The dynamics of starting gas-particle jets: a volcanic scenario

Explosive volcanic eruptions are a threat for a large part of global population and infrastructures. Explosive eruptions are the results of energetic magma fragmentation, where only gas exsolved in the magma drive the eruption, or of the interaction with external water. The mechanisms of fragmentation are complex and various, but despite that at explosive eruption onset the potential energy stored in gas bubbles in the magma always transforms into kinetic energy via gas expansion and produce the ejection of pyroclasts and/or non-juvenile material in the atmosphere. Particle ejection rate, velocity and trajectory differ depending on source conditions, e.g. magma composition, gas overpressure, conduit length, vent geometry, etc. Field observations, when possible, can help to characterize an ejection from which then the source conditions are indirectly retrieved. High-speed and infrared videos of volcanic ejections, seismic and acoustic measurements, as well as petrographycal and geochemical analysis on the pyroclasts ejected offer insight on the eruptive event. Nevertheless, to link observations and source parameters is not trivial and it still requires a certain number of assumptions. Therefore, the knowledge of source conditions stays uncertain. On the other hand, empirical studies can help linking observations and input parameters, since the latter are chosen experimental conditions. In general, laboratory experiments are far less complex than natural eruptions. However, the simplifications imposed benefit the investigation of single processes as well as the understanding of the effects of boundary conditions on such observed dynamics. The goal, at the end, is to learn the patterns of certain dynamics and possibly, to recognize certain characteristics of volcanic eruptions and be able to associate them to source conditions. Additionally, empirical results provide input parameters for numerical modelling and thus hazard assessment. I perform rapid decompression experiments of gas-particle mixtures generating starting jets. I use two different experimental apparatus, the first is the “fragmentation bomb” at the LMU facility and the second the “jet buster” at INGV Rome. With the two setups, it is possible to characterize the effect of boundary conditions such as: 1) vent geometry, 2) tube length, 3) particle load and size, 4) temperature, and 5) overpressure in the reservoir on the dynamics of the ejection of natural particles of different initial size distribution (from 0.125 to 4 mm). In particular, I focus the analysis on particle velocity and trajectory. Observations on particle fragmentation, mass ejection rate and lightning generation are also possible on experiments from the “fragmentation bomb”. The experiments are recorded with a high-speed camera, which provides visual observation of the dynamics. On the “jet buster” experiments, the video recordings are coupled with piezoelectric sensors providing microseismic signals of the related propagation dynamics. The two apparatus are different and complementary. The “fragmentation bomb”, a shock-tube made of metal, is 24 cm long, allows high overpressures (here 150 bar) and temperatures (here 500°C), gas and particles are pressurized in the same chamber and the observations are made at vent exit. The “jet buster”, on the other hand, with its 3 m of transparent PMMA tube allows the observation of the whole propagation and dynamics inside the pipe as well at vent exit. The overpressure threshold is in the order of few bar (here 2 bar), and the gas reservoir is separated and below the sample chamber. In the “fragmentation bomb” experiments, maximum particle velocity shows, in order of importance, 1) negative correlation with tube length; 2) positive correlation with particle load; 3) positive correlation with flaring vent walls, with peaks for funnel 15; 4) positive correlation with temperature, and 5) negative correlation with particle size. The evolution of particle velocity with time in non-linear and is mostly affected by particle load and tube length. Gas maximum initial spreading angle shows, in order of importance: 1) negative correlation with flaring vent walls; 2) negative correlation with experimental temperature; 3) positive correlation with tube length; 4) positive correlation with particle size, and 5) negative correlation with particle load. The gas spreading angle evolution with time shows a bell shape pattern and it is especially appreciable in setup 1 experiments, due to the particles later arrival. This is the main affecting parameter. The particle initial spreading angle shows: 1) positive correlation with particle load, 2) negative correlation with particle size; 3) negative correlation with vent geometry; 4) positive correlation with tube length, and 5) negative correlation with temperature. The particle spreading angle evolution with time shows patterns varying in particular with particle load and tube length. Estimations of the mass ejection rate (MER) and instantaneous mass or particle concentration show peaks of 26kg/s for setup 2 experiments, 7 kg/s for setup 3 and 4.6 kg/s for setup 1. The evolution of the MER with time reflects the evolution of particle velocity with time. Finally, mm to cm electrical discharges, i.e. lightning, are observed. Their appearance is positively correlated with particle load, and negatively correlated with tube length, temperature, particle size, and flaring of vent walls. In the “jet buster”, I perform both gas only and gas-particle mixture experiments. This to compare the elastic response of the system and jets’ dynamics. The gas only experiments includes a pinch of kaolin powder in order to make the flow front propagation visible in the camera. The gas flow front shows an initial fast propagation (up to 500m/s) in the pipe accompanied by an abrupt deceleration (to 150 m/s) at vent exit were it generates a vortex ring. On the other hand, particles show maximum velocities between 40 to 100 cm in the pipe in respect to initial sample position. In addition, in this case, maximum particle velocity shows negative correlation with particle size and the evolution of particle velocity displays a non-linear trend. Good correlation between microseismic signals and process occurring in the pipe is observed. The comparison of the experimental results with natural data collected on Stromboli volcano, Italy, is far from trivial. As mentioned above, volcanic eruptions are characterized by the interaction of several processes, thus making them far more complex. Nevertheless, I think the data set present here provides a promising link for both field volcanology (visual observations and quantitative monitoring) as well as numerical modelling in order to advance our understanding of explosive volcanic eruptions and assess the related hazard

