[4]: 718  The previous lava eruption from the Sundhnúkur crater row has been dated at 2350±90 BP,[4]: 719  and was of basaltic ʻaʻā type.

[4]: 718 On the evening of 18 December 2023, a volcanic eruption occurred at Sundhnúksgígaröð north of Grindavík,[7] with images showing lava spewing from fissures in the ground.

[8] The intensity of the eruption and accompanying seismic activity which preceded it decreased early on 19 December, with lava seen spreading laterally from both sides of the newly opened fissures.

[10] It pinpointed the origin of the eruption near Hagafell,[11] about 4 km (2.5 mi) north-east of Grindavík, and noted that the eruption stemmed from a fissure with a length of about 3.5 km (2.2 mi), with lava flowing at a rate of around 100 to 200 cubic metres per second, adding that seismic activity appeared to be moving towards the direction of Grindavík.

An Icelandic Civil Defence official told the public broadcaster RÚV that the eruption had happened quickly and appeared to be "quite a large event".

[12] By 19 December, the scent of smoke and ash was detected as far as 30 km (19 mi) from the eruption site, raising fears that volcanic gases could reach Reykjavík by the next day.