Ascent of Bubbles in Magma Conduits Using Boundary Elements and Particles

AbstractWe investigate the use of the Multipole-accelerated Boundary Element Method (BEM) and of the Singularity Method for studying the interaction of many bubbles rising in a volcanic conduit. Observation shows that the expression of volcanic eruption is extremely variable, from slow release of magma to catastrophic explosive manifestation. We investigate the application of the Fast Multipole Method to the solution of (i) the Boundary Element Formulation of the Stokes flow and of (ii) the particle formulation using the Stokeslets, the Green Function of the Stokes flow law, as a particle kernel. We show how these implementations allow for the first time to numerically model in a dynamic setting a very large number of bubbles, i.e few thousands with the BEM models, allowing investigating the feedback between the single bubble deformation and their collective evolution, and few hundred of thousands of bubbles with the particle approach. We illustrate how this method can be used to investigate the intense interaction of a large number of bubbles and suggest a framework for studying the feedback between many bubbles and a complex thermal nonlinear magmati

Magma emplacement and deformation in rhyolitic dykes:insight into magmatic outgassing

Exposed rhyolitic dykes at eroded volcanoes arguably provide in situ records of conduit processes during rhyolitic eruptions, thus bridging the gap between surface and sub-surface processes. This study involved micro- to macro-scale analysis of the textures and water content within shallow (emplacement depths <500 m) rhyolitic dykes at two Icelandic central volcanoes. It is demonstrated that dyke propagation commenced with the intrusion of gascharged currents that were laden with particles, and that the distribution of intruded particles and degree of magmatic overpressure required for dyke propagation were governed by the country rock permeability and strength, with pre-existing fractures playing a pivotal governing role. During this stage of dyke evolution significant amounts of exsolved gas may have escaped. Furthermore, during later magma emplacement within the dyke interiors, particles that were intruded and deposited during the initial phase were sometimes preserved at the dyke margins, forming dykemarginal external tuffisite veins, which would have been capable of facilitating persistent outgassing during dyke growth. It is further demonstrated that following initial dyke-opening, geochemically homogenous dykes grew via the incremental emplacement of magma, with fluctuations in the shallow-dyke permeability occurring via bubble collapse, and this is deemed to have been critical in dictating pressure within the deeper magma source region and fragmentation. Of further significance, it is also shown that shear deformation was localised during magma emplacement, with localised vesiculation occurring along emplacement boundary layers via viscous heating, which temporarily promoted magma ascent, but with later bubble collapse culminating in brittle failure of bubble-free magma, after shear zone migration. However, in some instances high strain rates during viscous bubble deformation resulted in ductile-brittle transitions, with resultant slip triggering micro-tensile failure of bubbly magma, as the slipped magmatic plug experienced decompression. This tensile failure probably occurred distal to shear zones, where bubbles where relatively isolated. Interlinking of the micro-cracks formed extensive internal tuffisite vein networks, which acted as efficient outgassing pathways, given their access to significant quantities of preexsolved volatiles. The models presented in this thesis are relevant to the conduit processes that take place during rhyolitic eruptions; insight is provided into how rhyolitic magma ascends through the shallow (<500 m deep) crust and also into how the magma deforms during its ascent and into the processes that govern magmatic outgassing

