GMPV9.6

GMPV9 EDI
Magma ascent, degassing and eruptive dynamics: linking experiments, models and observations 

Interaction between the different phases (exsolved and dissolved volatiles, liquid melt, crystals, and pyroclasts) that separate during magma evolution, ascent and storage as a result of interlinked fluid, thermodynamic and chemical processes have a dramatic influence on eruption dynamics, resulting in a plethora of explosive eruptions types.
On one side, constraining volatile budget in magmas and quantifying degassing processes is a fundamental task to better understand the role of volatile elements on eruption dynamics. On the other side, the complex shallow plumbing system dynamics produces seismic and acoustic events, ground deformation and changes in the hydrothermal system often preceding or follow the explosive activity and direct field observations can constrain individual eruptive processes.
For this reason, the session aims at gathering field observation and experimental and modeling studies on eruptive processes to unlock the complex dynamics of volcanic activity. We hereby invite contributions focusing on (but not restricted to) volatiles in magmas, crystallization dynamics, effusive/explosive transition, rheology of gas-liquid-solid mixtures, fragmentation processes.
Further, we like to stimulate discussion on how multidisciplinary approaches can be used to advance the interpretation of geochemical and petrological observations on magmatic products and more specifically on the quantification of disequilibria processes during volcanic eruptions.

Convener: Mattia de’ Michieli Vitturi | Co-conveners: Amelia BainECSECS, Mike Burton, Andrea Di Muro, Giuseppe La SpinaECSECS
vPICO presentations
| Wed, 28 Apr, 09:00–10:30 (CEST)

Session assets

Session materials

vPICO presentations: Wed, 28 Apr

Chairpersons: Andrea Di Muro, Giuseppe La Spina, Mike Burton
09:00–09:05
High-temperature experiments
09:05–09:07
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EGU21-60
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ECS
Renelle Dubosq, Pia Pleše, Brian Langelier, Baptiste Gault, and David Schneider

The nucleation and growth dynamics of gas bubbles and crystals play a vital function in determining the eruptive behaviour of a magma. Their rate and relative timing, among other factors, are controlled by the magma’s ascent rate. Investigating the kinetics of decompression-induced degassing and crystallization processes can thus give us insight into the rheology of magmas. For example, the rapid decompression of magmas inhibits microlite crystallization and bubble nucleation during ascent leading to crystallization and degassing at shallow levels. This results in a drastic increase in viscosity and an over pressured system, which can lead to violent eruptions. Although many experiments and numerical simulations of magma decompression have been carried out, nascent and initial bubble nucleation remain poorly understood. It is widely accepted that there are two ways bubbles can nucleate within a melt: heterogeneous (on a pre-existing surface) and homogeneous nucleation (within the melt), where homogeneous nucleation requires a higher volatile supersaturation. It has since been tentatively suggested that homogeneous nucleation is simply a variety of heterogeneous nucleation where nucleation occurs on the surface of submicroscopic crystals. However, evidence of these crystals is equivocal. Thus, we have combined novel 2D and 3D structural and chemical microscopy techniques including scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS) mapping, and atom probe tomography (APT) to investigate the presence of sub-nanometer scale chemical heterogeneities in the vicinity of gas bubbles within an experimental andesitic melt. The combined STEM and EELS data reveal a heterogeneous distribution of bubbles within the melt ranging between 20-100 nm in diameter, some of which have Fe and/or Ca element clusters at the bubble-melt interface. Element clusters enriched in Fe, Ca, and Na are also observed heterogeneously distributed within the melt. The reconstructed APT data reveals bubbles as low ionic density regions overlain by a Na-, Ca-, and K-rich cluster and heterogeneously distributed Fe clusters within the bulk of the melt. Based on these observations, our data demonstrate the existence of nano-scale chemical heterogeneities within the melt and at the bubble-melt interface of bubbles that were previously interpreted to be nucleated homogeneously within the melt, therefore contributing to the proposed hypothesis that homogeneous nucleation could in fact be a variety of heterogeneous nucleation. These results highlight the need to redefine homogeneous nucleation and revisit whether bubbles or crystals occur first within volcanic melts. 

How to cite: Dubosq, R., Pleše, P., Langelier, B., Gault, B., and Schneider, D.: Bubbles and element clusters in rock melts: A chicken and egg problem, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-60, https://doi.org/10.5194/egusphere-egu21-60, 2021.

09:07–09:09
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EGU21-3018
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ECS
Jenny Schauroth, Joshua Weaver, Jackie E. Kendrick, Anthony Lamur, and Yan Lavallée

Volcanoes can undergo rapid transitions between effusive and explosive eruptions that are often dependant on the melt’s ability to devolatilise and outgas. Eruptive products show widely contrasting permeability values for a given porosity owing to the fact that magma properties evolve over time and space, hence porosity and permeability vary depending on transport and deformation history, scale and orientation. The vesicularity that enables bubble coalescence and permeability development, termed the percolation threshold, is experimentally determined to be at ~30-80 %, depending on the microstructure of magma (i.e. bubble size and shape distribution, crystal content, dominant mode of rheological deformation during vesiculation and flow). During ascent of magma pressure decreases and the magma adapts to these new conditions by vesiculating and expanding against wall rocks. Friction between the vesicular magma and the conduit wall encourages shear, which modifies the architecture of the vesicular network. The geometrical constriction associated with conduits, dykes or fractures which host magma thus prevents or limits the isotropic growth of vesicles; we hypothesise that geometrical constraints instead lead to different ratios of isotropic to anisotropic expansion, which impacts vesicle coalescence and the onset and development of permeable gas flow in magma. We present experimental results detailing the impact of constricting geometry on the development of a permeable porous network, by combining various diameter basalt crucibles with different sized cylindrical cores of aphyric rhyolitic glass (0.12 wt.% H2O). We vesiculate the samples in a furnace at 1009 °C for different isothermal dwell increments, before cooling our sample assembly and determining porosity, strain and gas permeability. The vesiculated rhyolites host an impervious glass rind (due to near-surface bubble resorption via diffusion) surrounding a vesicular core; as such, we measure gas permeability of the assembly after cutting the upper and lower glassy rind, to expose the permeability of the internal porous network developed experimentally. The findings indicate that increasing anisotropy, caused by minimising the extent of isotropic vesiculation and maximising vesiculation under constricted conditions, lowers the porosity at which the percolation threshold occurs by ~30 %. We postulate that pure and simple shear, developed parallel to the constricting walls, increase bubble aspect ratios and enhance coalescence. This suggests magmatic foams form connected networks at lower porosities when they vesiculate in constricted conduits, dykes and fractures, thus impacting outgassing efficiency. This implies that the physico-chemical evolution of vesiculating magma may be more strongly linked to structural and rheological controls than previously anticipated, with important implications on ascending magma evolution and eruptive processes, such as degassing, outgassing and fragmentation.

