Eruptive Style Research Articles

Long-term change of the eruption activities of Sakurajima volcano, Japan, inferred from the fallout tephra deposits

Stratigraphic analysis of fallout tephra deposits in and around a volcano provides a framework for understanding the long-term temporal change in the volcano’s activities. Here, we reconstruct the evolution of the volcanic activities of Sakurajima volcano based on reconstructed tephra stratigraphy by original field surveys, compilations of geological and archeological data, and new 14C dating. We define three eruption stages of Stage 1 (30–24 ka), Stage 2 (12.8–4.8 ka), and Stage 3 (4.5 ka–present), based on a major hiatus and shifting of the volcanic centers. Stages 2 and 3 are further subdivided according to the predominant mode of volcanic activity. Revised distribution of tephra deposits indicates that a total of 14.5 km3 (5.8 km3 dense rock equivalent) of fallout tephra has erupted from Sakurajima. Among them, Stage 2a (13–8 ka) produced the largest volume of tephra fallout deposits, suggesting that Sakurajima peaked in magma discharge during Stage 2a (2.9 km3/kyr) and then decreased rapidly toward Stage 2b (8–4.8 ka; 0.07 km3/kyr). The eruption of large-volume tephra deposits in Stage 2a, followed by the development of a thick volcanic ash layer in Stage 2b, indicates the eruption style shifted from explosive pumice eruptions in Stage 2a to ash-producing Vulcanian activity in Stage 2b, with decreasing magma discharge rate. Thick volcanic ash deposits covered by several fallout tephra deposits during Stage 3 also reflect a shift in activity from repeated Vulcanian explosions with lava effusions in Stage 3a (4.5–1.6 ka) to predominantly pumice eruptions during the historical period (Stage 3b) with an increasing magma discharge rate. The case study of Sakurajima presented here demonstrates that the combined analysis of the distribution, stratigraphy, and age of pumice fall layers with the lava and pyroclastic cone deposits on the volcanic edifice is a powerful tool for deciphering the growth history of complex stratovolcanoes.

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.

A chemical threshold controls nanocrystallization and degassing behaviour in basalt magmas

An increasing number of studies are being presented demonstrating that volcanic glasses can be heterogeneous at the nanoscale. These nano-heterogeneities can develop both during viscosity measurements in the laboratory and during magma eruptions. Our multifaceted study identifies here total transition metal oxide content as a crucial compositional factor governing the tendency of basalt melts and glasses towards nanolitization: at both anhydrous and hydrous conditions, an undercooled trachybasalt melt from Mt. Etna readily develops nanocrystals whose formation also hampers viscosity measurements, while a similar but FeO- and TiO2-poorer basalt melt from Stromboli proves far more stable at similar conditions. We therefore outline a procedure to reliably derive pure liquid viscosity without the effect of nanocrystals, additionally discussing how subtle compositional differences may contribute to the different eruptive styles of Mt. Etna and Stromboli.

Degassed versus pristine: Evaluating melt inclusions with a new ATR-FPA-FTIR calibration and water imaging method in rhyolitic melts

The concentration of pre-eruptive volatiles is a key parameter that controls the eruptive behaviour of volcanoes. Among these, water is the most abundant volatile and has a major influence on magma chamber dynamics and a primary role in driving eruptions. While accurately quantifying the pre-eruptive dissolved water concentration of magmas is a crucial step in understanding eruptive styles, we still encounter problems in this regard. One of the main challenges is in discriminating pristine melt inclusions from those that lost water due to syneruptive and post-emplacement processes. To this end, we have calibrated a Fourier Transform Infrared (FTIR) setup with a Germanium crystal objective for Attenuated Total Reflectance analyses (ATR) coupled to a Focal-Plane Array detector (FPA), which allows the measurement of single-sided polished rhyolitic melt inclusions. The samples do not require any other special preparation routine, making it a versatile method (i.e. it can be applied to samples prepared for microprobe analysis). The setup generates high-resolution (4 μm) water distribution images that allow the quantification of dissolved water with an uncertainty of ±0.35 wt%. Water diffusion inside the melt inclusions can be identified on the spectral images, allowing for a qualitative analysis that, in most cases, can easily distinguish between pristine and degassed inclusions. We apply this setup to eight units (four explosive and four effusive) from the Nisyros-Yali volcanic center, in the South Aegean Sea, and compare the results with previously published hygrometry data. The results show that even in explosive units, which are generally considered unaffected by gas loss, up to 75% of the melt inclusions lost water. Because of this, the measured water contents show a spread over a range of ~2 wt%. Dissolved water contents in pristine melt inclusions match those from previous hygrometry calculations, indicating ~4 to 5 wt% H2O. In effusive units, all the melt inclusions are degassed, yielding water contents that vary from 0 to 3.6 wt%, as compared to ~5.6 wt% H2O given by mineral-melt hygrometry. The new setup and calibration provide a fast and cheap way for measuring water contents in calc-alkaline rhyolitic melt inclusions. Furthermore, our results indicate the importance of determining the pristine versus degassed nature of inclusions before any interpretations are made.

