Small crystals discovered in South Africa contain evidence of an abrupt shift on the planet’s surface 3.8 billion years ago.
These crystals, each no larger than a grain of sand, show that at about that time, LandThe crust broke down and began to move – a precursor to a process known as plate tectonics.
The findings provide clues about the evolution of Earth as a planet, and can help answer questions about possible links between them tectonic plates “The evolution of life,” said lead study author Nadia Drapon, professor of Earth and planetary sciences at Harvard University.
“Earth is the only planet that has life,” Drapon told Live Science. Earth is the only planet with plate tectonics.”
life engine
Presently, jigsaw pieces of hard crust are floating on a hot, sticky ocean of magma in the mantle, the Earth’s middle layer. These pieces of crust grind against each other, sink under each other in what are called subduction zones and push each other, resulting in the formation of mountains and ocean ridges, rifting volcanoes And cause earthquakes that shake the planet regularly. The sinking of tectonic plates also produces new rock in subduction zones, 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 weren’t always this way. When the Earth was young and hot, during the Hadian era (4.6 billion to 4 billion years ago), the planet was first covered by an ocean of magma and then, as the planet cooled, a solid, rocky surface.
Exactly when that surface cracked and parts of it started moving has been hotly debated. Some studies estimate that plate tectonics began only 800 million years ago, while other studies indicate that the age of this system is at least 2 billion years, Live Science previously reported.
But since the planet is constantly recycling its crust into the mantle, there are almost no ancient rocks at 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 old makes up only 5% of the rocks at the surface. And 4 billion years ago, there were no preserved rocks.
sudden transition
That changed in 2018, when Drapon and her colleagues discovered zircon crystals in a bed of green sandstone in South Africa, in the Barberton-Greenstone mountain range. The team found 33 zircons, ranging in age from 4.1 billion to 3.3 billion years.
In the new study published April 21 in the journal ancestor AGUThe team analyzed different isotopes, or different forms of elements with different numbers of neutrons, in those 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 dating back about 3.8 billion years. This result indicates that by then, in at least one place on the planet, a minor form of subduction has begun. Whether or not this occurred globally, Drapon said, it is likely that the “really efficient engine for plates moving against each other” that exists today has yet to emerge.
Isotope analysis of elements such as oxygen, niobium and uranium It also showed that rocks at the surface held water 3.8 billion years ago, suggesting that zircon was once trapped in oceanic crust buried on the primordial sea floor. Extrapolation from the oldest samples, 4.1 billion years ago, suggests that the planet had a solid crust no later than 4.2 billion years ago, Drapon said.
This means that the sea of magma on Earth only lasted until the late modern period. Previously, Darabon said, “People believed that the Earth was only covered by an ocean of magma until 3.6 billion years ago.”
The new study indicated that molten lava oceans existed at most for a few hundred million years before the solid crust formed.
So what led to this shift? One theory is that plate tectonics appeared once the Earth had cooled enough. It’s also possible, like a dessert spoon cracking the crisp top of a creme brulee, that massive space rocks may have smashed into Earth and shattered its crust.
Drapon adds another interesting question of whether Earth’s transition to early plate tectonics somehow aided the evolution of life.
While early fossil Evidence of life on Earth dates back about 3.5 billion years, The chemical fingerprints of biological processes, found in the ratio of carbon isotopes, are older. Some can be found 3.8 billion years ago — around the same time that early plate tectonics appeared, Drapon said.
Originally published on Live Science.
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