction between magma and external water was most intense, and it is thought that a large eruption column covered eastern Hokkaido. In addition, Kp I eruption deposits commonly contain glass shards derived from fragmentation via both magma degassing and Molten Fuel Coolant Interaction (MFCI). To account for this observation, we infer that the conduit penetrated a large aquifer, and the margin of the ascending magma came into contact with this external water source. Due to repeated caldera-forming eruptions, intra-caldera filled deposits (hosting a large aquifer) likely played a key role in supplying external caldera lake water to a level near the fragmentation depth of H2O-saturated felsic magma. The occurrence of these intra-caldera conduit and caldera-lake systems may provide the required conditions for phreatoplinian eruptions at continental arc caldera volcanoes in Japan and globally.

Pleistocene to recent evolution of Mocho-Choshuenco volcano during growth and retreat of the Patagonian Ice Sheet

Mocho-Choshuenco volcano (39.9°S, 72.0°W) produced ∼75 explosive eruptions following retreat of the &amp;gt;1.5-km-thick Patagonian Ice Sheet associated with the local Last Glacial Maximum (LGM, from 35 to 18 ka). Here, we extend this record of volcanic evolution to include pre- and syn-LGM lavas that erupted during the Pleistocene. We establish a long-term chronology of magmatic and volcanic evolution and evaluate the relationship between volcanism and loading/unloading of the Patagonian Ice Sheet via twenty-four 40Ar/39Ar and two 3He age determinations integrated with stratigraphy and whole-rock compositions of lava flows and glass compositions of tephra. Our findings reveal that the edifice is much younger than previously thought and preserves 106 km3 of eruptive products, of which 50% were emplaced immediately following the end of the penultimate glaciation and 20% after the end of the LGM. A period of volcanic inactivity between 37 and 26 ka, when glaciers expanded, was followed by the eruption of incompatible element-rich basaltic andesites. Several of these syn-LGM lavas dated between 26 and 16 ka, which crop out at 1500−1700 m above sea level, show ice contact features that are consistent with emplacement against a 1400- to 1600-m-thick Patagonian Ice Sheet. Small volume dacitic eruptions and two explosive rhyolitic eruptions dominate the volcanic output from 18 to 8 ka, when the Patagonian Ice Sheet began to retreat rapidly. We hypothesize that increased lithostatic loading as the Patagonian Ice Sheet grew prohibited dike propagation, thus stalling the ascent of magma, promoting growth of at least three discrete magma reservoirs, and enhancing minor crustal assimilation to generate incompatible element-rich basaltic andesitic to dacitic magmas that erupted between 26 and 17 ka. From an adjacent reservoir, incompatible element-poor dacites erupted from 17 to 12 ka. These lava flows were followed by the caldera-forming eruption at 11.5 ka of 5.3 km3 of rhyolite from a deeper reservoir atop which a silicic melt lens had formed and expanded. Subsequent eruptions of oxidized dacitic magmas from the Choshuenco cone from 11.5 to 8 ka were followed by andesitic to dacitic eruptions at the more southerly Mocho cone, as well as small flank vent eruptions of basaltic andesite at 2.5 and 0.5 ka. This complex history reflects a multi-reservoir plumbing system beneath Mocho-Choshuenco, which is characterized by depths of magma storage, oxidation states, and trace element compositions that vary over short periods of time (&amp;lt;2 k.y.).

Exploiting the Greenland volcanic ash repository to date caldera-forming eruptions and widespread isochrons during the Holocene

Polar ice-cores have long been recognised as unrivalled repositories of past volcanic events. Although tephra products from local eruptions tend to dominate these records, improvements in micro-sampling and analytical techniques are uncovering a growing number of cryptotephras erupted from exceptionally distant volcanoes. We present a series of nine Middle Holocene cryptotephra deposits detected within the NGRIP ice-core that originate from five different volcanic regions across the Northern Hemisphere (Alaska, Cascades, Iceland, Japan, Kamchatka). Unique compositional signatures are employed to identify ash from three large caldera-forming events in Kamchatka (KS2 from Ksudach), the Cascades (Mazama) and North East Japan (Mashu), along with ash from the Hekla 4 eruption in Iceland. High-precision ice-core ages (adopting a 1950 CE datum for the GICC05 timescale assigned to the Greenland ice cores) are derived for each eruption: Hekla 4 (4325 ± 8 a b1.95k), KS2 (7089 ± 26 a b1.95k), Mashu (i-f) (7473 ± 33 a b1.95k) and Mazama (7562 ± 35 a b1.95k), all of which can be employed as chronological fix-points in other proxy records where these deposits are also preserved. Four further cryptotephra deposits and one macro-deposit are also identified and traced to sources in Iceland and Alaska. The cryptotephra originating from Alaska is correlated to a deposit identified in lake records from the Kenai Peninsula, thought to originate from Redoubt Volcano. The remaining four deposits are typical of the products of Katla, Grímsvötn and Veiðivötn in Iceland. This ensemble of Middle Holocene tephra deposits highlights the pivotal position of the Greenland ice-sheet and its ice-cores to capture deposition from the convergence of several far-travelled ash clouds. Precise age estimates derived from the annually resolved ice-core record greatly enhances the value of these tephra isochrons.