Conduit processes in crystal-rich dacitic magma and implications for eruptive cycles at Guagua Pichincha volcano, Ecuador

Stratovolcanoes are commonly characterised by cyclic eruptive activity marked by transitions between dome-forming, Vulcanian, Subplinian and Plinian eruptions. Guagua Pichincha volcano (Ecuador) has been a location of such cyclicity for the past ~ 2000 years, with Plinian eruptions in the first and tenth centuries AD (Anno Domini/after Christ), and CE (Common Era) 1660, which were separated by dome-forming to Subplinian eruptions, such as the recent 1999–2001 eruption. These cycles are therefore a prominent example of effusive-explosive transitions at varying timescales. Here, we investigate the reasons for such shifts in activity by focusing on degassing and outgassing processes within the conduit. We have coupled a petrophysical and textural analysis of dacites from the CE 1660 Plinian eruption and the 1999–2001 dome-forming/Vulcanian eruption, with different percolation models in order to better understand the role of degassing on eruptive style. We demonstrate that the transition from dome-forming to Plinian activity is correlated with differences in phenocryst content and consequently in bulk viscosity. A lower initial phenocryst content and viscosity is inferred for the Plinian case, which promotes faster ascent, closed-system degassing, fragmentation and explosive activity. In contrast, dome-forming phases are promoted by a higher magma viscosity due to higher phenocryst content, with slower ascent enhancing gas escape and microlite crystallization, decreasing explosivity and yielding effusive activity.ResumenLos estratovolcanes se caracterizan comúnmente por presentar actividad eruptiva cíclica, marcada por transiciones entre erupciones formadoras de domos y erupciones de tipo Vulcanianas, Subplinianas y Plinianas. El volcán Guagua Pichincha (Ecuador) ha dado lugar a tal ciclicidad durante los últimos ~ 2000 años, con erupciones Plinianas tanto en los siglos Primero y Décimo, como en el año 1660, las cuales estuvieron intercaladas por erupciones formadoras de domos y de tipo Subplinianas, tal como ocurrió durante la erupción reciente de 1999–2001. Estos ciclos son, por lo tanto, ejemplos destacados de transiciones eruptivas de tipo efusiva-explosiva a escalas de tiempo variadas. En este trabajo, investigamos las razones de tales cambios de actividad enfocándonos en procesos de exsolución y pérdida de gases del magma en el conducto (desgasificación en sistemas cerrado y abierto). Hemos acoplado análisis petrofísicos y texturales tanto de dacitas de la erupción Pliniana de 1660, como de la erupción formadora de domos/Vulcaniana de 1999–2001, junto con diferentes modelos de percolación, para así comprender mejor el rol de la exsolución de volátiles en el estilo eruptivo. Demostramos que la transición desde una actividad efusiva formadora de domos a una Pliniana está correlacionada con diferencias en el contenido de fenocristales y, subsecuentemente, con la viscosidad total del magma. Un contenido inicial menor de fenocristales y una menor viscosidad se infiere para el caso Pliniano, lo que promueve un ascenso más rápido, desgasificación en sistema cerrado, fragmentación y finalmente actividad explosiva. Por el contrario, las fases formadoras de domos son promovidas por una viscosidad mayor debido a un contenido mayor de fenocristales, con ascenso más lento promoviendo a su vez el escape de gases y la cristalización de microlitos, disminuyendo la explosividad y produciendo actividad efusiva.

