Fragile rocks allow plates to slide under each another (Image: WestEnd61/REX)

Fragile things can be useful. Earth’s surface is a lively place, made up of shifting plates of rock. Now it seems the surface only moves because it is partly made of flimsy rocks that have been damaged in the planet’s heart.

Uniquely in the solar system, Earth’s crust is divided into several sections. These tectonic plates move over millions of years, throwing up mountains and triggering earthquakes. Often, one plate gets forced under another and sinks into the mantle beneath, a process called subduction. Elsewhere, new material is brought to the surface. If the plates did not recycle themselves like this, our planet might not have such a stable climate, or be so rich in the chemicals vital for life.

So the beginning of plate tectonics is a critical event in our planet’s history. But it is unclear how the plates first started to move, or when. The first subduction seems to have happened about 4 billion years ago, but clear evidence that all the plates were moving and subducting only appears 3 billion years ago.


To explain this delay, David Bercovici of Yale University and Yanick Ricard of the University of Lyon in France studied how rocks behave on a small scale, and then extrapolated to the planet as a whole.

Crumbling rocks

Bercovici and Ricard modelled what happens to rocks in the upper mantle, just below the plates. There, the strong currents shrink the grains that make up rocks, a bit like the way stirring foam makes smaller bubbles. As a result, weak zones form and grow.

Over time, more and more of these damaged rocks build up in the upper mantle, and eventually are thrust back up to form new sections of plate.

Because these new plates are partly made of fragile rocks, their edges are more breakable, making it easier for one plate to be forced under another.

Bercovici suggests that the first rocks underwent subduction 4 billion years ago, and over the next billion years were gradually damaged in the mantle and got reincorporated into the surface plates. By 3 billion years ago, the plates were fragile enough that subduction could get going in earnest.

Only on Earth

“Their model makes intuitive sense,” says Catherine McCammon of the University of Bayreuth in Germany. “They take things one step further and show quantitatively that it can work.”

McCammon says that, although we will probably never know for sure how plate tectonics began, Bercovici and Ricard’s model offers a plausible story.

The model could help to explain why Earth is the only planet known to have active plate tectonics. The key is that true plates can only evolve from weak zones in rocks. Venus’s surface atmosphere, for example, is much hotter than Earth’s, so its weak zones heal faster, stopping tectonic activity before it starts.

Journal reference: Nature, DOI: 10.1038/nature13072