Subplinian Events Research Articles

The last 1100 years of activity of La Fossa caldera, Vulcano Island (Italy): new insights into stratigraphy, chronology, and landscape evolution

A detailed study of past eruptive activity is crucial to understanding volcanic systems and associated hazards. We present a meticulous stratigraphic analysis, a comprehensive chronological reconstruction, thorough tephra mapping, and a detailed analysis of the interplay between primary and secondary volcanic processes of the post-900 AD activity of La Fossa caldera, including the two main systems of La Fossa volcano and Vulcanello cones (Vulcano Island, Italy). Our analyses demonstrate how the recent volcanic activity of La Fossa caldera is primarily characterized by effusive and Strombolian activity and Vulcanian eruptions, combined with sporadic sub-Plinian events and both impulsive and long-lasting phreatic explosions, all of which have the capacity to severely impact the entire northern sector of Vulcano island. We document a total of 30 eruptions, 25 from the La Fossa volcano and 5 from Vulcanello cones, consisting of ash to lapilli deposits and fields of ballistic bombs and blocks. Volcanic activity alternated with significant erosional phases and volcaniclastic re-sedimentation. Large-scale secondary erosion processes occur in response to the widespread deposition of fine-grained ash blankets, both onto the active cone of La Fossa and the watersheds conveying their waters into the La Fossa caldera. The continuous increase in ground height above sea level, particularly in the western sector of the caldera depression where key infrastructure is situated, is primarily attributed to long-term alluvial processes. We demonstrate how a specific methodological approach is key to the characterization and hazard assessment of low-to-high intensity volcanic activity, where tephra is emitted over long time periods and is intercalated with phases of erosion and re-sedimentation.

Volcanic Ash Classification Through Machine Learning

AbstractVolcanic ash provides information that can help understanding the evolution of volcanic activity during the early stages of a crisis and possible transitions toward different eruptive styles. Ash consists of particles from a range of origins within the volcanic system and its analysis can be indicative of the processes driving the eruptive activity. However, classifying ash particles into different types is not straightforward. Diagnostic observations for particle classification are not standardized and vary across samples. Here we explore the use of machine learning (ML) to improve the classification accuracy and reproducibility. We use a curated database of ash particles (VolcAshDB) to optimize and train two ML‐based models: Extreme Gradient Boosting (XGBoost) that uses the measured physical attributes of the particles, from which predictions are interpreted by the SHapley Additive exPlanations (SHAP) method, and a Vision Transformer (ViT) that classifies binocular, multi‐focused, particle images. We find that the XGBoost has an overall classification accuracy of 0.77 (macro F1‐score), and specific features of color (hue_mean) and texture (correlation) are the most discriminant between particle types. Classification using the particle images and the ViT is more accurate (macro F1‐score of 0.93), with performances varying from 0.85 for samples of dome explosions, to 0.95 for phreatic and subplinian events. Notwithstanding the success of the classification algorithms, the training dataset is limited in number of particles, ranges of eruptive styles, and volcanoes. Thus, the algorithms should be tested further with additional samples, and it is likely that classification for a given volcano is more accurate than between volcanoes.

Scenario-based probabilistic hazard assessment for explosive events at the San Salvador volcanic complex, El Salvador

