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

Abstract

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.

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

“…imperfect opening of the diaphragms). The reproducibility of gas and gas-particle jet spreading angles was demonstrated by Cigala et al ( 2021 ) and Schmid et al ( 2020 ). The subjective error by optically and manually measuring the spreading angles was quantified by letting three individuals analyse the same experiment and comparing the results (Cigala et al 2021 ).…”

“…The reproducibility of gas and gas-particle jet spreading angles was demonstrated by Cigala et al ( 2021 ) and Schmid et al ( 2020 ). The subjective error by optically and manually measuring the spreading angles was quantified by letting three individuals analyse the same experiment and comparing the results (Cigala et al 2021 ). Since the measuring methodology in the present study is the same and the same experimental setup was used, we assume negligible operator subjectivity as well.…”

“…Cigala et al 2017 ; Kieffer 1989 ; Ogden 2011 ; Wilson and Head 1981 ; Wilson et al 1980 ), jet radius (e.g. Jessop et al 2016 ; Koyaguchi et al 2010 ; Woods and Bower 1995 ), jet inclination (Schmid et al 2020 ) and gas and gas-particle spreading (Cigala et al 2021 ). Furthermore, vent geometry influences the trajectories of volcanic ballistics (Dürig et al 2015 ) and the likelihood of column collapse (Jessop et al 2016 ).…”

“…Cigala et al ( 2017 ) empirically correlated the temporal evolution of particle exit velocity from radially symmetric vents with internal vent geometry, particle load, grain size distribution, conduit length and temperature. High-speed video footage of these experiments was used to analyse the temporal evolution of the angular deviation of particles from the vertical (Cigala et al 2021 ).…”