Earth’s first stable continents did not form by subduction

Early Earth’s single solid shell

23 August 2017

Earth’s first continents probably encased the planet in a single solid shell, an Australia-US research team finds.

Continents are massive slabs of granite floating on heavier basalt that spews forth from volcanic rifts. As basalt trundles along like a conveyor belt and meets a tectonic plate, it slides underneath, process known as subduction, and is forced down to the crushing pressures and heat of Earth’s mantle. There, it melts and forms granite, which blobs up towards the surface, building continents from below.

But could granite form at lower pressures beneath a stagnant shell with no subduction required?

To find out, Curtin University’s Tim Johnson and colleagues modelled the behaviour of 3.5-billion-year-old basalt from Western Australia’s Pilbara region under such conditions. The resulting granite’s composition was consistent with that of Pilbara granite samples from the same era, indicating Earth lacked tectonic plates at the time.

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References

1. Nature 543, 239–242 (2017). doi: 10.1038/nature21383