Figure 4 Fagradalsfjall effusion characteristics described by the ratio
of initial effusion rate Effusioninitial divided by the
Mean Output Rate, MOR (Harris et al., 2007) compared to other recent
Icelandic eruptions (Gudmundsson et al., 2004, 2012, Jude-Eton et al.,
2012, Hreinsdóttir et al., 2014, Pedersen et al., 2017, 2018,
Thorarinsson, 1964,1967). Note the y-axis is logarithmic. Fagradalsfjall
2021 is clearly an outlier with a ratio below 1, while all other
eruptions plot above 1. The effusion rate evolution of types I and III
from Harris et al (2000, 2011) has been indicated in red and blue. Type
II should ideally plot along the dashed line, while the pulsating nature
of type IV could plot everywhere in the plot.
For the first time we observe an eruption that primarily has
characteristics of a type III eruption, while recent Icelandic eruptions
show characteristics that resemble type I eruptions. Based on the
interpretation of type I eruptions, it makes sense that eruptions in
Hekla, Grímsvötn and Bárðarbunga (Holuhraun 2014–2015) all show
effusion rate evolution controlled by pressurized reservoirs, since
these volcanic systems show evidence of having magma chambers (e.g.,
Ofeigsson et al., 2011, Geirsson et al., 2012, Hreinsdóttir et al.,
2014, Gudmundsson et al., 2016). Less information exists for the
Vestmannaeyjar volcanic system responsible for the Surtsey 1963–1967
eruption, but based on the available data (Thorarinsson, 1964), the
effusion rate evolution suggests that Fagradalsfjall is unlike this
eruption as well.
The evolution of the effusion rate of type III eruptions has been linked
to the ascent of a single magma batch, pushing a volume of degassed
magma ahead (Harris et al., 2011, Steffke et al., 2011). Interestingly,
geochemical evidence suggests that in phase 1–2 the magma plumbing
system gradually changed from being fed from a depleted shallow mantle
source to being fed by more enriched discrete melts from greater depth
(Marshall et al., 2021). Nevertheless, this gradual geochemical change
happened during the first 40 days of the eruption, where the TADR is
stable and the increase in TADR happen around day 50. However, during
phase 2, new vents opened at locations which were 60 m above the initial
vents in Geldingadalir (Fig. 1) suggesting an increase in pressure
during this time (in the order of 1–1.5 MPa based on higher lithostatic
load corresponding to 60 m increase in elevation of vents). By the end
of phase 2, the vents at the higher elevations had shut off and the
effusion from vent 5, which is located in similar elevation as vent 1a,b
increased from 7 to 13 m3/s in phase 3, while
displaying fire fountain activity.
The delay between the geochemical change and the increased effusion is
intriguing. In the 2018 Kīlauea eruption the increase in effusion
started within a day of an observed change to more mafic magma
increasing the effusion from 6.5 m3/s to 110
m3/s (Gansecki et al., 2019, Dietterich et al., 2021).
Furthermore, this change was associated with observed deformation and
earthquake activity. Thus, there was a clear link between a change in
geochemistry and a substantial increase in effusion.
If the effusion increase in Fagradalsfjall is related to ascent of a
magma batch pushing a degassed magma ahead (which would be ∼20 ×
106 m3 based on the erupted bulk
volume estimates at day 40), then it is clear that it is a more subtle
process compared to the 2018 Kīlauea eruption, potentially involving
lower overpressure and slower increase in effusion.
Another possible model used to explain increase in effusion with thermal
erosion. The eruption in phase 1–2 displayed type II characteristics
consistent with overflow in a non-pressurized system. The effusion rate
and the successive vent openings suggest that the system was not highly
pressurized. Over time the heating of the conduit walls enabled
sufficient thermal erosion to increase the effusion rate, which for a
cylindrical conduit is proportional to r4, where r is
radius (e.g., Turcotte and Schubert, 2002). This process may have been
enhanced by the increased temperature of the magma due to an increase in
MgO from 8.8–9.7% (Marshall et al., 2021). We consider this
conduit-controlled flow a plausible model for Fagradalsfjall because it
explains the sharp contrast with the behavior to other Icelandic
eruptions (e.g., Hekla, Grímsvötn and Bárðarbunga) where pressure in a
magma chamber is considered the main control of flow (e.g., Hreinsdóttir
et al., 2014).