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Magma viscosity strongly controls the style (for example, explosive versus effusive) of a volcanic eruption and thus its hazard potential, but can only be measured during or after an eruption. The identification of precursors indicative of magma viscosity would enable forecasting of the eruption style and the scale of associated hazards1. The unanticipated May 2018 rift intrusion and eruption of Kīlauea Volcano, Hawai'i2 displayed exceptional chemical and thermal variability in erupted lavas, leading to unpredictable effusion rates and explosivity. Here, using an integrated analysis of seismicity and magma rheology, we show that the orientation of fault-plane solutions (which indicate a fault's orientation and sense of movement) for earthquakes preceding and accompanying the 2018 eruption indicate a 90-degree local stress-field rotation from background, a phenomenon previously observed only at high-viscosity eruptions3, and never before at Kīlauea4-8. Experimentally obtained viscosities for 2018 products and earlier lavas from the Pu'u 'Ō'ō vents tightly constrain the viscosity threshold required for local stress-field reorientation. We argue that rotated fault-plane solutions in earthquake swarms at Kīlauea and other volcanoes worldwide provide an early indication that unrest involves magma of heightened viscosity, and thus real-time monitoring of the orientations of fault-plane solutions could provide critical information about the style of an impending eruption. Furthermore, our results provide insight into the fundamental nature of coupled failure and flow in complex multiphase systems.

Brittle fragmentation, generating small pyroclasts from magma, is a key process determining eruptive style. How low-viscosity magma fragments within a rising fountain in a brittle manner, however, is not well understood. Here we describe a fragmentation process in Hawaiian fountains on the basis of observations from the 2018 lower East Rift Zone eruption of Kīlauea Volcano, Hawai’i. The dominant fragmentation mechanism is inertia driven and produces a population of large fluidal pyroclasts. However, when sufficient volcanic gas is released in the fountain, a subpopulation of smaller and more vesicular pyroclasts is generated and entrained into the gas-dominant convective plume. The size distribution of these pyroclasts is similar to that of brittlely fragmented solid materials. The erupted high-vesicularity pyroclasts sometimes preserve a deformed shape. These observations suggest that late-stage rapid expansion lowers the gas temperature adiabatically and cools the outer surface of liquid pyroclasts below the glass transition temperature. The rigid crust fragments as the hot interior attempts to expand due to further volatile diffusion from the melt into bubbles. Adiabatic expansion of volcanic gas occurs in all eruptions. Brittle fragmentation induced by rapid adiabatic cooling may be a widespread process, although of varying importance, in explosive eruptions. In a Hawaiian fountain eruption, rapid gas expansion cools the melt below the glass transition temperature and causes brittle magma fragmentation, producing small, vesicular pyroclasts, according to observations of the 2018 eruption of Kīlauea.

The 2018 summit and flank eruption of Kīlauea Volcano was one of the largest volcanic events in Hawaiʻi in 200 years. Data suggest that a backup in the magma plumbing system at the long-lived Puʻu ʻŌʻō eruption site caused widespread pressurization in the volcano, driving magma into the lower flank. The eruption evolved, and its impact expanded, as a sequence of cascading events, allowing relatively minor changes at Puʻu ʻŌʻō to cause major destruction and historic changes across the volcano. Eruption forecasting is inherently challenging in cascading scenarios where magmatic systems may prime gradually and trigger on small events.

Microlite crystallization in erupting basalt can occur in the conduit, in flight, or in situ after deposition. Distinguishing the products of primary versus secondary (post-fragmentation) crystallization can be challenging in near-vent environments, but is vital for interpreting shallow conduit conditions from pyroclast textures. Here, we examine pyroclasts of the 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand, to assess the roles of primary versus secondary crystallization of microlites. Lapilli and ash were selected from (a) an ultra-proximal section (T47)

The 2021 Fagradalsfjall basaltic eruption in Iceland was effusive, but a different eruptive scenario could have unfolded if its location had been shifted a few kilometres to the south to an offshore setting. Namely a shallow marine event similar to the phreatomagmatic stages of the 1211 CE Younger Stampar eruption. The 1211 CE eruption was the initial event of the 1211-1240 Reykjanes Fires and its first stage was a Surtseyan eruption just offshore of the point of Reykjanes. It constructed the ~0.006 km3 Vatnsfellsgígur tuff cone that featured a short-lived dry phase towards the end. A second phreatomagmatic stage took place ca. 500 m off the current Reykjanes coastline to produce the larger Karlsgígur tuff cone (~0.044 km3), with a combined cone/tephra volume of ~0.15 km3. Later, the activity migrated onshore onto a 4km-long fissure with an effusive eruption that generated the Yngri-Stampar crater row and associated lava flow fields. The Vatnsfellsgígur and Karlsgígur tuff cones consist of alternating pyroclastic surge-tephra fall units, intercalated with units formed by simultaneous deposition from surge and fall. The 3.5m-thick Vatnsfellsgígur section is composed of 8 units, whereas the 5.5m-thick Karlsgígur section consists of 9 units. Chemical analysis reveals that the cones are tholeiitic basalt (MgO 6.0-7.5 wt%) with sporadic olivine phenocrysts (Fo78 to Fo84) and dispersed plagioclase macrocrysts with core composition of An87 to An91. Two compositionally distinct groups of plagioclase-hosted melt inclusions are identified: one with composition comparable to the host magma and another more primitive in composition with lower FeO, TiO2 and K2O and higher MgO (ranging from 9-10 wt% and 9-11.5 wt% for Vatnsfellsgígur and Karlsgígur, respectively). This suggests that whilst upper crustal storage zones may have facilitated melt evolution, the erupting magma originated from a deeper, crystal-mush-dominated storage zone. Original and residual sulfur contents of ~2221.7 ± 150 ppm and ~966.2 ± 120 ppm respectively, indicate that ~0.658 ± 0.034 Tg of SO2 were released into the atmosphere during these two stages of phreatomagmatic activity. Moreover, vesicularity measurements on lapilli reveal unimodal, left-skewed vesicularity distributions with modes of 90% and 95% and a range of ~40% for Vatnsfellsgígur and Karlsgígur, respectively. These results indicate that magma had gone through vesicle nucleation to free growth and coalescence and probably initial dry (magmatic) fragmentation prior to contact with external water. The evidence strongly suggests that expansion of exsolved magmatic gases was the driver of explosivity and that the role of external water in these phreatomagmatic stages of the 1211 CE eruption was confined to secondary quench granulation. The analysed juvenile clasts also displayed sharp-bound domains of contrasting vesicularity with boundaries that cross-cut the clast margins. This confirms early mingling of melt batches with different histories of ascent and/or stalling in the shallow conduit. Given such heterogeneity, regions of contrasting vesicularity were analysed separately to construct two vesicle size and number distribution (VSD/VND) datasets. Results from the ongoing micro-textural and additional analysis of volatile degassing shall also be presented here.

