The young Earth may have undergone immense deformations because of a long-neglected change: its own rotation.
Today, our planet completes one rotation on its axis in about 24 hours. But just after the formation of the Moon, around 4.5 billion years ago, a day may have lasted only two to five hours. Earth was therefore rotating much faster, with consequences for its shape.
On the young Earth, rotation pushed more matter toward the equator, the region halfway between the two poles. Our planet was then flatter than it is today. In some scenarios studied, its equatorial radius may even have reached twice its polar radius.
The Moon was also much closer to us. Under the effect of their mutual attraction, it gradually moved away while Earth's rotation slowed. As it lost speed, our planet became rounder. But what happened to its surface during this transformation?
Simon Lock, a researcher at the University of Bristol, studied this question using calculations describing Earth's interior and the evolution of the Moon's orbit. His results indicate that Earth's first rocky shell, the crust, may have been much thicker at the equator than at the poles.
The slowdown would then have forced this shell to change shape. Near the equator, the rocks would have been compressed against one another. Toward the poles, they would instead have been stretched.
These stresses could have buried rocks transformed by water, triggered volcanic episodes and brought molten materials back to the surface. They would thus offer an additional explanation for the diversity of rocks that appeared very early in Earth's history.
The difficulty lies in the age of these events. Almost all of the crust from that time has disappeared, reshaped by billions of years of geological movements and erosion. The researcher therefore compares several possible scenarios, without being able to determine which one exactly matches Earth's past.
A next step will be to incorporate this change in shape into models of the formation of the first continental rocks. These calculations could notably examine how compression, stretching and the presence of water acted together during the first tens of millions of years.