Sveinagjá is a crater row extending from Mt. Gjáfjöll in the south almost to Route 1 on Mývatnsöræfi. It is located in a 1.5-2 km wide graben of the same name. The Sveinagjá eruption is one of few fissure eruptions where eye witnesses described events in which magma was expelled via dikes from a shallow magma chamber along a fissure swarm. This eruption occurred on the fissure swarm of Askja. Subsurface lateral magma flows and lava extrusions took place over an 8 month period, beginning in February 1875 and ending in October of that year. A lateral magma flow without an eruption had occurred in the autumn of 1874. Then a gradual extension of ground fissures was evident where the main road to eastern Iceland lay across Sveinagjá. The crater row is discontinuous, 24 km long in total. The lava, Nýjahraun, covers about 30 km2.