How to cite: Schauroth, J., Weaver, J., Kendrick, J. E., Lamur, A., and Lavallée, Y.: The effect of anisotropic vesiculation on the porous-permeable evolution in magmatic foams, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3018, https://doi.org/10.5194/egusphere-egu21-3018, 2021.

09:09–09:11
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EGU21-10227
Dennis Eul, Anja Allabar, and Marcus Nowak

The non-in-situ analysis of H2O degassing of silicate melt at high temperature and pressure is conducted using synthetic, decompressed melt samples quenched to glass. Interpretations regarding the degassing behavior are based on the number of H2O filled vesicles and the porosity of the vitrified samples. These properties of the glass samples may not represent the vesiculation at experiment temperature Texp and target pressure Pfinal. Even at high quench rates q, a decrease of vesicle volumes during cooling occurs, facilitated by resorption of H2O fluid back into the melt (McIntosh et al., 2014) and by the decrease of molar volume of H2O (Marxer et al., 2015) in the vesicles. This vesicle shrinkage introduces uncertainty regarding the true q-dependent “freezing” temperature Tf, at which shrinkage stops, represented by the vesiculated glass sample. While often neglected, knowledge of Tf is useful for improved sample interpretation.

McIntosh et al. (2015) developed a computer tomography (CT) based method to determine Tf. This approach infers Tf from the volume fraction of liquid H2O in vesicles (whose volumes are comprised of a liquid and a gaseous H2O phase) which decreases for increasing Tf.

Using their theoretical foundations, we developed a simple, transmitted light microscopy (TLM) image-based approach for the determination of this intra-vesicle phase ratio, applying two different model calculations: 1) Approximation of phase boundaries using polynomial functions. 2) Calculation of total vesicle and gas-phase volumes from ellipsoid axes measurements, approximating the vesicle and gas-phase volumes with symmetrical spheroids. In our analyzed hydrous, haplogranitic samples, we found mean Tf’s up to ~250 to ~300 K lower than Texp, at which quench was initiated, for q’s of ~40 and ~90 K/s. These values are close to the estimated Tf’s obtained using an independent glass porosity equation (Gardner et al., 1999). The large scatter of volume fractions and thus Tf for individual vesicles cannot be attributed to our image-based approach as data obtained from phonolitic samples using the CT method (Allabar et al., 2020) depict a similar scatter. At present, no correlation of Tf with vesicle size or position within the sample could be made. The method is, for the range of vesicle sizes investigated (20 to 50 µm in diameter), limited to liquid volume fractions larger than ~10 vol% as a distinction between phases is limited by optical resolution.

Nevertheless, our TLM based approach provides a simple, readily available method to constrain Tf of vitrified vesiculated samples which significantly improves the quality and comparability of derived interpretations. Our method uses standard polished sections for FTIR, making it even applicable to already existing samples.

 

 

Allabar, A. et al. (2020), Contrib. Mineral. Petrol, 175, 21, 1-19

Gardner, J.E., Hilton, M. and Carroll, M.R. (1999), Earth Planet. Sci. Lett, 168, 201-218

Marxer, H., Bellucci, P. and Nowak, M. (2015), J. Volcanol. Geotherm. Res, 297, 109-124

McIntosh, I.M. et al. (2014), Earth Planet. Sci. Lett, 401, 1-11

McIntosh et al. (2015): ‘Practical’ glass transition temperatures of vesicular glasses: a combined FTIR-XRCT approach. Abstract, 10th Silicate melt workshop. La Petite Pierre, France

How to cite: Eul, D., Allabar, A., and Nowak, M.: An image-based technique to determine the freezing temperature Tf of vesicle volumes in decompressed, synthetic melt samples, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10227, https://doi.org/10.5194/egusphere-egu21-10227, 2021.

09:11–09:13
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EGU21-6511
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ECS
Patricia Petri, Anja Allabar, and Marcus Nowak

Explosive eruptions of silicic magmas depend mainly on the amount and the degassing behavior of soluble volatile components like H2O and CO2. The injection of a hot mafic magma into a cooler volatile-rich rhyolitic magma chamber might initiate mingling and mixing processes at the interface of the two melt reservoirs (Paredes-Marino et al. 2017). An accompanying increase in temperature and a buoyant ascent of the H2O-saturated rhyolitic melt may cause a sufficiently high decrease in solubility at pressures < 300 MPa (e.g. Holtz et al. 1995) to trigger vesicle formation. Furthermore, the interface between different melt compositions might act as a site for enhanced vesicle formation. To test this hypothesis, bimodal decompression experiments were conducted. Basaltic and rhyolitic compositions similar to the Askja eruption 1875 in Iceland (Sparks and Sigurdsson 1977) were used for this purpose. For the preparation of the experiments, rhyolitic and basaltic glass cylinders were molten and hydrated separately in an internally heated argon pressure vessel with H2O excess at 200 MPa and 1523 K for 96–168 h and then isobarically quenched with 16 K∙s‑1. The hydrated glass samples were cut perpendicular to the cylinder axis. The cylinder faces were polished to enable a perfect contact of the rhyolite cylinder with the basalt cylinder. An additional decompression experiment with two contacted hydrated rhyolite cylinders was conducted as a reference to test the experimental setup.

Each pair of cylinders was heated isobarically with 25 K·s-1 to 1348 K at 210 MPa and equilibrated for 10 min. To simulate the magma ascent, three bimodal samples and the reference sample were decompressed with rates of 0.17 MPa∙s-1 or 1.7 MPa∙s-1 to the final pressure of 100 MPa and then quenched with 44 K∙s-1. H2O vesicle number and spatial distribution as well as the H2O contents in the decompressed samples were analysed by microscope, quantitative BSE image analysis, and FTIR-spectroscopy, respectively.

All decompression experiments resulted in vesiculated samples. In the rhyolite reference experiment, the H2O vesicles are homogeneously distributed within the whole sample. The former interface of the cylinders is no longer visible. This confirms that the former contact plane of the cylinders does not influence the degassing behaviour during decompression.

