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The geothermal gradient (dT/dz) is typically 25–30 K/km in stable continental crust but varies with crustal thickness, composition, and age. Heat flow q = −k(dT/dz) depends on thermal conductivity k; high heat flows mark mid-ocean ridges and hot-spots, while low flows occur in cold subducting slabs. The global heat budget is dominated by mantle convection, radiogenic heat from the core and mantle, and cooling of the lithosphere away from spreading centers.
You already know from studying Earth's interior structure that temperature increases with depth — the core is far hotter than the surface. The geothermal gradient quantifies this increase: it is the rate of temperature change with depth, typically expressed in degrees per kilometer. In stable continental crust, this gradient averages about 25–30 K/km, meaning that for every kilometer you descend into a mine or borehole, the temperature rises by roughly 25–30°C. But this average hides enormous variation. Near mid-ocean ridges, where hot mantle material rises close to the surface, the gradient can exceed 100 K/km. In old, cold continental shield regions, it may drop below 15 K/km.
The geothermal gradient alone tells you how fast temperature changes with depth, but to understand how much thermal energy is actually flowing through the crust, you need heat flow. Heat flow (q) relates the gradient to the rock's ability to conduct heat through Fourier's law: q = −k(dT/dz), where k is the thermal conductivity of the rock. A high gradient in a poor conductor might produce the same heat flow as a low gradient in an excellent conductor. Measuring heat flow requires both a temperature profile from a borehole and knowledge of the thermal conductivity of the rocks encountered — this is why heat flow measurements are more informative than temperature readings alone.
Earth's surface heat flow averages about 87 mW/m², but the pattern is far from uniform. The highest heat flows occur at mid-ocean ridges (often exceeding 200 mW/m²), where new lithosphere is being created and hot mantle material is close to the seafloor. As oceanic lithosphere ages and moves away from the ridge, it cools conductively and heat flow decreases — 50-million-year-old ocean floor typically shows about 50–60 mW/m². This cooling relationship is so predictable that it forms the basis of thermal models of oceanic lithosphere. On continents, heat flow varies with the concentration of radiogenic elements — uranium, thorium, and potassium — in crustal rocks. Granitic upper continental crust is enriched in these elements, so continental heat flow has a significant contribution from radioactive decay within the crust itself, unlike oceanic crust where most heat comes from below.
Understanding crustal heat flow matters for everything from predicting the depth at which rocks become ductile rather than brittle (which controls earthquake depth), to estimating the thermal maturity of sedimentary basins for petroleum exploration, to evaluating geothermal energy potential. The global heat budget — roughly 46 terawatts total — is powered by two main sources: primordial heat left over from Earth's formation and ongoing radiogenic heating. Mantle convection is the dominant mechanism for transporting this heat from the deep interior to the base of the lithosphere, where conduction takes over for the final journey to the surface. This thermal framework connects your knowledge of Earth's interior to the surface processes and tectonic features that the thermal regime ultimately controls.