Earth was a water world for hundreds of millions of years after it formed, almost entirely devoid of continental crust, an analysis of some of the oldest crystals on Earth confirms. Although the discovery fits with the picture many geologists had formed of the planet's early days, some recent work challenged that view, prompting deeper investigations.
“The history of the Earth is like a book with its first chapter ripped out with no surviving rocks from the very early period,” said the Australian National University's (ANU) Dr Antony Burnham in a statement. The oldest surviving remnants are 4.4 billion-year-old mineral grains called zircons from the Jack Hills of Western Australia. The Hills are sandstone rocks in which the zircons became embedded after eroding out of even older rocks.
Burnham examined these zircons, comparing them to younger equivalents from eastern Australia formed in better-known circumstances, to establish their origins. Burnham found the Jack Hills zircons are very low in phosphorus. Among the eastern Australian zircons, those that came from igneous rocks (solidified from lava or magma) were similarly low in phosphorus, whereas crystals formed from the remelting of sedimentary rocks (formed from sediment transported by water) had much higher concentrations. Concentrations of some heavy metals further re-enforced the igneous origins of these zircons, Burnham reports in Nature Geoscience.
Mineral grains that looked sedimentary had been found inside Jack Hills zircons. If they were indeed sedimentary in origin, it would suggest the world at the time was very different from how we imagine it. “Sediment melting is characteristic of major continental collisions, such as the Himalayas, so it appears that such events did not occur during these early stages of Earth’s history,” Burnham said.
These sandstone rocks contain tiny grains that really are from the dawn of time. ANU
Instead, Burnham told IFLScience the small landmasses the early Earth supported were over volcanic hotspots like modern-day Hawaii or Iceland. The basaltic rocks formed in these conditions eventually experienced transformations to become the first granites, slowly aggregating to become continental crust.
In the early years, however, these proto-continents were so minor and isolated the Earth didn't experience the mountain ranges produced when continents collide, a state that continued for quite some time. “Our research indicates there were no mountains and continental collisions during Earth’s first 700 million years or more of existence – it was a much more quiet and dull place,” Burnham said, although he admitted to IFLScience that this is only true if you include being frequently bombarded with asteroids as “quiet and dull”.
Although there is little doubt the Earth, like the rest of the Solar System, copped many large asteroid strikes during this period, Burnham says there is no hint of this in the Jack Hills zircons. This contradicts the theory the first granites were produced by asteroid bombardment.
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