Linking gas and particle ejection dynamics to boundary conditions in scaled shock-tube experiments

Predicting the onset, style and duration of explosive volcanic eruptions remains a great challenge. While the fundamental underlying processes are thought to be known, a clear correlation between eruptive features observable above Earth's surface and conditions and properties in the immediate subsurface is far from complete. Furthermore, the highly dynamic nature and inaccessibility of explosive events means that progress in the field investigation of such events remains slow. Scaled experimental investigations represent an opportunity to study individual volcanic processes separately and, despite their highly dynamic nature, to quantify them systematically. Here, impulsively generated vertical gas-particle jets were generated using rapid decompression shock-tube experiments. The angular deviation from the vertical, defined as the \textquotedblspreading angle\textquotedbl, has been quantified for gas and particles on both sides of the jets at different time steps using high-speed video analysis. The experimental variables investigated are 1) vent geometry, 2) tube length, 3) particle load, 4) particle size, and 5) temperature. Immediately prior to the first above-vent observations, gas expansion accommodates the initial gas overpressure. All experimental jets inevitably start with a particle-free gas phase (gas-only), which is typically clearly visible due to expansion-induced cooling and condensation. We record that the gas spreading angle is directly influenced by 1) vent geometry and 2) the duration of the initial gas-only phase. After some delay, whose length depends on the experimental conditions, the jet incorporates particles becoming a gas-particle jet. Below we quantify how our experimental conditions affect the temporal evolution of these two phases (gas-only and gas-particle) of each jet. As expected, the gas spreading angle is always at least as large as the particle spreading angle. The latter is positively correlated with particle load and negatively correlated with particle size. Such empirical experimentally derived relationships between the observable features of the gas-particle jets and known initial conditions can serve as input for the parameterisation of equivalent observations at active volcanoes, alleviating the circumstances where an a priori knowledge of magma textures and ascent rate, temperature and gas overpressure and/or the geometry of the shallow plumbing system is typically chronically lacking. The generation of experimental parameterisations raises the possibility that detailed field investigations on gas-particle jets at frequently erupting volcanoes might be used for elucidating subsurface parameters and their temporal variability, with all the implications that may have for better defining hazard assessment. Supplementary Information The online version contains supplementary material available at 10.1007/s00445-021-01473-0

