Geothermal gradient
The rate at which temperature increases with depth in the Earth's crust, averaging about 25 to 30 °C per kilometre away from tectonic plate boundaries.
Heat flows outward from the Earth's interior, from residual primordial heat and radioactive decay of uranium, thorium and potassium in the crust, at a global average of about 87 mW/m² (Davies and Davies 2010). The temperature profile in the upper crust follows Fourier's law:
where q is heat flux (W/m²) and k rock thermal conductivity (W/m·K). With typical continental values q ≈ 65 mW/m² and k ≈ 2.5 W/m·K, the gradient is about 26°C/km.
Typical ranges:
- Stable continental shields: 10–20°C/km
- Average continental crust: 25–30°C/km
- Sedimentary basins with insulating cover (Paris, Pannonian, Upper Rhine Graben): 35–60°C/km
- Volcanic and rift zones (Iceland, Kenya Rift, New Zealand, Indonesia): above 100°C/km, with 250–350°C reservoirs at 1–3 km
The gradient determines how deep a well must go for a given temperature, and drilling cost rises steeply with depth. At an average 30°C/km, reaching 150°C for power generation takes about 5 km; in a volcanic field it may take less than 1 km. Conventional hydrothermal power is therefore concentrated at plate boundaries, while low-gradient regions rely on district heating from moderate depths, ground-source heat pumps near the surface, or enhanced geothermal systems.
Related terms
All terms →- Enhanced geothermal system (EGS)
A geothermal reservoir created by stimulating hot, low-permeability rock, usually by hydraulic fracturing or shearing, so that water can be circulated between wells to extract heat.
- Binary cycle power plant
A geothermal power plant in which hot brine heats a separate low-boiling-point working fluid that drives the turbine, enabling generation from resources of roughly 100 to 180 °C.
- Ground-source heat pump
A heat pump that exchanges heat with the ground through buried loops or boreholes, benefiting from stable soil temperatures for high seasonal efficiency.
- District heating
A network of insulated pipes distributing hot water or steam from central heat sources to many buildings for space heating and hot water.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.