The Influence of Magma Storage and Ascent Conditions on Laguna del Maule Rhyolite Eruptions

AbstractThe scarcity of historical rhyolite eruptions means that volcanological and petrological studies of past eruptions are a key tool for assessing the potential for future hazardous activity and improving interpretations of unrest signals. For the last 18 ky, the Laguna del Maule (LdM) volcanic complex in Chile has erupted primarily rhyolites but with differing magma compositions, eruption styles and eruptive volumes. Rapid surface uplift and episodic seismic activity at LdM over the last two decades has emphasized the need to understand both the recent evolution of the magmatic system and the most likely future eruption scenarios. Using mineral composition, geothermobarometry and MELTS modeling, we assess the influence of the magma storage and ascent conditions on the magnitude and styles of three LdM rhyolites. Magmas of the first and largest Plinian-ignimbrite eruption (LdM: rdm; >17 km3 DRE) form a distinct mineral assemblage with An28–60 plagioclase, amphibole and quartz derived from a magma plumbing system over a large pressure range (90–350 MPa). We suggest that the rdm eruption was triggered by magma recharge and overpressure within a ~ 90 MPa magma chamber of high-silica (>76.5 wt.% SiO2) rhyolitic melt. The rdm eruption appears to have reset the LdM storage conditions, such that subsequent rhyolite eruptions have been smaller (<4 km3 DRE), involved less evolved melt (<75.8 wt.% SiO2) and produced (sub)Plinian-effusive eruption sequences. Of these, we have studied the earliest (Los Espejos: rle) and the most recent (Las Nieblas: rln). Both have mineral assemblages with An20–34 plagioclase and biotite formed in water-saturated crystal-poor rhyolite batches over limited pressure ranges (ΔP ~ 50 MPa) within a rhyodacitic mush reservoir. We suggest that rle and rln were triggered by injection of water-rich magma and volatile exsolution; the transition to effusive behavior occurred because of changes in magma temperature (rle) or magma decompression rate (rln). Similarities in the limited pressure range and high volatile content inferred for the current low-density magma body with the petrologically determined source conditions of magmas that fed past subplinian-effusive eruptions suggest that the next LdM eruption will be similar.

Controls of crystal shape on degassing mechanisms in crystal-rich magmas with rhyolitic groundmass melts

Understanding the mechanical effects of crystals on degassing kinetics and permeability development in silicic magmas is important for modeling eruptions and examining first order controls on eruption style. We conducted high-pressure-high-temperature isothermal decompression experiments to investigate the role of crystal shape on permeability development and pore pathway geometry. Experiments were performed on hydrous rhyolitic glass (76.3 wt.% SiO2) seeded with variable amounts of equant (aspect ratio ∼1.8 ± 0.6) corundum crystals and elongate (aspect ratio ∼10 ± 5.5) wollastonite crystals to approximate natural phenocryst and microphenocryst/microlite populations, respectively. We measured total porosity, connected pore volume and permeability directly from the experimental charges by applying Archimedes' principle to determine bulk density, helium-pycnometry to measure connected porosity, and a custom-made permeameter to measure permeability. The experimental samples developed permeability at a critical melt porosity (ϕc-melt, above which degassing is enhanced due to bubble coalescence) of ∼55 vol.% vesicles for the corundum experiments and ∼48 vol.% for the wollastonite-bearing experiments; these values are considerably lower than the ϕc-melt>63 vol.% for prior crystal-free experiments (Lindoo et al., 2016; deGraffenried et al., 2019). Critical porosity is reduced when crystals comprise at least ∼20 vol.%, regardless of shape. Connected porosity increases and average bubble size decreases with increasing abundance of elongate wollastonite crystals, explained by the onset of yield strength behavior induced by loosely touching crystal frameworks that form at decreasing crystallinities with increasing elongation of crystals comprising the network. When the population of high-aspect-ratio crystals reaches random loose packing, the resulting reduction in interstitial melt available for unimpeded bubble expansion forces the bubbles to connect at lower total vesicularity. It therefore seems likely that crystal-bearing intermediate magmas experience an abrupt increase in degassing efficiency when crystallinity attains random loose packing. In hydrous magmas, the efficiency of decompression-driven degassing and resulting formation of anisotropic groundmass crystals is controlled by magma composition and decompression rate. Enhanced gas loss in slowly ascending (and crystallizing) magma may aid the development of dense conduit plugs, thus increasing the possibility of violent Vulcanian explosions.

