Vulcanian Research Articles

Eruptive Volume and Age of Large-Scale Pumice Eruptions of the Asama-Maekake Volcano, Central Japan, as Revealed by Detailed Trench Surveys: Implications for Future Volcanic Hazards

The eruptive volumes and ages of the large-scale pumice eruptions of the Asama-Maekake volcano were obtained from extensive trench-based surveys. The calibrated radiocarbon age unit (CRAU) was adopted instead of each calibrated radiocarbon age. From the high-resolution tephro-stratigraphy of large-scale pumice eruptions and CRAU dating, the eruptive history of the Asama-Maekake volcano is divided into three active and two moderately active stages as follows: active stage I (9430 to 7260 ycalBP), moderately active stage 1 (7261 to 6446 ycalBP), active stage II (6447 to 3160 ycalBP), moderately active stage 2 (3161 to 1819 ycalBP), and active stage III (1820 ycalBP to present). The eruptions of the Asama-Maekake volcano consists of small-scale (phreatic to phreato-magmatic), intermediate-scale (Vulcanian), and large-scale pumice eruptions (sub-Plinian). The active stages were characterized by the occurrence of large-scale pumice eruptions. In the moderately active stages, pumice producing eruptions were lacking but instead Vulcanian eruptions dominated. A step diagram showing the relationship between eruption volumes (DRE) and ages (CRAU) is proposed, which indicates that active stage III is not time-predictable but is volume-predictable; if the large-scale pumice eruption occurs in 2022 AD, the forecasted eruptive volume is approximately 0.21 km3. The eruption rate is not constant and changes in each stage, and the average eruption rate of active stage III (0.0011 km3/year) is larger than those of active stage I (0.00006 km3/year) and II (0.0001 km3/year).

The 2.6-2.3ka explosive eruptive period of the Pululahua dome complex, Ecuador: insights from pyroclast analysis.

Pululahua is an active volcano located 15 km north of Quito, Ecuador, that comprises sixteen dacitic-andesitic lava domes and a 13 km2 sub-rectangular depression formed between ~ 2.6 and ~ 2.3 ka. We use a detailed study of 70 flow and fall deposits that make up the pyroclastic sequence to show that the depression, previously classified as a caldera, was formed by numerous Vulcanian to (sub-) Plinian eruptions that destroyed both earlier and co-eruptive lava domes. We support this interpretation with field work, analysis of grain size distributions, density and components of 24 key deposits, supplemented by textural and petrologic analyses of 16 juvenile pyroclasts from throughout the pyroclastic sequence. These data document an alternation of (sub-) Plinian and Vulcanian eruptions dominated by denser juvenile material that preserves microtextural variations indicating changes in shallow level magma storage accompanying Vulcanian explosions. An exploratory examination of phenocryst textures and plagioclase and amphibole rim compositions suggests that much of the eruptive activity was driven by repeated inputs of less evolved magma into the Pululahua magmatic system. The inferred sequence of events provides a new hypothesis for the formation of the current morphology of Pululahua, including multiple episodes of both effusive and explosive eruptions accompanied by vent migration. Our findings offer an important insight into Pululahua’s potential future hazard scenarios, which could affect millions of people.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00445-022-01590-4.

Continuously Operable Simulator and Forecasting the Deposition of Volcanic Ash from Prolonged Eruptions at Sakurajima Volcano, Japan

At Sakurajima volcano, frequent Vulcanian eruptions have been seen at the summit crater of Minamidake since 1955. In addition to this eruption style, the eruptive activities of Strombolian type and prolonged ash emission also occur frequently. We studied the design of a simulator of advection-diffusion-fallout of volcanic ash emitted continuously. The time function of volcanic ash eruption rate is given by a linear combination of the volcanic tremor amplitude and the volume change of the pressure source obtained from the ground motion. The simulation is repeated using discretized values of the eruption rate time function at an iteration time interval of the simulation. The integrated value of the volcanic ash deposition on the ground obtained from each individual simulation is used to estimate the value of the ash fallout. The plume height is given by an empirical equation proportional to a quarter of the power of the eruption rate. Since the wind velocity field near the volcano is complicated by the influence of the volcanic topography, the predicted values published by meteorological organizations are made in high resolution by Weather Research and Forecasting (WRF) for the simulation. We confirmed that an individual simulation can be completed within a few minutes of iteration interval time, using the PUFF model as the Lagrangian method and FALL3D-8.0 as the Eulerian method on a general-purpose PC. Except during rainfall, the radar reflectivity, the count of particles per particle size, and fall velocity obtained by the disdrometers can be used for the quasi-real time matching of the plume height calculated from the eruption rate and the ash fall deposition rate obtained from the simulation.