Silicic submarine volcanic eruptions can produce large volumes of pumices that may rise buoyantly to the ocean surface and/or sink to the seafloor. For eruptions that release significant volumes of pumice into rafts, the proximal to medial submarine geologic record is thus depleted in large volumes of pumice that would have sedimented closer to source in any subaerial eruption. The 2012 eruption of Havre volcano, a submarine volcano in the Kermadec Arc, presents a unique opportunity to study the partitioning of well-constrained rafted and seafloor pumice. Macro- and microtextural analysis was performed on clasts from the Havre pumice raft and from coeval pumiceous seafloor units around the Havre caldera. The raft and seafloor clasts have indistinguishable macrotextures, componentry, and vesicularity ranges. Microtextural differences are apparent as raft pumices have higher vesicle number densities (109 cm−3 vs. 108 cm−3) and significantly lower pore space connectivity (0.3–0.95 vs. 0.9–1.0) than seafloor pumices. Porosity analysis shows that high vesicularity raft pumices required trapping of gas in the connected porosity to remain afloat, whereas lower vesicularity raft pumices could float just from gas within isolated porosity. Measurements of minimum vesicle throat openings further show that raft pumices have a larger proportion of small vesicle throats than seafloor pumices. Narrow throats increase gas trapping as a result of higher capillary pressures acting over gas–water interfaces between vesicles and lower capillary number inhibiting gas bubble escape. Differences in isolated porosity and pore throat distribution ultimately control whether pumices sink or float and thus whether pumice deposits are preserved or not on the seafloor.

Magma permeability allows release of exsolved and pressurized volatiles during magma ascent, potentially modulating explosive volcanic eruptions. While porosity-permeability relationships in the clasts from silicic Plinian eruptions have received considerable interests in recent years, knowledge on magma permeability during Plinian eruptions of basaltic magma is lacking. In this study, we investigate the porosity-permeability relationships in pyroclasts from the well-studied Plinian eruptions of basaltic magma at Mt. Tarawera, New Zealand (1886 CE) and Mt. Etna, Italy (122 BCE). We find that Darcian permeabilities in the studied clasts range between approximately 10−12 m2 and 10−10 m2 over a range of total porosity between 48 and 82%. The vesicles are well connected with values of 45–82% for connected porosity. Pyroclasts from the studied Plinian eruptions contain abundant microlites (∼ 60–90 vol. %) in the matrix surrounding vesicles. At a given total porosity, the measured permeabilities are somewhat higher than that measured in the crystal-poor lapilli from less explosive basaltic eruptions, and about 1–2 orders of magnitude higher than that in silicic Plinian clasts. The estimated percolation threshold is ∼34% for the two basaltic Plinian eruptions. Using modeling of ascent of multiphase magma through volcanic conduits during eruptions, we find that the relative timing of crystallization and the onset of percolation might have played a key role in the development of the measured porosity-permeability relationships. Using scaling analysis, we further show that rheological stiffening of basaltic melt due to a high crystal content just prior to magma fragmentation should have stabilized the vesicle networks, preserving textures reflecting the eruptive conditions with insignificant post-eruptive modification.

Most basaltic explosive eruptions intensify abruptly, allowing little time to document processes at the start of eruption. One opportunity came with the initiation of activity from fissure 8 (F8) during the 2018 eruption on the lower East Rift Zone of Klauea, Hawaii. F8 erupted in four episodes. We recorded 28 minutes of high-definition video during a 51-minute period, capturing the onset of the second episode on 5 May. From the videos we were able to analyze the following in-flight parameters: frequency and duration of explosions; ejecta heights; pyroclast exit velocities; in-flight total mass and estimated mass eruption rates; and the in-flight total grain size distributions. Videos record a transition from initial pulsating outgassing, via spaced, but increasingly rapid, discrete explosions, to quasi-sustained, unsteady fountaining. This transition accompanied waxing intensity (mass flux) of the F8 eruption. We infer that all activity was driven by a combination of the ascent of a coupled mixture of small bubbles and melt, and the buoyant rise of decoupled gas slugs and/or pockets. The balance between these two types of concurrent flow determined the exact form of the eruptive activity at any point in time, and changes to their relative contributions drove the transition we observed at early F8. Qualitative observations of other Hawaiian fountains at Klauea suggest that this physical model may apply more generally. This study demonstrates the value of in-flight parameters derived from high resolution videos, which offer a rapid and highly time-sensitive alternative to measurements based on sampling of deposits post-eruption.

International audience; Due to their generally low eruptive melt viscosities and concomitant high diffusivities of volatiles, basaltic magmas degas relatively efficiently. This relative efficiency, combined with variations in style, extent, timing and length scales of degassing govern the range of eruptive styles observed at basaltic volcanoes. The result is a surprising complexity of degassing regimes and products in basaltic volcanism. In particular, the transition between closed- and open-system degassing at low pressure at the percolation threshold may strongly affect the type of eruption. Here we aim to better understand degassing and gas percolation processes in basaltic magmas and their implications for eruptive style. Combining new and literature data, we present a database of vesicle metrics in basaltic rocks including vesicularity, vesicle number density, vesicle size distribution (and its polydispersivity), vesicle connectivity and permeability. We combine these textural and petrophysical data with a numerical model of percolation for systems having polydisperse vesicle size distributions. Using this model, we also evaluate different definitions of vesicle connectivity inherent to different measurement techniques. Our results show that polydispersivity exerts a strong control on the percolation threshold of basaltic magmas and consequently on eruptive style. Intermediate to highly polydisperse bubble networks are more typical of Hawaiian activity and are characterized by higher values of percolation threshold. This results in delayed coalescence and an increase in magma vesicularity hindering the formation of large decoupled and buoyant bubbles, which in turn can promote magma acceleration, fragmentation by inertia below the percolation threshold and sustained fountaining activity. Bubble populations with lower polydispersivity, typical of Strombolian eruptions, promote early coalescence prior to fragmentation, which in turn may lead to the formation of large decoupled slugs or gas pockets and/or plugs at the surface via outgassing. Further, we discuss the implications of our findings for Plinian, violent Strombolian, Surtseyan, deep submarine and effusive basaltic eruptions.

