geology
Scientists Pin Down When Earth’s Crust Cracked, Then Came to Life
By Howard Lee
March 25, 2021
Igneous rocks that formed (from top) 3.43, 3.24, 3.18, 3.07, and 2.74 billion years ago span from before to after the onset of plate tectonics.
Igneous rocks that formed (from left) 3.43, 3.24, 3.18, 3.07, and 2.74 billion years ago span from before to after the onset of plate tectonics.
Quanta Magazine; photos courtesy of Jonas Tusch
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.
Jonas Tusch, left, and Carsten Münker sledgehammering rocks from the Archean eon in Pilbara, Australia.
Chris S. Marien
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.
“There’s a lot of different people, coming from very different perspectives, coming up with a convergence of 3.2 to 3 billion years,” said Collins.
Earth’s Engine
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.”
Samuel Velasco/Quanta Magazine; source: USGS
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.
A Rocky Record
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.
Geologists conduct fieldwork in the barren landscape of the Pilbara Craton in Northwest Australia, home to some of the best-preserved rocks from the early stages of the planet.
Jonas Tusch
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.
Signal From Earth’s Infancy
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.
Tusch and Münker developed a powerful new method for extracting tiny traces of tungsten from ancient rocks. Then they went looking for the rocks.
First they analyzed Archean rocks collected in the Isua region in western Greenland. Tusch spent 11 months analyzing the samples, but in the end his tungsten-182 data was flat, with no significant variation between samples. The researchers surmised that the Greenland rocks had been deformed and heated in their history, scrambling their geochemical information.
They needed better rocks, so they headed to Pilbara in Western Australia. “It has some of the best-preserved Archean rocks on the whole planet,” Münker said. “They haven’t seen much heating when compared to similar rocks of that age.”
“I was really keen on finding samples that do not display the same value over and over again,” said Tusch.
Guided by co-author Martin Van Kranendonk of the University of New South Wales, the team crisscrossed the Outback in off-road trucks, visiting rust-red outcrops where ancient volcanic rock and vegetation mimic each other: Spinifex bushes at the outcrops are part silica, making them spiky and inedible to everything but termites. They hammered off a promising half-ton of rocks and lavas that formed between 2.7 billion and 3.5 billion years ago.
Back in Germany, Tusch set to work. He used a rock saw to get at the fresh rock inside each sample, then polished some slices down to half the width of a human hair to make them translucent for microscopy. He crushed the rest and concentrated the tungsten, then analyzed the tungsten isotope ratios in a mass spectrometer.
Over nearly two years, the results trickled out. This time the isotope ratios were not flat. “It was really nice to see,” remarked Tusch.
The tungsten-182 concentrations started out high in rocks formed before 3.3 billion years ago, showing that the mantle wasn’t mixing yet. Then the values declined over 200 million years until they reached modern levels by 3.1 billion years ago. That decline reflects the dilution of the ancient tungsten-182 signal as the mantle beneath Pilbara began to mix. That mixing shows plate tectonics had begun.
Earth would quickly transform from a water world studded with Iceland-like volcanic islands into a world of continents with mountains, rivers and floodplains, lakes, and shallow seas.
A New World Made for Life
The start date of roughly 3.2 billion years ago helps clarify how plate tectonics impacted life on Earth.
Life started beforehand, more than 3.9 billion years ago, and was making hummocky little stacks in sediments at Pilbara called stromatolites by 3.48 billion years ago. This shows that plate tectonics isn’t a prerequisite for life at its most basic level. Yet it’s probably no coincidence that life diversified just as plate tectonics got underway.
With plate tectonics came shallow sunlit seas and lakes fertilized with nutrients weathered from continental rock. Bacteria evolved in these environments to harvest sunlight through photosynthesis, generating oxygen.
For another half-billion years, this oxygen remained barely a whiff in the sky, partly because it immediately reacted with iron and other chemicals. Also, every oxygen molecule generated in photosynthesis is matched by a carbon atom, and these easily recombine into carbon dioxide with no net gain of oxygen in the atmosphere, unless the carbon is buried.
Gradually, though, plate tectonics provided the land and sediments in which to bury more and more of the carbon (while also providing plenty of phosphorus to stimulate photosynthetic bacteria). The atmosphere eventually oxygenated 2.4 billion years ago.
Oxygen set the planet up for the emergence of plants, animals and almost everything else with an oxygen-based metabolism. Life larger and more complex than microbes requires more energy, and organisms can make much more of the vital, energy-carrying molecule called ATP with oxygen than they can without it. “Oxygen is really important for what we think of as complex life,” said Athena Eyster of the Massachusetts Institute of Technology.
Progress toward complexity stalled during the “boring billion” era, the roughly billion-year reign of the supercontinent Nuna-Rodinia. With the continents stuck in a jam, Ming Tang of Peking University and colleagues argue, mountains eroded completely, reducing the flow of nutrients into the ocean and lowering oxygen levels.
Eventually the supercontinent broke apart, and new mountains grew and exported nutrients again. Only then — around 600 million years ago — did complex organisms diversify and get bigger, riding Earth’s second rise in oxygen.
Complex animal life exploded in the oceans 540 million years ago, and on land soon after. Dry land was now habitable because oxygen in the stratosphere formed ozone that protected land life from ultraviolet radiation.
“Potentially, there are a lot of other planets that are analogs for an Archean world, maybe without plate tectonics, that might have life,” said Eyster, but “it might be much harder to have complex life on a planet without plate tectonics.”
Consider Mars. Mars and Earth were quite similar for their first billion years. But Mars never developed plate tectonics, possibly because it is smaller than Earth, so its interior pressure was insufficient to drive mantle convection on a large scale. Instead, it quickly developed a thick crust not conducive to forming mobile plates. Today, Mars is rusted red, with little surface water, no magnetic field and scant atmosphere.
But for plate tectonics that might have been Earth’s fate too.
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