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Source of Geothermal Energy
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The source of geothermal energy is the heat inside the Earth. About half of it comes from the continuing radioactive decay of uranium, thorium and potassium in the rocks of the crust and mantle; the rest is primordial heat left over from the planet’s formation about 4.5 billion years ago. This heat flows out through the surface at roughly 44-47 terawatts (TW). It becomes usable wherever it is concentrated near the surface in hot rock, steam or hot water.
What is the ultimate source of geothermal energy?
There are two ultimate sources, and both were set up when the Earth formed.
1. Radiogenic heat: radioactive decay
Rocks contain small amounts of long-lived radioactive isotopes. Each decay releases energy that ends up as heat in the surrounding rock. Four isotopes supply almost all of it:
| Isotope | Half-life | Note |
|---|---|---|
| Uranium-238 | About 4.5 billion years | Main uranium contributor |
| Uranium-235 | About 700 million years | Much of it has already decayed, so its share today is small |
| Thorium-232 | About 14 billion years | Longest-lived; will keep heating the Earth long after the others fade |
| Potassium-40 | About 1.25 billion years | Common in granite and other crustal rocks |
These elements are concentrated in the continental crust, especially in granites, which is why some granite regions have unusually high heat flow.
2. Primordial heat: left over from the Earth’s formation
The Earth formed as dust, rocks and small planets collided and stuck together. The kinetic energy of those impacts became heat. When heavy iron sank to the centre to form the core, the gravitational energy released added more heat. The planet has been cooling ever since, but slowly, because rock is a poor conductor. A smaller extra source is the heat released as the liquid outer core slowly freezes onto the solid inner core.
How do we know it is about half and half?
The answer comes from geoneutrinos, the ghostly particles given off when uranium and thorium decay. In 2011 the KamLAND detector in Japan reported in Nature Geoscience that decay of uranium-238 and thorium-232 together produces about 20 TW, with a wide uncertainty of roughly plus or minus 9 TW. Potassium-40 adds about another 4 TW. Measured total heat flow out of the Earth is about 44 TW (Pollack and others, 1993), so radioactive decay supplies about 24 / 44 = roughly half. The rest is primordial heat, which shows the Earth has not yet used up its original store.
How hot is the inside of the Earth?
Temperature rises steadily with depth. Approximate figures for each layer:
| Layer | Depth | Approximate temperature |
|---|---|---|
| Crust | 0 to about 35 km under continents (5-10 km under oceans) | Surface temperature to several hundred °C at its base |
| Mantle | To about 2,900 km | From roughly 1,000-1,300 °C near the top to about 3,500-4,000 °C at the bottom |
| Outer core (liquid iron-nickel) | 2,900-5,150 km | About 4,000 °C at the top, rising with depth |
| Inner core (solid iron-nickel) | 5,150-6,370 km | About 5,000-6,000 °C |
The core figures are estimates because nobody can measure them directly. A 2013 laboratory experiment that melted iron at core pressures put the temperature at the inner-core boundary at about 6,230 K (roughly 6,000 °C), give or take 500 degrees. That makes it about as hot as the surface of the Sun.
The geothermal gradient: a worked example
The rate at which temperature rises with depth is called the geothermal gradient. In most continental areas, away from volcanoes and plate boundaries, it is about 25-30 °C per km.
Temperature at 3 km depth, taking a surface temperature of 25 °C:
- At 30 °C/km: T = 25 + 30 x 3 = 115 °C
- At 25 °C/km: T = 25 + 25 x 3 = 100 °C
Depth needed to reach 150 °C, a useful temperature for generating electricity:
- At 30 °C/km: depth = (150 – 25) / 30 = about 4.2 km
- At 25 °C/km: depth = (150 – 25) / 25 = 5.0 km
- In a geothermal field with a gradient of, say, 100 °C/km: depth = 125 / 100 = 1.25 km
This is the whole economics of geothermal energy in one calculation. With a normal gradient you must drill 4-5 km to reach 150 °C, which is expensive. Where the gradient is several times higher, the same temperature sits within reach of an ordinary well.
Heat flow from the gradient. Heat conducted up through rock follows Fourier’s law, q = k x (dT/dz). For typical crustal rock with thermal conductivity k = 2.5 W/m·K and a gradient of 30 °C/km (0.03 K/m), q = 2.5 x 0.03 = 0.075 W/m2, or 75 mW/m2. The measured global average is about 87 mW/m2 (about 65 on continents, 101 under the oceans). Multiplied by the Earth’s surface area of about 5.1 x 1014 m2, 0.087 W/m2 gives about 44 TW, which matches the total quoted above.
How does the heat reach usable depths?
