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Why is Earth’s core so hot?

The center of the Earth is roughly as hot as the surface of the Sun, 4.5 billion years after the planet formed. Here’s where all that heat came from, and why it hasn’t leaked away.

At the boundary of Earth’s solid inner core, the temperature is somewhere around 5,000 to 6,000 °C. A 2013 laboratory experiment put it at about 6,000 °C. For comparison, the visible surface of the Sun is about 5,500 °C.

Earth formed about 4.5 billion years ago. So why hasn’t all that heat leaked out into space by now? The answer comes in three parts: heat left over from the planet’s birth, heat that’s still being made today, and the fact that rock is a really good blanket.

1. Leftover heat from a violent birth

Earth was built by collisions. Dust, rocks and small planets slammed together, and every impact turned the energy of motion into heat. The young Earth got so hot that much of it melted.

Then came an even bigger heat source. In the molten planet, heavy iron sank toward the center to form the core. As it fell, it released a huge amount of energy, like a rock dropped down a well, and that energy turned into heat.

Geologists call this primordial heat, and a lot of it is still down there.

2. Radioactivity: a slow-burning fuel

Rocks contain small amounts of radioactive elements, mainly uranium, thorium and potassium. As their atoms decay, they give off heat. It happens slowly, over billions of years, so this heater has been running since Earth formed.

Most of these elements are in the crust and mantle, not the core. But together they make a big difference to how fast the planet cools.

Fun fact: A detector buried deep under a mountain in Japan catches geoneutrinos, ghostly particles given off by radioactive decay inside the Earth. In 2011 its scientists reported that uranium and thorium make about 20 terawatts of heat. That’s roughly half of all the heat flowing out of the planet, so the other half must be leftover heat from Earth’s birth.

3. The core is freezing, and that releases heat

Here’s a strange one. As the Earth slowly cools, the liquid iron at the very center freezes, and the solid inner core grows outward, roughly a millimeter a year.

When a liquid freezes it gives off heat (the same “latent heat” that keeps a freezing lake from dropping below 0 °C). So the growing inner core releases heat as it solidifies. And because lighter elements get left behind in the liquid as the iron freezes, the liquid around the inner core becomes buoyant and rises.

That rising, churning liquid iron is what powers Earth’s magnetic field.

Why is the inner core solid if it’s so hot?

Pressure. At the center of the Earth the pressure is about 3.6 million times the air pressure at the surface. Squeezing iron that hard raises its melting point so much that, even at about 6,000 °C, the inner core stays solid. The outer core is a little cooler, but under less pressure, so it stays liquid.

Why hasn’t the heat escaped?

Rock is a poor conductor of heat, like a giant blanket thousands of kilometers thick. Heat does get out, mostly through the slow churning of the mantle, volcanoes and the spreading of the seafloor, but only slowly.

All together, about 44 to 47 terawatts of heat flows out of Earth’s interior. That sounds like a lot, but spread over the whole planet it’s tiny: only about 0.09 watts per square meter, far less than sunshine. Earth is cooling, but over billions of years, not thousands.

See how hot each layer gets →

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