Optical examination and electron microprobe analysis of oxide diffusion profiles of the decompressed bimodal samples expose the development of a hybrid melt zone between the rhyolite and the partially crystallized basalt, documenting mixing processes during the decompression experiments (Petri 2020). The hybrid zone in the rhyolitic compositional dominated region is decorated with an enhanced number of H2O vesicles compared to the rhyolitic and basaltic glass volumes. This suggests that the injection of a basaltic melt into a rhyolitic melt reservoir may lead to significantly enhanced homogeneous H2O vesicle formation in the hybrid zone and, therefore, enhanced degassing with the concomitant triggering of explosive eruptions.

 

Holtz F. et al. (1995) American Mineralogist 80: 84-108.

Paredes-Marino J. et al. (2017) Scientific Reports 7: 16897.

Petri P. (2020) Master thesis University of Tübingen.

Sparks S.R.J. and Sigurdsson H. (1977) Nature 267: 315-318.

How to cite: Petri, P., Allabar, A., and Nowak, M.: Melt degassing triggered by magma injection?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6511, https://doi.org/10.5194/egusphere-egu21-6511, 2021.

09:13–09:15
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EGU21-9650
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ECS
Ernestina Appiah, Paola Stabile, Fabio Arzilli, Alessandro Fabbrizio, and Michael Robert Carroll

The volcanic system of Pantelleria is an example of volcanism in a continental rift basin which over the years has attracted much researcher due to the different eruptive styles it exhibits, ranging from effusive to explosive. Investigating the cooling history as well as the magma transport dynamics of peralkaline rhyolitic magma is useful to understand the eruptive behaviour of the pantelleritic magma system.

The present work seeks to obtain information on the liquidus temperature of alkali feldspar in pantellerite from the Fastuca pumice fall unit (PAN13) under water-saturated conditions. Alkali feldspar is one of the most abundant crystalline phases in peralkaline rhyolitic melts as well as in evolved, alkali-rich magma compositions (e.g., trachyte, phonolite).

A series of water-saturated isobaric single-step cooling experiments were performed at reducing conditions (graphite filler rod, water P-medium, ~NNO-2) with final temperature range of 670 °C-880 °C and water pressure of 20-150 MPa. Phase equilibria show that clinopyroxene is the first liquidus phase always appearing by 750 °C, followed by alkali feldspar over the entire pressure and temperature (P-T) range investigated, with also the presence of aenigmatite crystallizing near the liquidus at P of 50 MPa. Providing experimental constraints on pre- and syn-eruptive magma crystallization is fundamental to better understand the eruptive dynamics of peralkaline rhyolitic magmas. This is important for volcanic hazard assessments of peralkaline rhyolitic magmatic systems.

How to cite: Appiah, E., Stabile, P., Arzilli, F., Fabbrizio, A., and Carroll, M. R.: Phase relations and pre-eruptive conditions at low f(O2) in Pantelleria peralkaline rhyolites, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9650, https://doi.org/10.5194/egusphere-egu21-9650, 2021.

09:15–09:17
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EGU21-8094
Mattia Pistone, Luca Caricchi, and Peter Ulmer

Volcano deformation and gas emissions provide insights into subsurface magmatic systems. Large discrepancies are observed between the volumes calculated from deformation data, mass of emitted gases, and volumes of erupted magmas. Such discrepancies hinder our capacity to predict the magnitude and intensity of imminent eruptions and are ascribed to the amount of excess fluids stored in magma reservoirs. High-pressure (1240 bar) and high-temperature (1200 °C) hot isostatic press experiments show that the amount of trapped excess fluids in haplogranitic magmas with variable crystal contents (30, 50, 60, and 70 vol.%) depends strongly on fluid composition. Magmas with CO2 excess fluids become permeable at much larger porosities (44% higher) with respect to the H2O-rich counterparts at equivalent crystallinity. Available excess gas geochemistry data calculated from volatile-saturated melt inclusion record, syn-eruptive SO2 emission, and erupted juvenile porosity data collected for crystal-rich andesite and crystal-poor dacite/rhyolite volcanoes with known eruption magnitude and intensity (Mt St Helens 1980, Pinatubo 1991, Soufrière Hills 1996, and Merapi 2010) reveal that the discrepancy between erupted magma volume and SO2 released during the eruption increases with CO2 excess in magmas. In agreement with our experiments, these data highlight that CO2-rich fluids enhance magma’s capacity to store excess volatiles and shed light on the largest discrepancies between pre-eruptive deformation, gas emissions, and eruption intensity and magnitude.

How to cite: Pistone, M., Caricchi, L., and Ulmer, P.: CO2 favours the accumulation of excess fluids in felsic magmas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8094, https://doi.org/10.5194/egusphere-egu21-8094, 2021.

Low-temperature experiments with natural samples or analogue materials
09:17–09:19
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EGU21-8568
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ECS
Adrian Hornby, Ulrich Kueppers, Benedikt Maurer, Carina Poetsch, and Donald Dingwell

Pyroclastic density currents (PDCs) present perhaps the greatest proximal primary hazard of volcanic activity and produce abundant fine ash that can present a range of health, environment and infrastructure hazards. However, direct, fully quantitative observation of ash production in PDCs is lacking, and little direct evidence exists to constrain the parameters controlling ash generation in PDCs. Here, we use an experimental approach to investigate the effects of starting mass, material density and ash removal on the efficiency of ash generation and concurrent clast rounding in the dense basal flow of PDCs. We employ a rotary drum to tumble pumice and scoria lapilli clasts over multiple transport “distance” steps (from 0.2 to 6 km). We observe increased ash generation rates with the periodic removal of ash during the experiments and with increasing starting mass. By scaling to the bed height and clast diameter we obtain a general description for ash production in all experiments as a function of flow distance, bed height and average clast diameter. We confirm that changes in lapilli shape factors correlate with the ash fraction generated and that the grain size of ash produced decreases with distance. Finally, we estimate shear rate in our experiments and calculate the inertial number, which describes the ratio between clast-scale and flow-scale rearrangement during flow. We show that, under certain conditions, fractional ash production can be calculated accurately for any starting mass solely as a function of the inertial number and the flow distance. This work sheds light on some of the first systematic and generalizable experimental parameterizations of ash production and associated clast evolution in PDCs and should advance our ability to understand flow mobility and associated hazards.

