Syn-eruptive Processes Research Articles

Sulfur isotopic variations in the products of the 1895 CE eruption at Zao volcano (NE Japan): Implications for connecting eruption source and syn-eruptive magmatic-hydrothermal processes

Sulfur isotopic ratio in sulfate and sulfide in subvolcanic hydrothermal systems is a valuable tracer to study the magmatic-hydrothermal processes from the magma source through to volcanic eruptions. Zao volcano is among the most active volcanoes in NE Japan, with historical explosive eruptions occurring during the last thousand years and unrest episodes since 2013. This necessitates a detailed assessment of the potential risk of future volcanic hazards. We investigated the magmatic-hydrothermal processes that occurred during the 1895 CE eruption sequence at Zao volcano by conducting mineralogical and sulfur isotope analyses in the exposed well: (i) six volcanic units (Layers 1–6) of the 1895 CE eruption products (clayish ash deposits with andesitic bombs, lapilli of scoria, and minor altered lithic fragments) deposited on the rim of Okama crater lake; and (ii) clay-altered and silicified rocks from the Nigorikawa alteration zone (NGA) surrounding the Goshikidake cone. Mineralogical data show that the samples mainly consist of alunite, pyrite, and gypsum. Alunite and pyrite occur as fine crystal mixtures associated with mineral assemblages of both advanced argillic alteration (i.e., those of cristobalite and kaolinite) and silicification (i.e., those of cristobalite, tridymite and native sulfur). Gypsum typically appears as isolated euhedral crystals of several millimeters in size. Samples of the 1895 CE eruption products have a narrow range of δ34S values from +3 ‰ to +5 ‰ for gypsum, from +9 ‰ to +13 ‰ for alunite, and approximately −10 ‰ for pyrite. For the NGA samples, the δ34Sgypsum, δ34Snative sulfur, and δ34Spyrite values range from −12 ‰ to −9 ‰, whereas for alunite, these range from +8 ‰ to +18 ‰. This indicates that alunite and pyrite in the 1895 CE eruption products were derived from the advanced argillic alteration and silicification zones that developed under Okama crater, which is exposed as the NGA. Estimated alteration temperatures based on the sulfur isotopic equilibrium between alunite and pyrite pairs are 200 °C–300 °C. By contrast, δ34Sgypsum values in the 1895 CE products are significantly higher than those in the NGA (which are derived from oxidation of pyrite or H2S, or both), ranging between an estimated parental fluid of δ34Sbulk-initial = ca. +1 ‰ and the Quaternary volcanic rocks of the Japan arc. This suggests that gypsum in the 1895 CE eruption products derived from magmatic vapor condensate (anhydrite) formed in the volcanic conduit during the eruption, thus becoming replacement of anhydrite by gypsum after or during the tephra deposition on the Zao summit surface. Our results on sulfur-bearing minerals provide new clues for better understanding (and monitoring) the syn-eruptive processes of volcanic eruptions focused on subvolcanic hydrothermal systems.

Volcanic lithofacies control the space in unconventional, rhyolitic hydrocarbon reservoirs: The Hailar Basin, NE China

Volcanic rocks are generally characterized by significant spatial variations of lithofacies. The characterization of volcanic lithofacies is an important tool to achieve accurate reservoir prediction during hydrocarbon exploration because of the type of lithofacies strictly controls the porosity and other physical and chemical properties of reservoir rocks. Studying the lithofacies of rhyolitic volcanic rocks is essential for uncovering the genesis and characteristics of volcanic reservoirs. The Lower Cretaceous Shangkuli Formation is located on the western boundary of the Hailar Basin in NE China. This basin hosts oil-bearing rhyolitic volcanic rocks and represents a natural laboratory for investigating the spatial distribution of different lithologies and volcanic lithofacies. Here, we present detailed field observations, TESCAN Integrated Mineral Analysis (TIMA), and fluorescence imaging to characterize seven types of rhyolitic volcanic rocks belonging to four main lithofacies associations. We examine the relationship between lithofacies associations and reservoir rocks. Field observations reveal the existence of volcanic conduit, pyroclastic, effusive, and extrusive lithofacies associations within the Hailar Basin. These lithofacies associations can be further subdivided into nine distinct subfacies. Among these, the extrusive lithofacies association is dominant and it is followed by pyroclastic, effusive, and volcanic conduit lithofacies associations. Intra-spherulite microfractures and pores, lithophysa cavities in the extrusive lithofacies associations, and inter-clast pores of the volcanic conduit lithofacies associations constitute the primary reservoir spaces of rhyolitic reservoirs. The lithofacies associations exert a control on the reservoir spaces of rhyolitic volcanic rocks by limiting the degree of supercooling (ΔT) and volatile content, which impact the formation of different textures in rhyolitic magma during syn-eruptive processes. We conclude that spherulitic and lithophysa rhyolites in the extrusive lithofacies associations, and the crypto-explosive breccias in the volcanic conduit lithofacies associations are promising targets for oil and gas exploration. The facies model we obtain from the Hailar Basin may be extended to similar rhyolitic rocks in other basins hosting unconventional reservoirs.

