The nature of low-temperature activity

Borholes in Laugarnes, ReykjavíkThe low-temperature systems are all located outside the volcanic zone passing through Iceland. The largest of these systems are located in southwest Iceland on the flanks of the western volcanic zone, but smaller systems can be found throughout the country. On the surface, low-temperature activity is manifested in hot or boiling springs, while no surface manifestations are observed on top of some such systems. Flow rates range from almost zero to a maximum of 180 liters per second from a single spring.

The heat-source for low-temperature activity is believed to be Iceland's abnormally hot crust, but faults and fractures, which are kept open by continuously ongoing tectonic activity, also play an essential role by providing channels for the water that circulates through the systems, and mines the heat. The temperature of rocks in Iceland generally increases with depth. Outside the volcanic zones the temperature gradient varies from about 150°C/km near the margin to about 50°C/km farther away.

The nature of low-temperature activity may be described as follows: Precipitation, mostly falling in the highlands, percolates down into the bedrock to a depth of one to three km, where the water is heated by the hot rock, and subsequently ascends towards the surface because of reduced density. Systems of this nature are often of great horizontal extent and constitute practically steady state phenomena. The most powerful systems are believed to be localised convection systems where the water circulates vertically in several kilometers deep fractures. The water then takes up the heat from the deep rocks at a much faster rate than it is renewed by conduction from the surroundings. These fields are therefore believed to be of transient nature, lasting some thousands of years.