Sveinagja is a volcanic fissure and a fault in the eastern Myvatnsoraefi area.  It is 2-3 km wide; its walls are 10-15 km long and 20-40 m high.  It is easily traceable all the way south to the central volcano Dyngjufjoll (Askja), and north across the Holssandur area, where the summer road passes through its crater row near the waterfalls Hafragilsfoss and Dettifoss.  At Hafragilsfoss it is possible to observe the cross section of one of the craters in the western wall of the canyon.  The western crater row is older and its northern part is called Sveinar and the southern part Rauduborgir.  Altogether this volcanic fissure and fault line is 70-80 km (42-46 miles) long and it is roughly dated 8,000 years back.

Most of the eruptions have created Pa Hoj Hoj lava fields and along it, near road no. 1, is one of the youngest crater row of the country, which erupted at the same time as the major eruption of the Dyngjufjoll central volcano created the explosion crater Viti and Lake Oskjuvatn, the deepest of the country.  The fissure eruption near the main road started in February and came to an end in April and created the so-called Nyjahraun lava field clearly visible from the road.

Still another eruption took place within the same eruptive system between Dyngjufjoll and Nyjahraun, i.e. just east of the shield volcano Ketildyngja.  Never before in the history of volcanism in Iceland had witnesses to eruptions describe them on paper.  The naturalist Thorvaldur Thoroddsen witnessed the fissure eruptions and the Englishman William Lord Watts, who made the first recorded crossing of the ice cap Vatnajokull at the same time, witnessed the Askja eruption.