Construction of obsidian during explosive-effusive eruptions: insights from microlite crystals in obsidian pyroclasts

Obsidian pyroclasts are common in deposits from silicic sub-Plinian eruptions and can record pre- and syn-eruptive processes in the volcanic conduit. Previous work focusing on dissolved volatiles and vesicle textures has been useful in extracting timescales of sintering, diffusion, and vesicle relaxation recorded by obsidian pyroclasts. Here we focus on microlite crystals (<100 µm in size) to augment previous work because they form at different rates than vesicles or than rates of volatile degassing. Hence, they have the potential to disclose additional information about processes occurring in an explosive conduit. We examine microlites in 72 samples from tephra deposits of the 1340 C.E. North Mono eruption, California, U.S.A., and complement these measurements with hydrothermal experiments at 800°C, 10–50 MPa, and durations from 1 to 7 h. Three observations of the natural obsidians further elucidate their formation. First, microlite number densities (MND) increased as the eruption progressed. Second, multiple microlite morphologies occur for feldspars (blocky, swallowtail, tabular, cluster, skeletal) and pyroxenes (individual rods or clusters of acicular crystals) in each obsidian, regardless of any other characteristic. Third, microlite orientations correlate with the dominant morphology of vesicles, being generally well aligned in samples with ellipsoid vesicles, generally poorly aligned in samples with spherical vesicles, and either unaligned or aligned into planes in samples with distorted vesicles. In hydrothermal experiments, MND increase with time, microlites display only one morphology, and microlites are randomly oriented at any given pressure or temperature. When compared to natural obsidians, our experiments suggest most of the microlites could have grown in ≤∼7 h. The variety of microlite morphologies and orientations argue for repeated in-conduit fragmentation and sintering, consistent with the idea that each individual obsidian pyroclast is the product of ash sintering at multiple depths in the conduit prior to finally being erupted. During most of the eruption, obsidian pyroclasts were extracted from many depths in the conduit, preserving an array of volatile contents and microlite textures. Near the end of the explosive phase, however, higher MND record longer periods of stalling while dissolved volatile contents record vapor-melt equilibration at shallow depths in the conduit.

Syn-Eruptive Processes During the January–February 2019 Ash-Rich Emissions Cycle at Mt. Etna (Italy): Implications for Petrological Monitoring of Volcanic Ash

Low-intensity emission of volcanic ash represents the most frequent eruptive activity worldwide, spanning the whole range of magma compositions, from basalts to rhyolites. The associated ash component is typically characterized by heterogeneous texture and chemical composition, leading to misinterpretation of the role of syn-eruptive processes, such as cooling and degassing during magma ascent or even magma fragmentation. Despite their low intensity, the ash emission eruptions can be continuous for enough time to create problems to health and life networks of the communities all around the volcano. The lack of geophysical and/or geochemical precursor signals makes the petrological monitoring of the emitted ash the only instrument we have to understand the leading mechanisms and their evolution. Formation of low-level plumes related to ash-rich emissions has increasingly become a common eruptive scenario at Mt. Etna (Italy). In January–February 2019, an eruptive cycle of ash-rich emissions started. The onset of this activity was preceded on 24 December 2018 by a powerful Strombolian-like eruption from a fissure opened at the base of the New Southeast Crater. A lava flow from the same fissure and an ash-rich plume, 8–9 km high a.s.l., from the crater Bocca Nuova occurred concurrently. After about 4 weeks of intra-crater strombolian-like activity and strong vent degassing at summit craters, starting from 23 January 2019, at least four episodes of ash-rich emissions were recorded, mainly issued from the Northeast Crater. The episodes were spaced in time every 4–13 days, each lasting about 3–4 days, with the most intense phases of few hours. They formed weak plumes, up to 1 km high above the crater, that were rapidly dispersed toward different directions by dominant winds and recorded up to a distance of 30 km from the vent. By combining observations on the deposits with data on textural and chemical features of the ash components, we were able to discriminate between clasts originated from different crater sources and suggest an interpretive model for syn-eruptive processes and their evolution. Data indicate the occurrence of scarce (<10 vol.%) fresh juvenile material, including at least four groups of clasts with marked differences in microlite content and number density, and matrix glasses and minerals composition. Moreover, a large amount of non-juvenile clasts has been recognized, particularly abundant at the beginning of each episode. We propose that the low amount of juvenile ash results from episodic fast ascent of small magma batches from shallow reservoirs, traveling within a slow rising magma column subjected to cooling, degassing, and crystallization. The large number of non-juvenile clasts deriving from the thick crater infill of variably sealed or thermally altered material at the top of the magma column is suggested to contribute to the ash generation. The presence of a massive, granular crater infilling accumulating in the vent area may contribute to buffer the different geophysical signals associated with the active magma fragmentation process during the low-energy ash eruptions, as already evidenced at other volcanoes.

