Ancient Rocks Reveal When Earth’s Plate Tectonics Began
New data indicating that the planet’s surface broke up about 3.2 billion years ago helps clarify how shifting plates drove the evolution of complex life.
IN 2016, THE geochemists Jonas Tusch and Carsten Münker hammered a thousand pounds of rock from the Australian Outback and airfreighted it home to Cologne, Germany.
Five years of sawing, crushing, dissolving, and analyzing later, they have coaxed from those rocks a secret hidden for eons: the era when plate tectonics began.
Earth’s fractured carapace of rigid, interlocking plates is unique in the solar system. Scientists increasingly connect it to our planet’s other special features, such as its stable atmosphere, protective magnetic field and menagerie of complex life. But geologists have long debated exactly when Earth’s crust broke into plates, with competing hypotheses spanning from the first billion years of the planet’s 4.5-billion-year history to sometime in the last billion. Those estimates have wildly different implications for how plate tectonics affects everything else on Earth.
The spreading, smashing, and plunging of tectonic plates shapes far more than just geography. The recycling of Earth’s surface helps to regulate its climate, while the building of continents and mountains pumps vital nutrients into the ecosystem. Indeed, plate tectonics, if it began early enough, may have been a major driver of the evolution of complex life. And by extension, shifting plates could be a prerequisite for advanced life on distant planets as well.
Now, a study of the rocks from the Australian Outback by Tusch, Münker and their co-authors, published in Proceedings of the National Academy of Sciences, has captured “a snapshot” of the advent of plate tectonics, said Alan Collins, a geologist at the University of Adelaide in Australia. The team’s analysis of tungsten isotopes in the rocks reveals Earth in the act of transitioning to plate tectonics around 3.2 billion years ago.
The findings buttress other circumstantial evidence accrued over the last decade pointing to that date, said Richard Palin, a petrologist at the University of Oxford. It “supports the growing consensus in the geological community that plate tectonics established itself at a global scale” sometime around 3 billion years ago, he said.
When the geologist Alfred Wegener first proposed the theory of continental drift in 1912, most of his colleagues thought it was preposterous. How could giant landmasses move? Wegener couldn’t identify a mechanism to drive his drifting continents. And indeed it would take another five decades for geologists to figure out how convection within Earth’s mantle—the thick layer of hot rock between the crust and core—propels the plates on the surface. They eventually showed that these plates—15 main ones and dozens of smaller ones—spread apart at mid-ocean ridges, move with the mantle’s flow, scrape against each other at their edges, and plunge back into the mantle at “subduction zones.”
“Plate tectonics gives a very organized way of moving the surface,” said Carolina Lithgow-Bertelloni, a geophysicist at the University of California, Los Angeles. “You can then understand why there are earthquakes where there are earthquakes, why there are mountains where there are mountains.”
In the decades since, scientists have come to realize that Earth’s atmosphere, magnetic field, stable climate and biodiversity are all linked to plate tectonics. “It makes our planet work the way it works,” said Lithgow-Bertelloni.
For starters, plate tectonics has helped Earth maintain a habitable climate for billions of years despite a gradually brightening sun. Our Goldilocks climate largely results from chemical reactions between carbon dioxide in the air and silicate minerals, which slowly reduces the level of the greenhouse gas in the atmosphere by burying it in sediments. Most of that silicate-carbon dioxide reacting happens on the slopes of mountains made by colliding plates.
Moreover, recycling of material between the mantle, crust, oceans and atmosphere ensures a continuous supply of elements that are crucial to life. Plate tectonics refines the mantle, causing elements like phosphorus to accrue on the surface as continental crust. These elements fertilize life in ocean waters when mountains are weathered and washed into the sea. And the continents themselves provide sunlit real estate for new species.
Just as important, mantle convection lets heat escape from Earth’s core, helping the core generate a magnetic field. The field extends far into space and protects the atmosphere from being eroded away by solar storms.
But Earth’s infancy was different.
Radioactive decay made early Earth’s interior much hotter than it is today, so its crust was flaccid. For decades, scientists have debated when the core cooled enough for the crust to harden into plates that began to move, break apart, collide and plunge. Knowing when that fateful transition took place “would let us understand better what led to certain changes in the evolution of life, how we got to the present system, … how our planet operates today,” said Lithgow-Bertelloni.
Deciphering our planet’s formative years is hard. Rocks from billions of years ago are not only rare, but also tortured by time and tectonics. They give disjointed and potentially misleading glimpses into the past.
Several scientists have argued that plate tectonics has operated since at least 4 billion years ago. They base this on tiny, 4-billion-year-old crystals whose chemistry resembles that of modern rocks produced in subduction zones. But other researchers counter that those crystals could have formed in other ways.
Others have hypothesized that plate tectonics began recently, geologically speaking. They point to rock types known to form in modern plate collision zones that never seem to be older than about 0.7 billion years. If there aren’t any old examples of these rocks, then plate tectonics must be young as well, the argument goes.
The appearance of those rocks may reflect changes that happened after the onset of plate tectonics, though, such as the slow cooling of Earth’s interior.
To some extent, researchers said, the disagreement over timing illustrates how plate tectonics itself has changed over time. Rather than experiencing a sudden switch from off to on, tectonic activity probably evolved gradually toward its modern form.
Nevertheless, significant data gathered over the last decade suggests that a major inflection point in that evolution happened around 3.2 billion years ago, in the middle of the Archean eon. The inflection shows up in several lines of evidence.
Geochemical tracers indicate that oxygen, carbon dioxide and water began to move between the atmosphere and mantle after that time. The volume of stable continental crust jumped as well. Only diamonds that formed after that date contain specks of eclogite, a rock forged from material dragged down from Earth’s surface. And lavas called komatiites, which were super hot when they erupted, start to disappear from the rock record, further signaling that the mantle had begun to circulate.
Two giant papers published in 2020 by different teams reviewed the evidence and independently concluded that plate tectonics got going around 3.2 billion years ago. Earth’s record remains ambiguous, and for some the debate continues. But the new tungsten findings provide a “chemical fingerprint,” Collins said, in support of the emerging consensus.
In 2015, at the University of Cologne, Tusch and Münker devised a new way to probe the onset of plate tectonics. They focused on tungsten-182, an isotope of tungsten that was formed by the radioactive decay of hafnium-182 within 60 million years of the solar system’s formation. “It’s a vestige of the Earth’s earliest 60 million years,” said Münker.
Tungsten-182 should be relatively abundant in rocks from early in Earth’s history. Once plate tectonics started, however, the convective churning of the mantle would have mixed up tungsten-182 with the other four isotopes of tungsten, yielding rocks with uniformly low tungsten-182 values.