Quantifying the link between magma ascent dynamics and tilt

Magma viscosity and its ascent rate are key factors in controlling eruption style. Shear stress exerted on the conduit walls as magma ascends pulls up the surrounding edifice, whilst overpressure pushes the edifice outwards. Magma fractures if shear stress exceeds its shear strength, triggering low-frequency seismicity. Shear stress is proportional to both the viscosity of magma and its ascent velocity. Hence, it provides an important link between ascent dynamics and both deformation and seismicity that can be recorded at the surface. Tiltmeters measure changes in inclination, and both shear stress and pressure have been linked conceptually to changes in tilt recorded close to the conduit. However, how much shear stress and pressure are produced as magma ascends, and the relative contribution of each to the tilt, has not previously been quantified. Firstly, flow and deformation modelling are combined using COMSOL Multiphysics to quantitatively link magma ascent and tilt. Despite shear stress being several orders of magnitude smaller than overpressure at most depths, shear stress generally dominates the tilt signal. Next, I systematically investigate how topography influences tilt, showing how topography controls both the amplitude and polarity of the tilt, and thus the relative contribution of shear stress and pressure. 3D deformation modelling is performed including real volcanic topography to show how a tiltmeter can be strategically deployed at the location most sensitive to changes in source stress. Finally, time-dependent flow modelling is used to show how magma ascent dynamics, and thus shear stress and overpressure, evolve through time due to transient volcanic processes. The growth of a lava dome exerts an increasing loading pressure at the conduit vent that impedes magma ascent, and can cause it to stall even if conditions at depth remain unchanged. By unloading, a full or partial dome collapse can therefore cause an eruption to recommence. By quantitatively linking magma ascent and deformation, and examining how ascent evolves through time, this work shows the importance of combining flow and deformation modelling in retrospectively investigating what drives temporal variations in seismicity and deformation. This is an important step towards being able to develop a combined forecasting tool using both seismicity and deformation that can be used to detect critical changes in ascent dynamics

Magma Pressure-Temperature-Time Paths During Mafic Explosive Eruptions

We have constrained syneruptive pressure-temperature-time (P-T-t) paths of mafic magmas using a combination of short-timescale cooling and decompression chronometers. Recent work has shown that the thermal histories of crystals in the last few seconds to hours of eruption can be constrained using concentration gradients of MgO inside olivine-hosted melt inclusions, produced in response to syneruptive cooling and crystallization of olivine on the inclusion walls. We have applied this technique to the study of melt inclusions erupted by arc and ocean island volcanoes, including the 1974 subplinian eruption of Fuego volcano; the 1977 fire-fountain eruption of Seguam volcano; and three eruptions of Kilauea volcano (episode 1 of the 1959 Kilauea Iki fire-fountain eruption, the 1500 CE vigorous fire-fountain eruption, and the 1650 CE subplinian eruption). Of the eruptions studied so far, melt inclusions from the 1959 Kilauea Iki eruption record the highest syneruptive cooling rates (3–11°C/s) and the shortest cooling durations (4–19 s), while inclusions from the 1974 Fuego eruption record the slowest cooling rates (0.1–1.7°C/s) and longest cooling durations (21–368 s). The high cooling rates inferred for the Kilauea Iki and Seguam fire fountain eruptions are consistent with air quenching over tens of seconds during and after fragmentation and eruption. Melt inclusions sampled from the interiors of small (∼6 cm diameter) volcanic bombs at Fuego are found to have cooled more slowly on average than inclusions sampled from ash (with particle diameters < 2 mm) during the same eruption, as expected based on conductive cooling models. We find evidence for a systematic relationship between cooling rates and decompression rates of magmas, in which rapidly ascending gas-bearing magmas experience slower cooling during ascent and eruption than slowly ascending magmas. Our magma P-T-t constraints for the Kilauea Iki eruption are in broad agreement with isentropic models that show that the dominant driver of cooling in the conduit is adiabatic expansion of a vapor phase; however, at Fuego and Seguam, our results suggest a significant role for latent heat production and/or open-system degassing (both of which violate assumptions required for isentropic ascent). We thereby caution against the application of isentropic conduit models to magmas containing relatively high initial water concentrations (e.g., arc magmas containing ∼4 wt% water). We note that several processes that have been inferred to occur in volcanic conduits such as magma stalling, magma mingling, open- and closed-system degassing, vapor fluxing, and vapor accumulation (in foam layers or as slugs of gas) are associated with different implied vapor volume fractions during syneruptive ascent. Given the sensitivity of magma P-T-t paths to vapor volume fraction, the syneruptive thermometer presented here may be a means of identifying these processes during the seconds to hours preceding the eruption of mafic magmas

The role of percolation threshold and water-magma interaction on volcanic eruptive style