We present a scenario-based, probabilistic hazard assessment for the San Salvador volcanic complex (SSVC), a volcanic field located in the vicinity of San Salvador that includes the El Boquerón stratovolcano and 25 monogenetic vents. We define a set of likely eruption scenarios for tephra fallout and pyroclastic density currents (PDCs). The eruption scenarios range from violent Strombolian eruptions with a significant uncertainty in source position to sub-Plinian and Plinian activity fed from the central cone. The adopted methodology is mainly based on numerical modeling using Tephra2 (adopting the software TephraProb) to study tephra fallout and the branching box model and the branching energy cone model (adopting the programs BoxMapProb 2.0 and ECMapProb 2.0) to describe inertial and frictional PDCs, respectively. Despite the dominant W-WSW-trending winds, numerical results show that Plinian eruptions at El Boquerón volcano are able to deposit thick tephra layers in the metropolitan area of San Salvador city, likely reaching mass loads of the order of 100 kg/m2 (conditional probability of 50%). The simulated sub-Plinian events highlight the seasonal influence of wind patterns. In fact, the conditional probability of significant tephra sedimentation in San Salvador city is strongly reduced when eruptions occur during the rainy season. Numerical modeling of violent Strombolian eruptions is performed considering uncertainty in vent position. Results show that the conditional probability of depositing tephra mass loads higher than 10 kg/m2 at a given point reaches a maximum value of ∼7% on the NW flank of the volcano, at about 8 km from the central crater. On the other hand, very low conditional probabilities (<1%) are obtained for San Salvador city for any relevant threshold (10 kg/m2 or more) of tephra mass load during violent Strombolian events. Regarding PDCs, results show that those produced during large-scale Plinian eruptions are able to invade significant areas of the volcano surroundings, including San Salvador city. PDCs generated from the partial collapse of a sub-Plinian eruption column exhibit maximum inundation probabilities on the N, W and S flanks of the volcano. Cerro El Picacho exerts a significant shield effect on the propagation of these PDCs, with low inundation probabilities for San Salvador city (<3%). Finally, coupling published vent opening probability maps and numerical modeling of small-scale PDCs yields maximum inundation probabilities on the NW flank of the volcano, reaching maximum conditional probabilities of the order of ∼10% and values of about 5% near the village of Nuevo Sitio del Niño.

A data driven approach to mineral chemistry unveils magmatic processes associated with long-lasting, low-intensity volcanic activity

The most frequent volcanic eruptions are of low-intensity and small magnitude. They produce abundant ash-sized (< 2 mm) clasts, which are too small to establish quantitative links between magmatic processes and eruptive dynamics using classic approaches. This inhibits our ability to study the past behaviour of frequently erupting volcanoes, essential to predict their future activity and mitigate their impact. The Palizzi unit (10–13th century, Vulcano, Italy) includes a prototype sequence of ash deposits resulting from prolonged Vulcanian eruptions punctuated by those of two larger sub-Plinian events. We apply Hierarchical Clustering to chemical analyses of clinopyroxene collected along the stratigraphy to decipher magma dynamics during this eruptive period. We identify periods of magma input and we link deep magmatic processes to eruptive dynamics, also showing that our approach can be used to connect magma and eruptive dynamics in any volcanic sequence. This is essential to track the processes occurring during frequent eruptions and to identify the build-up to larger explosive events.

The Late Pleistocene to Holocene tephra record of ND14Q site (southern Adriatic Sea): Traceability and preservation of Neapolitan explosive products in the marine realm

A tephrochronological investigation was carried out at site ND14Q (1013 m of water depth), located in the southern Adriatic Sea, offshore the Gargano promontory. It was drilled in the frame of The NextData Project (www.nextdataproject.it, 2011-2013), focused on paleoclimate research. To obtain a composite and well preserved deep marine record, three cores were raised at the study site. Twelve primary cryptotephra and one tephra were analysed in terms of major-element content and correlated with a total of sixteen eruptive events. The studied materials display K-alkaline and subalkaline features pointing to a correlation with the Somma-Vesuvius, Campi Flegrei and Lipari Island (Aeolian Arc) volcanic activity occurred during the Late Pleistocene-Holocene. In detail, major marker tephra of the central Mediterranean have been recognized, among which the Phlegraean Neapolitan Yellow Tuff (ca. 15 ka), Pomici Principali (ca. 12 ka BP) and Astroni-Agnano-Monte Spina (ca. 4.2 ka - 4.4 ka BP) eruptions and, for the first time in this area, the Vesuvian 79 CE event. The very well preserved Mercato (ca. 9 ka BP) and Fiumebianco-Gabellotto (ca. 8.4 ka BP) cryptotephra have also been found in the sapropel S1 interval of the succession, which definitely records the last ca. 22 kyr (Pomici di Base tephra). A number of other Plinian and sub-Plinian events also occur and the availability of multiple cores provided new insights into their temporal relation such as between AP2 and Astroni 6 eruptions. The identification of tephra sourced by well-dated volcanic events, along with several AMS 14C age results, allowed an accurate chronological framework for the composite tephrostratigraphic record, thus providing accurate ages for those deposits with an uncertain correlative event. In addition, stratigraphic evidences in the core replicates at ND14Q site showed that several intervening factors affect preservation and traceability of cryptotephra deposits in the marine realm posing main critical issues on volcanological and paleoclimate research.