Ultra and Very Long Period Seismic Signatures of Unsteady Eruptions Predicted From Conduit Flow Models

AbstractExplosive volcanic eruptions radiate seismic waves as a consequence of pressure and shear traction changes within the conduit/chamber system. Kinematic source inversions utilize these waves to determine equivalent seismic force and moment tensor sources, but relation to eruptive processes is often ambiguous and nonunique. In this work, we provide an alternative, forward modeling approach to calculate moment tensor and force equivalents of a model of eruptive conduit flow and chamber depressurization. We explain the equivalence of two seismic force descriptions, the first in terms of traction changes on conduit/chamber walls, and the second in terms of changes in magma momentum, weight, and momentum transfer to the atmosphere. Eruption onset is marked by a downward seismic force, associated with loss of restraining shear tractions from fragmentation. This is followed by a much larger upward seismic force from upward drag of ascending magma and reduction of magma weight remaining in the conduit/chamber system. The static force is upward, arising from weight reduction. We calculate synthetic seismograms to examine the expression of eruptive processes at different receiver distances. Filtering these synthetics to the frequency band typically resolved by broadband seismometers produces waveforms similar to very long period seismic events observed in strombolian and vulcanian eruptions. However, filtering heavily distorts waveforms, accentuating processes in early, unsteady parts of eruptions and eliminating information about longer (ultra long period time scale depressurization and weight changes that dominate unfiltered seismograms. Our workflow can be utilized to directly and quantitatively connect eruption models with seismic observations.

Explosive-effusive-explosive: The role of magma ascent rates and paths in modulating caldera eruptions

Abstract One of the biggest challenges in volcanology is assessing the role of magma properties (volatile budgets, storage depths, and ascent rates) in controlling eruption explosivity. We use a new approach based on apatite to estimate volatile contents and magma ascent rates from a sequence of sub-Plinian, effusive, and Vulcanian eruption deposits at Rabaul caldera (Papua New Guinea) emplaced in 2006 CE to probe the mechanisms responsible for the sudden transitions in eruption styles. Our findings show that all magmas were originally stored at similar conditions (2–4 km depth and 1.8–2.5 wt% H2O in the melt); only the magma that formed the lava flow stalled and degassed at a shallower level (0.2–1.5 km) for several months. A more energetic batch of magma rose from depth, bypassed the transient reservoir, and ascended within ≤8 h to Earth's surface (mean velocity ≥0.2 m/s), yielding the initial sub-Plinian phase of the eruption. The shallowly degassed magma was then able to reach the surface as a lava flow, likely through the path opened by the sub-Plinian magma. The magma of the last Vulcanian phase ascended without storage at a shallow depth, albeit more slowly (ascent rate 0.03–0.1 m/s) than the sub-Plinian magma. Our study illustrates how the complexity of plumbing systems may affect eruption styles, including at other volcanic systems, and have implications for interpreting volcano monitoring data.