Vulkanausbrüche werden durch den Aufstieg von Magma aus dem Erdinneren an die Oberfläche getrieben. Der Ausbruchsstil ist sehr variabel und reicht von heftigen, anhaltenden explosiven Eruptionen bis hin zur langsamen Extrusion von Lavaströmen oder Kuppeln. Folglich sind auch die damit verbundenen vulkanischen Gefahren sehr vielfältig. Der Eruptionsstil wird stark von Entga-sungsprozessen während des Magmenaufstiegs im Schlot oder in den flachen Schichten der Krus-te gesteuert, sowie der Umgebung (z.B. Luft, Wasser oder feuchte Sedimente), in die das Magma eruptiert wird. Die Art der Entgasung, die im Vulkanschlot (geschlossenes vs. offenes System) auftritt, hat eine große Bedeutung für den eruptiven Stil. Während der Entgasung im geschlossenen System führen Übersättigung und Ausfällung volatiler Phasen in Folge von Dekompression zu Blasenbil-dung und - wachstum. Dies resultiert in einem signifikanten Blasenüberdruck oder einer Beschleu-nigung des aufsteigenden Magmas, was zu einem explosiven Ausbruch führen kann. Bei der Ent-gasung im offenen System wiederum können die volatilen Phasen über die Schlotwände oder über miteinander verbundene poröse Wegsamkeiten aus dem Magma zur Atmosphäre entweichen. Die-se letztgenannte Art der Entgasung verhindert tendenziell die Ausbildung von signifikantem Blasen-überdruck oder Magmenbeschleunigung und begünstigt damit die effusive Aktivität. Der Übergang zwischen geschlossenem und offenem System erfolgt an dem Perkolationsschwellenwert, der die kritische Porosität beschreibt, bei der das Magma von inpermeabel zu permeabel (oder umgekehrt) übergeht. Dieser Schwellenwert kann z.B. durch Blasenkoaleszenz, Sprödbruch oder verdichten-dem Verschweißen im Schlot erreicht werden, wodurch das Magma zwischen überdruckgünstigen Bedingungen und Gasaustritt in der Leitung umschaltet. Der Perkolationsschwellenwert entspricht dem Beginn der Porenkonnektivität und Permeabilität und kann daher mit Hilfe dieser Parameter qualitativ und quantitativ eingeschränkt werden. Im Rahmen dieser Dissertation wurden die Konnektivität-Porosität-Beziehungen einer Serie von vulkanischen Gesteinen untersucht und in einer Datenbank aus Literaturwerten und eigenen Messungen zusammengestellt. Weiterhin wurde die Rolle von Kristallen auf den Perkolations-schwellenwert anhand von 4D-Synchrotron-Vesikulations- und Sinterexperimenten an kristallhalti-gen Magma-Analoga in Kombination mit Röntgenmikrotomographie durchgeführt. Die Kombination von Helium-Pyknometrie und Röntgen-Tomographie erlaubte es, die Methoden zur Quantifizierung des Perkolationsschwellenwert zu verbessern und den Unterschied zwischen den beiden Techni-ken gründlich zu untersuchen. Konnektivität-Porosität-Beziehungen wurden systematisch mit Permeabilität-Porosität-Beziehungen verglichen. Die Porenkonnektivität ist eine nützliche und bislang unzureichend genutzte Metrik, um eruptive Prozesse bei Vulkanausbrüchen zu untersuchen. Diese erlaubt zum Beispiel die Unter-scheidung zwischen vulkanischen Produkten, wie Gesteinen aus explosiver und effusiver Aktivität, Produkten der Vesikulation und Verdichtung und Scoria hawaiianischer oder strombolianischer Ak-tivität. Es ermöglicht auch eine bessere quantitative Bewertung des Perkolationsschwellenwertes im Vergleich zur Permeabilität, da dies bei Porositäten unterhalb und oberhalb dieser Schwelle er-mittelt werden kann, während dies bei der Permeabilität nicht möglich ist. Die Beziehungen zwi-schen Porenkonnektivität und Porosität, kombiniert mit einer texturellen Untersuchung der vulkani-schen Produkte, erlauben Rückschlüsse darauf, welche Entgasungsprozesse im Magma vor der Eruption vorherrschend waren. Diese Beziehungen bedürfen jedoch einer sorgfältigen Prüfung, da sich das Magma nach der Fragmentation und Ablagerung texturell weiterentwickeln kann. Außer-dem können unterschiedliche Entgasungsprozesse und