Volcano-tectonic activity on the Reykjanes Peninsula, Iceland, since December 2019: 13 dike injections in the Svartsengi and Fagradalsfjall volcanic systems reveal a wide range of magma flow rates into dikes

Following an increase in seismic activity in December 2019, a pressure increase began in the center of the Svartsengi volcanic system in January 2020, as inferred from geodetic observations. The first diking event occurred, however, in the nearby Fagradalsfjall volcanic system, about 10 km east of Svartsengi, 24 February – 19 March 2021, when an ~9 km long dike gradually formed with geodetically inferred initial volume increase rates up to 35 m³/s, during the first week of diking. The total dike volume was ~34 Mm³, based on joint inversions of InSAR and GNSS observations that have been extensively used to study this and later events in the area. This dike intrusion culminated in an eruption on 19 March 2021. The initial dike had minor incremental volume increase in association with opening of additional vents above the dike during the 6-month-long 2021 eruption, with near-surface opening in the top few hundred meters. Three additional dike intrusions occurred in the Fagradalsfjall area between December 2021 to July 2023, with initial magma flow rates between 22 to 70 m³/s. The Fagradalsfjall dikes were fed through a channel with an inferred cross-sectional area of about ~2-4  m², passing through the lower crust from a source near the crust-mantle boundary, with a geodetically imaged deflation source at about ~12-13 km depth. Since late 2023, activity has been focused at the Svartsengi system, with 9 diking events and 7 eruptions in 2023-24. Initial diking there occurred on 10-11 November 2023 with inferred peak flow rates of ~7400 m³/s when an ~15 km long dike formed, following magma accumulation near the brittle-ductile boundary at about 4-5 km depth. The inferred cross-sectional area of the limiting part of the channel from the Svartsengi magma domain feeding the zone where dikes have formed in 2023-24 is on the order of ~2000 m² or about 2-3 orders of magnitude larger than that inferred at Fagradalsfjall. This and the different depth of magma storage in the plumbing systems at Fagradalsfjall and Svartsengi explains their different behaviour in recent years, that are coupled in such a manner that only one of of the systems has been primarily magmatically active at each time since 2020.