Eruptive dynamics and fragmentation mechanisms during cyclic Vulcanian activity at Sakurajima volcano (Japan): Insights from ash texture analysis

Quantitative morpho-textural analysis of volcanic ash is one of the most effective tools to characterize the style of ash-dominated volcanic activity, and to investigate the complex interplay between conduit processes and the associated eruptive dynamics. In this framework, many questions still remain unanswered about the role of conduit processes, particularly of the magma fragmentation processes, in controlling the eruptive dynamics of the unsteady and highly-transient Vulcanian explosions. For this reason, we analyzed ash samples collected during a five days-long eruptive sequence at Sakurajima volcano (Japan), to derive information about ash morphometry and textural features over the entire sequence. During the observed sequence, eruptive activity showed high unsteadiness in the modalities of ash emission, which included all the main different eruptive styles typical of the recent period of Sakurajima activity. Three main intra-eruptive phases (Phase1, Phase2 and Phase3) were recognized based on visual observations and thermal data. Quantitative information about external ash morphometry (i.e., shape) and internal textures were measured for the particles associated with the different phases and discussed in terms of the observed eruptive variability. We quantified crystallinity and vesicularity of ash grains, the crystal size distribution (CSD) of microlites and the microlite number density (MND) of the groundmass for a representative set of ash particles. We discussed the links between the eruptive dynamics and the dominant processes of magma fragmentation, as shown by the combination of the morpho-textural features of ash throughout the whole eruptive sequence and the observed variations of the eruptive phases. All the evidence presented in this work confirm the constant presence at Sakurajima of a highly stratified and degassed magma within the conduit, suggesting that the transient dynamics of the eruption were strongly controlled by variations in the process of magma fragmentation driving the eruptions during the different phases. In particular, the morpho-textural characteristics of ash suggest that Vulcanian eruptions at Sakurajima can be controlled by the progressive pressurization of the upper portion of the magma conduit (between 10 and 50 m in depth). Moreover, using crystal textures, we inferred that the time needed for conduit refilling during intra-eruptive stages is comprised between 1 and 10 month. The resulting information on the eruptive dynamics of Sakurajima is of primary importance for a more exhaustive comprehension of the low-to-mid intensity, ash-dominated explosive activities.

Sequence of Volcanic Activity of Sakurajima Volcano, Japan, as Revealed by Non-Eruptive Deflation

Mechanisms that control fluctuation of long-term eruptive activity are not well understood. Since 1955, Sakurajima volcano has experienced repeated Vulcanian eruptions accompanied by deflation. However, ground deflations sometimes occurred even if no eruption took place. Characteristics of the non-eruptive deflation (NED) have not been investigated because of the related ambiguous ground vibration, infrasound and visual phenomena. In this study, we categorize deflation events contained in continuous deformation records covering eruptive activities at the Sakurajima volcano Minamidake crater during the period from November 2017 to December 2020. The deflation events are classified into eruptive and NED types by examining the seismic data, infrasound generations, and visual phenomena. Their event type occurrence characteristics are then further clarified based on their relationships to pressure source volume changes. Comparing the pressure source volume changes with the monthly mass of volcanic ash expelled from the crater, which is estimated from deposition data around the volcano, we found the contribution of NED events to volcanic ash mass eruption amounts to be much smaller than that for Vulcanian events, and visual observations confirm that NED events are phenomena that primarily emit volcanic gas. The NED event changes during the episode are interpreted by the magna transition from bubble-rich to bubble-poor conditions and are supported by the sulfur dioxide (SO2) discharge rate. The activity of NED events is helpful to understand long-term variations in eruptive activity.

Эксплозивная активность вулкана Чикурачки в январе–октябре 2022 г. (о. Парамушир, Северные Курильские острова)

Chikurachki volcano located in the southern part of Paramushir Island (Northern Kuriles) was in a state of increased activity from January to October 2022. According to satellite data and the results of visual observations, at least 5 episodes of explosive activity, lasting from 2 to 8 days: January 30 – February 03, June 23–24, June 30 – 01 July, August 21 – September 02, October 13–20 were recorded. All observed events were similar to previous volcanic eruptions in their nature and duration, among which weak and moderate vulcanian eruptions prevailed: both single emissions and series of explosions to a height of 2–5 km a.s.l., as well as periods of relatively calm ash and gas mixture emission of various intensity were observed. Ash plumes and clouds spread mainly in the east, south, southeast, northeast and southwest directions, their length reached 790 km, and the area was 25 408 km2 (15 October, 2022).