Konnektivitäts-Porositätspfade zu sehr ähnlichen Endprodukten führen. Effusivgesteine lassen sich durch einen sehr niedrigen Perkolati-onsschwellenwert erklären, der durch Blasenverformung, Sprödbruch, Verdichtung und Vesikulati-on in hochkristallinen Schmelzen hervorgerufen wird und die Ausgasung und Reduzierung des Blasenüberdrucks begünstigt. Produkte primär explosiver Tätigkeit wie Scoria und Bimsstein wie-derum stammen meist aus kristallarmen Magmen mit einer stark polydispersen Blasengrößenver-teilung und häufig ohne Deformation, was alles zu einem hohen Perkolationsschwellenwert führt. Diese Magmen-Fragmente entstehen durch hohen Blasenüberdruck oder Magmenbeschleunigung, da der hohe Perkolationsschwellenwertden Gasaustritt in den Schlot behindert oder verzögert. Zu-künftige Studien sollten die Zusammenhänge zwischen Perkolationsschwellenwert, Entgasung und Fragmentation weiter untersuchen und die komplexe Wirkung des Perkolationsschwellenwertes in numerische Modelle zur Magmenentgasung in einem Vulkanschlot einbeziehen. Sobald Magma an der Erdoberfläche explosionsartig oder effusiv ausbricht, können weitere Veränderungen des eruptiven Stils durch die Eruptionsumgebung hervorgerufen werden. Bei-spielsweise wird das Vorhandensein von Meerwasser über dem eruptiven Schlot die eruptiven Prozesse wie Kühlung, Fragmentierung, Vesikulation und Aggregation aufgrund der unterschiedli-chen physikalischen, thermischen und chemischen Eigenschaften von Meerwasser im Vergleich zu Luft dramatisch verändern. Surtseyanische Ausbrüche sind flache subaquatische Ausbrüche, die oft im Laufe der Eruption die Wasseroberfläche durchbrechen, subaerisch werden und Tuffke-gel bilden. Die eruptiven Prozesse sind bei diesen Eruptionen sehr komplex, da sich die Wasser-Magma-Wechselwirkungen während der fortschreitenden Konstruktion des Tuffsteinkegels räum-lich und zeitlich entwickeln. Es ist von hoher Wichtigkeit, die eruptiven Prozesse während sur-tseyanischer Aktivität zu verstehen, da diese Art von subaquatischen Ausbrüchen Gefahren für Bevölkerung und Störungen des Flugverkehrs durch die hohe Bildung feiner Aschepartikel verur-sachen kann. Diese Arbeit untersucht die Rolle der Wasser-Magma-Wechselwirkung auf die eruptiven Prozesse während solcher surtseyanischer Ausbrüche und die Auswirkungen auf die damit ver-bundenen Gefahren. Lapilli und Bomben von mehreren surtseyanischen Ausbrüchen an den Vul-kanen Hunga Tonga-Hunga Ha'apai und Capelinhos wurden mittels Messungen von Porenmetriken und 3D-Texturanalyse mit Hilfe von Röntgenmikrotomographie analysiert. Die strukturellen Merk-male und Porenmetriken wurden mit numerischen thermischen Modellen kombiniert, um die Abküh-lungsdynamik in den Lapilli und Bomben zu begrenzen. Die mit dem thermischen Modell ermittelten Abkühlraten wurden mit Literaturdaten von Abkühlraten verglichen, die mit Hilfe der Geospeedo-metrie an subaquatischen Produkten gemessen wurden. Lapilli und Bomben, die während surtseyanischer Ausbrüche gebildet wurden, zeigen all-mähliche strukturelle Variationen mit einer Zunahme der Vesikelkonnektivität von Rand zu Kern, die durch die Vesikulation nach der Fragmentation verursacht wird, die in verschiedenen Stadien durch Abschrecken im Wasser unterbrochen wird. Die Abkühlung der Ränder der Lapilli erfolgt durch Wärmeleitung bei direktem Kontakt mit Wasser und durch Strahlung und Konvektion bei Vorhan-densein eines stabilen Dampffilms (Leidenfrost-Effekt). In beiden Fällen führen die hohen Abkühlra-ten an den Rändern zu einer raschen Abschreckung und Unterbrechung der Vesikulation, was die niedrigen Vesikularitäten und Vesikelkonnektivität erklärt. Im Kern sind die Abkühlungsraten viel niedriger und die Zeit, die für die Vesikulation zur Verfügung steht, ist dramatisch höher, was die höheren Vesikularitäten und Vesikelkonnektivität erklärt. Die Abkühlraten in Pyroklasten aus sub-aquatischen