The influence of complex volcanic vent morphology on eruption dynamics

Vulkanausbrüche gelten als eine der spektakulärsten Naturgewalten unserer Erde. Gleichzeitig stellen sie jedoch auch eine Gefahr für die menschliche Gesundheit und Infrastruktur dar. Aufgrund ihrer Dynamik und ihres unberechenbaren Charakters geht von explosiven Vulkanausbrüchen eine besonders große Gefährdung des Menschen und seiner Umwelt aus. Im Zuge eines explosiven Ausbruchs werden heiße Gase und Pyroklasten in die Atmosphäre ausgeworfen. Obwohl das Monitoring aktiver Vulkane in den letzten Jahren immer weiter verbessert wurde, ist es immer noch schwierig eine konkrete Vorhersage zu den Ausbrüchen zu erstellen. Aufgrund ihrer Komplexität ist das Verhalten von Vulkanen nicht kalkulierbar. Bis heute ist weder eine Beobachtung, noch eine Messung der unterirdischen Rahmenbedingungen möglich, welche den Ausbruch steuern. Trotz dieser Unwägbarkeiten unterliegen Vulkanausbrüche dennoch physikalischen Gesetzmäßigkeiten, sodass die Möglichkeit besteht, die Prozesse im Untergrund eines Vulkans zu modellieren oder durch Experimente zu beschreiben. Aufgrund der Komplexität der Wechselwirkungen innerhalb des Systems Vulkan ist es erforderlich Experimente zunehmend realistischer zu gestalten. Sobald das ausgeworfene Material aus dem Krater austritt können wir den Ausbruch visuell Beobachten. In diesem Bereich ist das Verhalten des Ausbruchs vollständig von den Prozessen im Untergrund und von der Geometrie des Kraters abhängig. Im Vergleich zu den symmetrischen Kraterformen, welche in Experimenten und Modellen oft angenommen werden, sind die Krater in der Natur deutlich unregelmäßiger geformt. Ihre Geometrien sind oft eingekerbt und haben eine schräge Oberfläche. Zudem können sich die Kratergeometrien innerhalb kürzester Zeit verändern. Um den Einfluss der Prozesse im Untergrund zu verstehen müssen wir zuerst den Einfluss der beobachtbaren Parameter (z. B. Kratergeometrie) ergründen. Schlussendlich wird ein tiefergehendes Verständnis der Parameter, die Vulkanausbrüche steuern, zu einem Fortschritt und der Verbesserung der Gefährdungsanalysen führen. Um dies zu erreichen, habe ich Beobachtungen aus Feldkampagnen und Laborexperimenten kombiniert. Zunächst habe ich die Geometrien von Vulkankratern erfasst und deren zeitliche Entwicklung dokumentiert. Dazu haben ich die Geometrie der Krater in der Kraterterrasse des Strombolis in einer hohen Auflösung vermessen und die jeweils zugehörigen Explosionen beobachtet. Dabei konnte ich feststellen, dass sowohl die Intensität, als auch die Art und die Richtung der Ausbrüche durch Formveränderungen der Oberflächentopografie beeinflusst werden. Mittels Drohneneinsatz habe ich innerhalb eines Zeitraums von neun Monaten (Mai 2019–Januar 2020) fünf topografische Datensätze erstellt. In diesem Zeitraum war es möglich „normale“ Strombolianische Aktivität, starke Ausbrüche und sogar zwei Paroxysmen zu beobachten (3. Juli und 28. August 2019), sodass es möglich war, die verschiedenen Ausbruchstypen mit den vorherrschenden Ablagerungs- und Abtragungsprozessen zu verknüpfen. Zudem konnte ich die Anzahl der aktiven Krater, deren Positionen sowie deren Umgestaltung nachverfolgen. Da Veränderungen der Kratergeometrie und der Kraterposition auf eine Modifikation des Ausbruchsgeschehens hinweisen können, sind auch dies wichtige Faktoren für eine Gefährdungsanalyse. Die aus den Feldforschungen gewonnenen Daten zeigen deutlich die Komplexität, Vielseitigkeit und Variabilität der Formen vulkanischer Krater in einer nie da gewesenen zeitlichen und räumlichen Auflösung. Darüber hinaus haben die Beobachtungen der Vulkanausbrüche deutlich gemacht, wie stark die Beziehung zwischen dem Krater, der Kratergeometrie und dem Auswurf von pyroklastischem Material ist. Diese Erkenntnis hat eine große Bedeutung für die Gefährdungsanalyse, vor allem für Gebiete, die potentiell durch vulkanische Bomben und pyroklastischem Fallout bedroht sind. Im Anschluss habe ich eine Reihe von Dekompressionsexperimenten mit Kratergeometrien durchgeführt, welche auf den Beobachtungen am Stromboli aufbauen. Durch diese Experimente wurde der Zusammenhang zwischen Kratergeometrie und Ausbruchsdynamik bestätigt. Die verwendeten Geometrien haben eine geneigte Oberfläche mit einem Winkel von 5°, 15° und 30° und jeweils einer zylindrischen und einer trichterförmigen inneren Geometrie. Daraus ergeben sich sechs experimentelle Krater die mit folgenden experimentellen Bedingungen getestet wurden: Vier unterschiedliche Startdrücke (5, 8, 15 und 25 MPa) und zwei Gasvolumina (127.4cm3, 31.9cm3). Alle Experimente wurden bei Raumtemperatur und mit Argon durchgeführt. Trotz des vertikalen Aufbaus konnte man auf beiden Seiten des Kraters unterschiedlich große Winkel des austretenden Gases beobachten. Weiterhin war der Gasstrahl geneigt. Die Richtung der Neigung wurde durch die innere Geometrie be- stimmt. Bei einer zylindrischen Geometrie neigte sich der Gasstrahl in die Einfallsrichtung der geneigten Oberfläche. Im Falle einer trichterförmigen inneren