Fig. 3: Sketch of the lava fountaining cycle at Fagradalsfjall.

A Three-dimensional perspective of eruption site, with vent 5 (the locus of eruption activity at the time, yellow star) fed by a melt conduit connected to a partially-solidified dyke at ~1.5 km depth¹⁷. Topography⁵¹ along the axis of the dyke shown for pre-eruption conditions, 3 May 2021 and 21 Sep 2021. The cross-sections in (B–E) focus in on an area where a conduit. The color indicates the degree of outgassing of magma, with red colors indicating ‘fresh’ magma and black indicating outgassed magma. B At the beginning of repose, the crater is filled with outgassed lava, and the level of the lava “lake” decreases as pressure at the base of the conduit equilibrates with the pressure in the underlying cavity. C Melt-gas separation in the cavity causes bubbles accumulation on the roof of the cavity to form a foam. Some gas bubbles leak through the conduit to the surface, decreasing the average density of the magma in the conduit/lake and increasing the level of the lava lake. D Ascent of a large slug through the conduit destabilizes the foam layer, which empties from the cavity into the conduit, leading to slug coalescence and tall fountains (~100 m). E Lower pressures at the base of the conduit during fountaining promote gas exsolution in the shallow cavity and influx of volatile-rich melt from depth. The progressive degassing of magma in the shallow cavity drives shorter fountains (~30 m) with a gradual waning in the intensity of the fountaining until dense, outgassed melt drains back into the conduit. The lower panels show the spectroscopic measurements for one of these fountaining cycles (see Fig. 1), relating (F) H₂O/CO₂ (blue) and (G) SO₂/HCl (red) to the fountaining cycle.