How Geothermal Energy Taps Earth's Internal Heat for Power

Beneath Your Feet, a Reactor 4.5 Billion Years Old

Thirty kilometers below the Earth's surface, temperatures exceed 1,000°C. The heat comes primarily from two sources: residual energy from the planet's formation 4.5 billion years ago, and the ongoing radioactive decay of uranium, thorium, and potassium in the mantle and crust. This internal heat engine generates approximately 47 terawatts of thermal power continuously—roughly three times humanity's total energy consumption. Geothermal energy taps into this heat, converting it to electricity or using it directly to warm buildings, greenhouses, and industrial processes. Unlike solar or wind, it runs 24 hours a day, 365 days a year, regardless of weather.

Three Ways to Turn Underground Heat Into Electricity

Geothermal power plants come in three designs, each suited to different reservoir temperatures and conditions.

Dry steam plants are the simplest and most efficient but require rare geological conditions—steam-dominated reservoirs exist in only a few locations worldwide. Flash steam plants are the most common type globally. Binary cycle plants can operate at lower temperatures, dramatically expanding the number of viable locations.

The Geysers: World's Largest Geothermal Complex

The Geysers geothermal complex in Sonoma and Lake Counties, California, spans approximately 117 square kilometers and contains 18 power plants with a combined installed capacity of 1,517 MW. The facility has operated continuously since 1960, making it the oldest commercial geothermal operation in the United States.

Production at The Geysers declined in the 1980s and 1990s as steam pressure dropped from overextraction. The solution was unconventional: the Southeast Geysers Effluent Pipeline Project now pumps 42 million liters of treated wastewater daily from the city of Santa Rosa into injection wells. The water recharges the reservoir, maintaining steam pressure while disposing of wastewater that would otherwise require ocean discharge. The system stabilized production and became a model for sustainable geothermal management.

Iceland: A Geothermal Nation

Iceland sits on the Mid-Atlantic Ridge, where tectonic plates diverge and magma rises close to the surface. This geological gift has made Iceland the world's most geothermally powered nation.

• Approximately 90% of Icelandic homes are heated by geothermal district heating systems
• Geothermal provides roughly 25% of Iceland's electricity generation
• Hydroelectric power supplies most of the remainder, making Iceland nearly 100% renewable in electricity
• Geothermal greenhouses grow tomatoes, cucumbers, and peppers year-round despite the Arctic latitude
• The famous Blue Lagoon spa uses wastewater from the Svartsengi geothermal power station

Reykjavik's district heating system, serving 200,000 people, delivers water at 80°C through 2,700 kilometers of pipeline. The system eliminates the need for individual boilers and furnaces, dramatically reducing both cost and carbon emissions. Heating a typical Icelandic home costs a fraction of what comparable heating costs in fossil-fuel-dependent countries.

Enhanced Geothermal Systems: Drilling Into Hot Rock

Conventional geothermal requires three things in one location: heat, water, and permeable rock. Enhanced Geothermal Systems (EGS) need only heat. The water and permeability are engineered.

The process involves drilling into hot dry rock at depths of 3–10 kilometers, hydraulically fracturing the rock to create permeability, then circulating water through the fracture network to extract heat. The technology is sometimes called "fracking for heat," though the goals and operational parameters differ significantly from oil and gas hydraulic fracturing.

The U.S. Department of Energy's Frontier Observatory for Research in Geothermal Energy (FORGE) project in Milford, Utah, successfully created an EGS reservoir in 2022 and has been circulating fluid through engineered fractures since. Fervo Energy, a private company, achieved a successful horizontal well EGS demonstration in Nevada in 2023, producing 3.5 MW from a single well pair.

Global Capacity and the 90% Advantage

Global installed geothermal power capacity reached approximately 15.96 GW by the end of 2023, according to the International Renewable Energy Agency. The top producing countries reflect geological endowment.

• United States: 3.7 GW (largest installed capacity, concentrated in California, Nevada, Utah)
• Indonesia: 2.4 GW (world's second-largest capacity, expanding rapidly on volcanic islands)
• Philippines: 1.9 GW (geothermal provides ~11% of national electricity)
• Turkey: 1.7 GW (rapid expansion since 2010, mostly binary plants)
• New Zealand: 1.0 GW (geothermal provides ~17% of national electricity)

Geothermal's defining advantage is reliability. Capacity factor—the ratio of actual output to maximum possible output—exceeds 90% for most geothermal plants. Solar photovoltaic systems achieve 15–25%. Onshore wind reaches 25–45%. Geothermal produces power regardless of time of day, season, cloud cover, or wind speed. This baseload reliability makes geothermal uniquely valuable in grid planning, even at its current modest scale.

The Heat Beneath Every Country

Conventional geothermal is geographically constrained to regions with accessible underground heat—primarily the Ring of Fire, rift zones, and volcanic hot spots. EGS changes the map. At sufficient depth, temperatures suitable for power generation exist everywhere on Earth. The U.S. Geological Survey estimates EGS resources in the United States alone could provide over 100 GW of electrical capacity—enough to replace a significant fraction of the nation's fossil fuel fleet.

The barriers are economic, not physical. Drilling to 5–10 kilometer depths is expensive, and the risk of investing millions before knowing whether a well will produce adequately deters conventional financing. As drilling technology advances—borrowing innovations from the oil and gas industry—and as demonstration projects prove commercial viability, those costs are declining. The heat source runs continuously, the fuel costs nothing, and the reservoir replenishes itself on human timescales when managed properly. Earth has been generating this heat since before life existed on its surface. The engineering to harvest it is finally catching up.