Small crystals have been discovered in southern Africa that 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 that time, LandThe crust broke down and began to move – a precursor to a process known as plate tectonics.
The findings provide clues to the evolution of Earth as a planet and could 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 on which there is life; “Earth is the only planet with plate tectonics,” Drapon told Live Science.
life engine
Today, jigsaw pieces of hard crust are perched on an ocean of hot sticky magma in the mantle, Earth’s middle layer. These pieces of crust rub 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, and rifting volcanoes And cause earthquakes that shake the planet regularly. The sinking of tectonic plates also produces new rocks in subduction zones, which interact with the atmosphere to absorb 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 (from 4.6 billion to 4 billion years ago), the planet was first covered by an ocean of magma, then as the planet cooled, it was a solid, rocky surface.
Exactly when this surface cracked and parts of it began to move has been hotly debated. Some studies estimate that the movement of tectonic plates began only 800 million years ago, while others indicate that this system is at least 2 billion years old. Previously reported on Live Science.
But because the planet is constantly recycling its crust back 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: “And 4 billion years is only 5% of the rocks that are on the surface. And 4 billion years ago, no rocks were preserved.”
sudden transition
That changed in 2018, when Drapon and colleagues discovered zircon crystals at the bottom of a green sandstone in South Africa in the Barberton-Greenstone mountain range. The team found 33 zircons, ranging in age from 4.1 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 these ancient zircons, as well as many zircons from other times and places on Earth.
In radioisotopes, scientists have found evidence of a sudden transition to early plate tectonics dating back about 3.8 billion years. This discovery indicates that by this time, in at least one place on the planet, a minor form of subduction has begun. Whether or not this occurred on a global scale remains unclear, Drapon said, and it is likely that the “really effective 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 surface rocks held water 3.8 billion years ago, indicating that zircon was trapped in ocean crust buried on the early sea floor. Extrapolation from the first samples, which date back 4.1 billion years, suggests that the planet had a solid crust no later than 4.2 billion years ago, Drapon said.
This means that the Earth’s sea of magma only lasted until the end of the Hadean. Darabon previously said, “People believed that the Earth was only covered by an ocean of magma until 3.6 billion years ago.”
She added that the new study indicates that the Earth’s ocean of molten lava existed at most for a few hundred million years before the formation of the solid crust.
So what led to this shift? One theory is that plate tectonics arose once the Earth had cooled enough. It’s also possible, like a dessert spoon cracking the crunchy top of creme brulee, that huge space rocks smashed into Earth and shattered its shell.
Another interesting question, Drapon added, is whether Earth’s transition to early plate tectonics somehow helped the evolution of life.
While the first excavations The history of life on Earth goes back about 3.5 billion years, The chemical fingerprints of biological processes, found in the ratio of carbon isotopes, are older. Some could be found 3.8 billion years ago — around the same time plate tectonics emerged, Drapon said.
Originally published on Live Science.