Physical Characterization of Long‐Lasting Hybrid Eruptions: The 2021 Tajogaite Eruption of Cumbre Vieja (La Palma, Canary Islands)

AbstractLong‐lasting, hybrid eruptions can be of complex description and classification, especially when associated with multiple eruptive styles and multiple products. The 2021 Tajogaite eruption of La Palma, Canary Islands, was associated with a magma‐gas decoupled system that resulted in the simultaneous emission of lava flows and tephra plumes from various vents. Even though the tephra blanket (∼2 × 107 m3) represents only 7%–16% of the total erupted volume, it provides fundamental insights into the overall eruptive dynamics. Tephra was mostly dispersed NE‐SW due to a complex regional and local wind patterns and was subdivided into 3 units and 11 layers that well correlate at different distances from the vent and with both tremor data and lava emission rate. While plume height varied at the temporal scale of a few hours, the average mass eruption rate associated with the tephra blanket of the different units remained relatively constant (∼3–4 × 103 kg s−1). In contrast, the emission rate of lava largely increased after the first week and remained higher than the overall emission of tephra throughout the whole eruption (average value of ∼6 × 104 kg s−1). Based on a detailed characterization of the tephra blanket in combination with atmospheric wind, tremor, and lava emission trend, we demonstrate the need of (a) multidisciplinary strategies for the description of hybrid eruptions that account for both the duration of individual phases and the quantification of the mass of multiple products, and of (b) dedicated ash dispersal forecasting strategies that account for the frequent variations of eruptive and atmospheric conditions.

The recent volcanism of Flores Island (Azores): Stratigraphy and eruptive history of Funda Volcanic System

Monogenetic volcanoes occur in many different geotectonic settings and are usually small and short-lived. They can experience a variety of eruptive styles, even during the same eruption. In monogenetic volcanic fields, volcanism usually migrates to different locations over time, making volcanic hazard assessment very challenging. The eruptive history of a volcanic region, including the size, style, and location of previous eruptions, provides valuable information to help predict the behaviour of future volcanic events and their associated hazards. Here, we reconstruct for the first time the eruptive history of the Funda Volcanic System (FVS), one of the most recent (∼3 ka) monogenetic eruptive centres of Flores Island (Azores), based on a detailed tephrostratigraphic work coupled with geochemical analysis of glass shards and radiocarbon dating. We identified at least three volcanic events at FVS spaced by time intervals of ∼100 yr. The first event (3430 cal yr BP) was a small Strombolian eruption, the second event (3330 cal yr BP) started as a violent Strombolian eruption and may have ended as phreatomagmatic, and the third event (3250 cal yr BP) was exclusively phreatomagmatic. Our results demonstrate that volcanism at the FVS was more prolonged and recurrent than previously reported. Moreover, we show that the FVS experienced different eruptive styles in a short timeframe, ranging from small basaltic eruptions to violent explosive phreatomagmatic events. Such diversity of eruptive styles results in different volcanic products, which have different hazard implications. Our new results contribute to the knowledge of the recent volcanic activity of Flores Island, and we anticipate them to be of paramount importance for future volcanic hazard assessment studies.

Identification and Geomorphometric Characterization of Volcanic Cones in the Marius Hills, the Moon

