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Remnants of Earth’s original crust preserve time before plate tectonics

CLASSIC ROCK Rocks from an outcrop in northeastern Canada may have formed from material dating back more than 4.2 billion years ago. That’s older than the oldest rocks found on Earth’s surface today.
J. O’NEIL

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Remnants of Earth’s original crust preserve time before plate tectonics

Full-blown plate tectonics had late start, geochemical evidence suggests

By Thomas Sumner

Not all of the newborn Earth’s surface has been lost to time. Transformed bits of this rocky material remain embedded in the hearts of continents, new research suggests. These lingering remnants hint that full-fledged plate tectonics, the movements of large plates of Earth’s outer shell, began relatively late in the planet’s history, researchers report in the March 17 Science.

These revelations come from ancient continental rock in Canada that preserves geochemical traces of the even older, 4.2-billion-year-old rock from which it formed. “For the first time, we can say something about what kind of rock was a precursor to the first continental crust,” says study coauthor Jonathan O’Neil, a geochemist at the University of Ottawa.

Earth began as a molten ball around 4.54 billion years ago, and over the next tens of millions of years, its surface cooled and solidified. Almost all of Earth’s early rocky surface has been destroyed and recycled by geologic processes such as plate tectonics. The oldest known unaltered bits of the planet aren’t rocks but tiny zircon crystals formed nearly 4.4 billion years ago (SN Online: 2/23/14). The oldest actual rocks date back to only about 4 billion years. “We’re missing a lot of Earth’s history,” O’Neil says.

The new discovery fills in some of that history. In northeastern Canada, along the eastern shore of the Hudson Bay, O’Neil and geochemist Richard Carlson of the Carnegie Institution for Science in Washington, D.C., discovered 2.7-billion-year-old continental rocks that hinted at something much older. The rocks contain an unusually large abundance of an isotope of neodymium that formed only during the first few hundred million years of Earth’s history. To have so much of this neodymium, the rocks must have formed from material that was first created more than 4.2 billion years ago, the researchers calculate. That’s far older than the oldest rocks ever studied.

Based on the composition of the Canadian rocks, the researchers think that the precursor material was similar to the crust that underlies modern oceans. The finding affirms previous studies that suggested that the first continental crust arose from the partial melting of oceanic crust (SN Online: 9/20/16).

But unlike modern oceanic crust, which typically lingers for less than 200 million years before getting recycled into Earth’s interior by plate tectonics, the precursor crust survived for more than a billion years before being reworked into continental crust 2.7 billion years ago. Plate tectonics during the precursor crust’s life span must have therefore been nonexistent, sluggish or limited to certain regions, O’Neil concludes.

“If you ask five geologists the simple question of when did plate tectonics start, you’ll have answers from 4.3 billion years ago to 1 billion years ago,” he says. The new finding seems to rule out the idea that full-blown, global plate tectonics began early in Earth’s history.

The new work is exciting and sheds light on the processes that set the scene for Earth’s subsequent evolution and habitability, says geologist Tony Kemp of the University of Western Australia in Crawley. Other vestiges of early crust may lurk undiscovered elsewhere on Earth as well, he says. “It will be intriguing to see how this [research] unfolds with future studies of this type.”

source: Science News

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