Eruptive vent openings during the 2021 Fagradalsfjall eruption, Iceland, and their relationship with pre-existing fractures

The Fagradalsfjall eruption started on the 19^th of March 2021 on a ~180 m long eruptive fissure, following a dike intrusion which had been ongoing for approximately three weeks. The eruption focused shortly thereafter on two eruptive vents. In April, new fissure openings occurred northeast of the initial eruption on the 5^th, 6/7^th, 10^th, and 13^th of April. The northernmost eruption occurred on the 5^th of April, approximately 1 km northeast of the initial fissure, whereas the other fissure openings occurred between this and the initial eruptive vents. Stills from web cameras and time-lapse cameras are available for five of the fissure openings. These data show that the eruptions were preceded by steam emitted from cracks in the exact locations where the eruptions started. The time between the first steam observations and the visual appearance of glowing lava ranged between 15 seconds and 1.5 minutes during night observations and 9 to 23 minutes during daytime observations, the difference is likely explained by different lighting conditions. The eruptive vents are located where the north-easterly oriented dike intersected pre-existing north-south oriented strike-slip faults. These strike-slip faults could be identified on both pre-existing aerial photographs and digital elevation models. A high resolution ICEYE interferogram spanning the first day of the eruption in March reveals deformation where the later vent openings occurred in April. This indicates how Interferometric Synthetic Aperture Radar Analysis (InSAR) could be used to predict where subsequent vent openings are likely. This is of great importance for hazard assessment and defining exclusion zones during fissure eruptions.