Cooling of the 2021 & 2022 Fagradalsfjall lavas: surface deformation and magnetic signatures

Lavas are known to cool and contract following their emplacement, resulting in measurable subsidence at their surface. Magnetic surveying of the cooling lava can also provide insight into the causation of such subsidence, whether it be due to for example lava tunnel collapse and/or cooling of the lava. Repeated geodetic, photogrammetric, and magnetic measurements can be used to monitor the subsidence and can help determine the cooling rate of the lava. Here, we present initial results on subsidence and total magnetic field of the Fagradalsfjall lavas (Reykjanes Peninsula, Iceland), which were emplaced in March-September 2021 and August 2022. The post-emplacement deformation of the lavas is measured from comparison of Digital Elevation Models (DEMs) in 2x2 m derived from aerial photogrammetric surveys, in-situ Global Navigation Satellite System (GNSS) surveys of benchmarks in the lava flow, and Interferometric Synthetic Aperture Radar (InSAR). The DEM differences show subsidence of up to 7 m in the first year since the end of the 2021 eruption. Magnetic measurements were performed using drone surveys (MagArrow magnetometer suspended on DJI Matrice 600) and hiking profiles (GEM Systems GSM-19 Overhauser magnetometer). Our preliminary results show quite variable magnetization of the lavas. We suggest that the low magnetic anomalies are either associated with internal structures or show evidence of hot lava still above its Curie temperature and possibly even in liquid form and coincide roughly with the higher subsidence rates. During the August 2022 eruption, when the new lava was partly emplaced on top of the 2021 lava field, some of the older lava squeezed out from the western border of the 2021 flow, demonstrating that the 2021 lavas were still partly in liquid form. We expect the 2021-2022 lavas to continue to subside as the lava cools down and contracts, and plan further studies to provide insight into the cooling processes.