A latest Pleistocene and Holocene composite tephrostratigraphic framework for northeastern North America

Lakes and bogs in northeastern North America preserve tephra deposits sourced from multiple volcanic systems in the Northern Hemisphere. However, most studies of these deposits focus on specific Holocene intervals and the latest Pleistocene, providing snapshots rather than a full picture. We combine new data with previous work, supplemented by a broad review of the characteristics and ages of potential source regions and volcanoes, to develop the first composite tephrostratigraphic framework covering the last ∼14,000 years for this region. We report new cryptotephra records from three ombrotrophic peat bogs—Irwin Smith (Michigan), Bloomingdale (New York), and Sidney Bog (Maine)—as well as new analyses and age models from previously reported sites, Nordan's Pond Bog (Newfoundland) and Thin-Ice Pond (Nova Scotia). A new tephra (Iliinsky) from the NGRIP and GRIP ice cores is also presented as it can be correlated to new data from these terrestrial records and helps validate radiocarbon age models. We identify 21 new tephra in addition to the 15 already known, several of which cover the entire region – the White River Ash east, Newberry Pumice, Ruppert (NDN-230), and Mazama. For the first time we find Mount St. Helens Yn (ca. 3660 cal yr BP) and a set P tephra (∼3000–2550 cal yr BP), and confirm the presence of Jala Pumice from Volcan Ceboruco, Mexico, and KS1 from Ksudach volcano, Kamchatka. We describe new “ultra-distal” tephra, including the early Holocene KS2 eruption, and propose correlations to volcanoes Iliinsky and Shiveluch of Kamchatka, and Ushishir of the Kurile Islands. Not all of these tephra represent large eruptions, with several plausible correlations to sub-Plinian events. Using Bayesian age-modeling, we present new age estimates for the newly described tephra, for tephra with previously poor age control, and for several proximal correlatives. Overall, we demonstrate northeastern North America's importance for providing transcontinental linkages between paleoenvironmental records and providing insights into ash distribution from different styles and sizes of eruptions.

Geometric estimation of volcanic eruption column height from GOES-R near-limb imagery – Part 2: Case studies

Abstract. In a companion paper (Horváth et al., 2021), we introduced a new technique to estimate volcanic eruption column height from extremely oblique near-limb geostationary views. The current paper demonstrates and validates the technique in a number of recent eruptions, ranging from ones with weak columnar plumes to subplinian events with massive umbrella clouds and overshooting tops that penetrate the stratosphere. Due to its purely geometric nature, the new method is shown to be unaffected by the limitations of the traditional brightness temperature method, such as height underestimation in subpixel and semitransparent plumes, ambiguous solutions near the tropopause temperature inversion, or the lack of solutions in undercooled plumes. The side view height estimates were in good agreement with plume heights derived from ground-based video and satellite stereo observations, suggesting they can be a useful complement to established techniques.

Conduit dynamics of highly explosive basaltic eruptions: The 1085 CE Sunset Crater sub-Plinian events

Basaltic volcanoes produce a range of eruptive styles, from Strombolian to low-intensity fire fountaining to, much more rarely, highly explosive Plinian eruptions. Although the hazards posed by highly explosive eruptions are considerable, controlling mechanisms remain unclear, and thus improving our understanding of such mechanisms is an important research objective. To elucidate these mechanisms, we investigate the magma ascent dynamics of basaltic systems using a 1D numerical conduit model. We find that variations in magmatic pressure at depth play a key role in controlling modelled eruption characteristics. Our most significant result is that a decrease in pressure at depth, consistent with the emptying of a magma chamber, results in enhanced volatile exsolution and in deepening fragmentation depth. The corresponding decrease in conduit pressure ultimately produces a collapse of the conduit walls. This type of collapse may be a key mechanism responsible for the cessation of individual explosive eruptions, a notion previously explored for silicic eruptions, but never before for basaltic systems. Using previously published field and sample analysis to constrain model parameters, we simulate scenarios consistent with sub-Plinian eruptions, similar to those at Sunset Crater volcano in ~1085 CE in terms of mass eruption rates and duration. By combining these analyses with a chamber-emptying model, we constrain the size of the magma chamber at Sunset Crater to be on the order of tens of km3. During the 1085 CE Sunset Crater eruption, there were three main sub-Plinian events that erupted between 0.12 and 0.33 km3 of tephra (non-DRE), indicating that ~1% of the total chamber volume was erupted during each sub-Plinian pulse.

