While scientists have linked the movements of Earth’s tectonic plates — earthquakes, tsunamis, volcanic eruptions — they have struggled to explain exactly how they came to exist in the first place.

Now, in the journal Nature, two geophysicists have proposed that Earth’s outermost layer, or lithosphere, was microscopically weakened and brittled by movement in viscous layers below it billions of years ago.

Study authors David Bercovici of Yale University and Yanick Ricard of the Univeristy of Lyon note that Earth is the only planet in the solar system that appears to have tectonic plates that move freely on its surface, propelled by the motion of layers below.

“The emergence of plate tectonics is arguably Earth’s defining moment,” they write. “How our planet, alone amongst known terrestrial bodies, evolved the unique plate-tectonic form of mantle convection remains enigmatic.”

The authors have created a mathematical model for the breaking of the lithosphere into pieces, and it involves the convection of Earth’s molten mantle.

The authors argue that when cooling sections of mantle moved downward, they stretched the rocks in the overlying lithosphere and this deformation caused microscopic changes in their the crystalline structure.

From there, a “self-weakening feedback” occurred that made these deformed areas into weakened zones. These weakened areas became enlarged as the downwelling movement of the mantle shifted to other areas, they argue.

“Although this case is highly idealized, it shows that a fully developed plate can evolve from a downwelling only,” they wrote.

The process likely began about 4 billion years ago, and caused complete fractures 3 billion years ago, the authors write.

The authors also offer an explanation as to why at least one other planet, Venus, lacks similar plates.

Due to far higher temperatures, any damage caused to the surface would become healed over time, according to their model.

“Only very faint weak zones accumulate because damage itself is weaker while healing is stronger,” the authors wrote.