Ausbrüchen zeigen eine große Bandbreite, die hauptsächlich von der Partikelgröße, der Art der Abkühlung an der Oberfläche (direkter Kontakt Wasser oder Leidenfrost-Effekt), der Schmelztemperatur und der radialen Position im Partikel abhängen. Die hohen Abkühlraten an den Rändern und die niedrigen Abkühlraten in den Kernen der Lapilli verursachen eine hohe thermische Belastung an den Rändern. Dies führt zu thermischer Rissbildung und thermischer Granulierung der Ränder und zur Bildung von Aschepartikeln, die ascheumrandete Lapilli bilden, in denen die Aschepartikel puzzelmäßig ineinandergreifen. Lösungs-Experimente ergaben, dass die Bindung von Aschepartikeln in den Rändern der mit Asche umhüllten Lapilli durch erhebliche Salzausfällun-gen (meist NaCl und CaSO4) stabilisiert wird, die durch die Verdunstung von Meerwasser verur-sacht werden. Die Salzkonzentration kann möglicherweise als Indikator für den Grad der Wasser-Magma-Wechselwirkung bei surtseyanischen Ausbrüchen dienen. Die mit Asche umhüllten Lapilli galten früher als beschichtete oder gepanzerte Lapilli, die durch Aggregation in einer asche- und dampfreichen Umgebung gebildet wurden. Das neue Modell besagt, dass die Ummantelung nicht notwendigerweise ein Beweis für die Aggregation von Partikeln ist, sondern vielmehr aus der Bil-dung neuer primärer Asche resultieren kann, mit deutlichen Auswirkungen auf die damit verbunde-nen Gefahren. Die thermische Granulierung gilt als ein wichtiger Mechanismus, der bei surtseyani-schen Ausbrüchen zur Aschebildung führt. In zukünftigen Studien wird das Verständnis des Gleichgewichts zwischen der Aggregation in der Aschewolke und der subaquatischen Produktion von Asche durch thermische Granulation ein Schlüsselfaktor für eine bessere Abschätzung poten-zieller Gefahren im Zusammenhang mit der Aschedispersion in flachen subaquatischen Settings sein. Diese Arbeit kombiniert 3D-Röntgen-Tomographie, porenmetrische Messungen, experimen-telle Arbeiten an Magma-Analoga, numerische Modellierung und chemische Analysen, um unser Wissen über den Einfluss von Entgasungsprozessen und Wasser-Magma-Interaktion auf den Stil von Vulkanausbrüchen und die damit verbundenen Gefahren zu verfeinern. Dieser Ansatz führte zu innovativen Ergebnissen mit neuartigen Schlussfolgerungen über die Mechanismen, die effusi-ve-explosive Übergänge während des Magmaaufstiegs auslösen, sowie für die Modifikationen von eruptiven Prozessen und Gefahren, die durch die Abkühlung im Wasser an der Erdoberfläche her-vorgerufen werden.Volcanic eruptions are driven by the generation and ascent of magma from the earth interior to its surface. The style of volcanic eruptions is highly variable and ranges from violent, sustained explo-sive eruptions to slow extrusion of lava flows or domes. Consequently, the related volcanic hazards are also highly diverse. Eruptive style is strongly controlled by degassing processes in the conduit during magma ascent at shallow levels of the crust as well as by the nature of the eruptive, cooling environment (e.g., air, water or wet sediments). The type of degassing occurring in the conduit (closed- vs. open-system) has a major con-trol on the eruptive style. During closed-system degassing, volatile exsolution leads to bubble nucle-ation and growth during decompression, causing significant bubble overpressure or acceleration of the gas phase possibly resulting in explosive fragmentation of the magma. During open-system de-gassing, in turn, the volatiles can freely escape from the magma to the conduit walls or the atmos-phere through interconnected porous pathways. This latter mode of degassing tends to impede sig-nificant bubble overpressure or acceleration and hence promotes effusive activity. The transition between closed- and open-system conditions occurs at the percolation threshold, which is the criti-cal porosity at which the magma transitions