Asked by: Anonymous
In a word, radioactivity. We’re all living on top of a vast natural nuclear reactor, packed with uranium, thorium and potassium that ended up trapped inside the Earth during its formation around 4.5 billion years ago.
As they decay, the fast-moving particles they release smash into their surroundings, dumping their energy as heat. It’s this that makes the interior of the Earth so hot, and allows lava to reach temperatures in excess of 1000°C.