3D crystal size distributions of pyroxene nanolites from nano X-ray computed tomography: Improved correction of crystal size distributions from CSDCorrections for magma ascent dynamics in conduits

Abstract Groundmass crystals indicate syneruptive magmatic conditions, and thus their crystal size distributions (CSDs) are used to infer magma ascent histories. Three-dimensional (3D) CSDs are most commonly estimated from two-dimensional (2D) observations and plotted against long-axis length, L (hereafter referred to as “L-plot CSDs”). However, L-plot CSDs have two significant problems: the error because of the conversion from 2D to 3D and a lowered sensitivity to changes in the degree of effective undercooling (ΔTeff), which arises because a crystal’s growth rate varies with ΔTeff most strongly along its long dimension. Although these problems can result in false interpretations of magma ascent dynamics, there has been little discussion of the influence of the size criteria on CSDs. In this study, we investigated which 3D size criterion [i.e., long (L), intermediate (I), or short (S) axis length] is optimum for 2D-estimated CSDs of groundmass crystals from two perspectives: (1) conformity with the actual distributions, and (2) the sensitivity of CSD slopes to the magma ascent conditions in the conduit. We observed groundmass pyroxene crystals in pumice clasts from sub-Plinian and Vulcanian eruptive phases during the 2011 eruption of Shinmoedake (andesitic volcano, Japan) by using synchrotron radiation-based X-ray computed nanotomography (SR-XCT) and field-emission scanning electron microscopy (FE-SEM) and rein-vestigated the crystallization kinetics of pyroxene nanolites ranging in width from a few hundred nanometers to 1 µm. The SR-XCT observations provided the detailed 3D shapes and 3D CSDs (CT-CSDs) of these nanolites directly. The FE-SEM observations allowed us to estimate 3D aspect ratios (S:I:L) and CSDs (SEM-CSDs). L-plot SEM-CSDs, acquired using the program CSDCorrections, were used to calculate S-plot SEM-CSDs and I-plot SEM-CSDs. We compared the data from FE-SEM with those from SR-XCT to evaluate the accuracy of 3D aspect ratios and CSDs estimated from 2D data. The L-plot SEM-CSDs from the sub-Plinian pumice sample showed significant inconsistencies with the CTCSD, a result of the difficulty in estimating representative 3D aspect ratios from 2D observations for elongated groundmass crystals. In contrast, the S- and I-plot SEM-CSDs kept the effect of aspect ratio to a minimum and preserved their actual slopes, except for a vertical discrepancy between the CSDs. Moreover, the slopes of S- and I-plot CSDs of the nanolites differed more markedly between the two eruptive styles (by ~20% more) than those of L-plot CSDs. For estimating magma ascent dynamics, we propose that the optimum method for acquiring SEM-CSDs is to measure the cross-sectional widths of crystals and convert the resulting 2D data set into S-plot CSDs. Our new finding that the 3D shapes and CSDs of pyroxene nanolites differ according to eruptive style means that nanolites indicate distinct differences in ascent histories at the shallow conduit: increasing ΔTeff just before sub-Plinian eruptions and decreasing ΔTeff before Vulcanian eruptions. Given the similarity in CSDs of micro-lites, our results suggest that eruptive style was determined in the shallow conduit. Monitoring the condition of the shallow conduit may contribute to predicting the time evolution of eruptive activity.

