Grímsvötn is a basaltic volcano which has the highest eruption frequency of all the volcanoes in Iceland and has a southwest-northeast-trending fissure system. The massive climate-impacting Laki fissure eruption of 1783-1784 was a part of the same fissure system. Grímsvötn was erupting at the same time as Laki during 1783, but continued to erupt until 1785. Because most of the volcanic system lies underneath Vatnajökull, most of its eruptions have been subglacial and the interaction of magma and meltwater from the ice causes phreatomagmatic explosive activity.