The unprecedented cost of the 2018 eruption in Hawai’i reflects an intersection of disparate physical and social phenomena: widely spaced, highly destructive eruptions, and atypically high population growth. These were linked and the former indirectly drove the latter with unavoidable consequences.

Connecting caldera collapse The Kīlauea Volcano on the island of Hawai‘i erupted for 3 months in 2018. Neal et al. present a summary of the eruption sequence along with a variety of geophysical observations collected by the Hawaiian Volcano Observatory. The cyclic inflation, deflation, and eventual collapse of the summit was tied to lava eruption from lower East Rift Zone fissures. A total volume of 0.8 cubic kilometers of magma erupted, roughly the equivalent of 320,000 swimming pools, which matched the change in volume at the summit. Science , this issue p. 367

Tephra from the early Hawaiian fountaining episodes of the ongoing eruption of Pu'u 'Ō'ō in the East Rift Zone (ERZ) of Kīlauea provides an opportunity to study the vesicle microtextures of pyroclasts erupted from a single vent over a prolonged period of time. We report the results of microtextural analysis of pyroclasts from five of Pu'u 'Ō'ō's high (>200 m) Hawaiian fountaining episodes (episodes 32, 37, 40, 44 and 45) erupted during 1985–1986. This analysis was carried out to constrain the parameters that led to large variations in fountain height at Pu'u 'Ō'o, and the extent to which pyroclast residence times in the fountain modified microtextures. Our results confirm the finding of Stovall et al., 2011 , Stovall et al., 2012 that pyroclasts from a single Hawaiian fountain can vary greatly in texture (from bubbly to foamy), and have vesicle volume densities (Nmv) and vesicle to melt ratios (VG/VL) that vary by an order of magnitude. This range in vesicle texture and population is due to extensive growth and coalescence of vesicles within the fountain after fragmentation. Only one pyroclast from four of five episodes was found to have textures interpreted as indicative of the vesicle population near the moment of fragmentation: bubbly texture, high density (typically >500 kg m−3), high Nmv (2.2 × 106 to 4.4 × 106), and low VG/VL of 2.06 to 4.65. We demonstrate a linear correlation between Δ(VG/VL) and peak fountain height across a range of Hawaiian fountains from Kilauea. This correlation could be used to infer peak heights of unobserved Hawaiian fountaining eruptions after further testing using well-recorded events.

Constraining the syn-eruptive volatile contents of magmatic melt is critical to understanding the intensities and styles of deep submarine volcanic eruptions, for which direct observations are scarce. Quantifying residual magmatic water contents in volcanic glass is complicated by rehydration, i.e., late-stage addition of molecular water. The 2012 deep submarine silicic eruption of Havre volcano provides an unusual opportunity to quantify glass water contents from a recent, well-sampled stratigraphic sequence. Fourier-transform infrared and microRaman spectroscopy measurements of water concentration and water speciation across the Havre 2012 eruptive sequence reveal an unanticipated range of excess molecular water within pumice. This excess water requires rapid timescales of diffusion that are inconsistent with our current understanding of low temperature secondary rehydration in both subaerial and subaqueous eruptive products. Diffusion models applied to enrichment profiles at vesicle edges confirm that low temperature rehydration is an unlikely cause. We instead support higher temperature, syn-eruptive pumice rehydration by condensed magmatic water and seawater in a submarine plume. Hydroxyl concentrations suggest shallow quenching depths of Havre pumice hundreds of meters above the 900-meter-deep main vent. Our data also support the presence of a vapor-rich plume and consequent modification of ocean pressure above the vent. We combine this novel volatile data with textural information and cooling rate calculations to explore the conditions that would cause slower, shallow cooling of clasts from a deep submarine eruption. By exploring the physical conditions for the interaction between pumice and submarine plumes, we emphasize fundamental differences between subaerial and submarine clast-producing eruptions.

The 1845 eruption is commonly viewed as a typical Hekla eruption. It is a key event in the eruptive history of the volcano, as it is one of the best documented Hekla eruptions, in terms of contemporary accounts and observations. The eruption started on 2 September 1845 with an intense, hour long explosive Plinian phase that passed into effusive activity, ending on the 16 March 1846. The amount of tephra produced in the opening phase was 0.13 km3/7.5 × 1010 kg. The total grain-size distribution of the deposit is bimodal with a dominant coarse mode at − 2.5 φ (5.6 mm) and a broad finer mode at 3 to 4.5 φ (0.125 to 0.045 mm). At individual sites, the grain-size distribution of the tephra from the Plinian opening phase is also commonly (not always) bimodal. Deconvolved grain-size distributions exhibit distinctly different sedimentation patterns of the coarse and fine subpopulations. The lapilli-dominated subpopulation fines rapidly with transport, while the ash-dominated subpopulation shows less changes with distance, indicating premature sedimentation of fines by aggregation from the 1845 volcanic plume. Tephra deposition was to the ESE of the volcano from a 19 km (a.s.l.) high eruption plume. The plume front travelled at speeds of 16–19 m s− 1. Reports of ash deposition onto ships near the Faroe and Shetland Islands, 700 to 1100 km away from Hekla, demonstrate that even moderate-sized Hekla eruptions can affect very large parts of European air-space.