An average gradient spreads the heat thinly. Usable geothermal resources exist where geology concentrates it:
- Conduction: heat moves slowly through solid rock from hot to cold. This alone sets the normal 25-30 °C/km gradient.
- Mantle convection: over millions of years the mantle creeps like a very thick fluid. Hot rock rises and cooler rock sinks, carrying heat far faster than conduction could.
- Magma intrusions: molten rock rising to within a few kilometres of the surface heats the rock around it. Many geothermal fields sit above such bodies.
- Plate boundaries: at spreading ridges (Iceland), subduction zones (Indonesia, Japan, the Philippines, New Zealand) and rifts (East Africa), magma rises close to the surface. Most of the world’s geothermal power comes from these belts, especially the Pacific “Ring of Fire”.
- Hotspots: plumes of hot mantle rising under a plate, far from its edges, such as Hawaii and Yellowstone.
- Circulating groundwater: water sinks along faults, is heated at depth, and rises again as hot springs or collects in permeable rock beneath an impermeable cap. This convection moves heat upwards much faster than conduction.
Types of geothermal resources
The same underlying heat is found in five forms, which differ in how easy the energy is to extract.
Hydrothermal resources
Hot water or steam trapped in permeable, fractured rock under a cap rock. Vapour-dominated systems produce mostly dry steam; liquid-dominated systems produce hot water that partly flashes to steam. These are the only resources used widely today.
Geopressured resources
Hot brine sealed in deep sedimentary basins at pressures well above normal hydrostatic pressure, often with dissolved methane. They hold three kinds of energy: heat, pressure and gas. The US Gulf Coast is the best-studied example, but none is in commercial use.
Hot dry rock and Enhanced Geothermal Systems (EGS)
Hot rock with too little water or permeability to produce fluid by itself. Engineers drill wells, open fractures by pumping water under pressure, then circulate water through the hot rock to bring heat up. Because hot rock exists at depth almost everywhere, EGS is the form with the largest potential, but it is still in the demonstration and early commercial stage.
Magma resources
Molten or partly molten rock at 700-1,200 °C or more, at shallow depth. In 2009 the IDDP-1 well at Krafla, Iceland, hit rhyolite magma at about 2.1 km by accident; the magma was later estimated at about 960 °C. Extracting energy from magma directly is still experimental.
Shallow ground heat (for heat pumps): a common confusion
A few metres below the surface the ground stays close to the local average air temperature all year. Ground-source heat pumps use this for heating and cooling buildings. They are often called “geothermal”, but at these depths the heat is mostly stored solar energy, not heat from the Earth’s interior. Sunlight absorbed at the surface averages about 160 W/m2, nearly 2,000 times the Earth’s internal heat flow of about 0.087 W/m2. If a question asks for the source of geothermal energy, the answer is the Earth’s internal heat, not the sun.
Why geothermal energy counts as renewable
The Earth loses internal heat continuously at about 44 TW and will keep doing so for billions of years, because thorium-232 and uranium-238 have half-lives of billions of years. Human use is tiny compared with this flow, so on a planetary scale the source is effectively inexhaustible.
A single reservoir is a different matter. If hot water is pumped out faster than heat and water flow back in, the field cools and pressure falls. Operators therefore reinject the cooled water and limit extraction so the reservoir can recover. Geothermal energy is renewable when it is managed this way, and it has very low emissions compared with fossil fuels.
This page covers where geothermal heat comes from. For how it is turned into electricity, with dry steam, flash and binary plants and their pros and cons, see geothermal energy: advantages, disadvantages and applications. For India’s hot springs, Puga and the 2025 national policy, see geothermal energy in India.
FAQs
What is the ultimate source of geothermal energy?
The Earth’s internal heat. About half comes from radioactive decay of uranium-238, uranium-235, thorium-232 and potassium-40; the rest is primordial heat left from the planet’s formation and the sinking of iron to form the core.
Is the sun the source of geothermal energy?
No. Deep geothermal heat comes from inside the Earth. Only the heat in the top few metres of ground, used by ground-source heat pumps, is mostly stored solar energy.
What is the geothermal gradient?
The rate at which temperature rises with depth, typically 25-30 °C per km in most continental areas. At 30 °C/km and a 25 °C surface, rock at 3 km is at about 115 °C.
How much heat flows out of the Earth?
About 44-47 TW in total, or about 87 mW per square metre on average. The KamLAND geoneutrino study found that radioactive decay supplies roughly half of it.
What are the main types of geothermal resources?
Hydrothermal (hot water and steam), geopressured brines, hot dry rock (developed as enhanced geothermal systems) and magma. Only hydrothermal resources are used widely today.
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