3.1. Fagradalsfjall eruption: Volume, discharge and lava field evolution
The eruption from March 19, 2021 to September 18, 2021 can be divided into five phases.
Phase 1 of the eruption (March 19 to April 5) began when a 180 m long fissure opened on March 19 around 20:30 in the Geldingadalir valley, which is located east of Mt. Fagradalsfjall (Fig. 1). Soon the eruption concentrated on two neighboring vents and the lava started infilling the valley (Fig. 2a). During this phase, the TADR ranged from 7.9 to 0.7 m3/s with a mean for the entire phase of 4.9±0.1 m3/s (Fig. 2b). The lava area increased to 0.33 km2, while the mean thickness increased to 22 m reaching a lava volume of 7.1 × 106m3 before phase 2 started.
In phase 2 (April 5 to April 28) the active vent migrated (Fig. 1). Multiple eruption segments opened, starting on April 5, when two new fissure-segments opened 800 m northeast of the first fissure segments. Another fissure opened at midnight on April 7, another one on April 10 at and then again on April 13 when two new fissure segments opened. Each fissure segment concentrated into 1–2 circular vents, which over the following 10 days became inactive, except for southern the vents that developed from the April 13 fissure segments. Phase 2 had similar TADR as in phase 1 in the range 4.6–7.5 m3/s with the highest TADR observed just after new vent openings. The mean TADR in this period was 6.3±0.4 m3/s and the volume increased to 19.4 × 106 m3. With the migration of active vent locations, lava started to flow into the valleys of Meradalir (April 5), Geldingadalir and Syðri-Meradalur (April 14) covering an area of 1.1 km2 with a mean thickness of 16 m.
In phase 3 (April 28 to June 28) the vent activity stabilized at one location. Most of the time (May 2 to June 12) it exhibited cycles of short-term (ca. 8–9 minutes) pulsations. The TADR increased from 8.8 to a maximum of 13.0 m3/s with a phase mean of 11.4±0.5 m3/s. The “fill and spill” from one valley into another increased the area stepwise to 3.82 km2 with mean thickness of 20.8 m yielding a volume of 79.8 × 106 m3. The lava migrated to Nátthagi valley through Syðri-Meradalir (May 22) and through southern Geldingadalir (June 13).
Phase 4 (June 28 to September 2) was characterized by episodic activity with intense lava emplacement (ca. 12–24 hours) followed by inactive periods of similar length. Despite the episodic activity this period had only slightly lower TADR to phase 3 with a mean TADR for the whole phase of 11.0±0.4 m3/s ranging from 8.5 to 11.1 m3/s and the volume increased to 142.5 × 106 m3. The lava thickened to around 50 m northeast of the active crater due to episodic overflows and in Meradalir the lava thickened by ~25 m due to stacking and inflation.
The eruption’s rhythm changed again in the phase 5 (September 2 to 18), when a week-long pause from September 2–11, was followed by week-long period of activity from September 11–18. The measured TADR was 12.2 m3/s for September 9–17. The mean TADR for phase 5 is 5.6±0.6 m3/s and the volume increased to 150.8 × 106 m3. Most of the deposition was in Geldingadalir, where a 10–15 m thick lava pond was established north-northwest of the active crater between September 11 to 15, that partly drained through an upwelling zone towards south and into Nátthagi from September 15 to 18. At the time of writing (November 18), no eruptive activity has been observed since September 18.