When the Earth was formed 4.5 billion years ago it was a mass of molten rock. It has been cooling ever since- but very slowly.

The inner core remains at an estimated temperature of greater than 5,000ºC.

Interesting Fact: Moving up a mountain results in a temperature decrease of about 0.7ºC per 100 m of altitude, yet moving into our Earth you can experience a temperature increase of 2.5ºC per 100 m!

The heat originates from several sources: the ball of hot gases that condensed into our planet; friction as gravitational forces sorted denser materials to the core and lighter materials to the surface; and latent heat released as the inner core gradually cools and solidifies.

But 4.5 billion years is a very long cooling off period. The extremely slow rate of cooling doesn’t appear to add up.

Is it because the Earth’s mantle is such a great insulator? Or is there something else going on?

This was a problem Lord Kelvin faced when calculating the age of the Earth based on present day surface temperatures. Starting as a mass of molten rock, his best estimates placed the age of the Earth at only 20 to 40 million years old.

In fact, the core of the Earth is still cooling, but at an extremely slow rate: only about 100 degrees Celsius every 1 billion years. The inner core started to solidify 0.5 to 2 billion years ago. The outer core remains liquid.

What has allowed the core to maintain its extreme temperatures for billions of years?

It turns out the original and latent sources of heat account for only about 10% of the actual heat observed deep in the Earth.

The vast majority of the heat in the Earth’s interior is actually generated via radioactive decay of isotopes such as Potassium 40, Uranium 238, 235, and Thorium 232 contained within the mantle.

The isotopes decay in an effort to become more stable, releasing excess heat energy in the process. The total heat produced by this radiation is about the same as the heat measured emanating from the Earth. So the interior temperature of the Earth remains essentially constant over any meaningful timeframe.