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).