Geometrie neigt sich der Gasstrahl entgegen der Einfallsrichtung. Der Winkel des Gasaustritts war bei einer zylindrischen inneren Geometrie immer größer als bei der trichterförmigen Geometrie. Sowohl die Winkel des Gasaustritts als auch die Neigung des Gasstrahls zeigten eine starke Reaktion auf eine Veränderung der Druckbedingung und Oberflächenneigung. Dabei zeigten sowohl der Austrittswinkel als auch die Neigung eine positive Korrelation mit dem Druck und der Oberflächenneigung. Hohe Druckbedingungen haben außerdem dafür gesorgt, dass für einen längeren Zeitraum Überdruckverhältnisse am Kraterausgang herrschten. Ein höheres Gasvolumen hat größere Gasaustrittswinkel ermöglicht. Zuletzt habe ich die Dekompressionsexperimente durch den Einsatz von Partikeln ergänzt, um so den Auswurf von Gas und Partikeln während eines explosiven Vulkanausbruchs nachzustellen. Dabei habe ich die beiden experimentellen Kratergeometrien aus den vorangegangenen Experimenten ausgewählt, welche den stärksten Einfluss auf die Gasdynamik aufgezeigt haben. Zusätzlich habe ich eine dritte Kratergeometrie verwendet, die dem aktiven Krater S1 auf Stromboli nachempfunden ist. Die Geometrie entspricht der Kratergeometrie aus der Vermessung im Mai 2019. Die S1 Geometrie zeichnet sich durch einen asymmetrischen Öffnungswinkel aus (~10° auf einer Seite, ~40° auf der anderen Seite). Zusätzlich zu den drei Kratergeometrien wurden unterschiedliche Partikel verwendet (Schlacke und Bims), mit jeweils drei unterschiedlichen Korngrößen (0.125–0.25, 0.5–1 und 1–2mm) und zwei Druckstufen (8 und 15MPa). Die Partikeldynamik, in der Nähe des experimentellen Kraters, wurde anhand der Winkel des Partikelauswurfs und der Geschwindigkeit der Partikel definiert und beschrieben. Dabei wurde festgestellt, dass die Geometrie des Kraters die Richtung und Neigung des Partikelauswurfswinkels und die Geschwindigkeit der Partikel bestimmt. Bei allen Kratergeometrien kam es zu einem asymmetrischen Partikelauswurf und im Falle von Bimspartikeln zudem zu einer ungleichmäßigen Geschwindigkeitsverteilung. Die Kombination aus Daten aus Feldkampagnen, Experimenten mit Gas und Experimenten mit zusätzlichen Partikeln zeigte deutlich den starken Einfluss der Kratergeometrie auf Eruptionen. In der Natur, führt eine modifizierte Kratergeometrie zu einem verändertem Auswurfsmuster der Pyroklasten. Im Labor haben komplexe Kratergeometrien zu geneigten Gasstrahlen, asymmetrischen Auswurfswinkeln von Gas- und Gaspartikeln und einer asymmetrischen Verteilung der Geschwindigkeit von Partikeln geführt. Auf Basis dieser Beobachtungen komme ich zu dem Schluss, dass asymmetrische Vulkankrater eine asymmetrische Verteilung von pyroklastischem Auswurf hervorrufen. Das führt zu einer bevorzugten Richtung für vulkanischen Fallout — und falls es zu einer kollabierenden Ausbruchsäule kommt — zu einer bevorzugten Richtung für pyroklastische Ströme. Der technische Fortschritt durch Drohnen, Photogrammmetrie und 3D Druck bietet einige Chancen für die Vulkanologie. Luftaufnahmen durch Drohnen ermöglichen eine schnelle, günstige und sichere Vermessung von Vulkankratern, auch in Zeiten erhöhter Aktivität. Zusammen mit Photogrammmetrie und 3D Druck lassen sich realitätsnahe Kratergeometrien erzeugen, für zunehmend realistische skalierte Laborexperimente.Volcanic eruptions are among the most violent displays of the Earth’s natural forces and threaten human health and infrastructure. Explosive eruptions are hazardous due to their impulsive and dynamic nature, ejecting gas and pyroclasts at high velocity and temperature into the atmosphere. In recent years, monitoring efforts have increased, but forecasting eruptions is still challenging as volcanoes are complex systems with the potential for inherently unpredictable behaviours. To date, the underlying boundary conditions are beyond observation and quantification. Still, they are constrained by physical laws and can be described through models and experiments. The complexity and interdependency of the parameters governing the dynamics of volcanic eruptions ask for increasingly realistic experiments to investigate the sub-surface conditions driving volcanic eruptions. Above the vent, in the near-vent region, the dynamics of explosive eruptions can first be visually observed. The characteristics at this stage are purely the result of the underlying boundary conditions and the exit (vent) geometry. Volcanic vents are rarely the symmetric features that are often assumed in models and experiments. They often exhibit highly irregular shapes with notched or slanted rims that can be transient. To eventually understand the unobservable boundary conditions, it is necessary to initially gain knowledge about the effect of the observable factors (i.e. vent geometry). This knowledge will ultimately improve the understanding of the parameters affecting an explosive event to develop accurate probabilistic hazard maps. To this end, a combination of field observations and laboratory experiments was used. First, I characterised vent and crater shape