Hengill

The Hengill volcanic system, cutting through Thingvallavatn lake, consists of a series of NE-SW-trending fissure vents, crater rows, and small shield volcanoes occupying a strongly faulted graben. Hengill is the easternmost of a series of four closely spaced basaltic fissure systems that cut diagonally across the Reykjanes Peninsula and lies at the triple junction of the Reykjanes Peninsula volcanic zone, the Western volcanic zone, and the South Iceland seismic zone. Postglacial lava flows surface much of the volcanic system.

The latest eruption was radiocarbon dated about 1,900 years before present. An eruption in the Hellisheidi area once thought to have occurred around 1000 CE at the time of a meeting of the Icelandic parliament at Thingvellir is now known to have occurred at a vent about 5 km away in the Brennisteinsfjöll volcanic system. The high-temperature Nesjavellir geothermal area NE of the uplifted hyaloclastite ridge forming the Hengill central volcano and the Helllisheidi geothermal field SW of Hengill are major producers of geothermal energy for Reykjavik.