The dynamic effects of temperature and strain rate on rheology of crystal-bearing magma are investigated. We conducted high-temperature rheometry experiments in both isothermal and dynamic crystallization regimes and recovered textural data for the isothermal runs. We propose a framework for the parameterization of magma rheology, via an equation describing how the rheological cutoff temperature (the temperature at which magma stops flowing) varies as a function of cooling rate and strain rate. This equation may be used to inform rapid response in effusive crises. Cooling rate has the larger effect, with higher cooling rates yielding lower cutoff temperatures; higher strain rates yield higher cutoff temperatures. Textural analyses reveal differences in crystal aspect ratios, such that higher cooling rates produce only subequant crystals, whereas lower cooling rates also produce a second, higher aspect ratio crystal population. We identify this textural variation as the physical cause for the dependence of cutoff temperature on cooling rate. Plain language summary As lava cools, it crystallizes. Eventually, this crystallinity becomes so high that the lava can no longer advance. The temperature at which the crystals “lock up” the lava is called the “rheological cutoff temperature.” This depends, in principle, on the crystallization pathway, which is influenced by both the cooling rate and the strain rate of the lava flow. We conducted rheological experiments on 2018 Kilauea lavas along different crystallization pathways. We determined that higher cooling rates (5 °C/min) yield cooler cutoffs (983–1058 °C), and higher strain rates (8 s−1) yield hotter cutoffs (1058–1093 °C). Moreover, the cooling rate affects the cutoff temperature more than the strain rate. Through a complementary set of experiments, we found that the physical cause for the dependence of the cutoff temperature on the cooling rate is crystal aspect ratio (length/width). At any given crystallinity, crystals with a higher aspect ratio interact more, and lock at higher temperatures. Higher cooling rates produce only crystals with an aspect ratio of 1, whereas lower cooling rates produce higher aspect ratio crystals as well. Therefore, lava which has cooled more slowly, which crystallizes higher aspect ratio crystals, has a higher rheological cutoff temperature.

Plinian lapilli from the 1060 Common Era Glass Mountain rhyolitic eruption of Medicine Lake Volcano, California, were collected and analyzed for vesicularity and permeability. A subset of the samples were deformed at a temperature of 975°, under shear and normal stress, and postdeformation porosities and permeabilities were measured. Almost all undeformed samples fall within a narrow range of vesicularity (0.7–0.9), encompassing permeabilities between approximately 10−15 m2 and 10−10 m2. A percolation threshold of approximately 0.7 is required to fit the data by a power law, whereas a percolation threshold of approximately 0.5 is estimated by fitting connected and total vesicularity using percolation modeling. The Glass Mountain samples completely overlap with a range of explosively erupted silicic samples, and it remains unclear whether the erupting magmas became permeable at porosities of approximately 0.7 or at lower values. Sample deformation resulted in compaction and vesicle connectivity either increased or decreased. At small strains permeability of some samples increased, but at higher strains permeability decreased. Samples remain permeable down to vesicularities of less than 0.2, consistent with a potential hysteresis in permeability-porosity between expansion (vesiculation) and compaction (outgassing). We attribute this to retention of vesicle interconnectivity, albeit at reduced vesicle size, as well as bubble coalescence during shear deformation. We provide an equation that approximates the change in permeability during compaction. Based on a comparison with data from effusively erupted silicic samples, we propose that this equation can be used to model the change in permeability during compaction of effusively erupting magmas.

We describe the discrete ballistic and wind-advected products of a small, but exceptionally well characterized, explosive eruption of wall-rock-derived pyroclasts from Kīlauea volcano on 19 March 2008, and, for the first time, integrate the size distribution of the two subpopulations, to reconstruct the true size distribution of a population of pyroclasts as it exited from the vent. Based on thinning and fining relationships, the wind-advected fraction had a mass of 6.1 × 105 kg and a thickness half-distance of 110 m, placing it at the bottom end of the magnitude and intensity spectra of pyroclastic falls. The ballistic population was mapped, in the field and using structure-from-motion techniques, to a diameter >10–20 cm over an area of ~0.1 km2, with an estimated mass of 1× 105 kg. Initial ejection velocities of 50–80 m/s were estimated from inversion of isopleths. The total grainsize distribution was estimated using a mass partitioning of 98% of wind-advected material and 2% of ballistics, resulting in median and sorting values of -1.7ϕ and 3.1ϕ. It is markedly broader than those calculated for the products of magmatic explosive eruptions, because the grainsize of 19 March 2008 clast population is unrelated to a volcanic fragmentation event and instead was ‘inherited’ from a population of talus clasts that temporary blocked the vent prior to the eruption. Despite a conspicuous near-field presence, the ballistic subpopulation has only a minor influence on the grainsize distribution because of its rapid thinning and fining away from source.

Meter-scale vesicular blocks, termed “giant pumice,” are characteristic primary products of many subaqueous silicic eruptions. The size of giant pumices allows us to describe meter-scale variations in textures and geochemistry with implications for shearing processes, ascent dynamics, and thermal histories within submarine conduits prior to eruption. The submarine eruption of Havre volcano, Kermadec Arc, in 2012, produced at least 0.1 km3 of rhyolitic giant pumice from a single 900-m-deep vent, with blocks up to 10 m in size transported to at least 6 km from source. We sampled and analyzed 29 giant pumices from the 2012 Havre eruption. Geochemical analyses of whole rock and matrix glass show no evidence for geochemical heterogeneities in parental magma; any textural variations can be attributed to crystallization of phenocrysts and microlites, and degassing. Extensive growth of microlites occurred near conduit walls where magma was then mingled with ascending microlite-poor, low viscosity rhyolite. Meter- to micron-scale textural analyses of giant pumices identify diversity throughout an individual block and between the exteriors of individual blocks. We identify evidence for post-disruption vesicle growth during pumice ascent in the water column above the submarine vent. A 2D cumulative strain model with a flared, shallow conduit may explain observed vesicularity contrasts (elongate tube vesicles vs spherical vesicles). Low vesicle number densities in these pumices from this high-intensity silicic eruption demonstrate the effect of hydrostatic pressure above a deep submarine vent in suppressing rapid late-stage bubble nucleation and inhibiting explosive fragmentation in the shallow conduit.