changes at a frequently erupting volcano (Stromboli) to collect high-resolution geometric data of volcanic vents and observe the related explosion dynamics. As a result of topographic changes, variable eruption intensity, style and directionality could be detected. Five topographic data sets were acquired by unoccupied aerial vehicles (UAVs) over nine months (May 2019-January 2020). During this period, changes associated with "normal" Strombolian activity, "major explosions" and paroxysmal episodes (3 July and 28 August 2019) occurred. Hence, the topographic data made it possible to link the predominant constructive and destructive processes to these eruption styles. Furthermore, the number and position of active vents changed significantly, which is a critical parameter for hazard assessment as vent geometry and position can be linked to shifts in eruptive mechanisms. These field surveys highlight the geometric complexity and variability of volcanic vents at an unprecedented spatiotemporal resolution. Additionally, the observations of explosions suggested the paramount influence of crater and vent geometry on pyroclast ejection characteristics, a fact that has strong implications for areas potentially affected by bomb impact and pyroclastic fall out. Secondly, I designed a series of shock-tube experiments incorporating the geometry elements observed at Stromboli to quantify the influence of vent geometry and several boundary conditions. These experiments validated the link between vent geometry and explosion dynamics that was observed in the field. The novel geometry element is an inclined exit plane of 5°, 15° and 30° slant angle combined with a cylindrical and diverging inner geometry resulting in six vent geometries. All experiments were conducted with gas-only (Argon) at room temperature, four different starting pressures (5, 8, 15, 25 MPa) and two reservoir volumes (127.4 cm3, 31.9 cm3). Despite the vertical setup, the slanted geometry yielded both a laterally variable gas spreading angle and an inclination of the jets. The inner geometry controlled the jet inclination towards the dip direction of the slanted exit plane (cylindrical) and against the dip direction of the slanted exit plane (diverging). Cylindrical vents produced larger gas spreading angles than diverging vents. Both gas spreading angle and jet inclination were highly sensitive to the experimental pressure and the slant angle. They had a positive correlation with maximum gas spreading angle and jet inclination. Additionally, the pressure was positively correlated with the maximum duration of underexpanded characteristics of the jet. The gas volume only showed a positive correlation with the maximum gas spreading angle. Thirdly, I added particles to the experiments to mimic the ejection of gas-particle jets during explosive volcanic eruptions. For this set of experiments, the two geometries with the 30° slant angle from the previous experimental series were used as they exhibited the strongest effect on the gas ejection dynamics. They were supplemented by a third vent that resembled the "real" geometry of Stromboli’s active S1 vent as it was mapped in May 2019 and fabricated by 3D printing. The S1’s geometry is characterised by a ~ 10° divergence on one side and a ~ 40° divergence on the other side. Besides three vent geometries, two types of particles (scoria and pumice), each with three different grain size distributions (0.125– 0.25, 0.5–1, 1–2 mm) and two starting pressures (8, 15 MPa) were used. The near-vent vent dynamics were characterised as a function of particle spreading angle and particle ejection velocity. The vent geometry governed the direction and the magnitude of particle spreading, and the velocity of particles. All geometries yielded asymmetric particle spreading as well as a non-uniform velocity distribution in experiments with pumice particles. The combination of field observations, gas-only and gas-particle experiments demonstrated the prime control exerted by vent geometry. In nature, a modification of the vent led to modified pyroclast ejection patterns. In the laboratory the complex geometries facilitated inclined gas jets, an asymmetric gas and particle spreading angle, and an asymmetric particle ejection velocity distribution. These findings suggest that the asymmetry of volcanic vents and/or craters can promote the asymmetric distribution of volcanic ejecta.Which, in turn, will lead to a preferred direction of volcanic fallout and — in case a column collapse occurs — to a preferred direction of the ensuing pyroclastic density currents. The availability of new technology like unoccupied aerial vehicles, photogrammetry and 3D printing provides several opportunities for the volcanological community. Aerial observations allow a fast, inexpensive and safe way to collect geometrical data of volcanic vents and craters, even in times of elevated volcanic activity. In combination with photogrammetry and 3D printing, "real" vents can be produced for increasingly realistic scaled laboratory experiments