Reconciling mechanical models of caldera ring-fault nucleation within the transcrustal magmatic system paradigm

The formation of a collapse caldera requires the nucleation of circumferential ring-faults that connect an underlying magma chamber with the Earth's surface. The roof of the magma chamber collapses as a consequence of magma withdrawal due to either over- or under-pressure within the chamber. Recent inferences have suggested that calderas may form atop complex transcrustal magma systems with several individual or interconnected magma chambers residing at depth throughout the crust. Whilst there have been several attempts to define the mechanical conditions leading to caldera fault nucleation, the assumptions for these models often rely on a single shallow magma chamber or the combination of a single shallow magma chamber with doming of a deep-seated magma reservoir. There have, so far, been no attempts to reconcile the mechanical conditions leading to ring-fault nucleation and potential collapse caldera formation with inferred geometries and arrangements of complex transcrustal magmatic systems. Here we address this issue using Finite Element Method (FEM) to reconcile mechanical conditions for caldera ring-fault nucleation within the transcrustal magma system paradigm by modeling multiple magma pocket arrangements. Out of the 150 distinct combinations of shallow magma chambers and pressure conditions that were tested, only 15% yielded the necessary conditions for the successful formation of a caldera ring-fault, supporting the need for special or very specific conditions for the occurrence of calderas in nature. Results show that relatively small lateral distances in the position between magma chambers inhibit the stress conditions required for caldera fault nucleation and propagation. Changes in the vertical spacing between stacked magma chambers do not significantly alter the distribution of either tensile or shear stress. This implies that the specified criteria for initiating ring-faults are consistently met, regardless of the number or arrangement of magma chambers. However, vertical offsets between laterally distributed magma compartments lead to an uneven distribution of shear stress, potentially triggering a trapdoor-type collapse. Consistent with the results presented here, the concept of vertically stacked magma compartments has been proposed as an explanation for both contemporary and ancient volcanic systems.

Magma flux variations triggering shallow-level emplacement of the Takidani pluton (Japan): Insights into the volcanic-plutonic connection

Defining the physico-chemical evolution of felsic magmatic reservoirs in the middle-upper continental crust is crucial for understanding the connection between granitic plutons and silicic volcanic rocks and for research on the trigger mechanisms of volcanic eruptions. In this study, we investigate the formation and evolution of the Hotaka-Takidani volcano-plutonic complex (Japan) using a comprehensive approach that integrates zircon petrochronology with phase-equilibria and thermal modelling. The Takidani pluton is a Pleistocene tilted zoned intrusion of granodiorite-granite composition associated with at least two large caldera-forming eruptions. We have investigated the three uppermost units of the pluton and the oldest and more voluminous Nyukawa eruption, which deposited more than 400 km3 (DRE, dense rock equivalent) of crystal-rich dacitic ignimbrite. Amphibole thermobarometry, U-Pb zircon dates and geochemical data indicate that the Takidani pluton and the Nyukawa dacite were sequentially sourced from a 10–12 km-thick long-lived magmatic reservoir in the middle crust. Thermal evolution of the reservoir led to the extraction of the Nyukawa magma, followed by brief storage in the upper crust and eruption at ca. 1.706 ± 0.091 Ma. For two of the units of the pluton, we combine high-spatial resolution zircon U-Pb geochronology with zircon chemical data. Applying the Ti-in-zircon thermometer, we observe that magma within the reservoir was stored at a relatively constant temperature of ca. 700 °C for 300 kyr and subsequently, starting from 1.4 Ma, the temperature increased progressively reaching 820–850 °C at 1.1 Ma. This temperature shift is inconsistent with a constant magma flux and suggests an increase of the magma injection rate into the reservoir. Using the distribution of zircon ages and thermal simulations, we determined that the reservoir was built by a fairly low average magma flux of ca. 6 × 10−6 km3*km−2*yr−1, shifting to a magma supply rate which is three to four times higher during the last 300 ka of zircon crystallisation. This increase in magma flux caused the final emplacement of the Takidani magma in the upper crust, where it rapidly crystallized. The integration of geochronological data with phase-equilibria and thermo-chemical modelling demonstrates that multiple melt extraction events occurred during the evolution of the Hotaka-Takidani magmatic system. Only the first one triggered a major eruption, while subsequent events resulted in the emplacement of the Takidani pluton in the shallow crust.