How to cite: Hornby, A., Kueppers, U., Maurer, B., Poetsch, C., and Dingwell, D.: Experimental constraints on volcanic ash generation and clast morphometrics in pyroclastic density currents and granular flows, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8568, https://doi.org/10.5194/egusphere-egu21-8568, 2021.

09:19–09:21
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EGU21-2652
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ECS
Edgar Zorn, Thomas Walter, Michael Heap, and Ulrich Kueppers

Lava dome formation is a common process at stratovolcanoes involving the shallow intrusion or extrusion of viscous lava and may lead to the rise of spines. Spines are protrusions observed to extrude episodically during lava dome growth, yet the structural and mechanical factors controlling their formation are only partially understood. Here, we provide new, detailed insight into lava dome growth and the production of spines using a novel set of analogue experiments extruding sand-plaster mixtures from a fixed-diameter conduit under isothermal conditions. We trace displacement and strain with photogrammetric methods for precise and detailed monitoring of the extrusion process. Results show initial dome growth forming a steep-sided and flat-topped shape through extrusion of new material, leading to slumping of oversteepening slopes, forming a talus. Spines are found to protrude at a later stage through the dome surface along discrete circular faults that originate from the conduit walls, starting a cycle of spine growth and collapse. As our spines only appear after prolonged extrusion, we relate their appearance to the compaction and strengthening of material within the conduit. We find that spine diameter, height and volume are positively correlated with increasing cohesion and therefore material strength. The spine diameter was also observed to be smaller or equal to the diameter of the underlying conduit, as shear extrusion occurs along vertical to outward-dipping fault planes. For natural domes, our findings imply that spine growth may be the consequence of compaction and densification via porosity loss, shearing and/or outgassing of conduit magma during ascent. More efficient compaction will yield wider and taller spines as a result of increasing rock strength. Our study further highlights the relevance of analogue experiments in the study of lava domes and spines, which remain one of the most hazardous and unpredictable features at dome-forming volcanoes worldwide.

How to cite: Zorn, E., Walter, T., Heap, M., and Kueppers, U.: Insights into lava dome and spine extrusion using analogue sandbox experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2652, https://doi.org/10.5194/egusphere-egu21-2652, 2021.

09:21–09:23
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EGU21-10633
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ECS
Stefano Panunzi, Jacopo Taddeucci, Valeria Cigala, Ulrich Kueppers, Danilo Mauro Palladino, Juan José Peña Fernández, Piergiorgio Scarlato, and Jörn Sesterhenn

Explosive volcanic eruptions eject a mixture of gas and pyroclasts into the atmosphere at a range of velocities. Directly above the vent, in the gas-thrust region, a supersonic jet may be generated that strongly controls the eruptive dynamics. To improve our quantitative understanding of volcanic supersonic jets, the effect on particles within them, and their acoustic emission, we have performed small-scale explosive eruptions in the laboratory using a shock-tube. The shock-tube is composed of 3 parts, a bottom (5.6 m long, elevated pressure) and a top (48mm long, ambient pressure) plexiglass cylinder (5 mm inner diameter), separated by an electrovalve.

We have run experiments using ambient air as gas and sand, with diameter between 0.1 and 0.3 mm, as particles. The gas volume was fixed while the pressure ratio (the shock-tube reservoir to ambient pressure ratio) was varied from about 4 to 8.4 to obtain supersonic flows. During the experiments, the jet was recorded with a high-speed camera operating at 34660 fps, and the resulting noise acoustic emission with microphones (6 Hz-140 kHz; 1000 kfps) positioned at 90° from the jet axis.

Among the acoustic signals produced by a supersonic jet (jet noise) we have particularly focussed on the broad-band shock noise (BBSN) that is emitted by the interaction between shock cells and the turbulence in the jet. We estimated the jet velocity using an acoustic model based on the identification of the peak frequency of the BBSN. We also identified the BBSN frequency and its variation over time by applying the complex Morlet wavelet transformation. As expected, the BBSN frequency is inversely proportional to the gas velocity. Concerning the video recording, we analysed the shock cells behaviour and their temporal oscillation due to the presence of particles. Finally, the particle ejection rate was calculated in every video frame.

We found that the acoustic signal and shock cells are influenced by the presence of particles. In fact, fluctuations in particle concentration are well visible and decelerate the flowing gas. As a consequence, there is a temporary decrease of the stand-off-distance between the vent and the first shock-cell and concurrent rise of the BBSN frequency. We noticed, in some cases, that the shock-cells disappear during a short time interval. The BBSN frequency and the stand-off-distance behaviour over time follow the oscillation of the particle ejection rate confirming their sensitivity to particle load variation.

The future prospectives of this embryonal study could lead to new instruments for determining either the amount of pyroclasts inside the volcanic jets and their exit velocity on the basis of the recorded acoustic signals.

How to cite: Panunzi, S., Taddeucci, J., Cigala, V., Kueppers, U., Palladino, D. M., Peña Fernández, J. J., Scarlato, P., and Sesterhenn, J.: Volcanic supersonic jets: an experimental study of the effect of particles on the shock cell structure and acoustic emissions., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10633, https://doi.org/10.5194/egusphere-egu21-10633, 2021.

Analyses of natural samples
09:23–09:25
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EGU21-10275
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ECS
Mila Huebsch, Ulrich Kueppers, Guillaume Carazzo, Anne-Marie Lejeune, Audrey Michaud-Dubuy, and Donald B. Dingwell

Mt. Pelée is a historically active volcano, situated on the island of Martinique (Lesser Antilles), that has shown a variety of explosive styles in the recent past, ranging from dome-forming (Pelean) to open-vent (Plinian) eruptions.  The 1902-1905 eruption is infamous for the pyroclastic density currents (PDCs) that destroyed the towns of St. Pierre and Morne Rouge, killing 30 000 residents.  Since the last eruption (dome-forming) in 1929-1932, Mt. Pelée was quiet and considered dormant until recently.  In late 2020, the local Volcanological Observatory (OVSM) raised the alert level following a noticeable increase in seismicity, bringing into effect a reinforcement of monitoring resources.  As St. Pierre is long since re-established, along with several other towns along the volcano’s flanks, it is of utmost importance to understand the possible range of eruptive activity to improve the preparedness strategies of local communities.

