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).