Observations Show Charge Density of Volcanic Plumes is Higher Than Thunderstorms

AbstractWe analyze slow electric field change and lightning mapping measurements to provide insight into the characteristics of volcanic lightning and the associated implications on charging processes and the charge structure of a Vulcanian eruption plume. Data were obtained during a multi‐instrumental field campaign at Sakurajima volcano in 2015 when the Showa crater was active. We combine the electric field change and lightning mapping data from one explosive eruption on June 6, 2015 to identify individual flashes. From this, we interpret the flash type and polarity. In addition, the long‐time constant of the electric field change instrument allowed measurement of the quasi‐static field associated with charge separation in the eruption plume. We find that both intracloud and cloud‐to‐ground discharges occurred, and the polarity of cloud‐to‐ground discharges were all negative. The quasi‐static field measurement showed the plume carried a net negative charge. We calculate both the total charge transferred by cloud‐to‐ground discharges and the net charge density of the eruption plume. We find that cloud‐to‐ground discharges transfer an average of −0.41C per flash and the net charge density was −33C/. The percent error is at least 200%, due to uncertainty in the antenna gain. We show that these estimates are consistent with lightning that is 100 m in length. Further, the average flash rate during the first 8 s following the onset of eruption was five flashes per second. After that time, the flash rate abruptly decreased, which may be related to the end of gas‐thrust forcing.

Source models of long-period seismic events at Galeras volcano, Colombia

SUMMARY Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anticorrelated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit.

Using GIS to Create Hazard Maps and Assess Evacuation Routes around “The Gate to Hell”; Masaya Volcano, Nicaragua

Volcán Masaya in Nicaragua is made of a series of calderas and craters that lies 7 km from the city of Masaya with a population of over 100,000. Masaya is part of the Central American Volcanic Arc (CAVA) which contains hundreds of volcanoes. While many of the volcanoes of the arc produce small, quiescent eruptions, some are capable of large explosive events. The recent cycle of volcanism at Masaya began 7000 years ago. Initial eruptions were primarily lava, small ash flows, ashfalls, and degassing events which eventually gave way to climactic Vulcanian eruptions. The hazards presented from this system affect not only larger cities, but many underdeveloped communities surrounding Volcán Masaya. These communities require a simple map and action plan to use during an evacuation. While government-issued routes are in place, there are no known secondary options. The communities surrounding Masaya lack the education they need for having such a fierce geologic feature right in their backyard to base their own opinions on when and if to evacuate the area on their own accord. We visited the study area to gather data for the creation of an evacuation route and hazard map to help prepare the surrounding communities. By consolidating our research and field data, the maps we created give the communities surrounding Masaya more insight as to the behavior of the eruptions. This research gives the local population options for evacuation and more knowledge about their environment.

Mush remobilisation and mafic recharge: A study of the crystal cargo of the 2013–17 eruption at Volcán de Colima, Mexico

Volcán de Colima is a highly active stratovolcano at the western end of the Trans-Mexican Volcanic Belt. Present-day activity consists of lava dome growth and destruction cycles, lava flows, small explosions, and larger explosive Vulcanian eruptions. It has been postulated that an increased frequency of more mafic eruptions signals the run-up to the end of c. 100-year eruptive ‘cycles’, terminating with a Plinian eruption such as those in 1818 and 1913. It is therefore important to understand the role played by mafic recharge during interplinian activity. We present new petrological and geochemical data for lava and ash from the 2013–17 phase of eruption. The uniform paragenesis and geochemical homogeneity of bulk rocks indicate efficient long-term homogenisation of magmas within the plumbing system, similar to the previous 1998–2005 eruptive products. Mineral chemistry however preserves complex patterns of magma recharge and mixing. Chemical and textural information support the interpretation of two magmatic end-members – an evolved end-member, saturated with respect to Fe--Ti oxides and apatite and crystallising low-An plagioclase and pyroxenes in the Mg# 69–75 range; and a more primitive, mafic end-member, crystallising high-An plagioclase and pyroxenes in the Mg# 77–88 range. Pyroxene textures and zoning patterns suggest mixing of the mafic melts with the evolved magma and remobilisation of the crystal mush. Two-pyroxene geothermometry constrains magmatic temperatures to c. 980–1000 °C for the evolved end-member, and c. 1020–1080 °C for the mafic end-member. Pressure estimates suggest crystallisation at 4–6 kbar, or c. 12–18 km depth. We interpret this to reflect periodic injections of mafic melts and remobilised crystals into evolved reservoirs in a mushy magma storage system in the mid-crust, in agreement with geophysical data suggesting a semi-molten, partially crystallised body at this depth. An increase in reverse zoned crystals, indicative of mafic injection, from mid-2015 onwards suggests that these melts were injected into the system following the large eruption in July 2015. Our findings suggest that the intense July 2015 eruption may be linked to increased input of mafic magmas into the shallow system, indicating that mafic injections may be a key process governing the timing and style of interplinian eruption at Volcán de Colima.

