4. Discussion
Satellite and airborne photogrammetry provided flexible methods for near real-time monitoring of volume and TADR on a daily to weekly basis for Icelandic conditions, where low vegetation and very changeable weather prevails. Airplane surveys were possible for cloud cover down to 550 m.a.s.l and could be deployed quickly since the flight to the eruption only takes 10 minutes from Reykjavík Airport (RVK). The acquisitions were mainly limited by the low cloud, and occasionally by lack of available aircraft.
The data products (orthomosaics, DEM, lava outline, thickness maps, thickness change maps and volume and TADR estimates) provided critical information for disaster response and for the scientific community. The volume and TADR were used to evaluate the status of the eruption and as input parameters together with the thickness maps for lava flow simulation. The orthomosaics and lava outlines were important to responders providing base maps for infrastructure, planning, rescue missions, and for tourists visiting the eruption.
The effusion rate evolution for basaltic eruptions provides important insights to increase understanding of the source of the magma and the conduit properties. Different trends in effusion rate evolution have been classified into types and linked to specific plumbing system dynamics (Harris et al., 2000, 2011, Araveno et al., 2020). Type I is characterized by a phase of high initial effusion followed by an extended phase of waning effusion, which has been interpreted as a tapping of a pressurized reservoir (Wadge et al., 1981) and efficient magma ascent in early stages (Araveno et al., 2020). Type II has a low, near-constant effusion rate and has been related to low values of overpressure (5–10 MPa), consistent with overflow in a non-pressurized system (Harris et al. 2000, Araveno et al., 2020). In type III eruptions, the effusion rate increases with time and has been suggested to be linked with ascent of a magma batch, pushing a volume of degassed magma ahead (Harris et al., 2011). However, this trend has also been linked to conduit erosion caused by high erosion coefficients, high initial overpressures, and/or large magma reservoirs, that in its extreme case may lead to a sudden overpressure drop and eruption shutdown caused by high effusion rate and magma withdrawal (Araveno et al., 2020). The last type is type IV, which shows highly pulsating effusion rate and has been related to ascent of multiple batches of magma (Harris et al., 2011).
The Fagradalsfjall 2021 eruption started with low and stable effusion rate between 4–8 m3/s in phase 1–2 (Fig. 2) and initially had the characteristics of a type II eruption. However, in phase 3–4 the effusion rate increased to 8–13 m3/s changing the characteristics to resemble a type III eruption, whilst in phase 5 the TADR had pulsating characteristics similar to type IV but lasted only for a month. The low initial effusion rate at Fagradalsfjall is between 30 and 2500 times smaller than other recorded Icelandic eruptions in the last 75 years (Gudmundsson et al., 2004, 2012, Jude-Eton et al., 2012, Hreinsdóttir et al., 2014, Pedersen et al., 2017, 2018, Thorarinsson, 1964, 1967). When normalizing the initial effusion rate to the mean output rate (Harris et al. 2007) of each eruption, it becomes clear that it is not only a low initial effusion rate that is unusual, but the evolution of the effusion rate at Fagradalsfjall, which is unlike any observations from previous recent Icelandic eruptions (Fig. 4).