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To explore the effect of conduit roughness on volcanic jet dynamics and on the related seismo‐acoustic radiation we performed a series of shock‐tube experiments using pipes with variable inner surface fractal dimension D. Variable starting pressure produced subsonic to supersonic jets visualized using high‐speed shadowgraph and recorded with an array of accelerometers and microphones. At all starting pressures, increasing Dincreases the energy transfer from the gas to the conduit walls, decreasing the jet exit velocity (Mach number) and, for supersonic cases, the related shock‐cell spacing, and increasing the seismic to acoustic radiation amplitude ratio. The roughness‐induced changes in jet velocity and turbulence affect the dominant sources of the jet noise and modulates the spectral properties of the acoustic signals. From our study we show that conduit wall roughness is an important and yet largely neglected factor in the dynamics of explosive volcanic eruptions and their monitored geophysical signals. Volcanoes are amongst the most fascinating and mysterious subjects of science, for they allow no direct observation of what is happening within the conduit during eruptive activity. Indirect observations (such as measurements of the sound and vibration accompanying eruptions) are routinely performed for monitoring and research purposes. Laboratory studies mimicking the eruptive processes in small‐scale devices, are of great support for correctly interpreting such data. As such, we investigated the effect of the irregularity of conduit surface, amongst the most relevant and poorly known variables characterizing the eruptive processes, on volcanic jets and on their seismic and acoustic signals. We performed a series of laboratory experiments using conduits with different roughness of the internal surface and various starting pressures. Microphones and accelerometers, capable of measuring conduit sounds and vibration, respectively, in synch with high‐speed camera were used to constrain the characteristics of the generated subsonic and supersonic jets. Results show that conduit roughness controls: (a) the relative amplitude of seismic and acoustic signals; (b) the velocity, turbulence and properties of the sound of these jets. Our results will shed light on the link between observation at the surface and dynamic evolution of conduit geometry at depth. Elastic radiation of analogue experimental volcanic subsonic to supersonic jets was compared against high‐speed shadowgraph analysisWe quantified variation in the spectral features, energy partitioning and jet structure due to differences in conduit roughnessDifferent behavior in subsonic versus supersonic regime was observed due to distinct dominating noise sources at different roughness Elastic radiation of analogue experimental volcanic subsonic to supersonic jets was compared against high‐speed shadowgraph analysis We quantified variation in the spectral features, energy partitioning and jet structure due to differences in conduit roughness Different behavior in subsonic versus supersonic regime was observed due to distinct dominating noise sources at different roughness

During the summer 2001 eruption of Mt. Etna, Italy, we developed a method for monitoring explosive activity through the study of airborne ash. The method is based on two routine and fast techniques for ash characterization: i) component analysis, that quantifies the proportion of juvenile particles, crystals, and non‐juvenile lithics in the ash; ii) Scanning Electron Microscope semi‐quantitative description of the morphology of ash particles. With these techniques we daily analyzed airborne ash from the eruption plume. Temporal evolution of particle shape and vesicularity, and of component proportion in the ash, documented initial crater excavation by hydromagmatic explosions, strong magmatic activity due to volatile exsolution, and final fragmentation of a cooling top of magma column. Favorable conditions of the studied eruption allowed validation of these results by comparison with other monitoring techniques. We also evaluate the applicability of the method to other cases, as well as its limits.