The word geothermal comes from the Greek words geo (earth) and therme (heat), – ‘the heat of the earth’.
Geothermal energy is derived from the heat in the earth’s core and from radioactive decay within its mantle. At high temperatures and pressures within the mantle, mantle rocks melt forming magma, which rises towards the surface carrying the heat from below.
In some regions where the earth’s crust is thin or fractured, or where magma bodies are close to the surface, there are high temperature gradients. Deep faults, rock fractures and pores allow groundwater to percolate towards the heat source and become heated to high temperatures. Some of this hot geothermal water travels back to the surface through buoyancy effects to appear as hot springs, mud pools, geysers, or fumaroles.
If the rising hot water meets an extensively fractured or permeable rock zone, the heated water will fill pores and fractures and form a geothermal reservoir. These reservoirs are much hotter than surface hot springs, reaching more than 350°C, and are potentially an accessible source of energy.
High temperatures can be achieved in liquid-dominated reservoirs because increasing hydrostatic pressure with depth allows elevated temperatures without boiling. Many undisturbed geothermal reservoirs in New Zealand have temperature and pressure profiles where fluid is close to boiling point to depths of more than 1 km.
Geothermal areas are commonly close to the edges of continental plates. New Zealand’s location on an active plate boundary (between the Indo-Australian and Pacific Plates) has resulted in numerous geothermal systems and a world-class geothermal energy resource.
The characteristics of geothermal systems vary widely.