During the four month-long 2018 Kilauea Lower East Rift Zone (LERZ) eruption, the bulk chemical compositions of magma ranged from basalt to andesite. This compositional variety was reflected in eruptive style, which ranged from Hawaiian fountaining to Strombolian explosions. Here, we quantified the evolution of the melt viscosity of the eruptive products through high-temperature laboratory experiments performed on a representative sample set that was collected in the field immediately after the eruptive series. This suite of 18 samples comprises all major eruptive phases (early phase I, late phase I, phase II, phase III, fissure 17). The results illustrate the significant rheological variability of the eruptive products, and appear to link to variations in eruption dynamics. We propose a new standard for the rheological study of a multi-episode effusive eruption, whereby precise, near-real-time viscosity results are obtained during ongoing eruptions will become a routine component of volcano monitoring during future eruptive events. Plain language summary During the 2018 eruption of Kilauea, emerging magma spanned a wider compositional range than ever previously observed during a single eruption. This compositional diversity was matched by a variety in eruptive styles, which ranged from more persistent fountaining to short-lived explosions. Immediately after the eruption ceased, we collected a representative suite of 18 samples in the field, which comprises all major eruptive phases (early phase I, late phase I, phase II, phase III, fissure 17). We measured the melt viscosity of such samples through high-temperature laboratory experiments. The results illustrate a significant variability in viscosity, which is linked to the highly variable eruption dynamics. Here we propose a new standard for the study of multi-episode effusive eruptions from a viscosity standpoint. We hope and expect that this methodology will become routine practice during future eruption.

Strombolian eruptions are among the most common subaerial styles of explosive volcanism worldwide. Distinctive features of each volcano lead to a correspondingly wide range of variations of magnitude and erupted products, but most papers focus on a single type of event at a single volcano. Here, in order to emphasize the common features underlying this diversity of styles, we scrutinize a database from 35 different erupting vents, including 21 thermal infrared videos from Stromboli (Italy), Etna (Italy), Yasur (Vanuatu), and Batu Tara (Indonesia), from puffing, through rapid explosions to normal explosions, with variable ejection parameters and relative abundance of gas, ash, and bombs. Using field observations and high-speed thermal infrared videos processed by a new algorithm, we identify the distinguishing characteristics of each type of activity and how they may relate and interact. In particular, we record that ash-poor normal explosions may be preceded and followed by the onset or the increase of the puffing activity, while ash-rich explosions are emergent, i.e., with inflation of the free surface followed directly by emission of increasingly large gas pockets. Overall, we see that all Strombolian activities form a continuum arising from a common mechanism and are modulated by the combination of two well-established controls: (1) the length of the bursting gas pocket with respect to the vent diameter and (2) the presence and thickness of a high-viscosity layer in the uppermost part of the volcanic conduit.

Imaging, in general, and high speed imaging in particular are important emerging tools for the study of explosive volcanic eruptions. However, traditional 2-D video observations cannot measure volcanic ejecta motion toward and away from the camera, strongly hindering our capability to fully determine crucial hazard-related parameters such as explosion directionality and pyroclasts' absolute velocity. In this paper, we use up to three synchronized high-speed cameras to reconstruct pyroclasts trajectories in three dimensions. Classical stereographic techniques are adapted to overcome the difficult observation conditions of active volcanic vents, including the large number of overlapping pyroclasts which may change shape in flight, variable lighting and clouding conditions, and lack of direct access to the target. In particular, we use a laser rangefinder to measure the geometry of the filming setup and manually track pyroclasts on the videos. This method reduces uncertainties to 10° in azimuth and dip angle of the pyroclasts, and down to 20% in the absolute velocity estimation. We demonstrate the potential of this approach by three examples: the development of an explosion at Stromboli, a bubble burst at Halema'uma'u lava lake, and an in-flight collision between two bombs at Stromboli.

International audience; The 1959 eruption of Kīlauea Iki on the Island of Hawai'i is a principal example of powerful Hawaiian fountaining. Over 36 days (including repose periods), 16 fountaining episodes created a small cone, a downwind tephra blanket of approximately 0.003 km3 and a lava lake of about 0.04 km3 volume. During the explosive activity, the maximum fountain heights reached 600 m. Based on a dataset of more than 450 tephra grain size samples, we present both a total grain size distribution (TGSD) of the entire downwind tephra deposit, and also TGSDs for two eruptive subunits (the opening and the closing stages). The opening stage was characterized by persistent fountaining over a period of 8 days with fountain heights averaging ∼ 100 m; in contrast, the closing stage was characterized by two short (hours-long) but powerful fountaining episodes (up to 600 m). The significantly different fountaining intensities are reflected in the characteristics of the TGSDs. For the closing stages, we link bimodality of TGSDs to periods of simultaneous deposition of ballistics and fallout from the convective cloud, both of which are a function of the maximum fountain height. The 1959 Kīlauea Iki case study presents a well-constrained set of TGSD data linked with Hawaiian-style fountaining of two contrasting intensities and can be used as a valuable reference point for eruption source parameters in future modeling of pyroclast dispersal during Hawaiian fountaining eruptions.

We present the evolution over 3 months of a 2016–2017 pāhoehoe flow at Kīlauea as it changed from a narrow sheet flow into a compound lava field fed by a stable system of tubes. The portion of the flow located on Kīlauea's coastal plain was characterized using helicopter-based visible and thermal structure-from-motion photogrammetry to construct a series of georeferenced digital surface models and thermal maps on eight different days. Results reveal key influences on the emplacement and evolution of such long-lived pāhoehoe flows. This region of the flow grew by ~12 × 106 m3 with a near-constant time-average discharge rate of 1.2–2.7 m3/s. The development of two tube systems is captured and shows an initial nascent tube enhanced by a narrow topographic confinement, which later inflated and created a topographic inversion that modulated the emplacement of a second flow lobe with its own tube system. The analysis of breakouts at various stages of the field's life suggests that the evolution of the thermal and morphological properties of the flow surface reflect its maturity. Thermal properties of breakouts were used to expand the empirical relationship of breakout cooling to longer timescales. This study contributes to the long-term development and validation of more accurate predictive models for pāhoehoe, required during the management of long-lasting lava flow crises in Hawai'i and elsewhere. Published version

