Geothermal heat is found at the surface of the Earth where subterranean groundwater has come into close range with hot rock, intrusive dykes, or magma at shallow depths, causing heat transfer. As the water temperature rises, the geothermal fluids become lighter and rise up to the surface. Fluid–rock chemical interaction occurs in geothermal systems. Hot geothermal fluids contain high concentrations of dissolved chemical substances such as silica, calcium, sulphur species, and metal ions. As the water cools and the pressure drops, these substances precipitate into solids at the surface or in subterranean crevices and cavities. For geothermal areas to continue to exist, the heat source must be regularly renewed through new magma intrusions or new fissures in the bedrock.
Geothermal areas in Iceland are divided into low-temperature and high-temperature fields.
Low-temperature geothermal fields
Most of the largest low-temperature geothermal fields in Iceland appear to be situated in locations where active fracture zones intersect with ancient high-temperature geothermal fields that are today outside the active volcanic zones, such as the geothermal field at Mosfellsbær on the outskirts of Reykjavík. Within these deep fractures, the still-hot bedrock heats the water that percolates down. Hot springs and warm springs are characteristic surface features of low-temperature geothermal fields.
Geothermal water in these low-temperature areas is alkaline and contains low concentrations of dissolved chemical substances. Mineral deposits and altered rock are thus not common on the ground surface, although one can sometimes find geyserite. Water in low-temperature geothermal areas appears to come from a long distance away. Warm pools are common throughout all of Iceland except the Eastfjords. Deildartunguhver in West Iceland is Europe’s largest hot spring in a low-temperature geothermal area.
High-temperature geothermal fields
Around 20 delimited areas in Iceland are defined as high-temperature geothermal fields. All high-temperature areas are located within Iceland’s active volcanic zones and are connected to active volcanic systems with central volcanoes and fissure swarms. In these central volcanoes, one can find acid and intermediate rock and the density of intrusions is high. The main surface features of these high-temperature geothermal fields are mudpots and steam vents known as fumaroles. Geothermal steam is acidic at the surface and contains various gasses. Geothermal water is alkaline and contains high concentrations of dissolved chemical substances. The deposits that form on the surface as these substances precipitate are generally quite large. The most common of these are fumarolic salts, sulphur, silica, and calcium carbonate. Surficial geothermal deposits form on the ground where geothermal waters contain high concentrations of dissolved substances and where bulk flow of water occurs. The presence of large quantities of hydrothermally altered rock, where the interaction with geothermal fluids has altered their original composition, characterises the surface of most high-temperature geothermal fields. Water in high-temperature areas appears to originate from a short distance away and is generally precipitation that has fallen in the vicinity of the geothermal field.
Geysers are rare geothermal phenomenon, which may be found in both low-temperature and high-temperature areas where the groundwater level is high. Geysers erupt when gas-rich water superheats deep in the geyser’s vent. The pressurised water boils violently, causing a steam explosion as the water shoots out through the geyser’s vent. Geysir in Haukadalur is the best-known geyser in the world, even though it has not been active for a long time. The name “geyser” derives from Geysir.
Geothermal areas in Iceland are protected under the Nature Conservation Act no. 60/2013, encompassing geysers and other types of hot springs and vents, together with geothermal biota and active alteration and precipitation at geothermal sites, including geothermal encrustations.
The caldera of the Torfajökull volcano contains Iceland’s largest and most powerful high-temperature geothermal area, with both hot springs and fumaroles. The ground temperature at the surface is highly variable, and many rare geothermal phenomena are found here. Shallow pools of boiling water with a sand or gravel bottom (Icelandic soðpönnur) are a distinctive feature of geothermal activity in the Torfajökull area. Mudpots, mud pools, fumaroles, hillock-shaped sulphur fumaroles (Icelandic brennisteinsþúfur), and chemically altered ground can also be found in geothermal areas at Torfajökull. At many spots, one can find heated ground with steam vents and halotrichite (feather alum). Warm mineral springs are widely distributed, and there are soda springs and warm springs at the margin of the high-temperature zone.