Earth is the only planet known to have continents, although how they formed and evolved is unclear.
An artist’s concept of meteors impacting the ancient Earth. Some scientists think such impacts may have delivered water and other molecules useful to emerging life on Earth. Image credit: NASA’s Goddard Space Flight Center Conceptual Image Lab.
“The idea that the continents originally formed at sites of giant meteorite impacts had been around for decades, but until now there was little solid evidence to support the theory,” said lead author Dr. Tim Johnson, a researcher in the School of Earth and Planetary Sciences at Curtin University and the Centre for Global Tectonics at the China University of Geosciences.
By examining tiny zircon crystals in rocks from the Pilbara Craton in Australia, which represents Earth’s best-preserved remnant of ancient crust, Dr. Johnson and his colleagues found evidence of these meteorite impacts.
“Studying the composition of oxygen isotopes in these zircon crystals revealed a ‘top-down’ process starting with the melting of rocks near the surface and progressing deeper, consistent with the geological effect of giant meteorite impacts,” Dr. Johnson said.
“Our research provides the first solid evidence that the processes that ultimately formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, but which occurred billions of years earlier.”
Geodynamic cartoons illustrating the three-stage evolution of the Pilbara Craton, Australia: (a-c) a giant meteorite impacting hydrothermally altered primary crust (a) produces low δ18O shallow melts and triggers formation of a mafic-ultramafic continental nucleus by decompression melting of the mantle (b,c); (d-f) the nucleus fractionates to produce low-MgO basaltic protoliths (d) that melt to form TTG magmas (e), which themselves melt to form granite (f). Image credit: Johnson et al., doi: 10.1038/s41586-022-04956-y.
“Understanding the formation and ongoing evolution of the Earth’s continents is crucial given that these landmasses host the majority of Earth’s biomass, all humans and almost all of the planet’s important mineral deposits.”
“Not least, the continents host critical metals such as lithium, tin and nickel, commodities that are essential to the emerging green technologies needed to fulfill our obligation to mitigate climate change.”
“These mineral deposits are the end result of a process known as crustal differentiation, which began with the formation of the earliest landmasses, of which the Pilbara Craton is just one of many.”
“The data related to other areas of ancient continental crust on Earth appears to show patterns similar to those recognized in Western Australia.”
“We would like to test our findings on these ancient rocks to see if, as we suspect, our model is more widely applicable.”
The team’s paper was published in the journal Nature.
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T.E. Johnson et al. 2022. Giant impacts and the origin and evolution of continents. Nature 608, 330-335; doi: 10.1038/s41586-022-04956-y
