Conduit resonance modulation of volcanic puffing (and maybe more)

Regularly pulsating emissions are frequent during volcanic activity, especially at open vent mafic systems. Most typical of such emissions is puffing, i.e., the intermittent or periodic emission of pressurized gas volumes from a vent, with or without the ejection of pyroclasts. Puffing is usually interpreted as the result of the explosion of gas bubbles at the surface of a static magma column. Here, we experimentally demonstrate that a steady gas flux can be transformed into a pulsating flux by closed pipe resonance, and provide field evidence for this process occurring at Mt. Etna volcano (Italy) in 2023. In laboratory, we inject pressurized air through a valve system and into a pipe of variable length and 4 cm in diameter. At certain inlet pressures, pipe resonance is triggered and the air flow from the pipe opening (nozzle), visualized by the injection of fog, pulsates. In particular, high-speed imaging at the nozzle revealed the repeated formation of vortex rings alternating with air re-entering the pipe nozzle in a kind of ‘backwash’. Video analysis reveals that air fluctuations at the nozzle have characteristic resonance frequencies that agree with the closed-pipe resonant frequency of the pipe. The same frequencies appear in the power spectrum of the acoustic signal from the experiment, supporting the notion that standing pressure waves in the pipe control the temporal flux of outgoing air flow. Puffing activity at Etna in 2023 produced volcanic vortex rings (VVR) alternating with ‘backwash’ phases. Thermal video imagery displays two characteristic frequencies of temperature changes above the vent, at 0.25 and 0.5 Hz, with a possible third one at 0.75 Hz, in agreement with a resonance process. No peak appears at these frequencies in the spectrum of the infrasonic signal associated with puffing. We conclude that puffing activity and VVR emission at Etna was controlled by conduit resonance that modulated the flux from a steady source of volcanic gases. As far as we can tell from the volcanology literature, despite organ-pipe resonance invoked to explain seismic harmonic tremor and acoustic signals, this the first time that conduit resonance is observed to control volcanic emissions. Resonance modulation may potentially extend to other pressure-controlled volcanic processes, such as bubble explosion, fumarolic activity or control the unsteady flux of erupted material during sustained and larger explosive eruptions, thus representing a key factor to be considered in future investigations.