The precise controls on eruption dynamics vary across volcanic systems and cannot be constrained via direct observation. However, crucial inferences can be made based on petrophysical properties and mechanical behaviours of erupted materials.  For this study, we collected samples from PDC deposits of Mt. Pelée, from the two historic Pelean (1902-1905, and 1929-1932) and three pre-Columbian Plinian eruptions (1300 CE P1, 280 CE P2, and 79 CE P3). We measured petrophysical properties (density, porosity, permeability) of cylindrical samples drilled from bomb-sized clasts and investigated their fragmentation behaviour via grain size and high-speed video analysis. These results are used in comparison with field data of grain-size distribution (GSD) of individual outcrops and calculated total GSD data.  We investigated the effects of transport-related sorting or fining.

The “Pelean” samples are found to be denser (32-47% open porosity) than the pumiceous “Plinian” samples (55-66% open porosity).  Moreover, these two classes are distinctly different in their crystallinity as samples underwent different ascent conditions.  In our experiments, distinct fragmentation behaviour and resulting GSDs are observed for samples from each eruption style, regardless of experimental pressure conditions (5-20 MPa). Our results show the paramount importance of open porosity on fragmentation efficiency in pumiceous samples, alongside a strong influence of crystallinity.  The fractal dimension of fragmentation calculated from weight fractions, independent of grain shape, shows clear differences in fragmentation efficiency as a function of sample properties and experimental starting conditions.

Our results suggest that (i) the variability in porosity and permeability is too low to cause the increased explosivity exhibited during the 1902 eruption compared to the 1929 event, (ii) open porosity has a major control on fragmentation efficiency in pumiceous samples, (iii) fragmentation efficiency can be effectively evaluated by calculating the fractal dimension of the cumulative weight fractions of experimental products.

The influence of crystallinity and pore textures on fragmentation efficiency must be further investigated to aid hazard model development for future eruptions of Mt. Pelée. Future work will constrain these textural parameters of naturally and experimentally fragmented materials from Mt. Pelée, to further elucidate the controls on eruptive dynamics at this hazardous volcano.

How to cite: Huebsch, M., Kueppers, U., Carazzo, G., Lejeune, A.-M., Michaud-Dubuy, A., and Dingwell, D. B.: Experimental determination of fragmentation efficiency for Plinian and Pelean eruptions of Mt. Pelée, Martinique, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10275, https://doi.org/10.5194/egusphere-egu21-10275, 2021.

09:25–09:27
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EGU21-4545
Abigail Metcalfe, Séverine Moune, and Jean-Christophe Komorowski

Signals of volcanic unrest do not usually provide insights into the timing, size and style of future eruptions. However, analysis of past eruptions provides useful information in order to understand the evolution, magma storage and onset of future eruptions, Here, we examine basaltic-andesitic to andesitic eruption deposits from La Soufrière de Guadeloupe, covering a range of eruption styles, ages and magnitudes. Our work is timely given unrest at this system has increased over the last 25 years, with a potential eruption capable of directly impacting ~70,000 people in Southern Basse-Terre.

Here, we study the geochemistry of pre-eruptive magmas and timescales of magmatic processes preceding four explosive eruptions: 1657 Cal. CE (Vulcanian), 1010 Cal. CE (Plinian), ca. 341 Cal. CE (Strombolian) and 5680 Cal. BCE (Plinian). Using diffusion timescale studies of orthopyroxene phenocrysts, we constrain the timing of magma injections into the La Soufrière de Guadeloupe magmatic reservoir. These range from 35 ± 0.37 to 848 ± 0.4 days before eruption. Diffusion timescales do not appear to correlate with eruption style/size, but may correlate with other parameters (e.g., magma interactions in the reservoir and/or volatile content of the magma).

Major element concentrations in whole rock (WR), groundmass glasses (GM) and melt inclusions (MI) show a strong linear trend. However, this evolution cannot be resolved through fractional crystallisation alone, as there is no clear temporal trend. MIs reveal a relatively homogenous melt composition from the first to the most recent eruptions, ranging from 63.6 – 78.7 wt% SiO2. Volatiles, including H­2O (2.3-4.4 wt%), CO2 (35-866 ppm) and sulphur (30-202 ppm), are also consistent across the various eruptions. MIs are often more evolved than the GM, indicating they cooled prior to their entrainment. This, along with the different crystal populations observed, suggests a recharge magma has intruded through a mush system and entrained crystals stored there. Crystals in different regions of the mush therefore experience different interactions with magmatic processes.

The major element compositional homogeneity across the eruptions indicates that composition does not have a large control on eruption style at this system. However, MI pre-eruptive volatile contents are more concentrated in the larger Plinian eruptions (e.g., CO­2 – 866 ppm) than the smaller Vulcanian and Strombolian eruptions (e.g., CO­2 – 674 ppm). Volatile emissions calculated through the petrologic method also differ, with higher total volatile emissions observed in the Plinian eruptions (12 Mt) than the smaller eruptions (0.1 Mt). The Plinian eruptions also have a faster magma ascent rate (0.3-22 m/s) than the vulcanian eruptions (3 m/s) as calculated from mass flux estimates. 

Though the composition of the La Soufrière de Guadeloupe system has remained constant over time, changes in eruption style can result from variations: (i) in the way magma interacts with the mush system, (ii) in the pre-eruptive volatile contents and (iii) in the ascent rates. Understanding the controls on eruption style is important, especially during the current phase of unrest, in order to improve early-warning system efficiency, forecast models, eruption scenario crisis response and long-term risk reduction planning.

How to cite: Metcalfe, A., Moune, S., and Komorowski, J.-C.: Controls on Eruption Style at La Soufrière de Guadeloupe from Melt Inclusions and Mineral Diffusion Timescales., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4545, https://doi.org/10.5194/egusphere-egu21-4545, 2021.

