The geothermal gradient quantifies how temperature increases with depth within a planetary interior, commonly reported as °C km−1 (equivalently K km−1 or mK m−1). In stable continental crust away from plate boundaries this near-surface gradient is typically on the order of 25–30 °C km−1, implying a temperature rise of roughly 25–30 °C per kilometre. Atmospheric forcing strongly modulates the uppermost ground temperatures: seasonal and weather-driven variations imprint the soil and shallow subsurface, which approach the mean annual ground temperature (MAGT) within roughly 10–20 m. Because these transient signals penetrate only to that shallow depth, brief intervals of inverse (negative) gradient—where temperature decreases with depth—are possible near the surface.
Beneath the shallow weather-influenced zone, the subsurface thermal regime preserves a layered record: the top hundreds of metres retain palaeoclimatic information, while at greater depths temperature increases more uniformly as the influence of surface variability wanes and internal heat sources dominate. Within the mantle the gradient is generally below the melting point of mantle rocks except in localized regions such as the asthenosphere; schematic profiles also show pronounced temperature steps at the base of the lithosphere/uppermost mantle transition and again across the core–mantle boundary. Current estimates place the inner-core temperature between about 4,000 and 7,000 K, with central pressure near 360 GPa, subject to the planet’s radial density distribution.
Earth’s internal heat arises mainly from retained accretional heat, radiogenic decay (notably of 40K, 238U, 235U and 232Th), and latent heat released during core crystallization, with smaller contributions possible from other processes. The combined output of these sources governs surface heat flow and the internal gradient. Because radiogenic heat production decays over time, Earth emitted substantially more heat in the past—roughly double present values around 3 billion years ago—resulting then in steeper internal gradients, stronger mantle convection and plate tectonics, and the generation of very high-temperature magmas (e.g., komatiites) that are rare today.
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These aspects of the geothermal gradient have direct practical and interpretive consequences. Engineering and geothermal projects must account for the shallow (~10–20 m) climatic influence when selecting design depths, while palaeoclimate reconstructions can exploit thermal signals preserved in the upper hundreds of metres. Deeper subsurface exploration and modelling must consider the progressively increasing temperatures controlled by internal heat production and the distinct thermal discontinuities at lithospheric and core–mantle boundaries.
Heat within Earth increases systematically with depth, producing a geothermal gradient that is observable at crustal scales (for example by shallow boreholes) and that governs phenomena from partial melting at plate margins to large-scale mantle convection. Temperatures of roughly 650–1,200 °C occur in highly viscous or partially molten rocks near tectonic boundaries, generating locally elevated gradients. At greater depth only the outer core is inferred to be fluid; the inner–outer core boundary lies near 3,500 km depth with an estimated temperature of about 5,650 ± 600 K. Earth’s internal energy reservoir is enormous (on the order of 10^31 J) and is sustained by several heat sources.
Radioactive decay of naturally occurring nuclides is the dominant contemporary heat source, accounting for roughly 45–90% of the heat flux escaping Earth. Radiogenic heat is concentrated principally in the crust and upper mantle because lithophile, relatively heavy elements (notably uranium and thorium) preferentially enter crustal melts and continental lithologies; granitic and near-surface basaltic rocks thus constitute the planet’s richest repositories of heat-producing nuclides. The mantle, by contrast, is relatively depleted in these large lithophiles and is dominated by higher-density, small-radius cations such as Mg, Ti and Ca.
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Other non-radiogenic contributions include residual heat from planetary accretion and the release of gravitational potential energy during core formation (differentiation), latent heat released as the outer core crystallizes onto the inner core, and modest heating by tidal dissipation. Among radiogenic isotopes, decay of 238U and 232Th provides the largest share of present-day internal heating. Representative parameters for major heat-producing nuclides (heat production per kg of nuclide, half-life, mean mantle concentration and resulting heat production per kg of mantle) are: 238U — 9.46×10^−5 W kg^−1, t1/2 = 4.47×10^9 yr, mantle concentration ≈30.8×10^−9 kg kg^−1, heat ≈2.91×10^−12 W kg^−1; 235U — 56.9×10^−5 W kg^−1, t1/2 = 0.704×10^9 yr, concentration ≈0.22×10^−9 kg kg^−1, heat ≈0.125×10^−12 W kg^−1; 232Th — 2.64×10^−5 W kg^−1, t1/2 = 14.0×10^9 yr, concentration ≈124×10^−9 kg kg^−1, heat ≈3.27×10^−12 W kg^−1; 40K — 2.92×10^−5 W kg^−1, t1/2 = 1.25×10^9 yr, concentration ≈36.9×10^−9 kg kg^−1, heat ≈1.08×10^−12 W kg^−1. These values underline the central role of radiogenic decay in Earth’s heat budget and the importance of crustal enrichment for surface-proximal heat production.
