Crater Rim Collapses Affect the Lava Fountaining Frequency during the Fagradalsfjall Eruption, Iceland 2021

The Fagradalsfjall eruption on the Reykjanes peninsula, Iceland, lasted from 19 March to 18 September 2021. While it continuously effused lava at the beginning, it opened up 7 further vents in April and focused the activity from late April on Vent 5. Surprisingly the continuous effusion changed to pulses of lava effusion (as lava fountains or vigorous overflow) between 2 May and 14 June that was seismically recorded as tremor pulses. We examined the frequency of 6939 lava fountaining pulses based on seismological data recorded at NUPH at the SE corner of Núpshlíðarháls 5.5 km southeast of the active vent.

We subdivide the time period into 6 episodes based on sudden changes in the pattern. In this presentation we present the different fountaining patterns and systematic changes and discuss their origin. Our comparison with vent height, vent stability and lava effusion style, led us to conclude that the changes in the pulsing behaviour might be caused by collapses from the crater walls. The system is clearly unstable and evolving with time.