09:27–09:29
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EGU21-14665
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ECS
Balázs Kiss, Dávid Karátson, László Aradi, János Szepesi, Tamás Biró, Tamás Sági, Veronika Szilágyi, and Zoltán Kis

The Sf. Ana crater is the young volcanic crater of the dacitic Ciomadul volcano located at the SE end of the Călimani-Gurghiu-Harghita volcanic chain in the Eastern Carpathians. The crater was formed at ~60-30 kyr-s ago probably by several eruptions. The Sf. Ana also called as TGS eruption sequence was the main event that shaped the crater to the present form. The eruption produced fall and PDC deposits, but it is unclear what caused the change in the eruption style. The stratigraphically controlled analyses of the Mohos Layered Pyrolcalstic Sequence (MLPS) provide deep insight into the evolution of the eruption. Assuming that juvenile clast density is primarily controlled by the magma vesiculation within the conduit, the processes close to the fragmentation level can be studied. The vesicularity, vesicle texture, microlite texture, and glass H2O content of the juvenile pyroclasts were studied to reveal the conduit processes. The juvenile clasts show textural evidence for different stages of the vesiculation from bubble nucleation to collapse indicating degassing and outgassing processes in the conduit. The increase of the juvenile clast density upward in the MLPS and the sharp increase of the dense clasts in the PDC deposits indicate the effect of magma column heterogeneity on the eruption style. The conduit heterogeneity was induced by the effective outgassing of the slowly ascending magma portion due to the evolution of vesicle textures together with localized shearing. The eruption column collapse was preceded by a vent failure event which caused densification in the conduit. Banded pumices suggest that the observed conduit heterogeneity was small scale.

 

The study is supported by the PD130214 project National Research, Development, and Innovation Fund of Hungary.

How to cite: Kiss, B., Karátson, D., Aradi, L., Szepesi, J., Biró, T., Sági, T., Szilágyi, V., and Kis, Z.: Conduit processes of the Sf. Ana (TGS) sub-Plinian-Vulcanian eruption sequence of the Ciomadul volcano (SE Carpathians), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14665, https://doi.org/10.5194/egusphere-egu21-14665, 2021.

09:29–09:31
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EGU21-107
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ECS
Emily Mick, John Stix, J. Maarten de Moor, and Geoffroy Avard

Turrialba is a basaltic to andesitic Holocene stratovolcano that after decades of quiescence re-activated in 1996 and has been highly active ever since. Turrialba is characterized by a highly active magmatic-hydrothermal system, and we propose that hydrothermal sealing and volatile accumulation are the mechanisms responsible for the reactivation and persistent phreatic activity at Turrialba since 2010. Evidence of sealing is found in pyroclastic breccias from phreatic eruptions as high concentrations of hydrothermal minerals coupled with low intrinsic permeability. The suite of volcanic breccias studied erupted from the main vent between 2014 and 2019 and has an alteration mineral assemblage of SiO2polymorphs ± gypsum ± natroalunite ± pyrite. The mineral assemblage is indicative of acid sulphate alteration within the advanced-argillic alteration facies characterized by temperatures of approximately 200-350°C as indicated by the presence of gypsum and natroalunite, the high temperature endmember of the alunite series. Acid sulphate alteration is the result of extreme base leaching by acidic fluids (pH<4) with a high sulphate content. Measurements of permeability and porosity yielded variable porosity and very low to non-existent permeability in all hydrothermal breccia samples. Back-scatter electron (BSE) images reveal nano-, micro- and macro-scale fracture networks discontinuously filled with hydrothermal gypsum and pyrite which are responsible for diminished permeability, supporting the conclusion that hydrothermal sealing is active at Turrialba. Diminished permeability associated with the formation of a seal inhibits the escape of gases, causing them to accumulate below the seal and pressurize the system. Eventual seal failure releasing overpressure and possibly dynamic rapid seal formation result in the frequent phreatic eruptions seen at Turrialba.

How to cite: Mick, E., Stix, J., de Moor, J. M., and Avard, G.: Hydrothermal alteration and sealing at Turrialba as a mechanism for phreatic eruption triggering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-107, https://doi.org/10.5194/egusphere-egu21-107, 2021.

09:31–09:33
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EGU21-6916
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ECS
Olivier Bernard, Weiran Li, Fidel Costa, and Caroline Bouvet de Maisonneuve

One of the major challenges faced by volcanologists to investigate controls on eruption dynamics is to quantify both pre-eruptive volatile budgets and timescales of magma ascent. Indeed, petrological investigations of the two parameters usually rely on different methods/analytical techniques that are not always applicable/accessible. Recent studies have shown that the abundance and zoning pattern of F, Cl, and OH in apatite can be used to determine both pre-eruptive volatile budget and magma degassing rates that can, under some conditions, be related to magma ascent rates ([1],[2]).

Here we apply the two methods to apatite in the Rabaul 2006 eruption deposits (Papua-New-Guinea). This was a VEI-4 eruption and occurred in three main phases: (1) a sub-plinian onset followed 12h after its start by (2) a mixed strombolian-effusive phase, which subsequently evolved into (3) discrete vulcanian explosions. We sampled deposits of the three phases: (1) pumices, (2) fragments of lava flow, and (3) fragments of cow-pad bombs.

We calculated pre-eruptive water contents using apatite included in clinopyroxene as they keep a better record of reservoir conditions from the time of entrapment. We found that the magma that fed the sub-plinian phase contained the highest water content of about 2 wt.%, while magmas that fed the lava flow and the vulcanian phase were drier, with 0.2 to 0.5 wt.% less H2O. X-ray maps acquired with an EPMA show that only apatite crystals in the groundmass of the vulcanian and effusive deposits are zoned in F and Cl at the crystal rims, whereas those from the sub-plinian deposits and included in clinopyroxenes are not zoned. This indicates that the zoning is related to syn- or immediately pre-eruptive changes of Cl-F-H2O during magma ascent towards the surface and can thus be modelled as diffusive reequilibration of the crystal and the melt. We obtained maximum diffusion timescales of <8 hours for the unzoned apatite in sub-plinian deposits, timescales of 20–22 hours for apatite in vulcanian deposits, and 600–1500 hours for those in the lava flow. Thus, the time scales increase with decreasing explosivity of the eruptions, as it could be expected if magma ascent rate played the key role of eruption dynamics. However, the degassing timescales of the effusive phase are significantly longer than the eruption duration itself, which can be explained if the magma started rising in the system 1–3 months prior to the onset of the eruption. The volatile-rich, fast-rising magma that fed the initial sub-plinian phase propagated through, disturbed and remobilized the shallower, more degassed batch of magma, which was erupted during the following effusive phase. Deeper, volatile-poor magma that kept moving up the open conduit, was responsible for the late vulcanian explosions.

Our results show that apatite is a powerful tool for probing slight changes in magma volatile chemistry and ascent rates that can vary between different phases of the same eruption and produce different eruption styles.