This paper describes the step-by-step process of characterizing tephra-fall deposits based on isopach, isomass and isopleth maps as well as thickness transects at different distances from their source. It covers the most frequently used empirical methods of integration (i.e., exponential, power–law and Weibull) and provides a description of the key physical parameters that can be retrieved from tephra-fall deposits. To streamline this process, a Matlab function called TephraFits is proposed, which is highly customizable and also guides the interpretation of the results. The function calculates parameters such as the deposit volume/mass, the VEI/magnitude, and the rates of thickness–decay away from the source and assists in eruption classification using deposit–based schemes. The function also contains a stochastic mode that can be used to propagate the uncertainty from field data to the quantification of eruption source parameters. The use of this function is demonstrated using the the 1180 ±80 years B.P. andesitic subplinian/Plinian tephra deposit Layer 5 of Cotopaxi volcano, Ecuador. In addition, we constrain the often delicate choice of the distal integration limit of the power–law method from synthetic deposits produced with the advection–diffusion model Tephra2. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

In this work the morphologic features of Pele’s hair formed during three different eruptions of Kilauea volcano have been investigated: fountaining from Kilauea Iki’s 1959 Episode 1, weak explosive activity from Halemaumau lava lake and littoral explosions at Waikupanaha (2009). Morphological studies were performed by optical, stereo- and scanning electron microscopy. For the first time 3D image analysis was carried out by synchrotron radiation X-ray computed microtomography, which allowed a high-resolution 3D reconstruction of the internal structure of each Pele’s hair, highlighting several differences in terms of number density, elongation and shape of the vesicles between the samples from the three eruptions. We identified three main parameters determining these differences: initial size of the magma droplet, ejection velocity and magma viscosity. Pele’s hair erupted during the Kilauea Iki’s fountaining shows the highest thickness and the least elongated shape of the vesicles, though it is related to fast ejection of a low viscosity magma. We therefore suggest that the size of magma droplets is the main parameter influencing the morphology and inner textures of the Pele’s hair. The comparison with Pele’s hair of similar eruptions elsewhere demonstrates that there is no univocal correspondence between eruptive style and Pele’s hair texture.

The 1959 Kīlauea Iki eruption on Hawai’i generated a succession of fountains, most reaching hundreds of meters high. The 16 episodes of fountaining persisted intermittently (with repose periods of between 7 h and 4 days) for 36 days. They produced tephra deposits that were dispersed several kilometers downwind of the vent, and a much larger volume of clastogenic lava which drained into the Kīlauea Iki crater to form a > 100 m deep lava lake. Field data from 211 tephra sample pits downwind of the vent reflect imperfectly the episodic nature of the fountaining behavior: only five composite stratigraphic subunits from the total of 16 fountaining episodes can be mapped in the field. However, isopach maps of these subunits were generated and, by the application of empirical deposit thinning relationships, volumes of each subunit were estimated. In combination with detailed observations made in 1959, our field data allow us to assign stratigraphic subunits to either single or aggregates of several fountaining episodes. The most voluminous subunits are linked with the highest fountaining, not with the longest in duration. In fact, significant downwind dispersal was possible only if the top part of the fountains reached above the crater rim, which was located about 100 m above the vent. The end of high fountaining episodes produced ash-rich partings, which are found on top of the lapilli-sized products of some episodes. However, the ash-rich intervals are strongly attenuated by wind advection and so are only present along the dispersal axes. The subunits follow slightly different dispersal axes. The maximum spread of 28 degrees between various dispersal axes is predominantly controlled by changing wind directions and spatter cone collapse into the vent, which modified its geometry. This study outlines the dependency of tephra dispersal on fountain height and emphasizes the capability of an episodic Hawaiian fountaining eruption to generate a seemingly monotonous downwind tephra deposit.

During the climactic Plinian phase of the 1886 basaltic eruption of Tarawera, New Zealand, vents along the 17 km fissure erupted explosively with a wide range of dispersal. The 8 km long segment of the fissure which cuts across Mt Tarawera contains approximately 50 vents and includes the sources of both the weakest and most intense activity of the 5 h eruption. We seek to explain (1) what allowed the intensity to reach Plinian values that are rarely achieved by basaltic magma, and (2) what caused adjacent vents to erupt with very different dispersals and intensities despite identical magma composition. AU juvenile clasts stuthed from this eruption have relatively high vesicle number densities (c. 106cm-3) and exceptionally high microlite crystallinities (60-90% of the groundmass), unlike the typical products of weaker Hawaiian and Strombolian basaltic explosions. Textural analysis of juvenile pyroclasts suggests that all the erupted magma experienced the same decompression history through to fragmentation. The Tarawera magma experienced a sudden, large, decompression producing nucleation of bubbles and microlites. The high microlite content was the primary means by which the magma's viscosity increased, which kept the bubble population well coupled to the magma and allowed it to fragment explosively in a manner analogous to that postulated for silicic Plinian eruptions. The main differences at different sites along the Mt Tarawera fissure segment are in the amount and grain size of the wall rock lithic component of the deposits. We suggest that conduit/vent erosion and incorporation of significant volumes of cold wall rock into the eruptive jet prevented some vents from achieving Plinian intensity. Bubble size analysis suggests that coalescence led to open-system degassing, ending the Plinian phase. © IAVCEI2009.

© 2018 Elsevier B.V. A long-standing conceptual model for deep submarine eruptions is that high hydrostatic pressure hinders degassing and acceleration, and suppresses magma fragmentation. The 2012 submarine rhyolite eruption of Havre volcano in the Kermadec arc provided constraints on critical parameters to quantitatively test these concepts. This eruption produced a >1 km3raft of floating pumice and a 0.1 km3field of giant (>1 m) pumice clasts distributed down-current from the vent. We address the mechanism of creating these clasts using a model for magma ascent in a conduit. We use water ingestion experiments to address why some clasts float and others sink. We show that at the eruption depth of 900 m, the melt retained enough dissolved water, and hence had a low enough viscosity, that strain-rates were too low to cause brittle fragmentation in the conduit, despite mass discharge rates similar to Plinian eruptions on land. There was still, however, enough exsolved vapor at the vent depth to make the magma buoyant relative to seawater. Buoyant magma was thus extruded into the ocean where it rose, quenched, and fragmented to produce clasts up to several meters in diameter. We show that these large clasts would have floated to the sea surface within minutes, where air could enter pore space, and the fate of clasts is then controlled by the ability to trap gas within their pore space. We show that clasts from the raft retain enough gas to remain afloat whereas fragments from giant pumice collected from the seafloor ingest more water and sink. The pumice raft and the giant pumice seafloor deposit were thus produced during a clast-generating effusive submarine eruption, where fragmentation occurred above the vent, and the subsequent fate of clasts was controlled by their ability to ingest water.