The difference in heat transport mechanisms between the lithosphere and the convecting mantle explains characteristic gradient variations: the mantle approximates an adiabat because convective transport dominates, whereas the lithosphere behaves as a conductive thermal boundary layer, producing a steeper temperature increase with depth.
Earth’s outward heat loss is large but spatially heterogeneous. Total global heat transfer from the interior to the surface is estimated at 44.2 TW (4.42 × 10^13 W), corresponding to a planetary mean surface flux of about 0.087 W m−2 (87 mW m−2). Mean surface heat flux varies systematically with crustal type: continental regions average ~65 mW m−2 while oceanic regions average ~101 mW m−2. Despite its global significance, this geothermal flux is small relative to solar input, amounting to roughly 0.03% of the solar energy absorbed by Earth.
The pattern of heat loss is strongly controlled by lithospheric thickness and tectonic setting. Where the lithosphere is thin—most notably at mid‑ocean ridges and above mantle plumes—conductive and especially advective heat transfer are elevated, concentrating heat loss at spreading centers and associated hydrothermal systems. Plate‑tectonic processes, particularly mantle upwelling at ridges, therefore act as primary pathways by transporting deep heat to the base of the lithosphere and enabling its escape. Conduction through the lithosphere is a dominant mode of heat transfer on a global basis; because oceanic lithosphere is both thinner and younger than continental lithosphere, most of the conductive heat flux is observed beneath the oceans, producing systematically higher oceanic heat‑flow values.
The solid crust functions as a substantial thermal insulator, so rapid or localized surface heat release commonly requires fluid-mediated pathways. Magma ascent, circulating groundwater, and other fluid conduits permit advective and convective transfer that yields volcanism, hydrothermal vents, hot springs, and similar phenomena that pierce the insulating crust. Internal radiogenic heat production—decay of heat‑producing isotopes in mantle and crust—partially compensates for outward loss, contributing about 30 TW to Earth’s internal heat budget.
Empirical quantification of these patterns relies on systematic measurements of heat‑flow density; the International Heat Flow Commission (operating within the IASPEI/IUGG framework) curates the global dataset used for mapping and analysis. Taken together, global geothermal flow rates exceed current human primary energy consumption by more than a factor of two, underscoring the large magnitude of Earth’s internal heat relative to anthropogenic energy use.
Geothermal energy comprises heat from Earth’s interior that can be exploited for human purposes by tapping the subsurface temperature gradient. Its use dates back millennia for bathing and space heating and in the modern era has expanded to include electrical generation as well as numerous direct-heat applications. Interest in geothermal resources has been strengthened by rising energy demand and concerns over emissions from conventional fuels, since geothermal systems can displace fossil‑based generation and thereby reduce greenhouse‑gas outputs.
Electricity production is feasible where subsurface temperatures are sufficiently high to yield steam or hot fluids that can be delivered to a plant; conversion is achieved by transferring reservoir heat to a working fluid and driving turbines coupled to generators. Because no combustible fuel is required, geothermal plants can provide firm baseload power with operational reliability typically exceeding 90%. The thermodynamic efficiency of conversion is governed largely by the temperature differential between the hot working fluid and the ambient sink, so deeper and hotter reservoirs permit greater conversion efficiencies. Efficient heat transfer between reservoir and plant is therefore a critical engineering requirement.
Despite demonstrated commercial deployment, Earth’s internal heat remains a relatively unconventional energy resource at the global scale. As of 2007 roughly 10 GW of geothermal electric capacity were installed worldwide (about 0.3% of global electricity supply), while direct‑use installations amounted to approximately 28 GW for applications such as district and space heating, spas, industrial processes, desalination, and agricultural uses.
