Earths Heat FluxEdit
Earth's heat flux refers to the flow of heat from the planet's interior to its surface. This energy transfer drives a wide range of geologic processes, from the movement of tectonic plates to the generation of Earth's magnetic field. The global average heat flux is small on a per-square-meter basis, but across the entire planet it amounts to a substantial tens of terawatts of power. Measurements show strong geographic variation: oceanic regions, especially near mid-ocean ridges, yield higher fluxes than stable continental interiors. The magnitude and distribution of this heat flux emerge from a combination of primordial heat leftover from accretion, radiogenic heat produced by the decay of long-lived isotopes, and ongoing cooling of the mantle and core Geothermal gradient Heat flow.
The heat that escapes from the interior is not uniform in time or space. A contemporary view emphasizes three major components: residual heat from Earth's formation and differentiation, radiogenic heat from isotopes such as 40K, 238U, and 232Th, and cooling of the deep interior, including the outer core. The global heat loss is typically cited as around 44–47 terawatts (TW), with global surface heat flux averaging about 0.04–0.09 watts per square meter (W/m^2) when averaged over the entire globe. However, the distribution is highly anisotropic: heat flux at mid-ocean ridges can exceed 0.6 W/m^2 locally, while cratonic regions of the continents often show lower values, closer to 0.02–0.04 W/m^2. These patterns reflect variations in crustal thickness, tectonic setting, and mantle dynamics. The balance among these energy sources and sinks remains an active area of research as scientists refine the global budget Heat flow Radiogenic heat Mantle convection.
Sources of heat
Earth’s interior stores heat from several sources, which together set the planet’s thermal evolution and current geodynamics. The major components are:
Primordial and residual heat: Heat left over from planetary formation and differentiation, often referred to as accretional or secular cooling heat. This heat gradually dissipates as the mantle and core convect and conduct heat outward. The rate of cooling and the efficiency of heat transport influence the long-term evolution of Earth’s internal structure Geodynamics.
Radiogenic heating: The decay of long-lived isotopes—primarily 40K, 238U, and 232Th—produces heat within the mantle and crust. Radiogenic heat production is estimated to contribute a substantial portion of the total heat loss, though estimates vary with rock composition and the distribution of heat-producing elements. Ongoing work seeks to narrow uncertainties in the global radiogenic contribution through geochemical and geophysical studies Radiogenic heating.
Core cooling and core–mantle interactions: Heat flowing from the outer core, together with the secular cooling of the core, contributes to the total heat that must be transported through the mantle to the surface. This component is particularly important for sustaining the geodynamo that generates Earth’s magnetic field and for driving deep mantle convection Earth's core Geodynamo.
Mechanisms of heat transfer
Heat leaves Earth's interior primarily through conduction and mantle convection, with advection in magmatic systems and hydrothermal circulation playing important local roles. The lithosphere acts as a thermal lid that slows, but does not fully prevent, heat loss. In the mantle, convection parcels transport heat upward, with hotter material rising and cooler material sinking, a process that shapes continental rifting, subduction, and plume activity. At the surface, oceanic crust commonly exhibits higher fluxes along spreading centers, whereas continental regions show lower average fluxes due to thicker crust and different thermal structure. The outer core’s convective motions create the magnetic field and contribute heat that must be conducted through the mantle to the surface over geological timescales. These processes are studied in the framework of Mantle convection and Plate tectonics.
Regional patterns and implications
Spatial variation in heat flux is tied to tectonic regime and crustal architecture. Mid-ocean ridges, where new oceanic crust forms as tectonic plates diverge, are hot spots of heat loss, generating high local fluxes and driving seafloor spreading. In contrast, stable continental interiors (cratons) typically exhibit lower heat flux due to thick, cold continental roots and slow mantle convection. Subduction zones introduce local anomalies as sinking slabs modify thermal structure in the mantle and influence mantle plume formation and melting processes. The global heat flux budget interacts with plate motions, mantle viscosity, and the history of crust formation, affecting long-term geologic activity and the thermal evolution of the planet Mid-ocean ridge Continental crust Subduction.
Measurements and uncertainties
Direct measurements come from boreholes and, in marine environments, from ocean-bottom heat-flow probes. These data are integrated with geophysical methods, such as seismic imaging and magnetotellurics, to infer mantle temperature fields and heat production rates. Despite substantial progress, uncertainties remain in the global average and in the relative contributions of radiogenic heating versus primordial cooling, particularly in the deep mantle. Efforts to map heat flux continue with improved instrumentation, modeling, and wide geographic sampling, including near-transform faults and in cratonic regions where data have historically been sparse Geophysical methods Heat flow.
Implications for Earth’s evolution
Understanding Earth’s heat flux is essential for explaining plate tectonics, volcanic activity, and the longevity of Earth’s magnetic field. The rate of heat loss constrains models of mantle convection, crustal formation, and the thermal history of the planet. If radiogenic heating is higher in the mantle than some models suggest, it could alter interpretations of mantle viscosity and the vigor of convection over geological time. Conversely, a predominantly primordial cooling signal would accentuate the importance of secular cooling in driving mantle dynamics and core–mantle interactions that influence long-term geodynamo behavior and seismicity patterns Mantle convection Geodynamo.