Conduit processes during the climactic phase of the Shinmoe-dake 2011 eruption (Japan): Insights into intermittent explosive activity and transition in eruption style of andesitic magma

The climactic phase of the Shinmoe-dake 2011 eruption was characterized by sub-Plinian events (January 26p.m., 27a.m., and 27p.m.) and lava accumulation in the crater (late on January 27 to January 29), all of which were accompanied by Vulcanian events. The magma discharge rate for lava accumulation was close to that of the sub-Plinian events. We discuss evolution of syneruptive magma ascent through the climactic phase on the basis of decompression-induced crystallization of groundmass microlites, in order to reveal how the final eruption style was determined at the explosive–effusive transition. We examined pumice and lava-like pyroclasts from three sub-Plinian events, the January 28 Vulcanian event, and a lava block ballistically ejected during the February 1 Vulcanian event. The samples exhibit variable groundmass textures due to variable syneruptive ascent conditions, not due to pre-eruptive inhomogeneity of the andesitic magma and different degrees of syneruptive volatile exsolution preceding quenching.Eruptive units were first characterized according to bulk density, assuming that slower ascent resulted in higher density. A decrease in the magma ascent rate led to a gradual shift to an effusive eruption style. There was little variation in ascent rate among magmas erupted simultaneously during the first and second sub-Plinian events (bulk density 0.8–2.1g/cm3), but differences increased in the third sub-Plinian and January 28 Vulcanian eruptions with minor appearance of slow magma (0.9–2.8g/cm3). Only slow magma occupied the shallow conduit at the completion of lava accumulation (February 1 lava; 2.1g/cm3). Among the first and second sub-Plinian units, the early stage of the second sub-Plinian event had the greatest variation in ascent rate, with extension to a slightly higher bulk density (1.0–2.1g/cm3) than the other events (0.8–1.7g/cm3). Ascent-rate variations in the early stage of the second sub-Plinian event, the third sub-Plinian event, and the January 28 Vulcanian event are all due to the preceding calm phase in the intermittent explosive activity. The moving magma column, which showed a vertical zonation in the degree of degassing, formed in the shallow conduit during the calm phase due to a decrease in the discharge rate, and this resulted in the co-ejection of magmas with variable degassing upon the start of explosive events.The crystal size distribution (CSD) of plagioclase microlites constrains the syneruptive branching (i.e., a point of divergence) in ascent rate and eruption style, which occurred after the start of microlite crystallization. The CSDs of the different samples show differences only in small crystal sizes; slope at these size fractions is smallest for low-density (<2.0g/cm3) pyroclasts from all eruptive phases, medium for high-density pyroclasts and lava (>2.0g/cm3) of the January 28 and February 1 events, and greatest for high-density pyroclasts (>2.0g/cm3) of the third sub-Plinian event. We propose, based on the similarity in CSD, that the shallow magma-feeding system was common for the two craters active during the January 28 explosive–effusive hybrid activity, where the Vulcanian event and emplacement of lava of the later February 1 event occurred in different parts of the original crater. The samples have variable plagioclase microlite number densities (1.4×105–3.5×106mm−3), based on variations in small crystal sizes among different CSDs. This variability resulted not only from the usual increasing trend in number density with increasing degree of undercooling, but also the decreasing trend associated with the strong undercooling during explosive eruptions.

