Where does the Earth's internal heat come from and how does it drive geological processes?
The sources of the Earth's internal heat, the geothermal gradient and heat flow, and how heat drives mantle convection, melting and metamorphism as the internal limb of the rock cycle.
A focused answer to WJEC and Eduqas A-Level Geology F2 on internal processes, covering the sources of the Earth's internal heat (radioactive decay and residual heat), the geothermal gradient and heat flow, and how heat drives mantle convection, partial melting and metamorphism as the deep limb of the rock cycle.
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What this dot point is asking
WJEC wants you to explain where the Earth's internal heat comes from, what the geothermal gradient and heat flow are, and how this heat drives the deep processes (mantle convection, melting, metamorphism) that form the internal limb of the rock cycle. This complements the surface processes in F2 and sets up plate tectonics in F4: the internal engine is what raises, melts and reworks rock at depth.
The answer
Where the heat comes from
The Earth has two main internal heat sources. Radioactive decay of long-lived isotopes (uranium-238, thorium-232 and potassium-40) in the mantle and crust releases heat continuously and is the dominant present-day source. Residual (primordial) heat is left over from the planet's formation: the energy of accretion, of impacts, and of core differentiation when dense iron sank to form the core. Together these keep the interior hot enough to convect and to melt rock locally.
Heat flow and the geothermal gradient
Heat flow is the rate at which heat escapes through the surface, conducted from the hot interior. The geothermal gradient and heat flow are not uniform: they are high at constructive plate margins and over hot spots (where hot mantle is near the surface) and low over old, cold continental shields. Local factors matter too, since insulating sediment piles and heat-producing granites raise the gradient. These variations control where rock melts and where metamorphism is most intense.
Mantle convection drives the deep processes
Because the deep mantle is hotter than the shallow mantle, hot material expands, becomes less dense and rises, while cooler material sinks, setting up slow convection currents in the solid but ductile mantle. These currents, together with the pull of dense subducting slabs, drive plate motion. Convection therefore powers the internal limb of the rock cycle: it carries crust to depth where it is metamorphosed and partly melted, and it brings hot material up where decompression melting generates magma.
Examples in context
Hot springs and geothermal power. Iceland sits on a constructive margin over a plume, so its steep geothermal gradient brings heat near the surface and is harnessed for geothermal electricity, a direct use of high heat flow. Metamorphic aureoles. A granite intrusion carries heat into the surrounding rock, raising the local gradient and baking a contact metamorphic aureole, the internal heat engine working on a small scale. Decompression melting at ridges. Rising mantle beneath a constructive margin melts as pressure falls, generating basaltic magma, an example of convection feeding the igneous part of the rock cycle.
Try this
Q1. Name the two main sources of the Earth's internal heat. [2 marks]
- Cue. Radioactive decay of long-lived isotopes (uranium, thorium, potassium-40) and residual (primordial) heat of formation.
Q2. State an average value for the geothermal gradient in the upper crust. [1 mark]
- Cue. About 25 to 30 degrees Celsius per kilometre.
Q3. Explain why mantle convection can occur even though the mantle is solid. [2 marks]
- Cue. The mantle is solid but ductile, so over millions of years it flows plastically; hotter, less dense material rises and cooler, denser material sinks, setting up slow convection.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20194 marksDescribe the sources of the Earth's internal heat and explain how this heat drives mantle convection.Show worked answer →
Name the sources first, because the marks reward both the origin of the heat and the convection mechanism.
The two main sources are the decay of long-lived radioactive isotopes (such as uranium, thorium and potassium-40) in the mantle and crust, and residual (primordial) heat left from the Earth's formation and core differentiation.
This heat makes the lower mantle hotter than the upper mantle, so deep mantle material expands, becomes less dense and rises, while cooler, denser material near the top sinks.
The rising and sinking sets up slow convection currents in the solid but ductile mantle, and the drag of these currents, together with slab pull, drives the motion of the overlying plates.
Markers reward naming radioactive decay and residual heat, explaining the density difference that drives rising and sinking, and linking convection to plate motion.
WJEC Eduqas 20213 marksExplain what is meant by the geothermal gradient and why it is not the same everywhere in the crust.Show worked answer →
The geothermal gradient is the rate at which temperature increases with depth in the Earth, averaging about 25 to 30 degrees Celsius per kilometre in the upper crust.
It is not uniform because heat flow varies with the geological setting: at constructive margins and over rising mantle plumes the gradient is steep, as hot material is close to the surface.
At stable continental interiors and over cool, old crust the gradient is gentler, and thick insulating sediment or radioactive granite can also raise local gradients.
Markers reward defining the gradient with an approximate value and explaining that variations in heat flow (tectonic setting and rock type) make it vary from place to place.
Related dot points
- The compositional and mechanical layering of the Earth (crust, mantle, outer and inner core; lithosphere and asthenosphere) and the seismic evidence (P and S wave behaviour, shadow zones, discontinuities) used to deduce it.
A focused answer to WJEC and Eduqas A-Level Geology F4 on Earth structure, covering the compositional layers (crust, mantle, outer and inner core) and mechanical layers (lithosphere and asthenosphere), and the seismic evidence (P and S wave behaviour, shadow zones and discontinuities) that reveals them.
- The development of plate tectonic theory from continental drift, and the evidence for it (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and palaeomagnetic stripes).
A focused answer to WJEC and Eduqas A-Level Geology F4 on plate tectonic theory, covering the development from continental drift, and the evidence (continental fit, matching geology and fossils, palaeoclimate, sea-floor spreading and the symmetry of palaeomagnetic stripes) that confirmed it.
- The three types of plate boundary (constructive, destructive and conservative), the processes and features at each, and the driving forces of plate motion (mantle convection, ridge push and slab pull).
A focused answer to WJEC and Eduqas A-Level Geology F4 on plate boundaries, covering constructive, destructive and conservative margins, the processes and landforms at each (ridges, subduction zones, ocean trenches, volcanic arcs, fold mountains, transform faults), and the driving forces of plate motion.
- The rock cycle as the set of processes (weathering, erosion, transport, deposition, burial, lithification, metamorphism, melting and crystallisation) that recycle material between the three rock classes, driven by internal heat and surface energy.
A focused answer to WJEC and Eduqas A-Level Geology F1 on the rock cycle, covering how weathering, erosion, transport, deposition, lithification, metamorphism, melting and crystallisation recycle material between igneous, sedimentary and metamorphic rocks, and the internal and external energy sources that drive it.
Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)