Tiny crystals found in South Africa contain evidence of a sudden transition to the planet’s surface 3.8 billion years ago.
These crystals, each no larger than a grain of sand, show that the Earth’s crust broke and began to move around that time, a precursor to the process known as plate tectonics.
The findings offer clues to the evolution of Earth as a planet and could help answer questions about the potential links between plate tectonics and the evolution of life, said study lead author Nadja Drabon, professor of earth sciences and planetary at Harvard University.
“Earth is the only planet that has life; Earth is the only planet that has plate tectonics,” Drabon told LiveScience.
Engine of life
Nowadays, stiff-crust jigsaw pieces float on an ocean of viscous, hot magma in the mantle, the Earth’s middle layer. These pieces of crust grind against each other, plunge under each other in so-called subduction zones and push each other upward, creating mountains and ocean ridges, forging volcanoes and triggering the earthquakes that regularly rock the planet. The sinking of the tectonic plates also produces new rocks in the subduction zones, which interact with the atmosphere to suck up carbon dioxide. This process makes the atmosphere more hospitable for life and keeps the climate more stable, Drabon said.
But this has not always been the case. When the Earth was young and hot, during the eon of Hades (4.6 billion to 4 billion years ago), the planet was first covered by an ocean of magma and then, when the planet cooled, by a solid rock surface.
The exact moment when that surface cracked and pieces of it began to move has been hotly debated. Some studies estimate that plate tectonics only began 800 million years ago, while others suggest this system is at least 2 billion years old, Live Science previously reported.
But as the planet constantly recycles its crust into the mantle, there are hardly any ancient rocks on the surface to help settle the debate. Prior to this study, “rocks that are between 2.5 [billion] and 4 billion years make up only 5% of the rocks on the surface, “said Drabon.” And before 4 billion years, there are no rocks preserved. “
That changed in 2018 when Drabon and his colleagues discovered zircon crystals in the green sandstone bed in South Africa’s 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 AGU Advances, the team analyzed several isotopes, or variants 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 the isotopes, scientists have found evidence of a sudden transition to primitive plate tectonics dating back about 3.8 billion years. This discovery suggests that a simple form of subduction had begun at least at one point on the planet. Whether or not this happened globally is still undetermined, and it is likely that the “really efficient engine of plates moving against each other” that exists today has not yet emerged, Drabon said.
Isotope analysis of elements such as oxygen, niobium and uranium also showed that rocks from the surface were holding water as early as 3.8 billion years ago, suggesting that zircons were once encased in the oceanic crust buried in a primeval seabed. . And extrapolation from the earliest samples, 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 the Earth’s magma sea only persisted until late Hadean. Previously, “people thought the Earth was just covered by an ocean of magma up to 3.6 billion years ago,” Drabon said.
The new study suggests that Earth’s molten lava ocean existed for at most a few hundred million years before the solid crust formed, he added.
So what triggered this transition? One theory is that plate tectonics simply emerged once the Earth cooled enough, he said. It’s also possible that, like a dessert spoon breaking the crunchy top of a crème brûlée, massive space rocks have slammed into the Earth and shattered its crust.
Another intriguing question is whether Earth’s transition to early plate tectonics has somehow helped life evolve, Drabon added.
While the earliest fossil evidence of life on Earth dates back to around 3.5 billion years ago, the chemical signatures of biological processes, found in the carbon isotope ratio, are even older. Some can be found as far back as 3.8 billion years ago, around the same time that the first plate tectonics emerged, Drabon said.
Originally published in Live Science.