The weakest explosive volcanic eruptions globally, Strombolian explosions and Hawaiian fountaining, are also the most common. Yet, despite over a hundred years of observations, no classifications have offered a convincing, quantitative way of demarcating these two styles. New observations show that the two styles are distinct in their eruptive time scale, with the duration of Hawaiian fountaining exceeding Strombolian explosions by ∼300–10,000 s. This reflects the underlying process of whether shallow-exsolved gas remains trapped in the erupting magma or is decoupled from it. We propose here a classification scheme based on the duration of events (brief explosions versus prolonged fountains) with a cutoff at 300 s that separates transient Strombolian explosions from sustained Hawaiian fountains.

A submersible study of the products of a large submarine eruption demonstrates the influence of the ocean on eruption dynamics., The 2012 submarine eruption of Havre volcano in the Kermadec arc, New Zealand, is the largest deep-ocean eruption in history and one of very few recorded submarine eruptions involving rhyolite magma. It was recognized from a gigantic 400-km2 pumice raft seen in satellite imagery, but the complexity of this event was concealed beneath the sea surface. Mapping, observations, and sampling by submersibles have provided an exceptionally high fidelity record of the seafloor products, which included lava sourced from 14 vents at water depths of 900 to 1220 m, and fragmental deposits including giant pumice clasts up to 9 m in diameter. Most (>75%) of the total erupted volume was partitioned into the pumice raft and transported far from the volcano. The geological record on submarine volcanic edifices in volcanic arcs does not faithfully archive eruption size or magma production.

The size distribution of the population of particles injected into the atmosphere during a volcanic explosive eruption, i.e., the total grain-size distribution (TGSD), can provide important insights into fragmentation efficiency and is a fundamental source parameter for models of tephra dispersal and sedimentation. Recent volcanic crisis (e.g. Eyjafjallajökull 2010, Iceland and Córdon Caulle 2011, Chile) and the ensuing economic losses, highlighted the need for a better constraint of eruption source parameters to be used in real-time forecasting of ash dispersal (e.g., mass eruption rate, plume height, particle features), with a special focus on the scarcity of published TGSD in the scientific literature. Here we present TGSD data associated with Hekla volcano, which has been very active in the last few thousands of years and is located on critical aviation routes. In particular, we have reconstructed the TGSD of the initial subplinian–Plinian phases of four historical eruptions, covering a range of magma composition (andesite to rhyolite), eruption intensity (VEI 4 to 5), and erupted volume (0.2 to 1 km3). All four eruptions have bimodal TGSDs with mass fraction of fine ash (

On March 19, 2008 a small explosive event accompanied the opening of a 35-m-wide vent (Overlook vent) on the southeast wall of Halema‘uma‘u Crater in Kīlauea Caldera, initiating an eruptive period that extends to the time of writing. The peak of activity, in 2008, consisted of alternating background open-system outgassing and spattering punctuated by sudden, short-lived weak explosions, triggered by collapses of the walls of the vent and conduit. Near-daily sampling of the tephra from this open system, along with exceptionally detailed observations, allow us to study the dynamics of the activity during two eruptive sequences in late 2008. Each sequence includes background activity preceding and following one or more explosions in September and October 2008 respectively. Componentry analyses were performed for daily samples to characterise the diversity of the ejecta. Nine categories of pyroclasts were identified in all the samples, including wall-rock fragments. The six categories of juvenile clasts can be grouped in three classes based on vesicularity: (1) poorly, (2) uniformly highly to extremely, and (3) heterogeneously highly vesicular. The wall-rock and juvenile clasts show dissimilar grainsize distributions, reflecting different fragmentation mechanisms. The wall-rock particles formed by failure of the vent and conduit walls above the magma free surface and were then passively entrained in the eruptive plume. The juvenile componentry reveals consistent contrasts in degassing and fragmentation processes before, during and after the explosive events. We infer a crude ‘layering’ developed in the shallow melt, in terms of both rheology and bubble and volatile contents, beneath a convecting free surface during background activity. A tens-of-centimetres thick viscoelastic surface layer was effectively outgassed and relatively cool, while at depths of less than 100 m, the melt remained slightly supersaturated in volatiles and actively vesiculating. Decoupled metre-sized bubbles rising through the column burst through the free surface frequently, ejecting fragments of the outgassed upper layer. When the surface was abruptly perturbed by the rock-falls, existing mm-sized bubbles expanded, leading to the acceleration of adjacent melt upward and consecutive explosions, while renewed nucleation created a minor population of 10-micron-sized bubbles. After each explosive event in September–October 2008, this layering was re-established but with decreasing vigour, suggesting that the magma batch as a whole was becoming progressively depleted in dissolved volatiles.

Hawaiian basaltic eruptions commonly initiate as a fissure, producing fountains, spattering, and clastogenic lava flows. Most fissures rapidly localize to form a small number of eruptive vents, the location of which may influence the subsequent distribution of lava flows and associated hazards. We present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of Mauna Ulu, Kīlauea, Hawai‘i. Exceptional preservation of the deposits allows us to reconstruct vent-proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization. Through detailed field mapping, including measurements of the height and internal depth of lava tree moulds, we reconstruct high-resolution topographic maps of the pre-eruption ground surface, the lava high-stand surface and the post-eruption ground surface. We calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure. Aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface. By comparing with the inundation and drainage maps, we find that fissure segments that were inundated with lava to greater depths (typically 1–6 m) during the eruption later became foci of lava drainage back into the fissure (internal drain-back). We infer that, in these areas, lava ponding over the fissure suppressed discharge of magma, thereby favouring drain-back and stagnation. By contrast, segments with relatively shallow inundation (typically less than ~ 1 m), such as where the fissure intersects pre-eruptive topographic highs, or where flow away from the vent (outflow) was efficient, are often associated with sub-circular vent geometries in the post-eruption ground surface. We infer that these parts of the fissure became localization points for ongoing magma ascent and discharge. We conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity.