AbstractInvestigation of the detailed geological characteristics of lunar volcanic cones can peer into their formation and emplacement processes, thus deepening our understanding of pyroclastic eruptions on the Moon. In this work, we focus on the Marius Hills region (centered at 13°N, 55°W), a volcanic plateau in south Oceanus Procellarum, which hosts the most abundant volcanic cones on the Moon, along with other volcanic features. The range of volcanic features indicates various eruption styles and complicated sequences of regional magmatic activities. Based on the SLDEM2015 topography data, with assistance from other multisource topography and image data sets, we identified 360 known and suspected volcanic cones and 22 volcanic domes in the Marius Hills. Following detailed morphological characterizations, the volcanic cones show various morphology types, including C‐shaped, elliptical‐shaped, elongated‐shaped, irregular‐shaped, and rounded‐shaped. The cones (median basal width ∼2.7 km, height ∼115 m, and volume 0.25 km3) contain summit pit craters (15%; median width ∼358 m), exhibiting smaller basal widths and summit pit crater widths compared with lunar mare domes. The mean slope (3.5°–15.6°) and rock abundance (0.3%–3.8%) of volcanic cones in Marius Hills are larger than those of the mare dome, suggesting different eruption mechanisms. Marius Hills cones also show larger basal widths and smaller height/width ratios than terrestrial volcanic cones, which are attributed to multiple factors, including subaerial environment, volatile content of the magma, eruption rate, and lava viscosity.

Erratum: Pre-eruptive excess volatiles and their relationship to effusive and explosive eruption styles in semi-plugged volcanoes

An erratum on: Utami SB, Andùjar J, Costa F, Scaillet B, Humaida H and Carn S (2022) Pre-eruptive excess volatiles and their relationship to effusive and explosive eruption styles in semi-plugged volcanoes. Front. Earth Sci. 10:882097. doi: 10.3389/feart.2022.882097Error in Figure/TableDue to a production error, there was a mistake in Table 3 as published. The table headings were incorrect. The corrected Table 3 appears below. The publisher apologizes for this mistake.The original version of this article has been updated.

Late Tonian explosive volcanism and hyaloclastites in northern Paraguay Belt, Central Brazil: A record of Rodinia break-up in western Gondwana

Explosive volcanism and oceanic crust generation are key aspects to understanding the break-up history of the Rodinia. Its impact on the extreme glaciations of the Cryogenian was marked by ocean chemistry fluctuations in these times, which are still poorly known and preserved in western Gondwana. Here we describe in detail and provide a series of U-Pb ages for the Nova Xavantina Metavolcanosedimentary Sequence, where extrusive rocks, such as hyaloclastite and metabasalts, as well as mafic and ultramafic intrusive rocks and ignimbrites, are associated with diamictites, limestones, and iron formations in the Paraguay belt, Central Brazil. Two styles of pyroclastic eruptions were identified in the studied area. Basic-ultrabasic Surtseyan eruption was enhanced by magma-water interaction and is represented by hyaloclastites and incorporated bombs of scoria. Felsic Plinian eruption, in its turn, was formed by violent explosions in a subaerial environment and is represented by widely exposed acidic tuffs and ignimbrites. Shallow subaqueous environments, such as lakes, salt lakes, and possibly proto-oceans, developed in this precursor basin concomitant with bimodal volcanism. High resolution U-Pb geochronology results indicate they were formed in at least three magmatic events, starting with the formation of ca. 745 Ma seafloor metabasalts and felsic volcanics dated at ca. 735 Ma, and intrusive gabbros at ca. 714 Ma. Alkaline ultramafic rocks of ca. 577 Ma intrude the region and may be associated with similar rocks that crop out regionally as the Planalto da Serra Alkaline Complex. Our results from Nova Xavantina volcanism explain the main Neoproterozoic source until now unrecognized and previous recorded in detrital zircons from the Paraguay belt sediment provenance.

Review of geology and geomorphology of the Xalapa Monogenetic Volcanic Field, eastern Trans-Mexican Volcanic Belt