Hazardous explosive eruptions of a recharging multi-cyclic island arc caldera

Caldera-forming eruptions of silicic volcanic systems are among the most devastating events on Earth. By contrast, post-collapse volcanic activity initiating new caldera cycles is generally considered less hazardous. Formed after Santorini’s latest caldera-forming eruption of ~1600 bce, the Kameni Volcano in the southern Aegean Sea enables the eruptive evolution of a recharging multi-cyclic caldera to be reconstructed. Kameni’s eruptive record has been documented by onshore products and historical descriptions of mainly effusive eruptions dating back to 197 bce. Here we combine high-resolution seismic reflection data with cored lithologies from International Ocean Discovery Program Expedition 398 at four sites to determine the submarine architecture and volcanic history of intra-caldera deposits from Kameni. Our shore-crossing analysis reveals the deposits of a submarine explosive eruption that produced up to 3.1 km3 of pumice and ash, which we relate to a historical eruption in 726 ce. The estimated volcanic explosivity index of magnitude 5 exceeds previously considered worst-case eruptive scenarios for Santorini. Our finding that the Santorini caldera is capable of producing large explosive eruptions at an early stage in the caldera cycle implies an elevated hazard potential for the eastern Mediterranean region, and potentially for other recharging silicic calderas.

Simultaneous normal and reverse faulting in reactivating caldera faults: A detailed field structural analysis from Campi Flegrei (southern Italy)

Models of volcanic collapses proposed in the literature rely on combining field examples with analogue and numerical modelling to connect superficial observables to sub-surface volcano-tectonic processes. However, the behaviour of such collapses in an already faulted and fractured medium needs to be better explored.We studied a complex array of normal and reverse faults within the central sector of the active Campi Flegrei caldera, where faults with centimeters-to-meters displacements are hosted in the La Pietra tuff (∼13.5 ka) and the overlying pyroclastic succession of the last ∼5.5 kyr. We analyzed the attitude, kinematics and throw of these structures, employing a UAV-based digital outcrop model. The analysis shows that antithetic normal and reverse faults form in the hanging wall of a pre-existing WNW striking, NNE-dipping master normal fault. Moving northward, the strike of the antithetic faults rotates from WNW to NNW directions, with the latter showing a right-lateral oblique component. The simultaneity and coherence of both kinematics and attitudes are verified by the throw analysis. We associated the formation of this array with the caldera-collapse phase of the Agnano-Monte Spina eruption (∼4.55 ka), which caused off-caldera faulting beyond the main collapsed area.Based on field data, we suggest that during peak caldera-forming phases, wider areas beyond the main caldera scarps can be involved in volcano-tectonic collapses in the presence of a pre-existing fault network and complex tapped reservoirs. This highlights the role of inherited structures in weakening the crust above the magma reservoir that can potentially increase the magnitude and duration of caldera-forming eruptions, as they may induce a broader roof rock subsidence, pressurizing wider regions of the sub-caldera magma system.

Geological evidence of extensive N-fixation by volcanic lightning during very large explosive eruptions

Most of the nitrogen (N) accessible for life is trapped in dinitrogen (N2), the most stable atmospheric molecule. In order to be metabolized by living organisms, N2 has to be converted into biologically assimilable forms, so-called fixed N. Nowadays, nearly all the N-fixation is achieved through biological and anthropogenic processes. However, in early prebiotic environments of the Earth, N-fixation must have occurred via natural abiotic processes. One of the most invoked processes is electrical discharges, including from thunderstorms and lightning associated with volcanic eruptions. Despite the frequent occurrence of volcanic lightning during explosive eruptions and convincing laboratory experimentation, no evidence of substantial N-fixation has been found in any geological archive. Here, we report on the discovery of a significant amount of nitrate in volcanic deposits from Neogene caldera-forming eruptions, which are well correlated with the concentrations of species directly emitted by volcanoes (sulfur, chlorine). The multi-isotopic composition (δ18O, Δ17O) of the nitrates reveals that they originate from the atmospheric oxidation of nitrogen oxides formed by volcanic lightning. According to these first geological volcanic nitrate archive, we estimate that, on average, about 60 Tg of N can be fixed during a large explosive event. Our findings hint at a unique role potentially played by subaerial explosive eruptions in supplying essential ingredients for the emergence of life on Earth.

