The Eruption in Fagradalsfjall (2021, Iceland): How The Operational Monitoring and The Volcanic Hazard Assessment Contributed to Its Safe Access

Abstract After more than a year of unrest, a small effusive eruption commenced in Fagradalsfjall on 19 March 2021. The eruption lasted six months with multiple fissure openings characterizing the first six weeks. During the eruption lava and low-level gases propagated over the complex terrain: a hyaloclastite massif with mountain peaks up to about 350 m asl with valleys in the between. It is uninhabited, but easily accessible at about 30 km distance from the Reykjavík capital area. While the eruption was on-going, more than 356,000 accesses were counted. Monitoring the onset, the eruption in real-time, forecasting the transport of gas and emplacements of lava flows, and assessing the hazards were instrumental in maintaining safe access to the area. In addition to data accessibility and interpretation, managing this volcanic crisis was possible thanks to a strong collaboration between the scientific institutions and civil protection agencies. The eruption presented an opportunity to tune, test and validate a variety of numerical models for hazard assessment as well as to refine and improve the delivery of information to the public, community members and decision makers. The monitoring team worked long hours during both the pre- and syn-eruptive phases for a safe access to the eruption site and to provide a regular flow of information. This paper reviews the eruption and its associated hazards. It also provides an overview of the monitoring setup, the adopted numerical tools and communication materials made available for informing the general public of possible eruption scenarios and exclusion zones.