Earth's energy flows: sun, wind, water, heat and life: The restless Earth: heat from below

3.1 Earth's interior releases about 45 TW, a dilute flow of under a tenth of a watt per square metre

Earth's core and mantle are hot. Part of that heat is primordial, left from the collisions that assembled the planet 4.5 billion years ago and from the sinking of iron to form the core. The rest is continuously generated by the radioactive decay of three elements spread through the rocks: uranium, thorium and potassium. The same uranium and thorium appear on the Energy table under Uranium and thorium as nuclear fuels; inside the Earth they act as a slow, natural reactor that never stops.

The total heat flowing out through the surface is about 44 to 47 TW. Smil uses the lower figure; a widely cited 2010 study, compiling tens of thousands of measurements, gives 47 TW plus or minus 2. Averaged over the planet that is roughly 85 to 90 milliwatts per square metre. Set beside the 165 to 170 W/m2 of sunlight absorbed at the surface, it is about one two-thousandth, or 0.05%.

The flow is not uniform. It is highest along mid-ocean ridges, where new crust forms, and in volcanic regions, and lowest over old continental shields. Underground temperature rises with depth everywhere, typically by about 25 to 30 C per kilometre in ordinary continental crust, which is why deep mines are hot and why drilling deep enough will reach usable heat almost anywhere.

The important distinction is between the stock and the flow. The heat stored in hot rock is enormous. The rate at which nature replenishes it near the surface is tiny. Geothermal projects mostly mine stored heat faster than it is renewed and then rely on the surrounding rock to refill it slowly, which is why reservoir management matters.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• Davies and Davies, 'Earth's surface heat flux', Solid Earth 1, 5-24 (2010) · preferred estimate 47 +/- 2 TW; https://se.copernicus.org/articles/1/5/2010/

3.2 Plate tectonics is that heat at work, and it sets the stage for everything else

Heat escaping from the mantle drives slow convection, and the surface expression of that convection is plate tectonics. Earth's outer shell is broken into about a dozen large plates that move at a few centimetres a year, about the speed at which fingernails grow. New oceanic crust forms where plates pull apart at mid-ocean ridges; old crust sinks back into the mantle at subduction zones; where continents collide, mountains rise.

India is the textbook example. The Indian plate broke away from the old southern supercontinent, travelled north and collided with Eurasia about 50 million years ago. It is still pushing, and the Himalayas are still rising by up to about a centimetre a year, a growth story Smil uses to open his book Growth. The same collision raised the Tibetan plateau, which in turn strengthens the monsoon described in the previous part.

Tectonics shapes energy resources too. It creates the subsiding basins where thick layers of plant debris and marine sediments accumulated and were buried, cooked and compressed into coal, oil and gas. It concentrates metal ores along ancient plate boundaries. It determines where mountains wring rain from moist air and where rivers fall steeply enough for hydropower.

The same forces also release stored energy suddenly. Earthquakes and volcanic eruptions are local in most cases, but very large eruptions can alter the climate of the whole planet. The eruption of Tambora in Indonesia in 1815 put enough sulphur into the stratosphere to cause the year without a summer in 1816, with crop failures in Europe and North America. Toba in Sumatra, about 74,000 years ago, was far larger. In Global Catastrophes and Trends Smil groups such mega-eruptions with asteroid impacts: rare, impossible to prevent and potentially devastating.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• Smil, The Earth's Biosphere: Evolution, Dynamics, and Change (2002) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• Smil, Global Catastrophes and Trends: The Next Fifty Years (2008) · via Smil Mastery Course, M2 'The restless Earth: internal heat'

3.3 Geothermal energy works where heat is concentrated, and shallow ground heat is mostly solar

Because the average flow is so dilute, geothermal electricity is practical only where heat comes close to the surface, usually near plate boundaries or volcanic hotspots. Iceland, New Zealand, Kenya's Rift Valley, Indonesia, the Philippines, Türkiye, Italy and California host most of the world's plants. These Geothermal power plants tap reservoirs of hot water or steam a few kilometres down, run the steam through turbines and usually reinject the cooled water. They run day and night at high capacity factors, which is their great advantage over wind and sunlight. The world had only about 15 GW of geothermal power capacity at the end of 2024, roughly one percent of wind capacity.

The Advanced geothermal tile covers efforts to break that geographical limit by drilling deeper or creating artificial reservoirs in hot dry rock: fracturing it and circulating water through it, borrowing techniques from the oil and gas industry. If drilling costs fall far enough, usable heat exists under most of the land surface, but this remains an emerging technology.

Lower-temperature heat has many direct uses: bathing, greenhouses, fish farming, drying crops and heating whole districts, as in Reykjavik. These belong to the Geothermal direct use and ground-source heat pumps tile.

Ground-source heat pumps deserve a careful note. A few metres below the surface the ground stays at roughly the local annual average air temperature all year. A heat pump uses that stable temperature as a source in winter and a sink in summer. The heat it moves, however, is overwhelmingly solar energy stored in the soil from the seasons, recharged from above, not heat flowing up from the Earth's core.

• Smil, Energy in Nature and Society: General Energetics of Complex Systems (2008) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• Smil, Energy: A Beginner's Guide (2006) · via Smil Mastery Course, M2 'The restless Earth: internal heat'
• IRENA, Renewable Capacity Statistics 2025 (Highlights), International Renewable Energy Agency (2025) · end-2024 capacity: solar 1,865 GW, hydropower 1,283 GW, wind 1,133 GW, bioenergy 151 GW, geothermal 15 GW, marine 0.5 GW; https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2025/Mar/IRENA_DAT_RE_Capacity_Highlights_2025.pdf

See it in the table

Check your understanding

Roughly how does Earth's internal heat flow compare with sunlight absorbed at the surface?

What is the main source of the heat a shallow ground-source heat pump extracts?

Why do geothermal power plants cluster in places like Iceland, Kenya and Indonesia?