Bam! There’s a subduction zone Marc Ward/Stocktrek Images/Getty

Plate tectonics – the process that shapes Earth’s surface and causes earthquakes – may have begun at least half a billion years earlier than thought. And it may have been triggered by staggeringly violent impacts from space rocks.

Earth’s surface is divided into more than a dozen tectonic plates that constantly move and jostle against each other. Occasionally one is forced under its neighbour and destroyed, in a process called subduction.

The world we live in is shaped by plate tectonics. It creates landscapes, including high mountain ranges like the Himalayas. It also provides a flow of chemicals from Earth’s interior, some of which are essential to life.


Yet nobody knows how or when plate tectonics got started. Other planets like Mars and Venus do not have tectonic plates.

Two new studies propose both a date for plate tectonics’ origins, and an explanation for how it kicked off.

Ancient rocks

Nicolas Greber of the University of Geneva, Switzerland, and his colleagues set out to trace the origin of global plate tectonics by determining which rock types were present at different stages of Earth’s history.

They focused on two rock types. Mafic rocks like basalt are typically dark, while felsic rocks like granite are lighter, sometimes verging on white.

“It’s more difficult to produce felsic rocks like granite,” says Greber, because they only form in the presence of water and heat. “The key process that produces them nowadays is subduction zones… because they transport water down into the mantle, the melting region.” So felsic rock is a sign that subduction zones were active during their formation.

Greber determined how early in Earth’s history felsic rocks were present by looking at shales, which are formed when rocks on land are eroded and carried into the sea, dating back 3.5 billion years. Mafic and felsic rocks carry different mixtures of titanium isotopes, which were preserved in the shales. This revealed that, as far back as 3.5 billion years ago, 55 to 60 per cent of the continental crust was made of felsic rocks.

Shifting plates

For Greber, that means subduction zones and modern plate tectonics must have been in action at least 3.5 billion years ago. It could be even earlier, but he has no older shales to verify that.

This runs counter to the prevailing wisdom, which is that global plate tectonics started around half a billion years later. There has been “an unusual convergence recently on an age of about 3 billion years ago from a number of sources,” says Craig O’Neill of Macquarie University in Australia.

For instance, in 2016 Ming Tang of Rice University in Houston, Texas, used different chemical traces in rocks to argue that Earth’s crust was largely free of felsic rocks until after 3 billion years ago, and that this marked the onset of plate tectonics (Science, doi.org/cdgr). Tang says Greber’s titanium isotopes might not be telling the whole story – but he adds that his analysis might not either. “No one so far can claim they know when plate tectonics started,” he says.

The key issue is that tectonics might have started more than once. “The geological record seems, or so we have argued, to indicate a few abortive attempts at plate tectonics earlier, and perhaps some shutdowns later as well,” says O’Neill.

Whack whack whack

When Earth was a young planet, its interior was probably too hot to drive plate tectonics, says O’Neill. That is, unless it had help. He has now shown that plate tectonics could have got started much earlier, thanks to massive rocks from space smashing into the planet.

O’Neill and his colleagues simulated what would happen to Earth’s crust and interior if rocks of different sizes collided with it. They found that a single large impact could trigger a subduction zone that remained active for 10 million years. By itself that is quite a short-lived period of tectonic activity, but young Earth was hit by a lot of space rocks.

The first spate of impacts happened when the planet was still forming and ended around 4.4 billion years ago. Then there was a lull until 4 billion years ago, followed by an intense bombardment that lasted until 3.5 billion years ago, with occasional big impacts until as recently as 2 billion years ago (Annual Review of Earth and Planetary Sciences, doi.org/cdgp).

Combined with Greber’s findings, this suggests that the history of plate tectonics on Earth can be split into two phases. From Earth’s birth until 3 billion years ago, there was intermittent subduction driven by impacts, which produced the felsic rocks Greber found. After that, the planet’s interior had cooled enough for global plate tectonics to begin in earnest.

Journal reference: Nature Geoscience, DOI: 10.1038/NGEO3029; Science, DOI: 10.1126/science.aan8086