from impermeable to permeable (or vice versa). This threshold can be achieved for instance via bubble coalescence, brittle fracturing or densification in the conduit, causing the magma to switch between conditions favourable for overpressure and gas escape in the conduit. The percolation threshold corresponds to the onset of pore connectivity and permeability and can therefore be qualitatively to quantitatively constrained using these metrics. I studied the connectivity-porosity relationships of a suite of volcanic rocks, compiled in a database from literature and own measurements. I also examined the role of crystals on the perco-lation threshold by performing 4D synchrotron vesiculation and sintering experiments on crystal-bearing magma analogues combined to X-ray micro-tomography. Combination of Helium pyc-nometry and X-ray tomography techniques allowed to improve the methods for quantification of the percolation threshold and the difference between the two techniques was thoroughly examined. Connectivity-porosity relationships were systematically compared to permeability-porosity relation-ships. Pore connectivity is a useful and underutilized metric to study the eruptive processes during volcanic eruptions. First, it allows distinguishing between subsets of volcanic products, including rocks derived from explosive and effusive activity, products of vesiculation and densification and scoria of Hawaiian and Strombolian activity. It also allows a better quantitative assessment of the percolation threshold compared to permeability because it can be constrained at porosities below and above this threshold, whereas permeability cannot. Pore connectivity-porosity relationships, combined with a textural study of the volcanic products, allow to infer which degassing processes were dominant in the parent magma prior to the eruption. However, these relationships require careful consideration because the magma can continue to evolve texturally after fragmentation or emplacement. Besides, different degassing processes and connectivity-porosity paths can lead to very similar final erupted products. Effusive rocks can be explained by very low percolation thresh-old due to bubble deformation, brittle fracturing, densification and vesiculation in highly crystalline melts, which all promote outgassing and reduction of bubble overpressure. In turn, products from explosive activity such as scoria and pumices originate mostly from crystal-poor magmas with a highly polydisperse bubble size distribution which all tend to increase the percolation threshold. These magmas fragment due to high bubble overpressure or acceleration of the gas phase due to a high percolation threshold which impedes or delays gas escape in the conduit. Future studies should further examine the relationships between percolation threshold, degassing and fragmentation and incorporate the complex effect of the percolation threshold in numerical models of conduit degas-sing. Once magma is erupted explosively or effusively at the earth surface, further modifications of the eruptive style can be induced by the cooling environment. For instance, the presence of seawater above the eruptive vent in subaqueous settings will dramatically alter the eruptive pro-cesses such as cooling, fragmentation, vesiculation and aggregation because of the different phys-ical, thermal and chemical properties of seawater compared to air. Surtseyan eruptions are shallow subaqueous eruptions becoming progressively emergent and leading to the formation of tuff cones. The eruptive processes are highly complex during these eruptions because the water-magma inter-actions evolves spatially and temporally during progressive construction of the tuff cone. It is of paramount importance to understand the eruptive processes during Surtseyan activity because this type of subaqueous eruptions can cause hazards