In historical times there have been two enormous eruptions in the area. In 939 Katla erupted and threw ash over the surrounding countryside. A large fissure, Eldgjá, opened to the northeast. It is 75 km long, reaching almost to the Vatnajökull ice cap. Many sections of the fissure were volcanically active, although the southern section was most productive. This was undoubtedly the largest eruption in the history of Iceland.

The name Eldgjá is normally used for a part of the volcanic fissure, that part is about 8 km long, 600 m wide and up to 150 m deep. Eldgjá‘s main attraction is the waterfall Ófærufoss, located in a short walking distance from the area‘s car park.