Multi-Parametric Field Experiment Links Explosive Activity and Persistent Degassing at Stromboli

Visually unattainable magmatic processes in volcanic conduits, such as degassing, are closely linked to eruptive styles at the surface, but their roles are not completely identified and understood. To gain insights, a multi-parametric experiment at Stromboli volcano (Aeolian Islands, Italy) was installed in July 2016 focusing on the normal explosive activity and persistent degassing. During this experiment, gas-dominated (type 0) and particle-loaded (type 1) explosions, already defined by other studies, were clearly identified. A FLIR thermal camera, an Ultra-Violet SO₂ camera and a scanning Differential Optical Absorption Spectroscopy were deployed to record pyroclast and SO2 masses emitted during individual explosions, as well as persistent SO₂ fluxes, respectively. An ASHER instrument was also deployed in order to collect ash fallouts and to measure the grain size distribution of the samples. SO2 measurements confirm that persistent degassing was far greater than that emitted during the explosions. Further, we found that the data could be characterized by two periods. In the first period (25–27 July), activity was mainly characterized by type 0 explosions, characterized by high velocity jets. Pyroclast mass fluxes were relatively low (280 kg/event on average), while persistent SO2 fluxes were high (274 t/d on average). In the second period (29–30 July), activity was mainly characterized by type 1 explosions, characterized by low velocity jets. Pyroclast mass fluxes were almost ten times higher (2,400 kg/event on average), while persistent gas fluxes were significantly lower (82 t/d on average). Ash characterization also indicates that type 0 explosions fragments were characterized by a larger proportion of non-juvenile material compared to type 1 explosions fragments. This week-long field experiment suggests that, at least within short time periods, Stromboli’s type 1 explosions can be associated with low levels of degassing and the mass of particles accompanying such explosive events depends on the volume of a degassed magma cap sitting at the head of the magma column. This could make the classic particle-loaded explosions of Stromboli an aside from the true eruptive state of the volcano. Instead, gas-dominated explosions can be associated with high levels of degassing and are indicative of a highly charged (with gas) system. We thus suggest that relatively deep magmatic processes, such as persistent degassing and slug formation can rapidly influence the superficial behavior of the eruptive conduit, modulating the presence or absence of degassed magma at the explosion/fragmentation level