to populations and disturbance of air traffic due to extensive generation of fine ash particles. This thesis investigates the role of water-magma interaction on the eruptive processes dur-ing Surtseyan eruptions and the implications for related hazards. Lapilli and bombs from several Surtseyan eruptions at Hunga Tonga-Hunga Ha’apai and Capelinhos volcanoes were analysed via measurements of pore metrics and 3D textural analysis via X-ray micro-tomography. The textural features and pore metrics were combined with numerical thermal modelling in order to constrain the cooling dynamics in the lapilli and bombs. Cooling rates obtained with the thermal model were compared to literature data of cooling rates measured by geospeedometry on subaqueous prod-ucts. Leaching experiments were then performed in order to constrain the role of salts on particle binding in Surtseyan deposits. Lapilli and bombs formed during Surtseyan eruptions exhibit gradual textural variations with increase of vesicle connectivity from margin to core caused by post-fragmentation vesiculation in-terrupted at different stages by quenching in water. Cooling of the margins of the lapilli occurs by conduction in the case of direct contact with water and by radiation and convection in the case of the presence of a stable vapour film (Leidenfrost effect). In both cases, the high cooling rates at the margins induce rapid quenching and interruption of vesiculation, explaining the low vesicularities and vesicle connectivities. In the core, the cooling rates are much lower and the time available for vesiculation is dramatically higher, explaining the higher vesicularities and vesicle connectivities. Cooling rates in pyroclasts from subaqueous eruptions show a wide range of values, depending mostly on the particle size, type of cooling at the surface (direct contact of Leidenfrost effect), melt temperature and radial position in the particle. The high cooling rates at the margins and low cooling rates in the cores of the lapilli cause high levels of thermal stress at the margins. This leads to ther-mal cracking and in situ thermal granulation of the margins and generation of ash particles that are kept in place in a jigsaw fit pattern, forming ash-encased lapilli. Leaching experiments revealed that the binding of ash particles in the ash rims of the ash-encased lapilli is stabilized by substantial salt precipitation (mostly NaCl and CaSO4) caused by seawater evaporation. Salt concentrations can potentially serve as an indicator of the degree of water-magma interaction during Surtseyan erup-tions occurring in seawater settings. Ash-encased lapilli were previously considered coated or ar-moured lapilli formed by aggregation in an ash and vapour-rich environment. The new model pre-sented here implies that encasement is not necessarily evidence of particle aggregation, but can instead result from new primary ash formation, with distinct implications for related hazards. Ther-mal granulation is considered an important disruption mechanism causing the generation of ash during Surtseyan eruptions. In future studies, understanding the balance between in-plume aggre-gation and subaqueous production of ash by thermal granulation will be a key for better assessment of potential hazards related to ash dispersal in Surtseyan settings. This thesis combined 3D X-ray tomography, pore metric measurements, experimental work on magma analogues, numerical modelling and chemical analysis to refine our knowledge of the influence of degassing processes and water-magma interaction on the style of volcanic eruptions and their related hazards. This approach yielded innovative results with novel implications for the mechanisms triggering effusive-explosive transitions during magma ascent and the modifications of eruptive processes and hazards induced by magma cooling in water at the earth surface

