Source: Science News

Modern plate tectonics may have gotten under way as early as 3.2 billion years ago, about 400 million years earlier than scientists thought. That, in turn, suggests that the movement of large pieces of Earth’s crust could have played a role in making the planet more hospitable to life.

Geologist Alec Brenner of Harvard University and his colleagues measured the magnetic orientations of iron-bearing minerals in the Honeyeater Basalt, a layer of rock that formed between 3.19 billion and 3.18 billion years ago. The basalt is part of the East Pilbara Craton, an ancient bit of continent in Western Australia that includes rocks as old as 3.5 billion years.

This craton, the researchers found, was on the move between 3.35 billion and 3.18 billion years ago, drifting around the planet at a rate of at least 2.5 centimeters per year. That’s a speed comparable to modern plate motions, the team reports April 22 in Science Advances.

The basalt layer, which burbled up as lava and hardened during the journey, contains iron-bearing minerals that can act as tiny signposts pointing the way toward Earth’s magnetic poles. While the lava was still molten, the minerals rotated, orienting themselves to align with either the north or south magnetic pole. By tracking the changes in orientation within the lava as more basalt formed during the journey, the researchers were able to determine how quickly the craton was moving.

Scientists have long used such preserved magnetic signposts to reconstruct plate motions, retracing the steps of drifting bits of continent. But the constant grinding and shifting of Earth’s tectonic plates over the last few billion years have reworked Earth’s surface many times over, leaving few outcrops that are older than 3 billion years.

The Honeyeater Basalt, however, is a rare site, both ancient and relatively unworked by metamorphism, the heat and pressure from which could have altered the minerals and reset their magnetic orientation. The team examined 235 samples of the basalt using an instrument called a quantum diamond microscope that can detect traces of magnetism at the micrometer scale. From these analyses, the researchers created a high-resolution map of magnetic orientations within the rock.

Based on the map, the team estimates that about 3.2 billion years ago, the East Pilbara Craton was at a latitude of about 45°, but whether north or south isn’t certain, Brenner said April 21 in a video news conference. That’s because researchers aren’t sure whether Earth’s magnetic poles at the time were in their current orientation or reversed. Either way, this bit of ancient crust moved in a gradual, steady motion — a hallmark of modern plate tectonics, the researchers say. Today, the craton is located at about 21° S, just north of the Tropic of Capricorn. 

Plate tectonics is generally thought to have become a well-established global process on Earth no earlier than around 2.8 billion years ago. Before that, Earth’s interior was considered to be too hot for cold, rigid plates to form at the surface, or for deep subduction to occur, in which one crustal plate dives beneath another.

An earlier start to plate tectonics would have implications for the evolution of life on Earth, Brenner told reporters. Whether the process was in operation when the first single-celled organisms emerged, currently thought to be at least 3.45 billion years ago, isn’t clear, he said.

But it is clear that plate tectonics is currently closely tied to the biosphere, he added. It promotes chemical reactions between once-buried rocks and the atmosphere that can modulate the planet’s climate over millions to billions of years. “So if [plate tectonics] happened on the early Earth, these processes were likely playing a part in the evolution of life,” Brenner said.

Active, modern-style plate tectonics is the most likely explanation for the data, the researchers say. But they acknowledge other possible explanations can’t yet be ruled out, including an early, episodic, fit-and-start style of plate tectonics.

Some researchers have proposed that, during the Archean Eon that lasted from about 4 billion to about 2.5 billion years ago, there was a proto-plate tectonics process in which bits of crust moved in fits and starts as the planet began to cool after its formation. Sediment eroded from Earth’s earliest continents may also have helped grease the wheels, setting the stage for modern plate tectonics.

The researchers’ data could support episodic rather than gradual plate motion, perhaps as a precursor to modern plate tectonics, says Michael Brown, a geologist at the University of Maryland in College Park. Those data suggest that after its initial burst of speed, the Honeyeater Basalt’s progress slowed considerably, from 2.5 centimeters per year to 0.37 centimeters per year, he says.

It’s still unclear how similar proto-plate tectonics may have been to the modern process. “We know too little to answer this question with confidence,” says geophysicist Stephan Sobolev of the University of Potsdam in Germany. Sobolev has suggested previously that, for about a billion years during the Archean, plate tectonics occurred regionally: Plates could have been broken apart by large meteorite impacts or powerful plumes rising from the mantle, generating regional cells in which ancient continents formed and small blocks of crust subducted.

Such a regional cell may have formed the East Pilbara Craton in Australia, Sobolev suggests. But for that bit of ancient continent to have traveled so far so quickly, he says, “large-scale subduction must have been involved” — a surprising possibility for early Earth’s history.

Source: Science News