Monogenetic volcanic fields have associated volcanic hazards that are not always well constrained due to the lack of complete and well established geological and geomorphological studies, field boundaries, volcano inventories, and of detailed reconstruction of individual volcano eruptive histories. This work presents a detailed review and further insights into geology and geomorphology of the Xalapa Monogenetic Volcanic Field (XMVF), achieved by completing a comprehensive revision of all previous works on the XMVF, along with new data produced from extensive fieldwork, field and digital mapping, radiocarbon dating, and morphometric and density analyses. The main results are: (i) a renewed inventory of 72 XMVF monogenetic volcanoes - a total number larger than previously recorded (59); (ii) identification of six new and not previously identified monogenetic volcanoes: La Natividad, La Pedrera, El Puente, Rancho Sicomoro, and Puentecillas scoria cones, and the Tenampa maar; (iii) recognition of four possible new volcanoes integrating the Macuiltépetl cluster, recorded in historical documents; (iv) corrections and new volcanoes identified in the volcano clusters of Pextlan, La Joya, Ocotepec, and Cruz Negra; (v) geological mapping of all lava flows associated with the 72 monogenetic volcanoes; (vi) comprehensive morphometric analyses of all 72 reported volcanoes; (vii) three new radiocarbon ages corresponding to 780 ± 60 BP and 1340 ± 60 BP lava flows of El Volcancillo scoria cone, and to 1300 ± 60 BP and 1115 ± 80 BP lavas of La Pedrera volcano; (viii) new evidence of the late Pleistocene to Holocene age of the Acatlán scoria cone; (ix) new outline of the boundaries of the XMVF based on fieldwork and remote sensing; and (x) volcanoes and lava flows density and distribution maps. From these results, we conclude that, in case of renewed monogenetic activity in the XMVF, the most likely deterministic hazard model would comprise effusion of lavas of variable length (3–40 km), of up to extremely long tube-fed pahoehoe lava flows; explosive activity ranging from typical to violent Strombolian style eruptions producing scoria cones, extensive fallouts, and associated pyroclastic density currents; and phreatomagmatic style activity producing maars, tuff rings and/or tuff cones, and sequences of wet pyroclastic density currents. Hazard distributions in these scenarios would be mainly controlled by topography, local tectonics, groundwater level and basement rocks lithology, and would mostly affect the north and south flanks of Cofre de Perote volcano, as well as central XMVF sectors where the metropolitan area of the city of Xalapa and towns of Naolinco and Acatlán locate. From this review, we conclude that without any further studies of geology and chemistry of every single volcano, the western boundaries of the XMVF will remain unclear, and the associated volcanic risk will remain underestimated. This work provides a fundamental basis from which further and more detailed work could be attained.

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.

A reappraisal of explosive–effusive silicic eruption dynamics: syn-eruptive assembly of lava from the products of cryptic fragmentation

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cordón Caulle 2011–2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive–effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011–2012 Cordón Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity – explosive, effusive, and hybrid – was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cordón Caulle, and from other similar eruptions, we further argue that hybrid explosive–effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cordón Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent non-fragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

Varying processes, similar results: How composition influences fragmentation and subsequent feeding of large pyroclastic density currents

Unlike their silicic counterparts, mafic eruptions are known for being on the low-end of the explosivity spectrum with eruption styles commonly ranging from effusive to Hawaiian fire fountaining. However, there are increasing discoveries of large mafic Plinian eruptions, sometimes generating ignimbrites, suggesting that this phenomenon might not be so uncommon. So, what processes lead a mafic magma to fragment violently enough to generate extensive ignimbrites?We sampled pumices from ignimbrites and PDCs with a compositional range from basaltic-andesite (Curacautín ignimbrite, Volcàn Llaima, Chile), andesite (Marapi, Indonesia) to trachyte (Gunungkawi ignimbrite, Batur, Indonesia). We use SEM imagery and X-ray Microtomography on pyroclasts from these deposits to characterize phenocryst, microlite and vesicle textures. From vesicle number densities we estimate fragmentation decompression rates in the range of 0.4–1.6 MPa/s for the three deposits. With a combination of EPMA and SIMS analyses we characterise pre-eruptive storage conditions. Based on the bulk and groundmass compositions, the storage temperature (1,050–1,100°C), pressure (50–100 MPa) and phenocryst content (1.0–2.5 vol%), we conclude that the basaltic-andesitic Curacautín magma was at sub-liquidus conditions, which allowed fast and widespread disequilibrium matrix crystallization (0–80 vol%) during ascent to the surface. Combined with the important decompression rate, this intense crystallization led to a magma bulk viscosity jump from 103 up to >107 Pa s and allowed it to fragment brittlely. Conversely, for the Marapi PDC and Gunungkawi ignimbrite, similar decompression rates coupled with larger initial bulk viscosities of 105–106 Pa s were sufficient to fragment the magma brittlely. The fragmentation processes for these latter two deposits were slightly different however, with the Marapi PDC fragmentation being mostly driven by vesicle overpressure, while a combination of bubble overpressure and intense strain-rate were the cause of fragmentation for the Gunungkawi ignimbrite. We conclude that mafic ignimbrites can form due to a combination of peculiar storage conditions that lead to strongly non-linear feedback processes in the conduit, particularly intense microlite crystallization on very short timescales coupled with intense decompression rates. Conversely, the high viscosity determined by pre-eruptive storage conditions, including temperature and volatile-content, are key in controlling the formation of more evolved magmas PDCs'.