Relative Source Locations of Continuous Tremor Before and After the Subplinian Events at Shinmoe‐dake, in 2011

AbstractVolcano monitoring systems are not always ready to resolve signals at the onset of eruptive activity. This study makes use of stations installed later to calibrate the performance of the stations that had been operated before the eruption. Seven stations recorded continuous volcanic tremor before and during the subplinian eruptions of Shinmoe‐dake, Japan, in 2011. We estimated the source locations of the tremor using the amplitude distribution. The stability of the analysis was obtained by careful selection of time windows in which signals from a single source are dominated. The site effects and the regional attenuation factor were evaluated using tremor recorded after the major eruptions by a dense seismic array and a good number of stations. A tremor source changed its depth beneath the crater for 1 week before the major eruption, rising from a depth of a few kilometer to the water layer 3 times, each of which occurred following shallow inflation and minor eruptions. It is interpreted as migration of gas probably with magma, which further transported heat to the water layer and triggered the subplinian eruptions.

Tephra fall at Tungurahua Volcano (Ecuador) - 1999-2014: An Example of Tephra Accumulation from a Long-lasting Eruptive Cycle

Tungurahua volcano awoke in 1999 after 75 years of quiescence, and its activity continues at the time of writing (2015). After fifteen years of eruptions, roughly 0.13 km• of tephra have been released and deposited mainly to the west and southwest of the volcano. In order to describe the activity and understand the implications associated to its tephra emissions, we review the: 1) their distribution, 2) quantification of ejecta, 3) their components and 4) eruption dynamics. Most of the eruptions have consisted of short-lived explosions, Strombolian eruptions, hydro-volcanic interactions triggering Vulcanian eruptions, one Subplinian event and transitional style phases, which is supported by analysis of tephra components. Major tephra fallouts occurred in 2001, 2006, and 2014. The rate of tephra discharge increased almost 2.1 times from the pre-2006 period to the post-2006 period. Tungurahua poses a continuing geological hazard across the adjacent regions, but also it is a natural laboratory for studying long-lasting eruptions and their effect.

The frequency and magnitude of post-glacial explosive eruptions at Volcán Mocho-Choshuenco, southern Chile

Volcán Mocho-Choshuenco (39°55′S 72°2′W) is one of the most hazardous volcanoes in Chile′s Southern Volcanic Zone (SVZ). To better evaluate these hazards, we have reconstructed a high resolution eruption history, the most detailed to date for a volcano in Chile, from detailed field observations and geochemical analyses. Mocho-Choshuenco has experienced ca. 75 post-glacial (<18ka) explosive eruptions, including three large (volume ≥1km3) Plinian eruptions (Neltume, Pirehueico and Huilo) and a large pyroclastic density current and associated sub-Plinian event (Enco). Poor preservation and lack of exposure made it difficult to distinguish between medial to distal deposits solely on field observations. Instead, deposits have been correlated using a combination of glass chemistry and Fe–Ti oxide compositions. New radiocarbon dates were obtained and integrated with existing dates in a Bayesian age model (OxCal) to constrain the tempo of the volcanism. Based on preserved deposits, we derive a post-glacial eruptive frequency of one explosive eruption every ~220years with the youngest, confirmed eruption from Mocho-Choshuenco in 1864. The total post-glacial volume of tephra erupted is estimated at ≥20km3 (ca. 50% with a dacitic or rhyolitic glass composition) with ≥4km3 erupted from the monogenetic cones on the flanks, making Mocho-Choshuenco one of the most productive (ca. 1km3/kyr) and active volcanoes in the SVZ during post-glacial times. Many tephra deposits from Mocho-Choshuenco could be preserved in one or more lakes in the region, and have the potential to form regional tephra markers. In particular the Neltume deposits, which are dispersed to the NNE, are found interbedded with tephra deposits from the Villarrica–Quetrupillán–Lanín chain and should be preserved around Sollipulli and Llaima (~140km NNE of Mocho-Choshuenco). This study highlights how a multi-technique approach enables a complicated tephrostratigraphy to be unravelled so more robust estimates of the past eruptive frequency and size can be determined.

