Characterization of volcanic vortex rings: comparison between field observations and experimental simulations

Active volcanic degassing often includes puffing activity, i.e., the repeated emission of pressurized gas slugs, with inter-event duration of few seconds. Puffing may generate volcanic vortex rings (VVRs), which are toroidal vortices of volcanic gases moving through the surrounding ambient air. This study presents the first systematic attempt to characterise VVRs, aiming at better constraining the mechanisms of magma degassing and conduit dynamics in the shallow plumbing system. During summer 2023, the degassing activity of a new pit crater at Bocca Nuova (Mt. Etna, Italy) was monitored and recorded by means of the user-friendly and easy portable SKATE device. Thermal and high-speed videos were analysed to investigate the evolution and behaviour of VVRs over time. Both manual analysis in ImageJ and automated analysis in MATLAB were applied, the latter based on the brightness temperature of individual pixels of the thermal video imagery. Two classes of VVRs were identified according to their physical characteristics: shape, outer radius (r_out), distance from the emitting vent (h), residence time in air, vertical rise velocity (v_rise), radius expansion rate. Class 1 rings are well-defined and stable (h > 50 m; initial r_out = 6 - 12 m; initial v_rise = 8 - 20 m/s), while Class 2 rings are irregular shaped and short-lived (h < 50 m; initial r_out = 3 - 9 m; initial v_rise < 12 m/s). An automated statistical analysis confirmed the existence of these two clusters and assessed the relative probability of occurrence of each cluster. Class 2 rings dominate the sequence, while Class 1 rings are less frequent. The probability that a Class 1 ring follows a Class 2 ring is 0.10, roughly half the probability that it follows another Class 1 ring (0.24). The statistical analysis of their emission frequency may potentially provide additional insights into magma degassing processes. Given that vortex rings are well defined in the literature (e.g. fluid engineering, medical science, biology), both theoretically and experimentally, the minimum conditions required for VVRs formation at volcanic vents were investigated. Vortex rings were experimentally reproduced using a device consisting of a cylinder, a piston and a spring. The bursting of gas slugs of varying volumes was simulated under different piston accelerations. Comparison between field and experimental data allowed estimation of the source parameters (e.g. L/D, magma depth) associated with VVRs formation. Characterizing volcanic vortex rings provides a unique opportunity to better understand the source conditions at the surface of the magma column, degassing processes, gas flux and conduit dynamics during active degassing of volcanoes.