Volume, effusion rate, and lava transport during the 2021 Fagradalsfjall eruption: Results from near real-time photogrammetric monitoring

The basaltic effusive eruption at Mt. Fagradalsfjall began on March 19, 2021, ending a 781-year hiatus on Reykjanes Peninsula, Iceland. At the time of writing (January 7, 2022), no eruptive activity has been observed since September 18, 2021. To monitor key eruption parameters (i.e., effusion rate and volume), near-real time photogrammetric monitoring was performed using a combination of satellite and airborne stereo images.

By late September 2021, 32 near real-time photogrammetric surveys were completed, usually processed within 3–6 hours. The results are a significant achievement in full-scale monitoring of a lava flow-field providing temporal data sets of lava volume, thickness, and effusion rate. This enabled rapid assessment of eruption evolution and hazards to populated areas, important infrastructure, and tourist centers.

The lava pathways and lava advancement were very complex and changeable as the lava filled and spilled from one valley into another and short-term prediction of the timing of overflow from one valley to another proved challenging. Analysis of thickness maps and thickness change maps show that the lava transport into different valleys varied up to 10 m³/s between surveys as lava transport rapidly switched between one valley to another.

By late September 2021, the mean lava thickness exceeded 30 m, covered 4.8 km² and has a bulk volume of 150 ± 3 × 10⁶ m³. Around the vent the thickness is up to 122 m. The March–September mean effusion rate is 9.5 ± 0.2 m³/s, ranging between 1–8 m³/s in March–April and increasing to 9–13 m³/s in May–September. This is uncommon for recent Icelandic eruptions, where the highest discharge usually occurs in the opening phase. This behavior may have been due to widening of the conduit by thermo-mechanical erosion with time, and not controlled by magma chamber pressure as is most common in the volcanic zones of Iceland.