A retrospective analysis of the Shinmoedake (Japan) eruption of 26–27 January 2011 by means of Japanese geostationary satellite data

During the sub-plinian eruptions of Mt. Shinmoedake (Japan) on 26–27 January 2011 a significant amount of ash was emitted into the atmosphere, destroying thousands of hectares of farm land, causing air traffic disruption, and forcing the closure of four railroad lines located around the volcano. In this work, a retrospective analysis of these eruptive events is presented, exploiting the high temporal resolution of the Japanese Multi-functional Transport Satellites (MTSAT) data to study thermal volcanic activity, to identify and track volcanic ash, and to determine the cloud-top height, inferring information about eruption features and space-time evolution. We show that a strong and sudden increase in the thermal signal occurred at Mt. Shinmoedake as a consequence of above mentioned eruptive events, generating hot spots timely detected by the RSTVOLC algorithm for the first time implemented here on data provided by geostationary satellites. This study also shows that the emitted ash plume, identified by means of the RSTASH algorithm, strongly fluctuated in altitude, reaching a maximum height around 7.4km above sea level, in agreement with information provided by the Tokyo VAAC. The plume heights derived in this work, by implementing the widely accepted cloud-top temperature method, appear also compatible with the values provided by independent weather radar measurements, with the main differences characterizing the third sub-plinian event that occurred in the afternoon of 27 January. The estimates of discharge rate, the temporal trend of ash affected areas, and the results of thermal monitoring reported in this work seem to indicate that the third sub-plinian event was the least intense. In spite of some limitations, this study confirms the potential of Japanese geostationary satellites in effectively monitoring volcanoes located in the West Pacific region, providing continuous information also about such critical parameters of ash clouds as the plume height. Such information is useful not only for driving numerical models, forecasting ash dispersion into the atmosphere, but also for characterizing eruption features and dynamics.

The 2011 eruptive activity of Shinmoedake volcano, Kirishimayama, Kyushu, Japan—Overview of activity and Volcanic Alert Level of the Japan Meteorological Agency—

Following several years of small phreatic eruption events, Shinmoedake volcano in Kyushu, Japan, began a new phase of activity on 19 January 2011. The eruption commenced with a small phreatomagmatic event followed by subplinian events on 26 and 27 January. Lava emerged as a dome in the summit crater on 27 January and filled the crater until the beginning of February. During the subplinian events and lava dome growth, deflation of the deep magma chamber was observed by tiltmeters and GPS. Eruptive activity began to decrease in March 2011 and no eruptive event occurred after the 7 September 2011 event. In response to the eruption, the Japan Meteorological Agency (JMA) issued warnings repeatedly to prepare local people and infrastructures around Shinmoedake. JMA did not raise the Volcanic Alert Level before the first subplinian event, because of a lack of clear precursory signals. We conclude that a large amount of magma moved in the shallow Shinmoedake plumbing system for a short time, based on a number of geophysical observations. It is important to detect magma movement and understand eruptive phenomena quickly in order to mitigate risks of volcanic hazards.

The outline of the 2011 eruption at Shinmoe-dake (Kirishima), Japan

The climactic phase of the 2011 eruption at Shinmoe-dake was a mixture of subplinian and vulcanian eruptive events, successive lava accumulation (lava dome) within the crater, and repetition of vulcanian events after the dome growth. It was preceded by inflation and elevated seismicity for about one year and by phreatomagmatic explosions of one week before. Small pyroclastic flows and ash-cloud surges formed during the subplinian events, when the eruption column reached the highest level and the vent was widened. A lava dome, which was extruded close to the vent of subplinian events, grew by swelling upward and filling the crater. After the vent was covered by the lava, an intense vulcanian event occurred from the base of the dome and the swelled dome became deflated. After that, vulcanian events were repeated for three months. Simultaneous eruption styles in the crater (vulcanian events, continuous ash emission and dome growth) and some phreatomagmatic events in the vulcanian stage probably are due to a complex upper-conduit system developed in water-saturated country rock.

Syneruptive deep magma transfer and shallow magma remobilization during the 2011 eruption of Shinmoe-dake, Japan—Constraints from melt inclusions and phase equilibria experiments

