Oldest evidence of a moving tectonic plate found in Australia

Plate tectonics sculpted Earth’s surface and may have set the stage for the emergence of life. A new study offers clues about how this planetary churning began.

Monday, April 27, 2020,
By Maya Wei-Haas
This illustration shows what a slice through the Earth may have looked like more than 3 ...

This illustration shows what a slice through the Earth may have looked like more than 3 billion years ago. Whether or not plate tectonics existed during this time is hotly debated, and a new study deepens the argument with the the earliest direct evidence of plate motion yet found.

Photograph by Image by Alec Brenner, Harvard University

In the desolate landscape of western Australia, a rocky outcrop that formed more than three billion years ago is giving geologists an unprecedented look at the early churnings of our planet. These rocks—among the most ancient in the world—contain what may be the oldest direct evidence of the movement of tectonic plates.

The rocks formed when magma oozed up from beneath Earth’s surface into a now-vanished ocean, cooling and hardening into a bulbous mass. As detailed in a new study in Science Advances, magnetic signatures preserved in the rock suggest the region was inching across the planet 3.2 billion years ago at similar speeds to tectonic plates today—nearly half a billion years earlier than previous evidence of such movement.

“This is kind of the smoking gun,” says geochemist Annie Bauer of the University of Wisconsin-Madison, who was not part of the new study. “This is the most important evidence we can get [of early plate motion].”

Today, Earth’s tectonic plates continually shift and migrate—a process that builds mountains, carves basins, and drives volcanic eruptions. These motions sculpted a variety of ecological niches, including hydrothermal vents at the bottom of the sea and boiling pools of water on the surface—the types of environments where life is believed to have formed.

“While piecing together the story of plate tectonics, we’re helping to piece together our own origin story,” says the study’s lead author Alec Brenner, a Ph.D. student at Harvard University.

Hunting for ancient rocks

Our planet coalesced from a swirling cloud of gas and dust some 4.5 billion years ago, and initially it was scorching hot. Oceans of molten rock glowed on the surface, and volcanoes likely spit lava into the air. But Earth soon began to cool, and over tens of millions of years, the surface hardened into a crust.

Scientists believe this early crust was a singular cap enveloping the planet, much like the surface of Mars today. At some point—estimates vary from roughly four billion to a billion years ago—this cap fractured into a global jigsaw of crust, with pieces crashing into each other and driving rock down into the mantle or up into the sky. Plate tectonics was born.

But very little is known about how and when this transition took place. Plate tectonics as a process continually recycles Earth’s rock, melting crust and dredging up fresh lava, which erases evidence of the distant past. “Basically, the first half of Earth’s history is represented today by only about 5 percent of surface rocks,” Brenner says.

Many studies of early plate tectonics infer motion by identifying chemical clues, such as the composition of ancient minerals that point to formation within subduction zones—where one tectonic plate plunges beneath another. But to chart the movement of plates, scientists have to use other measures such as the preserved magnetic signatures of the rocks.

In 2016, Brenner’s future advisor at Harvard, palaeomagnetist Roger Fu, began poring over maps of Australia in search of ancient rocks where he might use these magnetic fingerprints to directly measure the early drift of Earth’s crust. Fu and a colleague eventually homed in on a site: The Honeyeater basalt of western Australia. In the summer of 2017, Brenner and Fu ventured into the Australian outback to hunt down the 3.2-billion-year-old rocks.

They drilled around a hundred cores of rock from various parts of the outcrop, noting the position and orientation for each and combining them with more than a hundred previously collected samples. Back in the lab, they analysed the magnetic signatures of each sample, encoded in iron-rich minerals that orient themselves like tiny compass needles as they crystallise.

After accounting for changes in the rock’s position since it formed—a process known as a fold test—the compass needles all aligned, suggesting they represented the true ancient magnetic signature of the rock. “Maybe we are on to something here,” Fu recalls thinking.

Tectonic beginnings

The team compared the calculated position of the Honeyeater basalt to a previously analysed outcrop of rock nearby, which is slightly older and contains an earlier magnetic signature. The analysis revealed that the crust was shifting about 2.5 centimetres each year at the time these rocks formed.

That rate “would be totally run-of-the-mill ordinary for a plate tectonic setting like what we have on the modern Earth,” Brenner says.

The motion may have occurred while Earth was still covered by a single cap of crust, although the speed is faster than what would be expected if that were the case. The find instead hints that just over a billion years after our planet formed, plate tectonics could have already been revving up.

However, the evidence from this one location does not necessarily mean that plates were moving all around the world, Brenner says. Plate tectonics likely began in fits and starts, with crust breaking apart and moving in some areas earlier than others.

“It might be kind of a patchy process,” says Bauer, who recently published a study demonstrating the uneven beginnings of early plate movements.

The mechanism driving this early movement is also unclear, says palaeomagnetist John Geissman of the University of Texas at Dallas, who was not involved in the new study. One major force behind modern plate motions is the tug of rocky slabs as they plunge into the mantle at subduction zones. But other processes could have been at play billions of years ago, such as rising plumes of magma forcing rocks apart at the surface.

If these early stirrings 3.2 billion years ago were indeed the beginnings of plate tectonics, they point to a remarkably early start for Earth’s geologic churn, which was a pivotal point for the evolution of life as we know it. Plate tectonics acts like a planetary thermostat, cycling greenhouse gasses from the deep Earth to the atmosphere. It drives volcanic eruptions, which dredge up fresh nutrients from deep underground. It may even have played a role in piping oxygen into the skies.

By understanding the origins of plate tectonics, “you can try to nail down timing of events that were crucial for the development of life on this planet,” says geochemist Val Finlayson from the University of Maryland, who was not part of the study.

To do that, scientists are continuing to scour the earth for more signs of ancient movement. Brenner says: “We are actually, as we speak, running through the data analysis for another [rock] unit."

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