 

[1] Li and Costa, 2020, GCA [2] Li et al. 2020, EPSL

How to cite: Bernard, O., Li, W., Costa, F., and Bouvet de Maisonneuve, C.: Using apatite records of volatile budget and magma ascent rates to investigate eruption dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6916, https://doi.org/10.5194/egusphere-egu21-6916, 2021.

09:33–09:35
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EGU21-13287
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ECS
Corin Jorgenson, Luca Caricchi, Michael Stueckelberger, Giovanni Fevola, and Gregor Weber

Melt inclusions provide a window into the inner workings of magmatic systems. Both mineral chemistry and volatile distributions within melt inclusions can provide valuable information about the processes modulating magma ascent and preceding volcanic eruptions. Many melt inclusions host vapour bubbles which can be rich in CO2 and H2O and must be taken into consideration when assessing the volatile budget of magmatic reservoirs. These vapour bubbles can be the product of differential volumetric contraction between the melt inclusion and host phase during an eruption or indicate an excess fluid phase in the magma reservoir. Thus, determining the distribution of volatiles between melt and vapour bubbles is integral to our fundamental understanding of melt inclusions, and by extension the evolution of volatiles within magmatic systems.

A large dataset of 79 high-resolution tomographic scans of clinopyroxene and leucite phenocrysts from the Colli Albani Caldera Complex (Italy) was recently acquired at the German Electron Synchrotron (DESY). These tomograms allow us to quantify the volume of melt inclusions and associated vapour bubble both glassy and microcrystalline melt inclusions. Notably, in the glassy melt inclusions the vapour bubbles exist either as a single large vapour bubble in the middle of the melt inclusion or as several smaller vapour bubbles distributed around the edge of the melt inclusion. These two types of melt inclusions can coexist within a single crystal. We suggest that the occurrence of these rim- bubbles is caused by one of two exsolution pathways, either pre-entrapment and bubble migration or post entrapment with preferential exsolution at the rims. By combining the analysis of hundreds of melt inclusions with the chemistry of the host phase we aim to unveil magma ascent rates and distribution of excess fluids within the magmatic system of Colli Albani, which produced several mafic-alkaline large volume ignimbrites.

How to cite: Jorgenson, C., Caricchi, L., Stueckelberger, M., Fevola, G., and Weber, G.: A myriad of melt inclusions: a synchrotron microtomography study of melt inclusions and vapour bubbles from Colli Albani (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13287, https://doi.org/10.5194/egusphere-egu21-13287, 2021.

09:35–09:37
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EGU21-15199
Pier Paolo Giacomoni, Federico Casetta, Virginia Valenti, Carmelo Ferlito, Gabriele Lanzafame, and Manuela Nazzari

The concomitant activation off all four summit craters of Mt. Etna during the December 2015 eruptive event allow us to investigate the chemical-physical crystallization conditions and magma dynamics in the shallower portion of the open-conduit feeding system. In this study, we discuss new petrological, geochemical and thermo-barometric data as well as the composition of major element and volatile content (H2O, CO2, F, Cl and S) of olivine-hosted melt inclusions from the explosive and effusive products emitted during the December 2015 eruptive event.

Results and rhyolite-MELTS thermodynamic modelling of mineral phase stability highlight the relatively shallow crystal equilibrium depth prior to the eruption ranging from 400-500 MPa for Central Crater and North East Crater, up to 200 MPa below the New South East Crater. The study of high-pressure and high-temperature homogenized olivine-hosted melt inclusions allowed us to identify the composition of the almost primary alkali-basalt magma (11.8 wt% MgO) containing up to 4.9 wt% and 8151 ppm of H2O and CO2 respectively. The results, together with those already reported for the previous paroxystic events of the 2011-2012 (Giacomoni et al., 2018), reinforce the model of a vertically extended feeding system and highlight that the activity at the New South East Crater was fed by a magma residing at significant shallower depth with respect to Central Craters and North East Crater, although all conduits are fed by a common deep (P = 530-440 MPa) basic magmatic refilling. Plagioclase stability model and dissolution and resorption textures confirm its dependence on H2O content, thus suggesting that further studies on the effect that flushing from fluids with different H2O/CO2 ratio are needed in order to understand the eruption triggering mechanisms of paroxystic fountaining.

 

References

Giacomoni P., Coltorti M., Mollo S., Ferlito C., Braiato M., Scarlato P. 2018. The 2011-2012 paroxysmal eruptions at Mt. Etna volcano: Insights on the vertically zoned plumbing system. JVGR 349, 370-391.

How to cite: Giacomoni, P. P., Casetta, F., Valenti, V., Ferlito, C., Lanzafame, G., and Nazzari, M.: Chemical-physical constraints of the 2015 eruptive activity of Mt. Etna: new insights from thermo-barometry and geochemistry of olivine-hosted melt inclusions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15199, https://doi.org/10.5194/egusphere-egu21-15199, 2021.

Analysis of monitoring data
09:37–09:39
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EGU21-6746
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ECS
Kathleen McKee, Eveanjelene Snee, Sean Maher, Cassandra Smith, Kevin Reath, Diana Roman, Robin S. Matoza, Simon Carn, Larry Mastin, Kyle Anderson, David Damby, Anna Perttu, Jelle Assink, Rodrigo de Negri Leiva, Artem Degterev, Alexander Rybin, Marina Chibisova, Ima Itikarai, Kila Mulina, and Steve Saunders

Volcanic jet noise is the sound, often below the human audible range (<20 Hz and termed infrasound), generated by momentum-driven fluid flow through a volcanic vent. Assuming the self-similarity of jet flows and audible jet noise extends to infrasonic volcanic jet noise, the Strouhal number, St=Djf/Uj, connects frequency changes, f, to changes in the jet length (expanded jet diameter, Dj) and/or velocity scale (jet velocity, Uj). We examine the infrasound signal characteristics from the June 2019 VEI 4 eruptions of Raikoke, Kuril Islands and Ulawun, Papua New Guinea volcanoes with changes in crater geometry. We use data from the International Monitoring System (IMS) infrasound network and pre- and post-eruption satellite data (RADARSAT-2 and PlanetScope imaging for Raikoke and Ulawun, respectively). During both eruptions we observe a decrease in infrasound peak frequency during the transition to a Plinian phase, which remains through the end of the eruptions. The RADARSAT-2 data show a qualitative increase in the crater area at Raikoke; quantitative analysis is limited by shadows. At Ulawun, however, we estimate an increase in crater area from ~35,000 m2 on May 25, 2019 to ~66,000 m2 on July 17, 2019. We assume a constant Strouhal number and use the crater diameter as a proxy for expanded jet diameter. Our analysis suggests that the increase in crater diameter alone cannot account for the decrease in peak frequency during the Ulawun eruption. This suggests that the jet velocity also increased, which fits satellite data, and or the fluid properties (e.g. particle loading, nozzle geometry and roughness, etc.) changed. This is reasonable as the Ulawun eruption went Plinian, which likely involved an increase in jet velocity and erosion of the crater walls. This is the first study to corroborate the decrease in infrasound peak frequency with documented increase in crater area. The fortuitous satellite overpass timing, clear skies, and high spatial resolution enabled the quantitative examination of the Ulawun eruption.