How long-term hazard assessment may help to anticipate volcanic eruptions: The case of La Palma eruption 2021 (Canary Islands)

Long-term hazard assessment is an essential action that should be undertaken at all active volcanoes and/or volcanic areas as an indispensable contribution to reduce volcanic risk. It allows anticipating new eruptions responding to two of the main questions of volcano forecasting: where and how the next eruption will be. In the case of the Canary Islands, an active volcanic region that receives millions of visitors every year, and which has been recently severely impacted by a small size eruption on the island of La Palma in 2021, a systematic long-term hazard assessment is still pending, despite this being foreseen in its management plan to face volcanic threats. An essential basic long-term volcanic hazard assessment, conducted six years before this last eruption, was able to anticipate the location of the new vents and the eruptive style, including the extent of its main potential hazards. This long-term research was developed in the frame of a Master thesis, which was aimed at justifying the viability of a particular systematic long-term volcanic assessment methodology to be applied to the Canary Islands and elsewhere. Despite this particular study was not used to manage the La Palma 2021 eruption, it demonstrates how beneficial such studies may be to forecast new eruptions and to reduce their associated risk, and represents a new contribution to develop adequate territorial and management planning where we provide a qualitative hazard map for the Island of La Palma.

Pre-eruptive excess volatiles and their relationship to effusive and explosive eruption styles in semi-plugged volcanoes

Understanding the role of various factors influencing eruption style is challenging, but it can aid in adapting different hazard mitigations and crisis responses for explosive or effusive events. Here, we focus on the role of magma storage conditions in controlling eruption styles at basaltic andesite volcanoes, and how they can be related to monitoring data. We study the cycle of explosive (1990, sub-Plinian) → effusive (2007, dome) → explosive (2014, sub-Plinian) eruptions from Kelud (Kelut) volcano, Indonesia. We conducted petrological analyses of the eruption products and phase equilibria experiments using pumice and explored a range of temperatures, pressures, oxygen fugacity, and volatile contents. We show that we can reproduce the main mineral assemblage (plagioclase ± pyroxenes ± magnetite ± amphibole ± olivine) and phenocryst content (30–50 wt%) of the magmas from the three eruptions at T = 975 ± 39°C, p = 175 ± 25 MPa, fO2 = nickel–nickel oxide buffer, and about 4–6 wt% water in the melt (ca. 3 to 5 wt% and ca. 4 to 7 wt% for the 1990 and 2014 eruptions, respectively). However, geothermobarometric results also indicate that some crystals of amphibole were sourced from higher pressures. We infer from a synthesis of our data and historical observations that the high phenocryst content of the 2007 dome (∼70 wt%) likely resulted from slow magma ascent toward the surface alongside progressive degassing and re-equilibration at a lower volatile content (∼1 wt% water in the melt). Mass balance calculations on the sulfur budget of the 1990, 2007, and 2014 magmas show that the explosive events contained an excess fluid phase at pre-eruptive conditions, and we propose that this led to their higher explosivity compared to the 2007 dome. The accumulation of excess fluids during decadal-long repose depends on how plugged the volcanic system is, or its ability to passively release magmatic fluids, prior to eruption. Such condition could be inferred from monitoring records, including changes in gravity of the plumbing system over time, and thus contribute to better anticipate eruptive style.