Syn-Eruptive Conditions of the AD 1530 Sub-Plinian Eruption of La Soufrière of Guadeloupe (Lesser Antilles)

A prerequisite in refining volcanic hazard at explosive volcanoes is a better quantification of the timescales of the syn-eruptive processes, such as magma degassing and crystallization prior to eruption. To this aim, new data on the matrices (microlites, residual glasses, and bubbles) of pumice, scoria, and dense clasts erupted during the AD 1530 andesitic eruption of La Soufrière of Guadeloupe are combined with published data from phase-equilibrium and kinetic experiments, in order to estimate pressures of microlite crystallization and magma ascent rates. From the timescale data, we infer that the AD 1530 eruption started with phreatomagmatic explosions tapping magmas that ascended during about 1 month (decompression rate of ∼50 Pa/s) from the coldest parts of the reservoir (∼825°C and a 74 wt% SiO2 melt). These magmas continuously crystallized microlites (∼25 vol% plagioclase, orthopyroxene, clinopyroxene, magnetite, quartz, and apatite), but did not outgas likely due to sealed conduit margins. The flank collapse (preexisting “cold” edifice) that followed the phreatomagmatic phase triggered a sub-Plinian eruption that progressively tapped the hotter main reservoir (∼875°C and 71 wt% SiO2 interstitial melt), emitting banded and homogeneous pumice. The banded pumice did not significantly outgas and mostly lack decompression-induced microlites, suggesting short ascent durations of the order of 0.5–1 day (decompression rates of 1,400–4,000 Pa/s). The following Strombolian phase emitted dark scoria that did not significantly outgas and only crystallized rare microlites, suggesting ascent duration of the order of 2 days (decompression rates of ∼550 Pa/s). The terminal lava dome growth involved fully outgassed magmas ascended during more than 1 month, giving time for microlite crystallization (∼40 vol% plagioclase, orthopyroxene, clinopyroxene, magnetite, and cristobalite). The detection of any shallow new magmatic intrusion is crucial, since it can trigger a sequence of conduit processes leading to an eruption marked by a succession of different and disastrous eruptive styles, following a scenario similar to the AD 1530 eruption. Overall, we provide a combined approach of petrological, geochemical, and experimental data that may be used to infer ascent conditions and rates at other volcanic systems.

Deciphering magma storage and ascent processes of Taranaki, New Zealand, from the complexity of amphibole breakdown textures

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides, and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasites entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950–1010 °C at mid- to deep crustal levels). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from <5 to >50 μm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent as well as small increases in temperature. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, identified as irregular and aligned. The former is indicative of slow reaction of amphibole with melt entrapped in fractures and cleavages during decompression-induced degassing, while the other is indicative of heating-induced breakdown of amphibole triggered by uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such crystals may be useful in constraining volcanic pre- and syn-eruptive processes and, when appropriate experimental data become available, their rates, which may be of the order of days to weeks.

Basaltic dyke eruptions at Piton de La Fournaise: characterization of the eruptive products with implications for reservoir conditions, conduit processes and eruptive dynamics

Small-volume proximal-summit eruptions, sometimes with only little precursors, are common at Piton de la Fournaise (La Reunion Island, France). Their eruptive style ranges from Hawaiian to Strombolian, but their eruptive mechanisms are still not completely understood. To gain insight, we combined syn-eruptive field work, textural and geochemical measurements on the short-lived eruption of July 2015 and we compared it with the 2014 eruption, which opened the ongoing eruptive cycle at Piton de la Fournaise. Crystal-poor and moderately vesicular components were predominant at the beginning of the eruptions, during the lava flux peak and the Hawaiian-style activity. Their abundance decreased with time in favour of less vesiculated and crystal-rich components during the decrease of the lava flux and the Strombolian-style activity. Physical, textural and chemical characterization of the pyroclasts, as well as thermometry calculations, permit relating the different texture and chemical features of the eruptive products with the pre- and syn-eruptive processes. Geophysical precursors detected several weeks before the July 2015 eruption, as well as some evidences of magmatic reheating recorded in bulk and crystal compositions highlight that the July 2015 magma results from a pre-eruptive mixing between more and less differentiated magmas. We finally deduced that the observed syn-eruptive textural variations are related to the same batch of magma undergoing different cooling and degassing dynamics.

