Where does the heat in Earth's core come from?

The full narration of the video.

A sweet potato fresh from the oven, and the skin has already cooled. But break it open, and the inside is still steaming. Deep in the middle of the Earth, there's a hidden heat like that too. So where does it actually come from? First, let's slice the Earth open. A thin shell on the outside, thick rock beneath it. Go down almost three thousand kilometers, and you reach the core: a giant ball of iron.

At the very center, it's around six thousand degrees. As hot as the surface of the Sun. This heat has three main sources. The first: it's left over from the day the Earth was born. More than four and a half billion years ago, countless rocks, big and small, kept crashing together. They grew bigger and bigger, until they became the Earth.

Every crash made heat. Just like when you clap hard, and your palms get warm. The second source: sinking iron. Back then, the Earth was hot and soft. Heavy iron sank toward the middle, and lighter rock floated up. All the way down, the iron squeezed and rubbed. That rubbing made even more heat.

The third source: tiny heaters inside the rock. Some rocks contain a little bit of something special. Bit by bit, it breaks apart all on its own. Every time a bit breaks apart, it gives off a little heat. Billions of years, and it still hasn't run out. As it breaks apart, it also shoots out a tiny, invisible particle. It can pass straight through the whole planet. Deep underground in Japan, a giant machine counts these particles.

The count showed: about half the heat leaking out of the Earth comes from these tiny heaters. The other half is old heat, left from its birth. But most of the tiny heaters are in the rock outside the core. The core's own heat is mostly that old heat. So why, after four and a half billion years, hasn't the core cooled down?

Because the Earth is enormous. And it's wrapped in nearly three thousand kilometers of rock. Rock passes heat along very slowly, like a very thick blanket. It's just like sweet potatoes: the bigger the potato, the slower it cools. And the Earth is one unbelievably big sweet potato. So what does this heat have to do with you? It keeps the liquid iron in the outer core churning. And that churning iron makes a huge, invisible magnetic field.

That field works like a giant umbrella, shielding us from many of the tiny particles blowing in from the Sun. It's also why a compass points north and south. So next time you see a volcano or soak in a hot spring, you'll know: that's the warmth left from Earth's birth, still slowly seeping out. What else do you wonder about what's under your feet? Tell me in the comments.