Atmospheric vertical velocity - a crucial component in understanding proximal deposition of volcanic ash

The simulation of volcanic ash transport and deposition (VATD) for distances within a few kilometres from the vent (proximal region) is challenging owing to a combination of unresolved volcanogenic effects and the impact of the volcano's orography. Due to the urgency of calculations or sometimes lack of access to computational resources or expertise, atmospheric vertical velocity (w) is often underestimated in VATD modelling. The error associated with this underestimation has, however, never been properly quantified. Here, we use a weak vulcanian eruption that occurred at Sakurajima volcano on 1/10/2017 as a first step to addressing this limitation. We combine deposit characteristics observed by disdrometer measurements with high-resolution atmospheric and VATD modelling to validate and illustrate the differences in modelled trajectories when w is either ignored or accounted for. The Weather Research and Forecasting (WRF) model is used to model the orogenic effects down to a resolution of 50 m. Eulerian and Lagrangian VATD models (FALL3D and LagTrack, a newly-developed Matlab code, respectively) are used to describe the particle trajectories. Results confirm the importance of w in the case of low-altitude eruptions for capturing the complex, near-vent trajectory of ash particles: when neglected fall velocities were seen to differ up to 2–5 m s−1 depending on the particle size. Although the impact of w is most notable within 10 km from the vent, the forced sedimentation of low terminal velocity particles can have a significant secondary effect at larger distances.

Tephra4D: A Python-Based Model for High-Resolution Tephra Transport and Deposition Simulations—Applications at Sakurajima Volcano, Japan

Vulcanian eruptions (short-lived explosions consisting of a rising thermal) occur daily in volcanoes around the world. Such small-scale eruptions represent a challenge in numerical modeling due to local-scale effects, such as the volcano’s topography impact on atmospheric circulation and near-vent plume dynamics, that need to be accounted for. In an effort to improve the applicability of Tephra2, a commonly-used advection-diffusion model, in the case of vulcanian eruptions, a number of key modifications were carried out: (i) the ability to solve the equations over bending plume, (ii) temporally-evolving three-dimensional meteorological fields, (iii) the replacement of the particle diameter distribution with observed particle terminal velocity distribution which provides a simple way to account for the settling velocity variation due to particle shape and density. We verified the advantage of our modified model (Tephra4D) in the tephra dispersion from vulcanian eruptions by comparing the calculations and disdrometer observations of tephra sedimentation from four eruptions at Sakurajima volcano, Japan. The simulations of the eruptions show that Tephra4D is useful for eruptions in which small-scale movement contributes significantly to ash transport mainly due to the consideration for orographic winds in advection.

Sedimentary System of Ash Deposits from Long-Term Vulcanian Activity at Sakurajima Volcano, Japan

Ash deposits from short-lived vulcanian eruptions are ubiquitous at active volcanoes and are important for understanding the development of a volcanic edifice. In this study, we investigated the sedimentary system of ash deposits produced by long-term vulcanian activity from Showa crater of Sakurajima volcano, Japan, based on the grain size characteristics of ash from individual eruptions. We collected falling ash samples from 20 individual vulcanian eruptions between March 3 and 11, 2010. The grain size distributions of the ash samples were unimodal, bimodal, and trimodal, depending on the event. The median particle size of the main subpopulation decreases with distance from vent. The low standard deviation of the grain size of the main subpopulation implies deposition from transient volcanic plumes. For the finest subpopulation in the polymodal samples, the median grain size is very fine ash (<30 μm), indicating that this subpopulation was derived from ash aggregates. Overall, the sedimentary system of the long-term vulcanian activity at Sakurajima volcano is explained by accumulation of ash particles from repeated transient plumes and ash aggregation, and the latter controlled the behavior of the very fine ash.

