The geothermal area at Þeistareykir

The geothermal area at Þeistareykir is connected with an active central volcano. Around it runs a fracture system that trends almost N-S, about 4-5 km wide and reaches from Mývatn in the south north to the sea at the westernmost part of Kelduhverfi. The fracture system is little volcanically active, but large earthquakes are fairly common within it. The build-up of lava has, however, been quite considerable, but the youngest lava, Þeistareykjahraun, is 2,700 years old and lies over Stóravítishraun, 2 km wide from the western margin of the geothermal area west of Bæjarfell north below Grísatungufjöll.

The geothermal area is on the southern part of Reykjaheiði between Lambafjöll in the west and Þeistareykjabunga in the east, for a large part on flat land at about 300 m height. Surface heat reaches, however, to 530 m height in Bæjarfjall and almost 500 m height in Ketilfjall. The immediate surroundings of the geothermal area are covered by lavas that have mainly flowed in the present. Active geothermal heat is over an area of about 11 km2, and the most active area is around the northern part of Bæjarfjall. In addition there is an altered area east of Lambafjöll.

The surface signs of geothermal heat at Þeistareykir are typical of a high-temperature area. There one can find bubbling mud pots, steam vents, sulphur mounds and alteration. There are no water springs.

Source: Halldór Ármannsson, ÍSOR.