Tiny crystals found in South Africa contain evidence of a sudden shift in the planet’s surface 3.8 billion years ago.
These crystals, each no bigger than a grain of sand, indicate that at that time, LandThe crust breaks and begins to move – the forerunner of a process known as plate tectonics.
These findings provide clues about Earth’s evolution as a planet, and may help answer questions about possible relationships between them tectonic plates “The evolution of life,” said study lead author Nadia Drapon, professor of Earth and planetary sciences at Harvard University.
“Earth is the only planet with life; Earth is the only planet with plate tectonics,” Drapon told Live Science.
life machine
Currently, jigsaw pieces of hard crust are floating in a sea of hot, sticky magma in the mantle, Earth’s middle layer. These pieces of crust grind against each other, sink under each other in what is called a subduction zone and push against each other, resulting in the formation of mountains and oceanic ridges, rifting. volcano And cause earthquakes that rock the planet regularly. The sinking of tectonic plates also produces new rock in the subduction zone, which interacts with the atmosphere to absorb it. carbon dioxide. This process makes the atmosphere more hospitable to life and keeps the climate more stable, Drapon said.
But things are not always like this. When Earth was young and hot, during the Hadian era (4.6 billion to 4 billion years ago), the planet was first covered by a sea of magma and then, as the planet cooled, a solid, rocky surface.
Exactly when the surface cracked and the parts started moving has been hotly debated. Some studies estimate that plate tectonics began only 800 million years ago, while other studies suggest that this system is at least 2 billion years old, Live Science previously reported.
But because the planet is constantly recycling its crust into the mantle, there is hardly any primeval rock on the surface to help settle the controversy. Prior to this study, the rock size was between 2.5 [billion] Drapon said that 4 billion years made up only 5% of the rock on the surface. And 4 billion years ago, no rock was preserved.
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sudden transition
That changed in 2018, when Drapon and his colleagues discovered zircon crystals in a layer of green sandstone in South Africa, in the Barberton-Greenstone mountains. The team found 33 zircons, ranging in age from 4.1 billion to 3.3 billion years.
In a new study published April 21 in the journal ancestor of AGUThe team analyzed different isotopes, or different forms of the element with different numbers of neutrons, in these ancient zircons, as well as in many zircons from other times and places on Earth.
In isotopes, scientists have found evidence of a sudden transition to primitive plate tectonics that is about 3.8 billion years old. These results suggest that by then, at least one place on the planet, a form of minor subduction had begun. Whether this is happening globally or not, says Drapon, it’s possible that the “truly efficient machines for plates moving against each other” that exist today have yet to emerge.
Isotope analysis of elements such as oxygen, niobium and uranium It also shows that the rock on the surface held water 3.8 billion years ago, suggesting that zircon was once trapped in oceanic crust buried on the ancient ocean floor. Extrapolation from the oldest sample, 4.1 billion years ago, suggests that the planet had a solid crust as late as 4.2 billion years ago, Drapon said.
That is, the magma ocean on Earth only lasted until the end of modern times. Previously, Drapon said, “People believed that the Earth was only covered by a sea of magma until 3.6 billion years ago.”
The new study suggests that seas of molten lava existed at most for several hundred million years before solid crust formed.
So what caused this shift? One theory is that plate tectonics emerged after the Earth had cooled enough. It’s also possible that, like a dessert spoon cracking the top of a crunchy creme brulee, a huge space rock may have crashed into Earth and crushed its crust.
Drapon adds another interesting question about whether Earth’s transition to early plate tectonics somehow aided the evolution of life.
While the early fossils Evidence of life on Earth dates back about 3.5 billion years, Chemical fingerprints of biological processes, found in carbon isotope ratios, are older. Some can be found 3.8 billion years ago — around the same time that the earliest tectonic plates emerged, Drapon said.
Originally published in Live Science.
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