The dynamics of starting gas-particle jets: a volcanic scenario

Explosive volcanic eruptions are a threat for a large part of global population and infrastructures. Explosive eruptions are the results of energetic magma fragmentation, where only gas exsolved in the magma drive the eruption, or of the interaction with external water. The mechanisms of fragmentation are complex and various, but despite that at explosive eruption onset the potential energy stored in gas bubbles in the magma always transforms into kinetic energy via gas expansion and produce the ejection of pyroclasts and/or non-juvenile material in the atmosphere. Particle ejection rate, velocity and trajectory differ depending on source conditions, e.g. magma composition, gas overpressure, conduit length, vent geometry, etc. Field observations, when possible, can help to characterize an ejection from which then the source conditions are indirectly retrieved. High-speed and infrared videos of volcanic ejections, seismic and acoustic measurements, as well as petrographycal and geochemical analysis on the pyroclasts ejected offer insight on the eruptive event. Nevertheless, to link observations and source parameters is not trivial and it still requires a certain number of assumptions. Therefore, the knowledge of source conditions stays uncertain. On the other hand, empirical studies can help linking observations and input parameters, since the latter are chosen experimental conditions. In general, laboratory experiments are far less complex than natural eruptions. However, the simplifications imposed benefit the investigation of single processes as well as the understanding of the effects of boundary conditions on such observed dynamics. The goal, at the end, is to learn the patterns of certain dynamics and possibly, to recognize certain characteristics of volcanic eruptions and be able to associate them to source conditions. Additionally, empirical results provide input parameters for numerical modelling and thus hazard assessment. I perform rapid decompression experiments of gas-particle mixtures generating starting jets. I use two different experimental apparatus, the first is the “fragmentation bomb” at the LMU facility and the second the “jet buster” at INGV Rome. With the two setups, it is possible to characterize the effect of boundary conditions such as: 1) vent geometry, 2) tube length, 3) particle load and size, 4) temperature, and 5) overpressure in the reservoir on the dynamics of the ejection of natural particles of different initial size distribution (from 0.125 to 4 mm). In particular, I focus the analysis on particle velocity and trajectory. Observations on particle fragmentation, mass ejection rate and lightning generation are also possible on experiments from the “fragmentation bomb”. The experiments are recorded with a high-speed camera, which provides visual observation of the dynamics. On the “jet buster” experiments, the video recordings are coupled with piezoelectric sensors providing microseismic signals of the related propagation dynamics. The two apparatus are different and complementary. The “fragmentation bomb”, a shock-tube made of metal, is 24 cm long, allows high overpressures (here 150 bar) and temperatures (here 500°C), gas and particles are pressurized in the same chamber and the observations are made at vent exit. The “jet buster”, on the other hand, with its 3 m of transparent PMMA tube allows the observation of the whole propagation and dynamics inside the pipe as well at vent exit. The overpressure threshold is in the order of few bar (here 2 bar), and the gas reservoir is separated and below the sample chamber. In the “fragmentation bomb” experiments, maximum particle velocity shows, in order of importance, 1) negative correlation with tube length; 2) positive correlation with particle load; 3) positive correlation with flaring vent walls, with peaks for funnel 15; 4) positive correlation with temperature, and 5) negative correlation with particle size. The evolution of particle velocity with time in non-linear and is mostly affected by particle load and tube length. Gas maximum initial spreading angle shows, in order of importance: 1) negative correlation with flaring vent walls; 2) negative correlation with experimental temperature; 3) positive correlation with tube length; 4) positive correlation with particle size, and 5) negative correlation with particle load. The gas spreading angle evolution with time shows a bell shape pattern and it is especially appreciable in setup 1 experiments, due to the particles later arrival. This is the main affecting parameter. The particle initial spreading angle shows: 1) positive correlation with particle load, 2) negative correlation with particle size; 3) negative correlation with vent geometry; 4) positive correlation with tube length, and 5) negative correlation with temperature. The particle spreading angle evolution with time shows patterns varying in particular with particle load and tube length. Estimations of the mass ejection rate (MER) and instantaneous mass or particle concentration show peaks of 26kg/s for setup 2 experiments, 7 kg/s for setup 3 and 4.6 kg/s for setup 1. The evolution of the MER with time reflects the evolution of particle velocity with time. Finally, mm to cm electrical discharges, i.e. lightning, are observed. Their appearance is positively correlated with particle load, and negatively correlated with tube length, temperature, particle size, and flaring of vent walls. In the “jet buster”, I perform both gas only and gas-particle mixture experiments. This to compare the elastic response of the system and jets’ dynamics. The gas only experiments includes a pinch of kaolin powder in order to make the flow front propagation visible in the camera. The gas flow front shows an initial fast propagation (up to 500m/s) in the pipe accompanied by an abrupt deceleration (to 150 m/s) at vent exit were it generates a vortex ring. On the other hand, particles show maximum velocities between 40 to 100 cm in the pipe in respect to initial sample position. In addition, in this case, maximum particle velocity shows negative correlation with particle size and the evolution of particle velocity displays a non-linear trend. Good correlation between microseismic signals and process occurring in the pipe is observed. The comparison of the experimental results with natural data collected on Stromboli volcano, Italy, is far from trivial. As mentioned above, volcanic eruptions are characterized by the interaction of several processes, thus making them far more complex. Nevertheless, I think the data set present here provides a promising link for both field volcanology (visual observations and quantitative monitoring) as well as numerical modelling in order to advance our understanding of explosive volcanic eruptions and assess the related hazard

Complex geometry of volcanic vents and asymmetric particle ejection: experimental insights

