AP® Environmental Science · Unit 6 · Topic 6.10
Geothermal Energy
Geothermal energy taps heat stored in the Earth's crust, either as steam to spin turbines or as mild underground warmth to heat and cool buildings.
A geothermal power plant finds a spot where underground rock is hot, often because magma sits close to the surface. Wells pump water down into that hot rock. The water flashes to steam, rises back up under pressure, and spins a turbine connected to a generator, same basic setup as a coal or nuclear plant except the heat source is the Earth itself instead of a fire or a reactor. This only works well where hot rock is shallow, which is why it is geography-dependent in a way solar and wind are not.
A ground source heat pump is a different, much smaller-scale trick. A few meters down, soil temperature stays near a constant 10 to 15 degrees C year-round, warmer than winter air and cooler than summer air. In January a heat pump circulates fluid through buried pipes, lets it absorb that 12 degree C warmth, and compresses the fluid to concentrate the heat before releasing it inside the house. It moves heat rather than generating it, so it uses far less energy than a furnace burning fuel.
Where it makes sense to build depends on plate tectonics. Mid-ocean ridges and subduction zones, where plates spread apart or collide, bring magma near the surface and create usable heat gradients; Iceland sits directly on the Mid-Atlantic Ridge, which is why it can heat whole towns this way while most countries cannot. Hot spots like Yellowstone offer the same advantage away from plate edges. This geographic limit is geothermal's main contrast with fossil fuels and connects the unit's broader theme: every energy source has a resource that constrains where and how fast it can be used.
Key terms
- Geothermal reservoir
- An underground pocket of hot rock and water that a plant draws heat from.
- Mid-ocean ridge
- A boundary where tectonic plates spread apart, bringing hot magma close to the surface.
- Hot spot
- A fixed region of volcanic activity in the mantle not located at a plate boundary, like Yellowstone.
- Ground source heat pump
- A system that moves heat between a building and the stable underground temperature rather than generating heat by burning fuel.
- Conduction
- The transfer of heat through solid rock from hotter to cooler regions, which naturally refills a drawn-down reservoir.
Watch it explained
- How it worksWalk me through hot rock, injected water, rising steam, and a turbine in a geothermal plant.
- How it worksHow does a ground source heat pump use a 12 degree C yard to heat a house in January?
- Ties togetherHow do plate boundaries and hot spots decide where geothermal power is worth building?
- Going furtherWhat happens to a geothermal reservoir drawn down faster than heat flows back into it?
Check yourself
End of topic.