Researchers have discovered “lost continents” from 4 billion years ago

The idea we have of the early Earth involves a huge ball of incandescent magma and conditions incompatible with life. The problem? That there are no rocks from 4.3 billion years ago to confirm this consolidated theory. What we do have are some microscopic crystals called zircons. And zircons are telling a different story, according to this study by a research team at the University of Wisconsin-Madison. published in Nature.

What zircon says. Regarding the behavior of the Earth’s surface, geology valued two ideas for that period known as Hadean: that there was a plate tectonics where one plate sinks under another or that the Earth had a kind of stagnant lid, a rigid and hot surface where heat only escaped through large columns of magma.

Well, neither one nor the other, both: zircons leave evidence of an Earth that already had oceans, liquid water and a crust that alternated both systems. John Valley, the University of Wisconsin-Madison geoscientist who leads the study explains that “There were about 800 million years of Earth’s history in which the surface was already habitable, although we have no fossil evidence and we do not know when life first emerged.”

Why it is important. Because they determine that the Earth did not choose a single model, but rather that both processes took place at the same time in different places. Of course, it was not a stable plate tectonics like the one that exists today, but rather it had violent and short episodes of sliding of the edges of one plate under another (subduction) that coexisted with large jets of magma that rose from the interior of the Earth.

This discovery is key to understanding how the Earth’s surface moved, the formation of continents and life. On the one hand, without tectonics, the felsic continental crust that floats on the mantle and makes up the lands on which we live would not exist. On the other hand, plate tectonics regulates the climate and recycles nutrients, so knowing when it started working helps understand when the Earth became a place compatible with life.

How they analyzed it. The John Valley team analyzed the popular zircons from Jack Hills (Western Australia). These sand-sized minerals are a kind of time capsule, housing the only direct record of Earth’s first 500 million years. They were looking for chemical “fingerprints” that would reveal where and how they were formed, for which they used technology WiscSIMS high resolution. They then compared the results of the analysis with other zirconiums from the Hadic Eon found in Barberton (South Africa). Each one told a different story.

Surprises in the “DNA” of the mineral. 47% of oceanic zircons had high levels of Uranium compared to Niobium, indicating that they formed in subduction zones where ocean water sinks into the mantle. On the other hand, the South African zircons show that they were born from virgin rock from the planet’s interior, confirming the classic ‘stagnant lid’ theory by which the Earth’s first solid surface was rigid and immobile.

Or what is the same: while in Australia the crust sank and created protocontinents, in what is now South Africa the Earth behaved differently, with a rigid and stagnant crust. That is, the early Earth was a mosaic of tectonic styles. The Earth did not go from being hell to what it is today overnight, but rather it was a hybrid process and generated the necessary conditions for life sooner than we thought.

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Cover | Tomáš Malík and Javier Miranda