Geothermal energy comes from heat stored deep inside the Earth. About half of that heat is left over from the planet’s formation more than 4.5 billion years ago, when Earth coalesced from a hot cloud of gas and dust. The other half is continuously generated by the radioactive decay of elements like uranium, thorium, and potassium in the crust and mantle. Together, these two sources push roughly 44 terawatts of heat toward the surface at all times.
What Produces the Heat
Earth’s interior is essentially a slow-burning furnace powered by two processes. The first is primordial heat, the thermal energy trapped when the planet formed under immense gravitational pressure. The second is radioactive decay: unstable atoms of uranium-238, thorium-232, and potassium-40 break apart over billions of years, releasing energy with each disintegration. Radioactive decay alone produces about 24 terawatts of power and accounts for roughly 54% of the heat flowing up through the surface. The remaining 46% is that ancient primordial heat, still slowly radiating outward after billions of years.
This is why the Earth gets hotter the deeper you go. Near the surface, temperatures increase by about 25 to 30°C per kilometer of depth. In volcanically active regions, the gradient can be much steeper, bringing usable heat much closer to the surface.
Where It’s Closest to the Surface
Geothermal heat exists everywhere beneath your feet, but it’s only practical to tap where that heat is concentrated near the surface. The best locations sit along the edges of tectonic plates, where the Earth’s crust is thinnest or most fractured. The Ring of Fire, a horseshoe-shaped belt circling the Pacific Ocean, is one of the most active geothermal zones on the planet. It runs through Indonesia, the Philippines, New Zealand, Japan, and the western coasts of North and South America.
Volcanic hotspots away from plate boundaries also work. Iceland sits on the Mid-Atlantic Ridge, where two plates are pulling apart, giving the country access to shallow, intense heat. In the United States, most geothermal power plants are concentrated in western states and Hawaii. The Geysers in Northern California is the largest known dry steam field in the world and has been producing electricity since 1960.
The five countries with the most installed geothermal capacity tell this story clearly:
- United States: 3,919 MW
- Indonesia: 2,384 MW
- Philippines: 1,952 MW
- Türkiye: 1,717 MW
- New Zealand: 1,055 MW
Every one of these countries sits on or near active plate boundaries.
What a Geothermal Reservoir Looks Like
A natural geothermal reservoir, called a hydrothermal system, needs three things: heat, water, and permeable rock that lets that water circulate. Rainwater or snowmelt seeps deep underground, gets heated by hot rock, and collects in porous formations. When engineers drill into these reservoirs, the hot water or steam rises to the surface under its own pressure.
Typical wells for conventional geothermal plants reach depths of 1.5 to 2.5 kilometers (roughly 5,000 to 8,000 feet). At 1.5 km, rock temperatures around 175°C are common. At 2.5 km, temperatures can reach 225°C or higher. These are the sweet spots for commercial power generation.
How the Heat Becomes Electricity
All geothermal power plants use the same basic principle: heat from underground creates steam, steam spins a turbine, and the turbine drives a generator. The differences come down to how hot the underground fluid is.
Dry steam plants are the simplest. Where underground reservoirs produce steam rather than liquid water, that steam pipes directly into a turbine. The Geysers operates this way. These sites are rare but efficient.
Flash steam plants handle reservoirs where the fluid is extremely hot liquid water, above 182°C (360°F). When this pressurized water is pumped into a low-pressure tank at the surface, it rapidly “flashes” into steam. That steam drives the turbine. Some plants use a double-flash design, running the leftover liquid through a second tank to extract even more energy.
Binary cycle plants work with moderate-temperature water, the most common geothermal resource. Instead of using the geothermal fluid directly, the hot water passes through a heat exchanger and transfers its energy to a secondary fluid with a much lower boiling point. That secondary fluid vaporizes and drives the turbine. The geothermal water never touches the turbine and is typically injected back underground. Most new geothermal plants use this design because it can work with lower-temperature reservoirs that exist in far more locations.
Tapping Heat Where There’s No Natural Reservoir
The vast majority of Earth’s underground heat sits in rock that has no natural water circulation, no cracks for fluid to move through. Enhanced geothermal systems (EGS) aim to create artificial reservoirs in this hot, dry rock. The process involves drilling deep wells, then injecting high-pressure water to open up tiny fractures in the rock. Once the rock is permeable enough, water can be circulated through it: pumped down one well, heated by the rock, and brought back up through a second well.
EGS wells typically go deeper than conventional ones, ranging from 3 to 7 kilometers. At those depths, rock temperatures of 175°C to 250°C or more are available in many parts of the world, not just along tectonic boundaries. This technology is still scaling up, but it has the potential to make geothermal energy accessible almost anywhere.
Environmental Footprint
Geothermal power produces far less carbon than fossil fuels. Life cycle emissions, including construction, drilling, and operation, range from about 11 to 47 grams of CO₂ equivalent per kilowatt-hour depending on the plant type. For comparison, natural gas plants emit roughly 400 to 500 g CO₂eq/kWh, and coal exceeds 800. Binary cycle plants using high-temperature reservoirs come in at the low end, around 11 g CO₂eq/kWh, making them among the cleanest electricity sources available.
Geothermal plants also have a small land footprint compared to wind or solar farms of equivalent output, and they produce power around the clock regardless of weather. The main environmental considerations are water use (binary plants recirculate most of their fluid, reducing this concern) and the potential for minor seismic activity from EGS operations, which is an active area of engineering research.