Measuring the degree of “nanotilization” of volcanic glasses: Understanding syn-eruptive processes recorded in melt inclusions

Iron and water content substantially affect the physical properties of natural silicate melts and may, therefore, influence magmatic and volcanic processes such as crystallization, degassing, flow behaviour and eruptive style. Here we present Raman spectroscopic data for a set of synthetic and natural multicomponent glasses with varying iron oxidation state and water content. We systematically study the effect of different Raman excitation sources on the spectral response of both crystal free and magnetite nanolite bearing glasses spanning basaltic to calc- and per-alkaline rhyolitic compositions. Based on these data we document changes in Raman spectra resulting from the formation of nano-scale crystals. We show that the peak located at ~970 cm−1 is directly related to the presence of Fe2O3 dissolved in the quenched melt structure and that this feature is present regardless of the chemical composition of the sample and the Raman excitation source. We further show that a peak between 670 and 690 cm−1, which is not related to any other spectral feature of the glass structure, reveals the presence of nanolites. Based on systematic spectral investigations of this feature, we present a new index that allows to identify if iron is present in the nanocrystalline state and/or bound in the glass structure. Since the melt structural and physical effects of the formation of nanolites can heavily affect the flow behaviour of melts and the eruptive style of volcanoes, the results presented in this study significantly broaden the application of Raman spectroscopy for investigations of nano-heterogeneity in synthetic and natural glasses.We apply this method to study both the degree of nanolitization as well as the H2O content and iron oxidation state of groundmass glasses as well as melt inclusions and glass embayments in explosive products from Pantelleria island (Italy). We observe that the process of nanotilization is not purely restricted to magnetite nanolites but that Raman spectroscopy may also identify the incipient crystalization of pyroxene and feldspar at sub-micron scale. The data document that nanolite formation correlates well with the observed intensity of the respective eruptions suggesting that structural changes in the melt, caused by incipient crystallization play an important role in defining the eruptive style of relatively low viscosity magmas.

Combining textural and geochemical investigations to explore the dynamics of magma ascent during Plinian eruptions: a Somma–Vesuvius volcano (Italy) case study

Trigger mechanisms and syn-eruptive processes of Plinian eruptions are poorly understood especially in the case of mafic powerful events. In the last decades, the combined geochemical and textural studies on volcanic rocks have proven to be fundamental tools for exploring the dynamics of magma ascent in volcanic conduits and for improving our ability to interpret volcano-monitoring signals and assess hazard. In this case study, we quantitatively investigate 2D and 3D micro-textural, geochemical, and isotopic features of pyroclastic rocks erupted during the Pomici di Base Plinian eruption (22 ka), the generally acknowledged first and most powerful event of the Somma–Vesuvius volcano. A peculiar aspect of this eruption is its high intensity that remained stable during the entire Plinian phase despite the strong magma compositional variation towards mafic terms. We infer that the transfer of magma towards the surface was intensified by the occurrence of rapid vesiculation pulses driven by limestone assimilation (skarn recycling) during magma ascent through the carbonatic bedrock. We conclude that limestone assimilation can hence be a syn-eruptive process, able to trigger further gas nucleation with deep impact on the eruption intensity, particularly crucial in the case of mafic/intermediate magma compositions.

The Use of Surveillance Cameras for the Rapid Mapping of Lava Flows: An Application to Mount Etna Volcano

In order to improve the observation capability in one of the most active volcanic areas in the world, Mt. Etna, we developed a processing method to use the surveillance cameras for a quasi real-time mapping of syn-eruptive processes. Following an evaluation of the current performance of the Etna permanent ground NEtwork of Thermal and Visible Sensors (Etna_NETVIS), its possible implementation and optimization was investigated to determine the locations of additional observation sites to be rapidly set up during emergencies. A tool was then devised to process time series of ground-acquired images and extract a coherent multi-temporal dataset of georeferenced map. The processed datasets can be used to extract 2D features such as evolution maps of active lava flows. The tool was validated on ad-hoc test fields and then adopted to map the evolution of two recent lava flows. The achievable accuracy (about three times the original pixel size) and the short processing time makes the tool suitable for rapidly assessing lava flow evolutions, especially in the case of recurrent eruptions, such as those of the 2011–2015 Etna activity. The tool can be used both in standard monitoring activities and during emergency phases (eventually improving the present network with additional mobile stations) when it is mandatory to carry out a quasi-real-time mapping to support civil protection actions. The developed tool could be integrated in the control room of the Osservatorio Etneo, thus enabling the Etna_NETVIS for mapping purposes and not only for video surveillance.

