The Reykjanes thermal brine area is in southwesternmost Iceland on the subaerial continuation of the Reykjanes ridge. This area is unique among thermal areas for its fluid composition. In the reservoir, where temperatures are between 250 and 290°C, the brine has the same salinity as seawater. However, the concentrations of some ions are different and can be explained by relatively simple interaction of seawater with the rock. The content of trace metals is low. In the upflow zone, hot brine has been exchanged many times during the lifetime of the hydrothermal system. Surface activity covers an area of 1 sq km, and a resistivity survey indicates that the system is of similar area above 900 m within the hyaloclastite formation where it is surrounded by cold groundwater of seawater composition. By contrast, the system is more extensive in the underlying basalt formation consisting of lava flows and thick interbeds of hyaloclastites and sediments. At 2,600 m depth P-velocity increases from 4.2 to 6.5 km/sec. At this depth the basalt formation is underlain by a denser formation considered to be similar to the "oceanic layer."

Because of its temperature and composition, feasibility studies indicate that the brine could be exploited economically for the production of NaCl, KCl, CaCl₂, and possibly other components. Aquifers are abundant in the basalt formation. It therefore is recommended that production wells penetrate to depths of about 2,000 m to withdraw brine within this permeable formation, so as to ensure highest mass flow and minimize risk of cold seawater intrusion.