The 2011 Shinmoe-dake eruption started with a phreatomagmatic eruption (Jan 19), followed by climax sub-Plinian events and subsequent explosions (Jan 26–28), lava accumulation in the crater (end of January), and vulcanian eruptions (February–April). We have studied a suite of ejecta to investigate the magmatic system beneath the volcano and remobilization processes in the silicic magma mush. Most of the ejecta, including brown and gray colored pumice clasts (Jan 26–28), ballistically ejected dense lava (Feb 1), and juvenile particles in ash from the phreatomagmatic and vulcanian events are magma mixing products (SiO2=57–58wt.%; 960–980°C). Mixing occurred between silicic andesite (SA) and basaltic andesite (BA) magmas at a fixed ratio (40%–30% SA and 60%–70% BA). The SA magma had SiO2=62–63wt.% and a temperature of 870°C, and contains 43vol.% phenocrysts of pyroxene, plagioclase, and Fe–Ti oxide. The BA magma had SiO2=55wt.% and a temperature of 1030°C, and contains 9vol.% phenocrysts of olivine and plagioclase. The SA magma partly erupted without mixing as white parts of pumices and juvenile particles.The two magmatic end-members crystallized at different depths, requiring the presence of two separate magma reservoirs; shallower SA reservoir and deeper BA reservoir. An experimental study reveals that the SA magma had been stored at a pressure of 125MPa, corresponding to a depth of 5km. The textures and forms of phenocrysts from the BA magma indicate rapid crystallization directly related to the 2011 eruptive activity. The wide range of H2O contents of olivine melt inclusions (5.5–1.6wt.%) indicates that rapid crystallization was induced by decompression, with olivine crystallization first (≤250MPa), followed by plagioclase addition. The limited occurrence of olivine melt inclusions trapped at depths of <5km is consistent with the proposed magma system model, because olivine crystallization ceased after magma mixing. Our petrological model is consistent with a geophysical model that explains whole crustal deformation as being due to a single source located 7–8km northwest of the Shinmoe-dake summit. However, even the shallowest estimated source of this deformation (7.5–6.2km) is deeper than the SA reservoir, which thus requires a contribution of deeper BA magmas to the observed deformation.Remobilization of mush-like SA magma occurred in two stages before the early sub-Plinian event. Firstly, precursor mixing with BA magma and associated heating occurred (925–871°C; stage-1 of ≥350h), followed by final mixing with BA magma (stage-2). MgO profiles of magnetite phenocrysts define timescales of 0.7–15.2h from this final mixing to eruption. The mixed and heated magmas, and stagnant mush that existed in the SA reservoir in the precursor stage, were finally erupted together. Magnetite phenocrysts in the Feb 18 ash reveal the occurrence of continuous erosion of the stagnant mush during the course of the 2011 eruptive activity.

Magma discharge variations during the 2011 eruptions of Shinmoe-dake volcano, Japan, revealed by geodetic and satellite observations

We present precise geodetic and satellite observation-based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of Kirishima-Shinmoe-dake volcano, Japan. During these events, the type and intensity of eruption drastically changed within a week, with three major sub-Plinian eruptions on January 26 and 27, and a continuous lava extrusion from January 29 to 31. In response to each eruptive event, borehole-type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface. These measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event. Erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater. Combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub-Plinian event. These precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics, and indicate that the Shinmoe-dake eruptions occurred in a critical state between explosive and effusive eruption.

Dynamics of explosive paroxysms at open-vent andesitic systems: High-resolution mass distribution analyses of the 2006 Tungurahua fall deposit (Ecuador)

Long-lasting andesitic eruptions sometimes include strong short-lived explosive events, which can pose significant hazards in populated regions. The origin and dynamics of such violent eruptions remain poorly known and may involve a combination of different factors. Tungurahua volcano, Ecuador, reawakens in 1999 and is an example of such an open-vent system that experienced a strong and deadly andesitic pyroclastic flow-forming event in August 2006. Inspection of the deposits suggested that the event could have been triggered by magma mixing (coexistence of both silicic pumices and andesitic scoria in the tephra), magma–water interaction (presence of xenolithic clasts) or deep andesitic magma reinjection (based on mineral chemistry). Here we investigate these options by performing a high-resolution mass budget analysis of the scoria fall deposit. This is achieved by analysing componentry compositions and their mass distribution pattern in the layer, which allow us to document and integrate exponential and power laws mass decay rates over wide areas. The results yield a total mass for the tephra layer of ∼2×1010kg. The pumice mass fraction is far too small (<0.4%) to account for the high explosivity of the 2006 event. Similarly, the xenoclastic mass fraction is unexpectedly small (0.2%) and suggests limited magma–water interaction. Instead, we interpret these xenoclasts as a result of upper conduit erosion at a rate of ∼30cm/h during the paroxysm. Altogether our results support an explosive event fed by a deep gas-rich andesitic reinjection, which would have incorporated a pocket of older differentiated magma and eroded the upper conduit during the sub-plinian event. The high-resolution mass-based approach reveals useful to decipher the origin of the violent 2006 paroxysm and has potential to improve magnitude determinations of ancient eruption by considering componentry mass instead of volume. It is also applicable for monitoring purposes in the context of ongoing crises at andesitic volcanoes worldwide

