Linking styles of active magma degassing with fragmentation dynamics for mafic magmas: a multiparametric approach

Explosive activity at mafic volcanoes — whether ejecting particles or particle-free — represents nearly half of the world’s most thermally active volcanoes. In such openvent systems, volcanic explosions release gas before, between, and after eruptive episodes, and their degassing dynamics exert a strong control on eruption style and transitions. Understanding these transitions requires a detailed characterization of each degassing regime, from fountaining activity to puffing activity. This PhD thesis aims to identify and quantify the fundamental parameters that govern the different modes of active degassing and outgassing in basaltic systems through highresolution, multiparametric field observations combined with data from the literature. To achieve this, a multispectral and multiparametric approach was adopted, integrating visible, thermal, and acoustic datasets acquired during field campaigns at Mount Etna, Stromboli, and Tajogaite volcano. Different eruptive styles have been studied; fountaining activity (the most explosive one), spattering activity, “normal” Strombolian explosions, and low energy strombolian explosions called puffing here without gas. The visible and high-speed imagery allowed for the estimation of ballistic velocities, and physical properties (shape, size, fragmenting or not and how), providing new insights into an unstudied fragmentation process: the in-flight fragmentation. Thermal imaging offered continuous, high-resolution monitoring of summit activity, enabling the measurement of gas jet velocities, temperatures, frequencies, and gas volumes. Acoustic data complemented these observations by capturing pressure fluctuations associated with degassing events, thus extending detection capabilities beyond visual observations, allowing us to define the depth of the outgassing events. This thesis addresses two major gaps in the current literature: (i) the physical mechanisms, quantification, and efficiency of in-flight fragmentation, which remain poorly constrained, and (ii) the lack of high-quality, high-resolution field data required to validate degassing models. The research is organized into two main themes: degassing with particle emission (bomb fragmentation, Chapter III) and degassing without particles (puffing and vortex ring release, Chapter IV and V). In Chapter III, I investigate the in-flight fragmentation of volcanic bombs ejected during explosive eruptions using ten seconds of high-speed and high-resolution video data. This study quantifies how much and how fragmentation occurs after ejection and assesses its impact on bomb dispersal and hazard. Four eruption sequences were analysed, encompassing Strombolian activity at Stromboli, spattering and fountaining phases at the Tajogaite eruption, and a fountaining episode at Mount Etna. By tracking individual bombs through time, four primary fragmentation mechanisms are identified: detachment, inflation, collision-induced fragmentation, and deformation-driven breakup. The first two account for a minor proportion of the overall fragmentation, while collisions and deformation dominate the process. Results indicate that in-flight fragmentation affects up to 73% of bombs larger than 0.2 m, depending on eruption style. Deforming, detaching, and inflating bombs highlight the pivotal role of aerodynamic drag, which increases with both velocity and size, preferentially fragmenting the largest and fastest projectiles. This mechanism acts as a self-limiting control on bomb range and impact energy. The findings provide a quantitative framework for integrating in-flight fragmentation processes into ballistic hazard modelling and for improving the interpretation of deposits produced by low-viscosity, bomb-bearing eruptions. In Chapter IV, I present the methodological framework developed to analyse lowenergy degassing activity at Mount Etna using combining thermal and acoustic sensors. The study focuses on two adjacent vents—Sbof and Rings—within Bocca Nuova crater, monitored over a 10-day period in July–August 2023. Both vents have a bimodal distribution of their degassing behavior and we decide to focus on the major degassing events, as they have a signature in the acoustic signal. Through synchronized thermal and acoustic analyses of the major degassing events, I identify two distinct degassing behaviours: at Rings, frequent and short-lived puffs (~0.25 Hz, 2 s) rise rapidly (21 ± 4 m.s-1 ) and often generate Volcanic Vortex Rings, while Sbof emits slower (12 ± 7 m.s-1 ), louder, and longer puffs (~9 s) linked to episodic gas accumulation. These contrasts are interpreted as the result of differing conduit geometries and vent shape. The study demonstrates that integrating waveform classification with vent-resolved thermal data provides a robust approach for tracking puffing dynamics, improving the detection and interpretation of low-energy degassing processes at mafic open-vent volcanoes. In Chapter V, I explore the dynamics of Volcanic Vortex Rings (VVRs) by analysing 10 min of thermal signals, generated at Mount Etna’s Bocca Nuova crater during the July 2023 field campaign, as part of an ongoing collaboration with Johan Gilchrist. Using high-resolution thermal imagery, ten ash-free and gas-dominated VVRs emitted from the Rings vent were analysed to quantify their ascent, entrainment, and dissipation. Two families are observed (supporting the bimodality observed on Chapter IV): shortlived thermals, characterized by rapid expansion and limited ascent, and long-duration events, sustained by strong initial momentum and coherent buoyancy-driven rise. Spectral analysis reveals a consistent f⁻³ power-law dissipation in both frequency and wavenumber domains, indicating buoyancy-controlled turbulent entrainment rather than isotropic turbulence. These results correlate impulsive jet start with sustained buoyant rise, establishing VVRs as natural analogues of buoyant thermals. They also present one of the first field-based of evidence of f⁻³ scaling in volcanic gas emissions, providing useful limits for model validation and plume dynamics. Chapter VI synthesizes the findings and discusses how these insights contribute to model development and are concluded in Chapter VII. Overall, this thesis provides a comprehensive, data-driven understanding of gas and bomb interaction in mafic volcanoes. It demonstrates how multiparametric field measurements can bridge the gap between visual observations and physical models, offering new constraints for hazard assessment and monitoring of low-energy eruptive activity.