Basaltic Plinian eruptions at Las Sierras-Masaya volcano driven by cool storage of crystal-rich magmas

Although rare, basaltic Plinian eruptions represent a considerable volcanic hazard. The low viscosity of crystal-poor basaltic magma inhibits magma fragmentation; however, Las Sierras-Masaya volcano, Nicaragua, has produced multiple basaltic Plinian eruptions. Here, we quantify the geochemistry and volatile concentrations of melt inclusions in samples of the Fontana Lapilli and Masaya Triple Layer eruptions to constrain pre-eruptive conditions. Combining thermometry and geochemical modelling, we show that magma cooled to similar to 1000 degrees C prior to eruption, crystallising a mush that was erupted and preserved in scoriae. We use these data in a numerical conduit model, which finds that conditions most conducive to Plinian eruptions are a pre-eruptive temperature &lt;1100 degrees C and a total crystal content &gt;30 vol.%. Cooling, crystal-rich, large-volume basaltic magma bodies may be hazardous due to their potential to erupt with Plinian magnitude. Rapid ascent rates mean there may only be some minutes between eruption triggering and Plinian activity at Masaya

Combining experimental volcanology, petrology and geophysical monitoring techniques

In general, an understanding of the complex processes acting before and during volcanic eruptions is approached from various different sides, e.g. laboratory experiments on fragmentation and/or bubble burst eruption mechanisms, petrological analysis of the eruptive products and various geophysical monitoring and source localization techniques. Each of these techniques can deliver valuable insights by adding pieces of information about the physical processes that drive the volcanic activity. However, often studies are focussing on a single aspect of the process, without setting the results in a more general context. Often, this strategy is absolutely valid, when the focus is laid on a single piece in the complex chain of processes taking place in volcanic eruptions. This must fail when the results aim to suggest a valid model for the combined observations at volcanoes using the above described techniques. The resulting models of volcanic source mechanisms and eruptive features can therefore lead to biased assumptions. This study aims to close this gap between laboratory experiments, petro-chemical analysis and modern geophysical monitoring and source localization techniques in a case study of Mt. Yasur (Vanuatu) volcano. The presented laboratory experiments on explosive volcanic eruptions upon rapid decompression show that decompression rate is the dening parameter in the experiments and that a scaling to large-scale processes is valid. Furthermore, a model is presented that correlates measured particle velocities to decompression rate and initial gas-overpressure. This model is used to estimate source volumes and overpressures at Volcan de Colima (Mexico) and Mt. Yasur (Vanuatu). A petrographically and geochemically characterization of Mt. Yasurs eruptive products suggests a shallow magma-mingling process at both of Mt. Yasurs active craters, perhaps due to rejuvenation of material slumped from the crater walls into an open conduit system. A study on the time-reversal imaging technique and its ability to detect the details of finite rupture (or time-variant) processes shows that the limitations of TR imaging start where the source stops being point-localised with respect to the used wavelength. Inversion of the source mechanisms of Strombolian explosions at Mt. Yasur are performed using a multi-parameter dataset consisting of seismic, acoustic and Doppler-radar data. Time-reversal imaging and moment tensor inversion are used to invert the source location of the seismic long-period (f < 1Hz) signals, which is supposed to refl ect fluid movement at depth. The source is located in the north-east of the crater region in a depth of several hundred meters. Furthermore, the source volume of the radiated infrasound signals is estimated from fundamental resonance frequencies. The results showed that the maximum particle velocity measured with the Doppler radar correlates nicely with the estimated source volumes lengths. The inverted seismic moment does not show any correlation with the estimated slug sizes, i.e. the slug size does not map in seismic moment. This is an important information, as it states that a larger source volume does not necessarily produces a larger seismic moment. From these combined results, a common feeder system for all active craters at Mt. Yasur is proposed. The differences in event recurrence rate at the three active craters are believed to be controlled by either the conduit geometry or variations in degassing or cooling rate. Strombolian-type eruptions at Mt. Yasur are suggested to be due to the burst of gas slugs with lengths and overpressures comparable to volcanoes showing similar eruptive patterns. The results illustrate the importance of combined studies that overcome the limitations of single disciplines. In this way, a more comprehensive view of volcanic eruptions and the associated observations is possible. Such a multi-disciplinary approach will contribute to a better understanding of volcanic processes and the associated hazards

A numerical model for temporal variations during explosive central vent eruptions

An axisymmetrical numerical model has been developed in order to find the temporal evolution of pressure, the position of the exsolution level, the velocity field, the eruption rate, and the amount of erupted material of a shallow, volatile‐rich, felsic magma chamber during a Plinian central vent eruption. The overpressure necessary to trigger the eruption is assumed to result from crystallization‐driven volatile oversaturation. We solve the resulting set of equations using a finite element method. The results obtained show that the pressure at the conduit entrance decreases exponentially as the eruption proceeds. This produces a shifting of the exsolution level, so that deeper parts of the chamber become progressively volatile oversaturated during the eruption. We assess the influence of chamber geometry and the physical properties of the magma on the computed parameters using several numerical examples. The results are also compared with those predicted by previous models from the literature and are found to be in good agreement with documented eruptions. The model constitutes a first attempt to numerically model the dynamics and the temporal evolution of the most relevant physical parameters during withdrawal from a closed magma chamber