A tale of two magmas, Fuego, Guatemala

Fuego volcano in Guatemala erupted in 1974 in a basaltic sub-Plinian event, which has been well documented and studied. In 1999, after a period of quiescence lasting 20 years, Fuego erupted again, this time less violently, but with persistent low-level activity. This study investigates the link between these episodes. Previous melt inclusion studies have shown magma erupted in 1974 to have been a volatile-rich hybrid tapped from a vertically extensive system. By contrast, magma erupted in 1999 and 2003 is similar in composition to that erupted in 1974, but melt inclusions are more evolved. Although melt inclusions from the later period are CO2 rich (up to ∼1,500 ppm), they have low H2O concentration (max 1.5 wt.%, compared to ∼6 wt.% in 1974). These melt inclusions have a modified H2O concentration due to diffusive re-equilibration at shallow pressures. Despite this diffusive exchange, both eruptions show evidence of recent mingling of the same low and higher K melts, one of which was slightly cooler than the other and as a result traversed the amphibole stability field. (210Pb/226Ra) data on selected bulk rock samples from 1974 suggest that whereas the cooler, more evolved end-member may have been degassing since the last major eruption in the 1930s, the warmer end-member intruded at most a decade prior to the 1974 eruption. The two end-members are thus batches of the same magma emplaced shallowly ∼30 years apart during which time the older batch was cooled and differentiated before mixing with the younger influx. The presence of the same two melts in the later eruptions suggests that magma in 1999 and 2003 is partly residual from 1974. The current eruptive activity is clearing the system of this residual magma prior to an expected new magma batch.

The 512 AD eruption of Vesuvius: complex dynamics of a small scale subplinian event

We describe the products of the hitherto poorly known 512 AD eruption at Vesuvius, Italy. The deposit records a complex sequence of eruptive events, and it has been subdivided into eight main units, composed of stratified scoria lapilli or thin subordinate ash-rich layers. All the units formed by deposition from tephra fallout, pyroclastic density currents of limited extent being restricted to the initial stages of the eruption (U2). The main part of the deposit (U3 and U5) is characterized by a striking grain size alternation of fine to coarse lapilli, similar to that often described for mid-intensity, explosive eruptions. The erupted products have a phonotephritic composition, with progressively less evolved composition from the base to the top of the stratigraphic sequence. Based on different dispersal, sedimentological and textural features of the products, we identify five phases related to different eruptive styles: opening phase (U1, U2), subplinian phase (U3 to U5), pulsatory phreatomagmatic phase (U6), violent strombolian phase (U7) and final ash-dominated phase (U8). A DRE volume of 0.025 km3 has been calculated for the total fallout deposit. Most of the magma was erupted during the subplinian phase; lithic dispersal data indicate peak column heights of between 10 and 15 km, which correspond to a mass discharge rate (MDR) of 5 × 106 kg s−1. The lower intensity, violent strombolian phase coincided with the eruption of the least evolved magma; a peak column height of 6–9 km, corresponding to an MDR of 1 ×10 6 kg s −1, is estimated from field data. Phreatomagmatic activity played a minor role in the eruption, only contributing to the ash-rich deposits of U1, U4, U6 and U8. The two most striking features of the 512 AD eruption are the recurrent shifting of the eruption style and the pulsatory nature of the subplinian phase. Basing on a large set of observational data, we propose a model to explain this complex dynamics, also observed in other eruptions of similar scale from Vesuvius and elsewhere. The inbalance between the rates of magma supply and magma eruption may have caused the frequent changes in the eruptive style. Conversely, the high frequency oscillations of magma discharge recorded by the deposits of the subplinian phase were possibly related to cyclic instabilities in the permeability of the low viscosity magma column, which modulated magma fragmentation and discharge.