The nature of high-temperature activity

 

High-temperature areas are located within the active volcanic zones or marginal to them. They are mostly on high ground. The rocks are geologically very young and permeable. As a result of the topography and high bedrock permeability, the groundwater table in the high-temperature areas is generally deep, and surface manifestations are largely steam vents. Hydrogen sulphide present in the steam tends to be oxidised at the surface by atmospheric oxygen, either into elemental sulphur, which is deposited around the vents, or into sulphuric acid, which leads to acid waters altering the soil and bedrock. 

The internal structure of fossil high-temperature systems can be seen in Tertiary and Quaternary formations, where erosion has exposed rocks that were formerly at a depth of 1-3 kilometers. The system's heat source is generally shallow magma intrusions. In the case of high-temperature systems associated with central volcanic complexes the intrusions often create shallow magma chambers, but where no central volcanoes have developed only dyke swarms are found. Intrusive rocks appear to be most abundant in reservoirs associated with central complexes that have developed a caldera.

The boiling point of water depends on the hydrostatic pressure. As the pressure increases with depth the temperature needed for the water to boil rises along a curve that is called the boiling point curve.

Temperatures in active, high-temperature systems generally follow the boiling point curve. The highest recorded downhole temperature is 386°C. Hydrological considera- tions and permeability data imply that the groundwater in the reservoir is undergoing a density driven vertical circulation. This groundwater is in most cases of meteoric origin. However, in three areas on the Reykjanes Peninsula it is partly or solely ocean water.