The mud pools and steam vents facing Kísilhóll hill to the northeast are collectively named Gunnuhver. They form where steam generated by boiling in a geothermal reservoir emanates and condenses and mixes with surface water. Accompanying gases such as carbon dioxide and hydrogen sulphide make the water acid, which causes alteration of the fresh lava rock to clay. Steaming of the ground at Reykjanes increased markedly as a consequence of pressure drawdown in the geothermal reservoir after exploitation began in 2006. Iceland´s largest mud pool at present is prominent among the hot springs, and located highest up in the Gunnuhver group. It is 20 m across with a rim of mud, boiling vigorously.
Reykjanes is a restless ground due to earthquakes, which occur in swarms but are rather small, the largest just over magnitude 5. Some have caused a minor slip on a fissure that passes through Gunnuhver. This last happened about 40 years ago. Such ground movements have revived the steam field intermittently and allowed the deep reservoir water to escape to the surface through geysers. The deep reservoir fluid is sea water by origin, rich in dissolved chlorides, but also in silica, which may precipitate as sinter. Thus Kísilhóll (Silica hill) has a layer of silica sinter at its top. The bowls of two extinct geysers are close by the viewing deck to the northwest and southwest. The collapsed shaft of the last active geyser, which formed in the year 1919, is located on a fissure east of the road about 100 m to the south.
Kristján Sæmundsson, 2010