While scientists have linked the movements of Earth's tectonic plates to earthquakes, tsunamis and volcanic eruptions, they have struggled to explain 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 made brittle by movement in viscous layers below it billions of years ago.
Study authors David Bercovici of Yale University and Yanick Ricard of the University of Lyon noted that Earth was the only planet in the solar system that appeared 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 wrote. "How our planet, alone amongst known terrestrial bodies, evolved the unique plate-tectonic form of mantle convection remains enigmatic."
The authors created a mathematical model for the breaking of the lithosphere into pieces, and it involves the convection of Earth's molten mantle.
The authors argued that when cooling sections of mantle moved downward, they stretched the rocks in the overlying lithosphere and this deformation caused microscopic changes in their crystalline structure.
From there, a "self-weakening feedback" occurred that made these deformed areas into weakened zones. These weakened areas became enlarged as the down-welling movement of the mantle shifted to other areas, they argue.
"Although this case is highly idealised, it shows that a fully developed plate can evolve from a down-welling only," they wrote.
The process was likely to have commenced about four billion years ago, and caused complete fractures three billion years ago, the authors wrote, offering an explanation as to why at least one other planet, Venus, lacked similar plates.
Due to far higher temperatures, any damage caused to the surface would become healed over time, they said.