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Variations in the geothermal gradient
Observed geothermal gradients are shaped by a combination of measurement artefacts, surface-climate history, near‑surface material redistribution, and fundamental changes in subsurface heat‑transport processes with depth. Practically, site gradients are usually inferred from bottom open‑hole temperatures recorded in boreholes, but temperature logs recorded immediately after drilling are biased by circulation of drilling fluids; reliable bottom‑hole estimates therefore require long‑term thermal equilibration of the well, a condition that is often unattainable in routine field practice.
Near‑surface gradients reflect recent surface boundary conditions. In tectonically stable tropical regions the temperature–depth profile commonly approaches the mean annual surface temperature within shallow depths, producing a predictable near‑surface signature tied to local climate. Conversely, past climatic extremes and ground‑thermal memory can produce long‑lived anomalies: areas that developed deep Pleistocene permafrost retain cold anomalies extending to several hundred metres (the Suwałki anomaly in Poland is a well‑documented example), and similar post‑glacial disturbances appear in boreholes across polar and subpolar regions.
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Shallow geothermal profiles also record recent vertical movements of the crust. Where Holocene uplift and erosion have exposed formerly deeper material, shallow gradients tend to be steeper than the stabilized deeper heat‑flow regime; extrapolating the stabilized gradient upward to meet the present mean surface temperature yields a vertical offset that can be interpreted as net uplift and erosion. By contrast, Holocene subsidence and sedimentation produce a subdued shallow gradient that rises and merges with the deeper regime, producing a characteristic concave temperature–depth curve diagnostic of depositional histories.
Temporal variability of surface temperature imposes oscillatory signals on the subsurface thermal field over timescales from daily and seasonal cycles up to orbital (Milankovitch) periods. These oscillations attenuate exponentially with depth: short‑period changes penetrate only metres to tens of metres, whereas the longest climatic cycles (tens of thousands of years) have characteristic scale depths on the order of kilometres. Some authors have further argued for a role of large‑scale ocean bottom meltwater flows in homogenizing near‑surface gradients globally, but this assertion appears in the literature as contested and requires further verification.
At greater depth the geothermal gradient cannot be extrapolated linearly from shallow borehole values. Simple extrapolation would predict mantle temperatures above rock solidus conditions, which contradicts seismic observations (S‑wave propagation indicates a largely solid mantle). The principal reason is a change in the dominant heat‑transport mechanism: conduction controls heat transfer in the rigid lithospheric plates and near the surface, whereas slow viscous flow (convection and advective transport) becomes important in parts of the mantle over geological timescales. In addition, radiogenic heat production is concentrated in the crust—especially the upper crust—because uranium, thorium and potassium are enriched there; this elevates near‑surface gradients relative to the deeper mantle. Within the convecting mantle the temperature gradient is much lower—of order 0.5 K km−1—and is controlled largely by the adiabatic gradient appropriate to mantle peridotite rather than by the conductive gradient measured in the crust.
A negative geothermal gradient describes sections of the crust where temperature decreases with depth, a reversal of the usual increase of temperature downward. Near the surface—typically within the upper few meters to a few hundred meters—such gradients commonly arise because rocks have low thermal diffusivity and therefore conduct heat slowly. As a result, short-term surface fluctuations (diurnal or seasonal) remain largely confined to the very near surface; at depths of a few meters ground temperatures converge on the annual mean surface temperature, while at depths of tens to hundreds of meters the subsurface integrates climate over much longer intervals (hundreds to thousands of years). Those deeper shallow profiles can therefore record past climatic conditions—sometimes yielding temperatures colder than today where cooling during the Last Glacial Maximum or other past events persists in the subsurface signal.
Beyond climate-driven surface effects, advective and convective transport in groundwater systems can produce local negative gradients. Moving water can carry heat upward, warming shallower strata relative to somewhat deeper rocks and producing a local decrease of temperature with depth where advection dominates conductive heat transfer. At tectonic scales, negative gradients are characteristic of subduction zones: an oceanic plate sinking into the mantle at rates of the order of centimeters per year remains anomalously cold because conductive heating cannot keep pace with its descent, so the downgoing slab is cooler than the surrounding mantle and creates a large-scale negative vertical temperature anomaly.
Thus, negative geothermal gradients arise from processes operating across a broad range of spatial and temporal scales—from near-surface thermal inertia set by material diffusivity and recent climate, through fluid-dynamic heat transport in aquifers, to slab dynamics at convergent plate margins. In all cases the contrast between heat-transport rates and the thermal properties of rocks and fluids controls whether temperature increases or decreases with depth.