Geothermal energy

Geothermal heat is the heat within the Earth that exceeds the temperature prevailing at the surface.

Geothermal systems form where the crust is hot enough and contains enough fractures and water-bearing strata for water to flow through it, carrying heat up from deeper, hotter layers to the surface. These conditions exist in volcanic countries such as Iceland, where tectonic plates meet and are created.

Iceland's crust is relatively hot because its lower part is formed largely from magma intrusions that never reached the surface but solidified on their way up. It is hottest beneath the volcanic and rift zones and cools with distance as the crust grows older.

Beneath most of the country's high-temperature areas, all within the volcanic belt, magma intrusions linked to volcanism are thought to be the main heat source. Low-temperature areas lie where the crust is cooler, yet still warm enough to heat water to 50–150°C.

Map of the main geothermal areas and sites in Iceland. — The main geothermal areas and sites in Iceland. Drawing: ÍSOR.

In this section

Geothermal, explained

Boreholes

The word "borehole" means, according to the Icelandic dictionary, a hole that has been drilled (for example, for hot water). There are many types of borehole and different methods are used in drilling.

Low-temperature geothermal

Where the temperature is below 150°C at 1-3 km depth, the area is described as low-temperature.

High-temperature geothermal

High-temperature geothermal is where a temperature of >200°C is measured at 1 km depth.

High-temperature areas

In Iceland the high-temperature areas number 25 to 40 depending on how they are counted, of which 5-6 lie beneath glaciers.

Renewable energy sources

By a renewable energy source is meant that energy comes in place of that which people take from the source, that is to say, the energy source does not diminish even when drawn upon. It is not entirely simple to define what counts as a renewable energy source, but internationally people have nonetheless agreed on which energy sources are to count as renewable and which not. Hydropower, wind power, solar power and ocean power are good examples of renewable energy sources; they all originate in solar radiation and will persist as long as it lasts. Despite that, these energy sources can change because of local changes in the flow of water, the radiation of the sun or the streaming of the sea and winds.

Sustainable use of geothermal energy

Geothermal energy counts as a renewable resource, and it can be used both sustainably and unsustainably.