What is the definition of geothermal energy? Learn the simple meaning, where Earth's heat comes from, and how scientists measure it. Classroom-friendly for all ages.
Geothermal energy is heat from inside the Earth. The word comes from Greek. Geo means earth. Therme means heat. Put them together and you get earth heat.
This heat has been inside our planet for more than 4 billion years. It never stops. It never runs out. And we can use it to power our lives.
Here is the simplest definition: geothermal energy is the heat that the Earth naturally makes and stores underground. We tap into it with wells and use it to generate electricity or warm buildings directly.
Geothermal energy is thermal energy that comes from the Earth’s interior. Scientists define it as the heat stored in the Earth’s crust. This heat moves from the core toward the surface. We can capture some of it along the way.
The deeper you go, the hotter it gets. Near the surface, the ground stays at 10 to 15 degrees Celsius. A mile down, temperatures can reach 45 degrees or more. In volcanic areas, it gets even hotter much faster.
Scientists call this the geothermal gradient. It measures how much the temperature rises with depth. In most places, it rises about 25 degrees Celsius for every kilometer you go down.
There are two main sources of Earth’s internal heat.
Primordial heat. When Earth formed 4.5 billion years ago, rocks and debris crashed together. The collisions created enormous heat. Some of that heat is still trapped inside the planet. It makes up about 20 percent of the geothermal energy we can access.
Radiogenic heat. This provides the other 80 percent. Certain elements deep inside Earth are radioactive. As they decay, they release heat. Uranium, thorium, and potassium are the main ones. This process is constant. It has been running for billions of years and will keep going for billions more.
These two sources together produce about 44 terawatts of heat flow. That is more than twice the world’s total energy consumption.
Have you ever put your hand on a warm sidewalk on a summer day? The sun heats the top. But dig down just a little and the ground feels cool.
Deep under your feet, the opposite happens. The farther down you go, the hotter it gets. At the very center, it is as hot as the sun.
Geothermal energy is just a fancy name for that underground heat. People drill deep holes to reach it. They use the heat to make steam. The steam spins a big fan. The fan makes electricity.
It is like a pot of boiling water with the lid jumping up and down. The steam wants to escape. When we let it out through a pipe, it has power.
Geothermal energy is a form of renewable thermal energy. Unlike solar or wind, it does not depend on weather. It runs all the time.
The definition matters because it shapes how we think about this resource. Geothermal is not stored sunlight. It is not ancient biomass. It is Earth’s own internal heat, produced by physical processes that have operated since the planet formed.
This makes it unique among renewable energy sources. Solar and wind capture energy from outside Earth. Geothermal captures energy from inside. It is always available. It does not fluctuate with seasons or daylight.
The challenges are practical. The best heat sources are deep underground. Drilling is expensive. But once a well is built, the energy is free. The heat keeps coming.
Scientists use several terms to describe geothermal energy.
Geothermal gradient. The rate at which temperature increases with depth. Average is 25 to 30 degrees Celsius per kilometer.
Heat flux. The amount of heat flowing from Earth’s interior to the surface. Average is about 0.1 megawatts per square kilometer.
Reservoir temperature. The temperature of underground water or rock that can be used for energy. For electricity generation, you typically need temperatures above 150 degrees Celsius.
Capacity factor. How often a power plant actually produces energy. Geothermal plants can achieve up to 96 percent. That is much higher than solar or wind.
Geothermal energy is the Earth’s natural heat. It is renewable. It is always available. It can be used for electricity or direct heating. The source is radioactive decay and primordial heat. The key is the geothermal gradient, which makes deeper areas hotter.
It is not a perfect energy source. Location matters. Cost matters. But the definition is clear: geothermal energy is heat from inside the Earth, and we can put it to work.
Common question: Is geothermal energy the same as solar?
No. Solar comes from the sun. Geothermal comes from inside Earth. They are completely different sources.
Common question: Can we use geothermal energy anywhere?
Geothermal heat pumps work almost anywhere. Deep geothermal for electricity needs the right geology. New technology is expanding the options.
Common question: How deep do we need to drill?
For heat pumps, only 6 to 10 feet. For power plants, 1 to 3 miles. The deeper you go, the hotter it gets.
Last updated: June 15, 2026
What does the word "geothermal" literally mean?
What percentage of geothermal heat comes from radioactive decay?
What is magma?
Why is geothermal energy called renewable?
What is the geothermal gradient?
Answers: B: Earth heat, C: About 80 percent, B: Hot melted rock beneath the crust, B: Earth keeps producing heat, B: How fast temperature rises as you go deeper
What is the simple definition of geothermal energy?
Geothermal energy is heat from inside the Earth. We can use that heat to make electricity and warm buildings. The word comes from Greek words meaning earth heat.
What are the two main sources of Earth's geothermal heat?
About 80 percent comes from radioactive decay deep inside the planet. Elements like uranium and thorium break down and release heat. The other 20 percent is leftover from when Earth formed.
How hot is the Earth's core?
The Earth's core is about 5,000 degrees Celsius. That is nearly as hot as the surface of the sun. The heat travels outward through the mantle toward the crust.
Is geothermal energy considered renewable?
Yes. The Earth keeps making heat through radioactive decay. This process has been going on for billions of years and will continue for billions more. Geothermal energy will not run out.
How do we capture geothermal energy?
We drill wells into underground hot water reservoirs. The hot water or steam rises to the surface. It spins turbines that turn generators, which produce electricity.