How to cite: McKee, K., Snee, E., Maher, S., Smith, C., Reath, K., Roman, D., Matoza, R. S., Carn, S., Mastin, L., Anderson, K., Damby, D., Perttu, A., Assink, J., de Negri Leiva, R., Degterev, A., Rybin, A., Chibisova, M., Itikarai, I., Mulina, K., and Saunders, S.: Decrease in Volcano Jet Noise Peak Frequency from Crater Expansion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6746, https://doi.org/10.5194/egusphere-egu21-6746, 2021.

09:39–09:41
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EGU21-12049
Mike Burton, Giuseppe La Spina, Catherine Hayer, and Benjamin Esse

Analysis of TROPOMI data with plume trajectory tools opens the possibility of new insights into volcanic / magmatic processes from two data sources: SO2 flux time series and plume height time series. In this paper we investigate results from explosive eruptions and attempt to explain the results with a magma ascent conduit model. The combination of plume height and gas flux data with a model of the magma ascent process provides a toolkit which allows us to constrain magma reservoir processes from satellite monitoring data. The combination of modelling and observations opens a new volcanological research frontier, because the TROPOMI sensor has daily global coverage, a high spatial resolution and is sensitive enough to detect many small-medium explosions globally, so that a large inventory of explosive activity can be characterised. 

How to cite: Burton, M., La Spina, G., Hayer, C., and Esse, B.: New insights into magmatic processes from integrated satellite observation, trajectory analysis and magma ascent modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12049, https://doi.org/10.5194/egusphere-egu21-12049, 2021.

Numerical modelling
09:41–09:43
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EGU21-10661
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ECS
Giuseppe La Spina, Fabio Arzilli, Ed Llewellin, Mike Burton, Amanda Clarke, Mattia de’ Michieli Vitturi, Margherita Polacci, Margaret Hartley, Danilo Di Genova, and Heidy Mader

Basaltic volcanoes exhibit a wide range of eruptive styles, from relatively gentle effusive eruptions (producing lava flows and lava domes) to highly explosive activity (where pyroclastic materials are ejected from the vent as a jet or plume). The difference between explosive and effusive eruptions is dictated by the ability of magma to fragment during ascent. For lava fountains the distinction is unclear, as the liquid phase in the rising magma may remain continuous to the vent, fragment in the fountain, then re-weld on deposition to feed rheomorphic lava flows.

Here we use a magma ascent model to constrain the controls on basaltic eruption style, using Kilauea and Etna as case studies. Following our results, we suggest that lava fountaining is a distinct style, separate from effusive and explosive eruption styles, that is produced when magma ascends quickly and fragments above the vent, rather than within the conduit. Performing sensitivity analyses of Kilauea and Etna case studies we found that high lava fountains (> 50 m high) occur when the Reynolds number of the bubbly magma is greater than ~0.1, the bulk viscosity is less than 106 Pa s, and the gas is well-coupled to the melt. According to our results, explosive eruptions (Plinian and sub-Plinian) are expected over a wide region of parameter space for higher viscosity basalts, typical of Etna, but over a much narrower region of parameter space for lower viscosity basalts, typical of Kilauea. Numerical simulations indicate also that the magma that feeds high lava fountains ascends more quickly than the magma that feeds explosive eruptions, thanks to its lower viscosity. For the Kilauea case study, a decreasing ascent velocity is expected to produce a progressive evolution from high to weak fountaining, to ultimate effusion. For the Etna case study, instead, small changes in parameter values lead to transitions to and from explosive activity, indicating that eruption transitions may occur with little warning.

How to cite: La Spina, G., Arzilli, F., Llewellin, E., Burton, M., Clarke, A., de’ Michieli Vitturi, M., Polacci, M., Hartley, M., Di Genova, D., and Mader, H.: Role of rheology, ascent rate and outgassing on fragmentation: implications for basaltic lava fountains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10661, https://doi.org/10.5194/egusphere-egu21-10661, 2021.

09:43–09:45
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EGU21-15923
Marielle Collombet, Alain Burgisser, Mathieu Colombier, and Elizabeth Gaunt

Previous studies of Vulcanian eruptive products have shown that the respective volcanic conduits were filled for the
most part with low-porosity magma prior to eruption. Comparison with the theoretical porosity distribution
expected from closed-system degassing suggests that gas loss must have taken place at great depth within the
magmatic column. At such high pressures, however, porosities are low enough to rule out traditional gas loss
mechanisms. We tested if channelling, an outgassing mechanism based on bubble connection due to high crystal
content proposed to occur in mushy magma reservoirs, could also happen in volcanic conduits. We reanalysed
phenocryst, microlite, and porosity data from recent eruptions of Merapi volcano, Indonesia, Soufrière Hills
volcano, Montserrat, and Tungurahua volcano, Ecuador. Overall, these magmas had crystal contents high enough
for outgassing to occur by channelling. Gases could be channelled out of the magma columns at various levels
during ascent to yield mostly gas-depleted magma columns prior to explosive behaviour. Such outgassing by
channelling has thus the capacity to influence eruptive style. Depending on the phenocryst content, microlite
growth during ascent can either foster, or impede gas escape by channelling. Considering the pervasive occurrence
of microlites and ensuing high crystal contents in volcanic conduits, the high likelihood of channelling implies that
other outgassing mechanisms might not be as dominant as previously envisioned.

How to cite: Collombet, M., Burgisser, A., Colombier, M., and Gaunt, E.: Open volcanic systems: evidence for deep gas loss, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15923, https://doi.org/10.5194/egusphere-egu21-15923, 2021.

09:45–10:30