Eruption of compositionally heterogeneous andesites from a complex storage region during the 2006 eruption of Augustine Volcano

Despite the common occurrence of heterogeneous andesitic eruptions, few studies have investigated the compositional effects on microlite crystallization and vesiculation in co-erupted natural samples. In 2006, Augustine Volcano erupted compositionally heterogeneous andesites that range from 56.4 to 63.3 wt% SiO2 and include two endmember lithologic groups: low-silica andesite (LSA) and high-silica andesite (HSA). Textural and compositional differences between LSA and HSA end members are explored for two discrete, sequential vulcanian explosions from January 17 (event 9) and 27 (event 10), 2006. Groundmass glass compositions of pyroclasts within LSA and HSA compositional suites are not colinear and do not correlate with plagioclase microlite crystallinities, indicating eruption from multiple isolated shallow magma reservoirs with various pressure-temperature pathways. HSA pyroclasts have lower crystallinities, 0.02–0.24, than most LSA pyroclasts, 0.16–0.39, demonstrating the influence of composition on crystallinity. Additionally, microlite textural and groundmass glass compositional differences exist between consecutive vulcanian explosions. The event 9 deposits have a typical bimodal density distribution and groundmass glass compositions range from 65 to 78 wt% SiO2. Plagioclase microlite number densities (MNV) are 104.6–6.4 mm−3 and crystallinities are 0.02 to 0.28, similar to products from other andesitic vulcanian eruptions. Deposits from the early phase of event 10 have a bimodal density distribution and contain a high proportion of LSA pyroclasts, similar to event 9. Groundmass glass compositions range from 72 to 79 wt% SiO2 and plagioclase MNV are 105.9–6.3 mm−3, forming narrower ranges compared to event 9. A transition occurred during the later phase of event 10 to a unimodal density distribution, a more homogeneous groundmass glass composition, 75–78 wt% SiO2, a higher proportion of HSA pyroclasts, and the highest MNV of 105.9–6.7 mm−3. We interpret this shift to reflect eruption from reservoir depths around 4–6 km and the cessation of pre-eruptive magma staging in the upper conduit, transitioning the eruption to continuous and effusive phases. Attention to heterogeneous andesitic erupted products reveals additional details about heterogeneous shallow magma storage, variable upper conduit magma staging, and a range of pressure-temperature paths prior to fragmentation.

Mechanism of the historical and the ongoing Vulcanian eruptions of Ebeko volcano, Northern Kuriles.

Ebeko is one of the most active volcanoes of the Kurile island arc, producing frequent mild Vulcanian explosions with eruption clouds up to 5 km high. The volcano poses a serious threat to the Severo-Kurilsk town with a population of around 2500 inhabitants, located at a distance of only 7 km on a fan of the volcano’s laharic deposits. Here, we report an overview of the activity of the volcano in the 20th–21st centuries and the results of our geological and petrological investigations of the ongoing Vulcanian eruption that started in 2016. We have found that eruptions of Ebeko span a range of mechanisms from purely magmatic to phreatic/hydrothermal. Three of its historical eruptions (the 1934–1935, 1987–1991, and the 2016–ongoing) involved fresh magma, while during the others (1967–1971, 2009–2011) fresh magma was not erupted. Juvenile material of the ongoing eruption represents highly crystalline and highly viscous (more than 108 pa s) low-silica (56–58 wt% SiO2) andesite. Historical data and our observations of the ongoing eruption allowed us to suggest a functional model of the volcano where Vulcanian explosions are caused by shallow intrusions of small diapir-like batches of strongly crystallized and highly viscous andesitic magma ascending into water-saturated, hydrothermally altered rocks composing the volcano summit. We suggest that the diapir’s ascent is governed by their positive buoyancy. Some of the diapirs reach and breach the ground surface producing magmatic eruptions of Ebeko, while the others are stuck at the shallow subsurface level and feed intensive hydrothermal activity as well as phreatic eruptions of the volcano. Positive buoyancy of the diapirs is too weak to allow them to extrude high above the ground surface to form lava domes.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00445-020-01426-z.

