Heat guide
Geothermal Heat: Where Earth’s Internal Heat Comes From
Earth’s interior remains hot because of residual heat from planetary formation and continuing heat production from radioactive decay, among other processes.
Earth stores enormous internal energy
The planet’s interior is much hotter than its surface. Heat moves outward through conduction, convection within the mantle, movement of magma and circulation of hot fluids.
Radioactive decay contributes heat
Long-lived radioactive isotopes inside Earth release energy as they decay. This radiogenic heating contributes substantially to the planet’s present internal heat budget.
Some heat is primordial
Earth also retains energy from accretion, compression, differentiation and other events associated with its formation and early history.
The geothermal gradient varies
Temperature generally rises with depth in the crust, but the rate varies by geology, tectonic setting and groundwater movement. A single global gradient cannot represent every location.
Geothermal systems use local heat flows
Geothermal power and direct-use systems exploit accessible hot water, steam or hot rock. Their practical value depends strongly on local geology and reservoir conditions.
Gradient and heat flow are related but not interchangeable
Geothermal gradient describes how temperature changes with depth, while conductive heat flux also depends on the thermal conductivity of the intervening rock. Two locations can therefore have different heat flows even when their measured gradients appear similar.
Groundwater circulation, magmatic systems, crustal structure and local geology can further disturb a simple one-dimensional conductive picture. Shallow measurements should not be extrapolated indefinitely without geological context.
Ground-source heat pumps use shallow ground temperature differently from deep geothermal systems
A ground-source heat pump exchanges heat with relatively shallow soil or groundwater and uses electrical work to move that heat. It does not require a naturally hot geothermal reservoir. Deep geothermal power or district heating instead relies on elevated subsurface temperatures that can provide useful heat directly or drive a power cycle.
The two technologies share the ground as part of the thermal system but operate on different temperature levels and resource requirements. Keeping them separate avoids treating every ground-coupled heat pump as geothermal power generation.
Earth loses internal heat continuously
Heat flowing from the interior eventually reaches the surface and is transferred to the atmosphere, oceans and space. The global average geothermal flux is small compared with absorbed solar energy at the surface, but concentrated geothermal resources can still provide useful local power and heating.
This difference in scale explains why geothermal heat is important for geology and energy resources without being the main driver of ordinary surface weather.
Geothermal resources differ by temperature and permeability
A hot rock body alone does not guarantee an easily usable resource. Productive hydrothermal systems also depend on fluid availability and pathways that allow useful flow rates. Engineered geothermal concepts may create or enhance fractures where natural permeability is insufficient.
Resource evaluation therefore combines temperature, depth, geology, permeability, chemistry and drilling considerations rather than relying on gradient alone.
The geothermal gradient is not one universal number
Temperature generally increases with depth in the crust, but the rate varies with regional heat flow, rock conductivity, groundwater circulation and geological setting. Volcanic and tectonically active regions can differ greatly from stable continental interiors.
A linear gradient is therefore useful for an educational estimate over limited depth ranges, but it should not be extrapolated blindly into the deep crust or mantle.
Heat flow and temperature gradient are connected
In a simplified conductive region, Fourier’s law links geothermal heat flux to thermal conductivity and the vertical temperature gradient. The same gradient can correspond to different heat fluxes in rocks with different conductivity.
Real geothermal systems can also transport substantial energy through moving fluids, fractures and hydrothermal circulation, so conduction alone does not describe every resource.
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Sources and further reading
Use the linked primary or authoritative resources for additional detail, standards and source-specific conditions.