Explosive volcanic eruptions eject a gas-particle mixture into the atmosphere. The characteristics of this mixture in the near-vent region are a direct consequence of the underlying initial conditions at fragmentation and the geometry of the shallow plumbing system. Yet, it is not possible to observe directly the sub-surface parameters that drive such eruptions. Here, we use scaled shock-tube experiments mimicking volcanic explosions in order to elucidate the effects of a number of initial conditions. As volcanic vents can be expected to possess an irregular geometry, we utilise three vent designs, two complex vents and a vent with a real volcanic geometry. The defining geometry elements of the complex vents are a bilateral symmetry with a slanted top plane. The real geometry is based on a photogrammetric 3D model of an active volcanic vent with a steep and a diverging vent side. Particle size and density as well as experimental pressure are varied. Our results reveal a strong influence of the vent geometry, on both the direction and the magnitude of particle spreading and the velocity of particles. The overpressure at the vent herby controls the direction of the asymmetry of the gas-particle jet. These findings have implications for the distribution of volcanic ejecta and resulting areas at risk

Real-Time Geophysical Monitoring of Particle Size Distribution During Volcanic Explosions at Stromboli Volcano (Italy)

Of all the key parameters needed to inform forecast models for volcanic plumes, real-time tracking particle size distribution (PSD) of pyroclasts leaving the vent coupled with plume modeling has probably the highest potential for effective management of volcanic hazard associated with plume dispersal and sedimentation. This paper presents a novel algorithm capable of providing syn-emission horizontal size and velocity of particles in real time, converted in mass discharge rates, and its evolution during an explosion, using thermal infrared videos. We present data on explosions that occurred at the SW crater of Stromboli volcano (Italy) in 2012. PSDs and mass eruption rate (MER) data, collected at frequencies of 40 Hz, are then coupled with particle and gas speed data collected with traditional image analysis techniques. The dataset is used to quantify for the first time the dynamics of the explosions and the regime of magma fragmentation. We find that explosive evacuation of magma from a Strombolian conduit during a single explosion proceeds at a constant rate while the explosive dynamics are marked by a pattern that includes an initial transient and short phase until the system stabilizes at equilibrium. These stationary conditions dominate the emission. All explosions begin with a gas jet (onset phase), with maximum recorded vertical velocities above 150 m/s. These high velocities are for small particles carried by the faster moving gas or pressure wave, and larger particles typically have slower velocities. The gas jets are followed by a particle-loaded plume. The particles increase in number until the explosion dynamics become almost constant (in the stationary phase). MER is either stable or increases during the onset to become stable in the stationary phase. The shearing at the interface between the magma and the gas jets controls fragmentation dynamics and particles sizes. Quantification of the Reynolds and Weber numbers suggests that the fragmentation regime changes during an explosion to affect particle shape. The algorithm proposed requires low-cost thermal monitoring systems, and low processing capability, but is robust, powerful, and accurate and is able to provide data with unprecedented accuracy. In general terms, its applicability is limited by the size of individual pixels recorded by the camera, which depends on the detector, the recording distance, and the optical system, particle temperature, which has to be significantly higher than the background

Volcanic plumes from explosive basaltic eruptions: the case of Mount Etna (Italy)

Explosive basaltic eruptions at Mount Etna, Italy, distinguished by a lava fountain surrounded by a tephra plume, have occurred frequently in recent decades. The associated injection of tephra into the atmosphere creates a hazard to local and regional communities. Despite this, the plume dynamics are poorly-understood. To improve the understanding of this phenomena, I investigate coupled tephra plumes – lava fountains through three approaches. First, I develop a new integral model that explicitly considers the denser, coarse inner lava fountain and its effect on the surrounding tephra plume. Depending on the grain-size distribution and partitioning of initial mass flow rate (MFR) into the lava fountain, a coupled tephra plume can go higher or lower than a standard tephra plume for a given initial MFR. Secondly, I examine the relationship between plume dynamics and eruption deposits. While neither the initial MFR from a standard or the newly-developed integral model correlate to the deposit-derived MFR, the modelled MFR at the point above the lava fountain in the newly-developed model does, suggesting that these plumes have significant fallout that is not captured in typical deposit measurements. Specifically, the cone-deposit itself must be considered to account for the discrepancy between the deposit-derived and modelled initial MFRs. Finally, these results are supported by visible-wavelength video analysis of these eruptions. Qualitative analysis shows that lava fountains and tephra plumes are not fully-coupled, that lava fountains occur in the centre of tephra plumes and that surrounding material (volcanic gas and loose particles) are entrained into the plumes. Rotation of the plumes in some eruptions is also examined, although I show that its effect on plume dynamics is insignificant. Determined wind and radial entrainment coefficients are also comparable to those of standard tephra plumes. Together, these findings highlight that lava fountains significantly affect the rise of coupled tephra plumes