The fissure rapidly extended from 300 metres to a length of , crossing the island from one shore to the other. Submarine activity also occurred just offshore at the northern and southern ends of the fissure. Spectacular lava fountaining 50 to 150 metres high occurred along the whole fissure, which reached a maximum length of about during the first few hours of the eruption, but activity soon became concentrated on one vent, about north of the old volcanic cone of Helgafell and just outside the eastern edge of the town.