Chemical zonation in olivine-hosted melt inclusions

Significant zonation in major, minor, trace, and volatile elements has been documented in naturally glassy olivine-hosted melt inclusions from the Siqueiros Fracture Zone and the Galapagos Islands. Components with a higher concentration in the host olivine than in the melt (e.g., MgO, FeO, Cr2O3, and MnO) are depleted at the edges of the zoned melt inclusions relative to their centers, whereas except for CaO, H2O, and F, components with a lower concentration in the host olivine than in the melt (e.g., Al2O3, SiO2, Na2O, K2O, TiO2, S, and Cl) are enriched near the melt inclusion edges. This zonation is due to formation of an olivine-depleted boundary layer in the adjacent melt in response to cooling and crystallization of olivine on the walls of the melt inclusions, concurrent with diffusive propagation of the boundary layer toward the inclusion center. Concentration profiles of some components in the melt inclusions exhibit multicomponent diffusion effects such as uphill diffusion (CaO, FeO) or slowing of the diffusion of typically rapidly diffusing components (Na2O, K2O) by coupling to slow diffusing components such as SiO2 and Al2O3. Concentrations of H2O and F decrease toward the edges of some of the Siqueiros melt inclusions, suggesting either that these components have been lost from the inclusions into the host olivine late in their cooling histories and/or that these components are exhibiting multicomponent diffusion effects. A model has been developed of the time-dependent evolution of MgO concentration profiles in melt inclusions due to simultaneous depletion of MgO at the inclusion walls due to olivine growth and diffusion of MgO in the melt inclusions in response to this depletion. Observed concentration profiles were fit to this model to constrain their thermal histories. Cooling rates determined by a single-stage linear cooling model are 150–13,000 °C h−1 from the liquidus down to ~1,000 °C, consistent with previously determined cooling rates for basaltic glasses; compositional trends with melt inclusion size observed in the Siqueiros melt inclusions are described well by this simple single-stage linear cooling model. Despite the overall success of the modeling of MgO concentration profiles using a single-stage cooling history, MgO concentration profiles in some melt inclusions are better fit by a two-stage cooling history with a slower-cooling first stage followed by a faster-cooling second stage; the inferred total duration of cooling from the liquidus down to ~1,000 °C ranges from 40 s to just over 1 h. Based on our observations and models, compositions of zoned melt inclusions (even if measured at the centers of the inclusions) will typically have been diffusively fractionated relative to the initially trapped melt; for such inclusions, the initial composition cannot be simply reconstructed based on olivine-addition calculations, so caution should be exercised in application of such reconstructions to correct for post-entrapment crystallization of olivine on inclusion walls. Off-center analyses of a melt inclusion can also give results significantly fractionated relative to simple olivine crystallization. All melt inclusions from the Siqueiros and Galapagos sample suites exhibit zoning profiles, and this feature may be nearly universal in glassy, olivine-hosted inclusions. If so, zoning profiles in melt inclusions could be widely useful to constrain late-stage syneruptive processes and as natural diffusion experiments.

Evaluation of morphometry-based dating of monogenetic volcanoes—a case study from Bandas del Sur, Tenerife (Canary Islands)

Morphometry-based dating provides a first-order estimate of the temporal evolution of monogenetic volcanic edifices located within an intraplate monogenetic volcanic field or on the flanks of a polygenetic volcano. Two widely used morphometric parameters, namely cone height/width ratio (Hmax/Wco) and slope angle, were applied to extract chronological information and evaluate their accuracy for morphometry-based ordering. Based on these quantitative parameters extracted from contour-based Digital Elevation Models (DEMs), two event orders for the Bandas del Sur in Tenerife (Canary Islands) were constructed and compared with the existing K-Ar, paleomagnetic and stratigraphic data. The results obtained suggest that the commonly used Hmax/Wco ratio is not reliable, leading to inappropriate temporal order estimates, while the slope angle gives slightly better results. The overall performance of such descriptive parameters was, however, generally poor (i.e. there is no strong correlation between morphometry and age). The geomorphic/morphometric mismatches could be the result of (1) the diversity of syn-eruptive processes (i.e. diverse initial morphologies causing geomorphic/morphometric variability), (2) contrasting, edifice-specific degradation that depends partly upon the inner facies architecture of the volcanic edifices, (3) various external environmental controls (e.g. tephra mantling from pyroclastic density currents unrelated to the edifice evaluated) and (4) differences in the scale/resolution of input data. The observed degradation trend and changes in morphometric parameters over time do not support a simple degradation model for monogenetic scoria cones volcanoes.

Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption

Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption

Syn-eruptive morphometric variability of monogenetic scoria cones

According to Wood's model, morphometric parameters such as slope angle can provide valuable information about the age of conical volcanic edifices such as scoria cones assuming that their initial slopes range from 30° to 33°, measured manually on topographic maps, and assuming that their inner architectures are homogenous. This study examines the morphometric variability of nine young (a few thousand years old) small-volume scoria cones from Tenerife, Canary Islands, using high-resolution digital elevation models in order to assess their slope angle variability. Because of the young age and minimal development of gullies on the flanks, their morphometric variability can be interpreted as the result of syn-eruptive processes including: (1) pre-eruptive surface inclination, (2) vent migration and lava outflow with associated crater breaching and (3) diversity of pyroclastic rocks accumulated in the flanks of these volcanic edifices. Results show that slope angles for flank sectors differ by up to 12° among the studied volcanoes, which formed over the same period of time; this range greatly exceeds the 2–3° indicated by Wood. The greater than expected original slope range suggests that use of morphometric data in terms of morphometry-based relative dating and detection of erosional processes and settings must be done with great care (or detailed knowledge about absolute ages and eruption history), especially in field-scale morphometric investigation.

Lithium concentration gradients in feldspar and quartz record the final minutes of magma ascent in an explosive supereruption

Pre- and syn-eruptive time-scales of magmatic processes in large-scale explosive eruptions can be quantified from compositional gradients established between (and within) crystals and their host melt using models of diffusive relaxation. The faster the elemental diffusion rate, the shorter the time periods that are measurable. Here we document Li compositional gradients from crystals in the rhyolitic magma of the ~27ka Oruanui supereruption (Taupo, New Zealand). In plagioclase feldspar and quartz from pumices in the first-erupted material, and loose feldspar crystals from late-erupted ignimbrite, the crystal rims display a ~50% increase in Li concentration over equilibrium values in the crystal cores. At appropriate magmatic temperatures, these gradients represent time-scales of 125 to 720s, equivalent to decompression rates of 30–180kPa/s and rise rates of frothy magma of 4–21m/s. We infer that these short time-scales reflect changes in lithium partitioning behaviour during decompression, and cannot be due to changes in Li concentration accompanying degassing and the growth of bubbles (which act in the opposite sense). The Li partitioning changes are due to the breakdown of the liquid–vapour equilibrium at the critical pressure of 22MPa, where the vapour phase transitions from a supercritical to a subcritical state. This breakdown causes Li to partition from melt into crystals, inducing a short-lived increase in effective Li activity of about 50%. The system is then quenched rapidly by cooling in air or water on eruption. These results capture syn-eruptive processes on unprecedentedly short time-scales in a type and size of eruption where information concerning such late-stage processes has never before been obtainable.

On the geophysical fingerprint of Vulcanian explosions

Extrusion of viscous magma and the subsequent formation of a lava dome is often interspersed by short-lived vigorous (Vulcanian) explosions. The causes for and the timing of the transition from effusive to explosive activity during dome formation are poorly understood and forecasting this transition remains a challenge. Here, we describe and interpret a robust and unique multi-parameter data set documenting the subsurface processes associated with Vulcanian explosions at Soufrière Hills Volcano, Montserrat (W.I.) in July and December 2008. We quantify explosion priming by processes in either the shallow (< 2 km depth) or the deep magmatic system and quantify syn-eruptive processes. The July 29 explosion has a signature related exclusively to shallow dynamics including conduit destabilisation, syn-eruptive decompression and magma fragmentation, conduit emptying and expulsion of juvenile pumice. In contrast, the December 3 explosion was triggered by unprecedented sudden pressurisation of the entire plumbing system from depths of about 10 km (including the magma chambers) resulting in surficial dome carapace failure, a violent cannon-like explosion, propagation of pressure waves and pronounced ballistic ejection of dome fragments. With timescales for explosion priming on the order of a few minutes, the precursory geophysical signatures are indicative of the nature of ensuing Vulcanian explosions. The short precursory phases characterise Vulcanian explosions as freak events triggered by abrupt rather than gradual changes in subsurface dynamics. Our findings provide important constraints for theoretical and experimental investigations of the effusive to explosive transition, forecasting of Vulcanian explosions and volcanic risk mitigation.