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting.

Three-dimensional resistivity structure in Toya caldera region, Southwest Hokkaido, Japan — Constraints on magmatic and geothermal activities

Southwestern Hokkaido, Japan, is characterized by numerous Quaternary volcanoes and geothermal areas. At the same time, the region hosts various critical infrastructures, and there is a need to assess the impact of volcanic hazards on them. Geophysics could provide scientific clues for the hazard assessment by elucidating the abundance of subsurface magma. To clarify the resistivity structure from the crust to uppermost mantle of the Toya caldera, a representative Quaternary volcanic area, a wideband magnetotellurics survey of 117 points over land, sea, and lake areas, as well as 3D inversion, has been conducted. In combination with petrological and seismological findings, quantitative interpretation of the inverted model has found that conductive bodies in the uppermost mantle (14–68 [Formula: see text]) suggest the presence of melts (0.25 vol%–3.4 vol%) or fluids (0.068 vol%–0.45 vol%). An extremely conductive body (&lt;10 [Formula: see text]) at a depth of approximately 3–14 km in the eastern geothermal area could be interpreted as a hydrothermal reservoir; below this body, the conductive column (1.8–15 [Formula: see text]), rising from the uppermost mantle, suggests fluid upwelling. In contrast, high resistivity (&gt;100 [Formula: see text]) beneath Usu Volcano, the center of active volcanism, suggests that no mobile magma was present. A columnar-shaped region of slightly low resistivity (44 [Formula: see text] at minimum) is observed below the Toya caldera, which was inferred as cooling magma or an altered or heated upper crust attributed to past magma intrusion. A resistivity structure observed below the volcanic edifice is considered to reflect the steady state of the dormant volcanic system in this area, and there is likely no large amount of melt that would be deemed imminent for a caldera-forming eruption. This information could be a valuable scientific contribution to the volcanic hazard risk assessments currently being conducted in Japan.

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

Gedemsa caldera is a peralkaline volcanic depression located in the Central sector of the Main Ethiopian Rift. An integrated volcanological study of stratigraphic sections was carried out in order to constrain the eruptive history of Gedemsa caldera volcano (GCV), Ethiopia. Textural analyses on plagioclase crystals together with field observations shed light on magma chamber processes feeding the volcanic complex. A multi-vent eruption is ascribed to the pre-caldera volcanic products, as can be seen from the varying caldera wall sequences comprising lavas and pyroclastic deposits. At least three major caldera-forming eruptions are identified producing: 1) a lower ignimbrite, 2) extensive pumice fall and pyroclastic density current deposits, and 3) an upper ignimbrite exposed in different areas of the caldera, indicating a sector collapse. We identified 20 individual pyroclastic deposits found outside and within the caldera that erupted after the climactic caldera collapse. The most recent volcanic activity at Gedemsa is characterized by rhyolitic lava domes and pyroclastic deposits from eruptions through numerous vents aligned with WNW-ESE pre-existing cross-cutting structures. Macrocryst disequilibrium textures such as fine-scale oscillatory zoning (FOZ), sieve textures, glomerocryst, and synneusis indicate that magma reservoir is characterized by repeated magma injection, convection flows, and mixing.

Reconciling complex stratigraphic frameworks reveals temporally and geographically variable depositional patterns of the Campanian Ignimbrite

Abstract The 39.8-ka Campanian Ignimbrite was emplaced during a large caldera-forming eruption of Campi Flegrei near Naples, Italy. The ignimbrite is found up to 80 km from the caldera, and co-ignimbrite ash-fall deposits occur 3200 km away. The proximal and distal stratigraphy of the Campanian Ignimbrite has not been definitively correlated due to the dissimilar appearance of the proximal and distal deposits, a lack of medial exposures, and the inconsistency and heterogeneity of the proximal stratigraphy. Here, we document the major-element glass-shard chemistry, matrix componentry, and lithic componentry of the proximal and distal stratigraphic sequences of the ignimbrite to attempt to correlate the units. The results of these disparate observations taken together suggest that the established stratigraphic units cannot be directly and uniquely correlated between the proximal and distal regions and that neither the proximal nor distal stratigraphy provides a record of the entire eruptive sequence. However, the characteristics studied can be used to demarcate eruptive phases that are connected to some of the defined units in the proximal and distal stratigraphy.