Evolving Shallow-conduit Container Affects the Lava Fountaining during the 2021 Fagradalsfjall Eruption, Iceland

Abstract

Pulsating behaviour is observed in volcanic phenomena ranging from caldera collapses to explosions, spattering or lava fountaining. The repeating processes can define irregular, regular or systematically changing patterns. These patterns yield information about the subsurface structure, which often is not considered in detail. We analyse the pattern of 7058 lava fountaining episodes that occur between 2 May and 14 June 2021 during the Fagradalsfjall eruption, Iceland. Our seismometer records the lava fountaining episodes as tremor pulses. We analyse the seismic tremor amplitude, the pulse duration, the repose time and the sum of pulse duration and repose time (cycle duration). We define six periods characterised by different patterns: Three periods feature long pulses that exponentially shorten with time. One period features coexisting long and short pulses in a haphazard sequence. One period shows a stable pulsing duration but increasing repose time and one period has stable, short pulses and repose times. We conclude that the episodic fountaining starts because a shallow-conduit container forms on 2 May shifting the magma degassing from sustained continuous to an episodic state. This situation evolves until 11 May when a semi-stable state is reached. The length of the repose times are most likely influenced by the amount of outgassed magma present in the uppermost part of the shallow conduit. Finally, we suggest that the vent is mechanically eroded and widens with time causing increasing seismic tremor amplitudes. However, the trends are frequently punctuated by partial crater wall collapses that temporarily disrupt the system.

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

“…This amount approximately correlates with analysis carried out using seismic data (Eibl et al, 2022). The duration and repose intervals along with their broad fluctuations over time also correlate with the seismic analysis but we see an apparent diurnal cycle in early May due to lower signal-to-noise ratios from wind noise.…”

“…This is supported by a steady increase in seismic tremor amplitudes during May 2021 (Fig. 4c, g; Eibl et al, 2022). Mechanical erosion of upper conduit and vents have been observed to affect acoustic frequencies during eruptions (e.g.…”

“…Nevertheless, it is clear that while seismic data can be used to visualise overall trends in lava fountaining (e.g. Eibl et al, 2022), it may overestimate activity durations and underestimate repose intervals due to 'overshooting' of vent activity (Fig. 5).…”

“…Variations in duration, repose intervals, and heights of the lava fountaining was observed throughout May and into early June, eventually transitioning into discontinuous lava outpouring that defined the remainder of the eruption (Barsotti et al, 2022). Seismic data analysis (Eibl et al, 2022) and FTIR gas emission measurements (Scott et al in prep. ) have suggested the presence of a shallow cavity (<500 m) beneath the vent where repeated foam collapse in the magma may be driving the distinctive lava fountaining activity.…”