Transient processes in Stromboli’s shallow basaltic system inferred from dolerite and magmatic breccia blocks erupted during the 5 April 2003 paroxysm

We describe the mineralogy, geochemistry, and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 April 2003 paroxysm of Stromboli (Aeolian Islands, Italy). These blocks represent ∼50 vol.% of the total erupted ejecta and consist of fine- to medium-grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias. The breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated (lava-like to scoriae) crystal-rich (CR) basalt. All of the studied blocks are cognates with the CR basalt of the normal Strombolian activity or lavas and they are often coated by a few-centimeter thick layer of crystal-poor (CP) basaltic pumice erupted during the paroxysm. We suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 April 2003 explosion, when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas-rich CP basalt that triggered the eruption. Dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the CR basalts erupted during normal Strombolian activity and effusive events at Stromboli volcano. Dolerites consist of (a) phenocrysts normally found in the CR basalts and (b) late-stage magmatic minerals such as sanidine, An60-28 plagioclase, Fe–Mn-rich olivines (Fo68-48), phlogopite, apatite, and opaque mineral pairs (magnetite and ilmenite), most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical Strombolian behavior. Subvolcanic crystallization of the Stromboli CR magma, leading to slowly cooled equivalents of basalts, could result from transient drainage of the magma from the summit craters to lower levels. Fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 December 2002 coupled with the short break in the summit persistent explosions between December 2002 and March 2003 permitted the CR magma pockets to solidify as dolerites, which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity. Crystal size distribution of the basaltic blocks and crystallization of the finer-grained (<0.1 mm) mafic minerals of the dolerites over a time interval of ∼100 days closely agrees with the above interpretation. Vesicle filling (miarolitic cavities) locally found in some dolerites, with minerals deposited as vapor-phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system. Poorly welded magmatic breccias formed during syn-eruptive processes of 5 April 2003, when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the CR magma. In contrast, the high degree of welding between the dolerite clasts and the CR basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the CR basalt into the dolerite when, after a 2-month break in activity, CR magmas started to rise again to the summit craters. Blocks similar to these subvolcanic ejecta of 5 April 2003 were also erupted during previous paroxysms (e.g., 1930) suggesting that changes in the usual Strombolian activity (e.g., short breaks in the persistent mild explosions and/or flank effusive activity) lead to transient crystallization of dolerites in the shallow plumbing system.

Magma ascent and eruptive processes from textural and compositional features of Monte Nuovo pyroclastic products, Campi Flegrei, Italy

Geochemical and textural studies were carried out on alkaline products of the AD 1538 Monte Nuovo eruption. Due to the integration of the volcanological study with eyewitness reports, the dynamics and timing of each phase of the eruption and the volume of emitted magmas are known in detail. On this basis, unique in Campi Flegrei, the relations between magma chamber mechanisms, eruptive styles, magma ascent dynamics and volatile exsolution processes have been explored. Glass and phenocryst compositions indicate that the erupted magma has a homogeneous phono-trachytic composition. Textures and compositions of phenocrysts indicate that they crystallised at equilibrium with the melt in the magma chamber, likely as a mushy boundary layer along the chamber wall, where the temperature was below the liquidus temperature of the crystal free-chamber core. The estimated crystallisation temperature is 850±40°C. The magma phase relations in Petrogeny’s Residua System suggest that phenocryst crystallisation occurred at \(\text{P}_{\text{H}_2 \text{O}}\) between 100 and 200 MPa, corresponding to depths ranging from 3 to 8 km. The microlite composition and their close genetic relations with vesicles indicate that groundmass crystallisation occurred during the eruption as a consequence of magma degassing and vesiculation induced by decompression during its ascent toward the surface. Crystal size distributions reveal that microlites grew in two stages of undercooling that we define as: (1) magma migration onset upward from the chamber and (2) magma rising through the conduit to the surface, possibly lasting tens of days and few days, respectively. These results provide information on the physical conditions that characterise pre- and syn-eruptive processes, which may be useful in order to define eruptive scenarios and to evaluate short-term precursors. Furthermore, the collected data provide for the first time information on degassing-induced crystallisation during the eruption of a highly evolved alkaline magma.