The summer 2019 basaltic Vulcanian eruptions (paroxysms) of Stromboli

Stromboli is an active, open conduit mafic volcano, whose persistent mild Strombolian activity is occasionally punctuated by much stronger explosions, known as paroxysms. During summer 2019, the volcano unexpectedly produced one such paroxysm on July 3, followed by intense explosive and intermittent effusive activity culminating in a second paroxysm on August 28. Visual observations and the analysis of the fall deposits associated with the two paroxysms allowed us to reconstruct ballistic exit velocities of up to 160 m s−1. Plume heights of ~ 8.4 km and 6.4 km estimated for the two events correspond to mass eruption rates of 1.1 × 106 kg s−1 and 3.6 × 105 kg s−1, respectively. This is certainly an underestimate as directional pyroclastic flows into which mass was partitioned immediately formed, triggering small tsunamis at the sea entrance. The mass of ballistic spatters and blocks erupted during the July 3 event formed a continuous cover at the summit of the volcano, with a mass calculated at ~ 1.4 × 108 kg. The distribution of fall deposits of both the July 3 and August 28 events suggests that pyroclasts characterized by terminal fall velocities < 10–20 m s−1 remained fully suspended within the convective region of the plume and did not fall at distances closer than ca 1700 m to the vent. Based on the impulsive, blast-like phenomenology of paroxysms as well as the deposit distribution and type, paroxysms are classified as basaltic Vulcanian in style. The evolution of the summer 2019 eruptive events was not properly captured within the framework of the alert level system which is focused on tsunamigenic processes, and this is discussed so as to provide elements for the implementation of the reference scenarios and an upgrade of the system to take into account such events. In particular we find that, although still largely unpredictable, at least at operational time scales, and not necessarily tsunamigenic, Vulcanian eruptions and the subsequent evolution of the eruptive phenomena should be considered for the alert level system. This serves as a warning to the implementation of alert systems where the unexpected needs to be taken into account, even at systems that are believed to be relatively “predictable” as is the case at many persistently active, open vent mafic systems.

Rheological change and degassing during a trachytic Vulcanian eruption at Kilian Volcano, Cha\xeene des Puys, France

Magma ascent during silicic dome-forming eruptions is characterized by significant changes in magma viscosity, permeability, and gas overpressure in the conduit. These changes depend on a set of parameters such as ascent rate, outgassing and crystallization efficiency, and magma viscosity, which in turn may influence the prevailing conditions for effusive versus explosive activity. Here, we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive-explosive transitions during Vulcanian phases of the 9.4 ka eruption of Kilian Volcano, Chaîne des Puys, France. Our results suggest that effusive activity at the onset of Vulcanian episodes at Kilian Volcano was promoted by (i) rapid ascent of initially crystal-poor and volatile-rich trachytic magma, (ii) a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization, and (iii) efficient degassing/outgassing in a crystal-rich magma at shallow depths. Trachytic magma repeatedly replenished the upper conduit, and variations in the amount of decompression and cooling caused vertical textural stratification, leading to variable degrees of crystallization and outgassing. Outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles, fractures, and tuffisite veins, fostering the formation of cristobalite in the carapace and talus regions. Build-up of overpressure was likely caused by closing of pore space (bubbles and fractures) in the dome through a combination of pore collapse, cristobalite formation, sintering in tuffisite veins, and limited pre-fragmentation coalescence in the dome or underlying hot vesicular magma. Sealing of the carapace may have caused a transition from open- to closed- system degassing and to renewed explosive activity. We generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity, rapid microlite growth, and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents. We show that trachytes may erupt with a similar style (e.g., Vulcanian) but at significantly higher ascent rates than their andesitic, dacitic, and rhyolitic counterparts. This suggests that the periodicity of effusive-explosive transitions at trachytic volcanoes may differ from that observed at the well-monitored andesitic, dacitic, and rhyolitic volcanoes, which has implications for hazard assessment associated with trachytic eruptions.