Tiny crystals unearthed in South Africa contain evidence of a sudden transition in the planet’s surface 3.8 billion years ago.
These crystals, each no larger than a grain of sand, show that at that time, land‘s crust broke apart and began to move, a precursor to the process known as plate tectonics.
The findings offer clues about the evolution of Earth as a planet and could help answer questions about possible links between tectonic plates and the evolution of life, said study lead author Nadja Drabon, a professor of Earth and planetary sciences at Harvard University.
“Earth is the only planet that has life; Earth is the only planet that has plate tectonics,” Drabon told WordsSideKick.com.
engine of life
Today, puzzle pieces of rigid crust float in a hot, viscous ocean of magma in the mantle, Earth’s middle layer. These pieces of crust grind against each other, plunge under each other in so-called subduction zones, and push against each other, creating mountains and mid-ocean ridges, forging volcanoes and triggering the earthquakes that regularly shake the planet. The sinking of tectonic plates also produces new rocks at subduction zones, which interact with the atmosphere to absorb Coal dioxide. This process makes the atmosphere more hospitable to life and keeps the climate more stable, Drabon said.
But things were not always like this. When the Earth was young and hot, during the Hadean eon (4.6 to 4.0 billion years ago), the planet was first covered with an ocean of magma and then, as the planet cooled, a solid rock surface.
It has been hotly debated when exactly that surface cracked and parts began to move. Some studies estimate that plate tectonics began as little as 800 million years ago, while others suggest that this system is at least 2 billion years old. Live Science previously reported.
But because the planet constantly recycles its crust into the mantle, there are almost no ancient rocks on the surface to help settle the debate. Before this study, “rocks that are between 2.5 [billion] and 4 billion years old they only make up 5% of the rocks on the surface,” Drabon said. “And before 4 billion years old, there are no preserved rocks.”
sudden transition
That changed in 2018, when Drabon and colleagues discovered zircon crystals in South Africa’s Green Sandstone Bed, in the Barberton Greenstone 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 AGU AdvancesThe team analyzed different isotopes, or variants of elements with different numbers of neutrons, in these ancient zircons, as well as in many zircons from other times and places on Earth.
In the isotopes, the scientists found evidence of a sudden transition to primitive plate tectonics dating to about 3.8 billion years ago. That finding suggests that by this time, in at least one place on the planet, a simple form of subduction had begun. Whether or not this happened globally has yet to be determined, and the “really efficient engine of plates moving against each other” that exists today has likely not yet emerged, Drabon said.
Isotopic analysis of elements such as oxygen, niobium and uranium it also showed that surface rocks contained water as early as 3.8 billion years ago, suggesting that zircons were once locked in oceanic crust buried in a primitive seafloor. And extrapolation from the oldest samples, from 4.1 billion years ago, suggests the planet had a solid crust no later than 4.2 billion years ago, Drabon said.
This would mean that Earth’s magma sea persisted only until the late Hadean. Previously, “people thought the Earth was covered by an ocean of magma until 3.6 billion years ago,” Drabon said.
The new study hints that Earth’s molten lava ocean existed for a few hundred million years before the solid crust formed, he added.
So what triggered this transition? One theory is that plate tectonics simply arose once Earth got cold enough, she said. It’s also possible that, like a dessert spoon breaking off the crusty top of a crème brûlée, massive space rocks have smashed into Earth and shattered its crust.
Another intriguing question addresses whether Earth’s transition to early plate tectonics somehow helped life evolve, Drabon added.
While the earliest fossils Evidence of life on Earth dates to about 3.5 billion years ago., the chemical signatures of biological processes, found in the ratio of carbon isotopes, are even older. Some can be found as far back as 3.8 billion years ago, around the same time the first tectonic plates emerged, Drabon said.
Originally published on Live Science.
