The division of geothermal heat into high- and low-temperature can be defined in two ways: by borehole temperature — where 200°C is measured above 1000 m depth it is called high-temperature — and by the substances dissolved in the water, under which definition acidic springs (steam and mud pots) belong to high-temperature areas.
High-temperature areas occur within Iceland's active volcanic belt and its continuation along the mid-ocean ridges to the south and north of the country. On land they number 25 to 40 depending on how they are counted, of which 5–6 lie beneath glaciers.
High-temperature areas form from hot magma intrusions deep underground. There, the groundwater of the geothermal systems — the deep water — is at boiling point.