The 1991 Hekla eruption started on 17th of January with an intense 50-min-long explosive phase that transitioned into fire fountain activity lasting for 2 days. The eruptive plume rose to maximum height in about 10 min and the total mass of tephra deposited from the opening phase was 8.6 × 109 kg (VEI 3 event). The principal axis of tephra fall is to the NNE of Hekla and grain-size analysis reveals a systematic decrease in grain-size away from source. Majority of sample sites show typically unimodal grain-size distributions, although a few have bimodal distributions where the secondary mode is a subtle finer peak. The calculated total grain-size distribution (TGSD) is bimodal, with a coarse primary peak (−3.5 to −2.5 φ) and a subordinate fine peak (2.5 to 3.5 φ). The coarse peak is lapilli-dominated and was deposited within the first 25 km of transport, whereas the fine peak is coarse-ash-dominated and fits well with the modal grain-size of samples deposited >65 km from Hekla. Ascent rate of the magma and conditions for vesiculation in the shallow conduit became increasingly uniform with time through the 1991 opening phase.

A large number of people died during an explosive eruption of Kīlauea Volcano in 1790 CE. Detailed study of the upper part of the Keanakāko‘i Tephra has identified the deposits that may have been responsible for the deaths. Three successive units record shifts in eruption style that agree well with accounts of the eruption based on survivor interviews 46 yr later. First, a wet fall of very fine, accretionary-lapilli–bearing ash created a “cloud of darkness.” People walked across the soft deposit, leaving footprints as evidence. While the ash was still unconsolidated, lithic lapilli fell into it from a high eruption column that was seen from 90 km away. Either just after this tephra fall or during its latest stage, pulsing dilute pyroclastic density currents, probably products of a phreatic eruption, swept across the western flank of Kīlauea, embedding lapilli in the muddy ash and crossing the trail along which the footprints occur. The pyroclastic density currents were most likely responsible for the fatalities, as judged from the reported condition and probable location of the bodies. This reconstruction is relevant today, as similar eruptions will probably occur in the future at Kīlauea and represent its most dangerous and least predictable hazard.

The 1959 Kīlauea Iki eruption provides a unique opportunity to investigate the process of shallow magma mixing, its impact on the magmatic volatile budget and its role in triggering and driving episodes of Hawaiian fountaining. Melt inclusions hosted by olivine record a continuous decrease in H2O concentration through the 17 episodes of the eruption, while CO2 concentrations correlate with the degree of post-entrapment crystallization of olivine on the inclusion walls. Geochemical data, when combined with the magma budget and with contemporaneous eruption observations, show complex mixing between episodes involving hot, geochemically heterogeneous melts from depth, likely carrying exsolved vapor, and melts which had erupted at the surface, degassed and drained-back into the vent. The drained-back melts acted as a coolant, inducing rapid cooling of the more primitive melts and their olivines at shallow depths and inducing crystallization and vesiculation and triggering renewed fountaining. A consequence of the mixing is that the melts became vapor-undersaturated, so equilibration pressures cannot be inferred from them using saturation models. After the melt inclusions were trapped, continued growth of vapor bubbles, caused by enhanced post-entrapment crystallization, sequestered a large fraction of CO2 from the melt within the inclusions. This study, while cautioning against accepting melt inclusion CO2 concentrations “as measured” in mixed magmas, also illustrates that careful analysis and interpretation of post-entrapment modifications can turn this apparent challenge into a way to yield novel useful insights into the geochemical controls on eruption intensity.

Silicic volcanic eruptions commonly show abrupt shifts between powerful and dangerous (Plinian) explosive episodes and gentle effusion of lava. Whether the onset of magma permeability and ensuing gas loss controls these transitions has been a subject of debate. We measured porosities and permeabilities in samples from the A.D. 1912 eruption of Novarupta volcano, Alaska, and analyzed them within the context of a well-constrained eruptive sequence that encompasses sustained explosive and effusive activity. For the explosive samples, we find that the degree of vesicle interconnectivity, measured as the ratio of connected to total porosity, decreases with phenocryst content and with increasing eruption intensity. Permeabilities of explosive samples show a weak dependence on porosity. Dome samples are not significantly different in permeability, but are of lower porosity, which together with abundant flattened vesicles is consistent with bubble collapse by permeable outgassing. Quantitative analysis indicates that outgassing alone was insufficient to affect the transition to effusive activity. Rather, the change from explosive to effusive activity was probably a consequence of high versus low magma ascent rates.

Explosive eruptions at basaltic volcanoes have been linked to gas segregation from magmas at shallow depths in the crust. The composition of primary melts formed at greater depths was thought to have little influence on eruptive style. Ocean island basaltic volcanoes are the product of melting of a geochemically heterogeneous mantle plume and are expected to give rise to heterogeneous primary melts. This range in primary melt composition, particularly with respect to the volatile components, will profoundly influence magma buoyancy, storage and eruption style. Here we analyse the geochemistry of a suite of melt inclusions from 25 historical eruptions at the ocean island volcano of Kīlauea, Hawai‘i, over the past 600 years. We find that more explosive styles of eruption at Kīlauea Volcano are associated statistically with more geochemically enriched primary melts that have higher volatile concentrations. These enriched melts ascend faster and retain their primary nature, undergoing little interaction with the magma reservoir at the volcano’s summit. We conclude that the eruption style and magma-supply rate at Kīlauea are fundamentally linked to the geochemistry of the primary melts formed deep below the volcano. Magmas might therefore be predisposed towards explosivity right at the point of formation in their mantle source region. The explosive style of volcanic eruptions has been linked to gas separation from magmas in the shallow crust. Geochemical analysis of magmas erupted over the past 600 years at Kīlauea Volcano, Hawai‘i, now reveal a link between eruption style and the geochemistry of magmas formed at greater depths, implying that some magmas are predisposed towards explosivity.

Aravena et al. (2018) breathed fresh air into understanding the role of external water in silicic phreatomagmatic eruptions with their hypothesis that significant involvement of external water is only feasible above the level of primary fragmentation of the magma. We discuss here strong supporting evidence that primary fragmentation does not involve external water from the microtextures of the pyroclasts erupted from the two ‘type’ examples of large-scale interaction of silicic magma with external water.