In situ modification of modern submarine hyaloclastic/pyroclastic deposits by oceanic currents: an example from the Southern Kermadec arc (SW Pacific)

Submarine volcaniclastic deposits, both modern and ancient, pose a conundrum in distinguishing between syn- and post-eruptive processes. High-standing, submarine volcanic edifices of the late Quaternary southern Kermadec arc (SW Pacific) are point sources of pyroclastic/hyaloclastic deposits that are bathed and modified by a complex current system of the South Pacific gyre flowing southeast along the northern margin of New Zealand, which in part comprises the anticyclonic flow of the warm-cored East Cape Eddy (ECE). Flow of the ECE across the southern Kermadec arc provides a present-day case of extensive and in situ, post-eruptive, textural modification of modern pyroclastic/hyaloclastic deposits on the crests and upper flanks of submarine stratovolcanoes. Photographic observations (and limited textural data) from seven Kermadec volcanoes reveal pervasive evidence of sediment winnowing (including crag and tail structures, scour and moating around volcanic blocks, coarse sand-granule lag deposits, epifaunal deflection, lineated mud streaking, and moulded bioturbation mounds) and asymmetric current-ripple bedforms at water-depths of at least 1500 m. All bedforms indicate increasing current speed at progressively higher elevations (decreasing water-depth) for each volcano. Current-ripples mostly have discontinuous, asymmetric, shorted-crested, linguoid–lunate forms below 1000 m water-depth, progressing to semi-continuous, asymmetric, shorted-crested, linear-sinuous forms above 500 m. Current elutriation of the Kermadec deposits progressively removes fines with decreasing water-depth resulting in relatively fines-depleted, volcaniclastic sands and granules. This post-eruptive process overprints syn-eruptive processes that notionally generate more comminuted fine-grained clasts with decreasing water-depth as phreatomagmatic explosive eruptions become more vigorous. Current-elutriation also modifies volcaniclastic detritus prior to subsequent removal by episodic, mass-gravity flow. In addition the sand-granule traction load, driven by current-flow, moves sediment nearly continuously to gully and rill heads for removal down-slope, independently of syn-eruptive sediment flux. The underlying observation is that volcaniclastic deposits rarely reflect just syn-eruptive processes, and that significant in situ current-elutriation of at the least surficial pyroclastic/hyaloclastic eruption products can occur on submarine volcanoes. Threshold current velocities, derived assuming unidirectional flow over cohesionless sand-lapilli grainsizes, and accounting for bed friction, yield current velocities (at 100 cm above the bed) of ?15 cm s?1 for water-depths >1500 m through to 70 cm s?1 for depths

Conduit flow and fragmentation

Abstract This chapter provides a critical overview of syn-eruptive processes between magma chamber and vent in magmatic, sustained, Plinian-type eruptions. Phreatomagmatic effects are largely neglected. The main sources of information derive from textural studies of ejecta, theoretical models and physical experiments of eruption dynamics. Textural studies commonly find bimodal vesicle populations, indicative of several discrete nucleation events. The degree of disorder increases with explosivity. Dynamical parameters, such as nucleation density and rate and bubble growth rate, can be inferred from studies of the moments of bubble size distributions. Vesicularity in pumices is observed to vary significantly both within and between deposits, which suggests that the vesicularity at fragmentation is affected by the flow dynamics. Vesicularity variations correlate most closely with changes in magmatic composition and viscosity, but not with discharge rate. Conduit flow models can be broadly grouped into first- and second-generation models; the former generally impose a lithostatic pressure gradient and a constant Newtonian viscosity, whereas the latter include equations for the rheological changes that take place during vesiculation and solve for the pressure. Second-generation models derive highly non-lithostatic pressure gradients with the result that most of the vesiculation occurs at a high rate over a short distance just prior to fragmentation. The mechanisms of brittle and ductile fragmentation have been investigated in separate studies in non-vesiculating magmas, but which mechanism operates in explosive eruptions is not known. Dynamical laboratory experiments provide observations of the physical processes operating in conduit flows. Gas-expansion experiments have shown that it is possible to generate violent explosions by unloading in cool magmatic materials. Expanding dusty flows are found to be stable only if the bulk density increases with height. Exsolution experiments have demonstrated that acceleration precedes fragmentation and that gas evolution is enhanced by advection and bubble deformation. Deformed vesicles similar to those found in ‘woody’ pumice have been generated in an analogue system that has similar rheology to that found in vesiculating magmas. Large-scale exsolution experiments suggest that explosive volcanic eruptions are inherently heterogeneous: the fluctuations in discharge rate and discrete pulses and shocks commonly observed are a consequence of the large physical scale of volcanic systems. The effect of the magma chamber and conduit geometry has also been investigated. Eruption of material from a spherical flask up a narrow cylindrical tube generates quasi-steady flow conditions after an initial transient during which the discharge rate grows, as frequently observed in volcanic eruptions. The fragmentation surface does not propagate down into the magma chamber.