Beneath the clouds, a magma ocean swayed, with partially molten rock pushed around by tides, Sleep hypothesizes.
These tides were due to the mutual attraction of the Earth and the moon, and were much stronger than those in today's watery oceans, as the moon was sitting much, much closer to the Earth back then.
The tides constantly stirred the ocean, causing the mantle to lose heat, similar to stirring and blowing on a bowl of soup. But once released from the Earth's depths, the heat was trapped at the surface, held back by the thick, opaque primordial atmosphere.
The heat could only escape the planet (and cool it down) at so-called cloud-top temperature levels - where it would be as cold as on a modern high mountain summit. But for the first 10 million years, the temperatures were much, much higher, Sleep said.
The energy loss caused by the mutual attraction of the Earth and the moon was also making the moon gradually pull away. This made the tides progressively weaker, so the molten rock was being stirred less and less, and the Earth's mantle began to solidify in stages.