Nesjavellir (64°06’49.7″N 21°17’46.3″W) lies on the south shore of Lake Þingvallavatn and consists of a young pahoehoe lava flow, which is called Nesjahraun and around 2,000 years old. The Nesjavellir Power Plant is located in a narrow graben, called the Hengill graben, with a distinct eastern normal fault. The movements on this fault are up to tens of meters. Geothermal fields tend to follow the fault system. The faults are still active, which is documented by traces in the youngest lava flows in the area (e.g., Nesjahraun) (Guðmundsson, 2017). The island Sandey, a double-crater tuff cone, was also formed during the Nesjahraun eruption. The Sandey cone represents the hydromagmatic phase of the Nesjahraun eruption (Thordarson and Höskuldsson, 2015). The fissure is up to 30 km long and the total area of the Nesjahraun lava flow is estimated to be around 14 km2, of which about half is on the bottom of Lake Þingvallavatn (Guðmundsson, 2017).
Hengill is the central volcano of the Hengill volcanic system, which has formed in numerous eruptions during the past several hundred thousand years. The oldest rocks in the Hengill area are about 800,000 years old. Hengill is up to 800 m above sea level (Guðmundsson, 2017).
The Nesjavellir Power Plant produces around 120 MW of electricity and supplies Reykjavík with about 1100 l/s at a temperature of about 84°C for space heating. It is the second largest geothermal plant in Iceland. The geothermal field in the Hengill area is with 100 km2 one of the largest in Iceland. The hot springs are mostly related to faults. The heat source at Hengill are magmatic intrusion and a shallow magma chamber (Guðmundsson, 2017).
Guðmundsson, Á. (2017). The glorious geology of Iceland’s Golden Circle. Springer. 334 pp.
Thordarson, T., and Höskuldsson, Á. (2015). Classic geology in Europe 3 – Second Edition. Dunedin. 256 pp.