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

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

Release characteristics of overpressurised gas from complex vents: implications for volcanic hazards

Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30° slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents

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

Fossil bubble in porphyritic basaltic pyroclasts produced by small and large strombolian eruption at Pacaya, Guatemala

We investigate compositionally monotonous, but energetically diverse, tephra samples from Pacaya to see if fossil bubbles in pyroclasts could reflect eruptive style. Bubble size distributions (BSD) were determined for four ash to lapilli size tephra samples using an adapted version of stereology conversion by Sahagian and Proussevitch (1998). Eruptions range from very weak to very energetic. Hundreds of ESEM BSEs images were processed throughout ImageJ software for a robust and statistically correct data set of vesicles (minimum 700 bubbles per sample). Qualitative textural analysis and major element chemical compositions were also executed. There is higher vesicularity for explosive pyroclasts and an inverse correlation between bubble number density (NV) and explosivity

Release characteristics of overpressurised gas from complex vents: implications for volcanic hazards

Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30° slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents.European Research Council http://dx.doi.org/10.13039/50110000078

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 “spreading angle”, 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.Seventh Framework Programme http://dx.doi.org/10.13039/501100004963Deutsche ForschungsgemeinschaftEuropean Research Council http://dx.doi.org/10.13039/501100000781https://doi.org/10.5880/fidgeo.2020.03

The Influence of Grain Size Distribution on Laboratory-Generated Volcanic Lightning

Over the last decades, remote observation tools and models have been developed to improve the forecasting of ash-rich volcanic plumes. One challenge in these forecasts is knowing the properties at the vent, including the mass eruption rate and grain size distribution (GSD). Volcanic lightning is a common feature of explosive eruptions with high mass eruption rates of fine particles. The GSD is expected to play a major role in generating lightning in the gas thrust region via triboelectrification. Here, we experimentally investigate the electrical discharges of volcanic ash as a function of varying GSD. We employ two natural materials, a phonolitic pumice and a tholeiitic basalt (TB), and one synthetic material (soda-lime glass beads [GB]). For each of the three materials, coarse and fine grain size fractions with known GSDs are mixed, and the particle mixture is subjected to rapid decompression. The experiments are observed using a high-speed camera to track particle-gas dispersion dynamics during the experiments. A Faraday cage is used to count the number and measure the magnitude of electrical discharge events. Although quite different in chemical composition, TB and GB show similar vent dynamics and lightning properties. The phonolitic pumice displays significantly different ejection dynamics and a significant reduction in lightning generation. We conclude that particle-gas coupling during an eruption, which in turn depends on the GSD and bulk density, plays a major role in defining the generation of lightning. The presence of fines, a broad GSD, and dense particles all promote lightning