Why is Earth hot inside?

We’ve seen that Earth is quite hot inside, but where did this heat come from? Scientists have identified three major sources of Earth’s internal heat.

The first source was heat that accumulated inside Earth as it formed. To understand this heat of formation, you need to think about gravity and energy. You know that a falling rock has energy due to its motion (which we call “kinetic energy”). But where did this energy come from? The answer is that it came from gravity, or more technically from what we call “gravitational potential energy.”

In other words, as objects fall downward, they convert gravitational potential energy into kinetic energy of motion. When Earth was forming, many smaller clumps of material combined together through gravity. In essence, all these clumps were pulled “downward” to make the Earth, which means they gained energy of motion as they fell. They stopped falling once they became part of the forming Earth, at which point the energy of motion was converted into heat. This process deposited an enormous amount of heat into Earth and was probably what caused the interior to be molten when Earth was young. The illustrations below summarize the ideas behind this first source of heating for Earth.

Credit: Zofostro Science.

The second heat source was a direct result of the separation by density that occurred in the young Earth’s molten interior. Because the separation meant that denser material sank while less dense material rose, it had the same energy effect as dropping heavy materials. Again, the falling of heavy material caused that material to gain energy of motion that was ultimately converted into heat.

The third heat source was the release of heat through radioactivity. Although radioactive elements are rare, some of them were mixed in with the material that formed the Earth and therefore became trapped within the Earth. Radioactive elements decay gradually over time, releasing heat as they do so.

Note that the first two heat sources deposited heat only when Earth was very young. In contrast, the third source (radioactivity) still adds heat to Earth’s interior today, because there are still radioactive materials (that have not yet fully decayed) within our planet.