The interplay between environmental and magmatic conditions in eruption style transitions within a fissure-aligned monogenetic volcanic system of Auckland, New Zealand

Recent conceptual geological frameworks of continental monogenetic volcanism highlight that the small magma volume eruptions, resulting volcanic geology and edifice architecture in such settings are sensitive to variations in external or environmental conditions. These conditions, along with fluctuations in magma flux, can change rapidly over short time frames and cause dramatic changes in eruption style. Understanding the drivers of transitions in explosive to effusive behaviour within the short timescales of eruptions at individual volcanic centres is essential to accurately assess volcanic hazards in continental monogenetic settings. Wiri Mountain Volcanic Complex is one of the largest and most complex volcanic centres in the active Auckland Volcanic Field. Despite the significant removal of much of the original volcanic deposits, present-day exposures and historical images provide a unique opportunity to examine the growth and evolution of the volcanic complex. Wiri Mountain deposited an initial basal tuff ring (covering an area of approximately 0.67 km2) by predominantly pyroclastic density currents, followed by at least two smaller tuff rings erupted through the outer flanks of the first, in a transition from phreatomagmatic to Strombolian eruptive style. A 90 m high central scoria cone was then produced within the initial tuff ring, partially capped by lava spatter, clastogenic lava flows and lava flows that mostly covered all tuff rings, the scoria cone, and the surrounding area. A high-resolution stratigraphic study of the well-exposed tuff ring to capping magmatic succession was conducted to determine the changes in eruptive style and their driving forces. The deposit architecture of Wiri Mountain can be described using three volcanic stratigraphic units: a basal unit comprised of tuff, lapilli tuff and tuff breccia deposits, a middle unit comprised of juvenile-rich transitional tuff deposits of black scoria ash and lapilli, red scoria and spatter, and a capping unit comprised of agglutinate and lava flow successions. Most volcanic materials were either fragmented and ejected at near-optimal scaled depth, or the transition between near-optimal and shallower/deeper depths. Wiri Mountain provides a striking example of a fissure eruption likely controlled by a pre-existing tectonic fault that was fed by relatively stable melt sources over a sustained period. We infer that despite eruption within a water-saturated coastal plain, the initial phreatomagmatic phase was overridden by subsequent explosive and effusive magmatic phases through the formation of an increasingly established conduit, thus allowing sustained magma flux and melt supply. The transitions, both gradual and rapid, from an initial phreatomagmatic to subsequent magmatic explosive and effusive phases fit well to the general understanding of eruption style changes over time from other larger and more complex volcanoes of Auckland, and elsewhere worldwide. Wiri Mountain showcases the fine balance between the external and internal conditions that control eruption style variations and govern the formation of complex monogenetic volcanoes.

Mapping the facies and geomorphology of igneous bodies by integrating multiple disciplines data: A case study of the BZ-A oilfield, Bohai Bay Basin, China

The BZ-A oilfield is one of the significant discoveries within the Huanghekou Sag, the southern Bohai Sea Basin, China, and the reservoirs of the BZ-A oilfield were deposited in the Paleogene period. Unfortunately, the spatial continuity of the Paleogene clastic reservoirs of the BZ-A oilfield was destroyed by the widely distributed igneous rocks. Characterizing those “isolated” clastic geobodies is the main task in the exploration and production phases. The exploration wells within the BZ-A oilfield demonstrate that the size of the clastic geobodies is one of the main factors that determine if a new drilled well is a commercial oil-producing well. The spatial distribution and the size of igneous rocks are other important factors affecting whether a drilled well is an oil-producing well. Thus, the analysis of 3D geomorphology of igneous rocks is among the most important tasks in well and development planning. We integrate 3D seismic data, well logs, and core data to characterize the Paleogene igneous bodies within the BZ-A oilfield. The analyzing procedure begins with determining the igneous rock types by analyzing the texture of cores and corresponding thin sections. Then, the volcanic/volcaniclastic facies are defined by integrating the well logs and igneous rock. Finally, the volcanic edifices are interpreted by integrating the seismic data, well logs, and core data. The analysis demonstrates that the volcanic edifice usually consists of five facies (volcanic conduit, effusive, explosive, extrusive, volcanic sedimentary, and subvolcanic facies). The overall shape of the edifice and the distribution of each facies are controlled by the volcanic eruption styles. In our study area, the seismic variance attribute can be used to locate the volcanic conduit facies, and the seismic root-mean-square amplitude can characterize the volcanic effusive facies.