Acoustic observations of lava fountain activity during the 2021 Fagradalsfjall eruption, Iceland

The 2021 eruption within the Fagradalsfjall volcanic system in Iceland provided a rare opportunity to record acoustic data generated by a basaltic fissure. Eruptive activity in May 2021 was defined by a sequence of repetitive lava fountaining activity. Here we describe key observations and analysis conducted on acoustic data recorded by a four-element infrasound microphone array near the eruption site. Detailed inspection of acoustic waveforms and comparisons with seismic data and lava fountain height measurements revealed a complex eruptive sequence during each lava fountain event: acoustic tremor during peak lava fountaining was followed by a transition to Strombolian-style activity with distinct high-amplitude impulsive waveforms. Quantitative comparisons to jet noise spectra find complex turbulence acoustics during each event, with evidence of variations in the wavefield centred on peak lava fountain heights. Strombolian explosions could mostly be modelled by oscillations of bursting gas slugs at the top of the magma column, with a minor number of events exhibiting Helmholtz resonance behaviour instead. We find an increase in bubble radii between early and late May, suggesting a widening of the upper conduit during the lava fountain sequence. Finally, we propose that higher acoustic amplitudes, in addition to a wider conduit in late May, indicate higher gas flux through the conduit culminating in shorter lava fountain events. This study highlights the value of deploying acoustic sensors for providing additional constraints on eruption dynamics and source parameters during effusive fissure eruptions in Iceland and elsewhere.