What is the source of geothermal energy?

Geothermal energy originates from the Earth’s core, which is estimated to have a temperature of about 5,000° C – almost as hot as the surface of the Sun. Scientists believe the core has cooled by only a few hundred degrees over 4.5 billion years since the planet was formed. This nearly constant temperature is possible because some new energy is continually released by the decay of radioactive elements and because the core is very well insulated.

Surrounding the core is a thick section of ductile rock called the mantle. (Ductile means capable of being shaped or stretched without fracturing.) The upper part of the mantle generates molten rock (magma) that rises and forms volcanoes at the surface. The solid crust rests atop the ductile mantle. The crust is a jigsaw puzzle of abutting sections called tectonic plates, and is about 35 kilometres thick under the continents but only averages about seven kilometres thick under the oceans. Where the crust is thin or fractured, as at the edges of tectonic plates, heat from deep in the Earth can be a significant source of energy. Volcanoes, geysers and hot springs are manifestations of this energy reaching the surface, directly or indirectly.

Average temperatures increase at a rate of 10° to 30° C for each kilometre of depth in the crust. The rate of increase, called the geothermal gradient, varies considerably among regions. It is very low in the Canadian Shield, for example, and much higher under the ocean floors. The geothermal gradient in Nova Scotia is about 15° C per kilometre of depth, but water in some flooded Nova Scotia coal mines has a temperature of 21° C just 50 metres below the surface because it circulates from greater depths. Temperatures of about 250° C have been recorded at a depth of two kilometres in a volcanic centre in British Columbia.