While plate tectonics are the dominant mechanism for generating new continental crust today, in the distant past Earth’s subsurface temperatures are widely thought to have been too hot to allow plates to form
is a poor archivist. The rigid tectonic plates of its outer layers are continuously in motion, sliding over one another to swallow almost all records of the past, melting them into the mantle and then casting them anew. This makes it difficult to reconstruct the 4.5bn-year-oldA particular problem is the crucial biographical detail of how the first continents came to be.
The new study offers evidence that sits uncomfortably with this theory. Dr Johnson and his colleagues looked at the Pilbara craton, an approximately 3.5bn-year-old slab of north-western Australia, roughly the size of Germany, which represents one of the few remaining scraps of the early Earth’s continental crust.
Analysis led the researchers to conclude that the oldest crystals on the Pilbara—those formed around 3.5bn years ago—must have originated close to the surface. This favours impact formation, as zircons transported by mantle plumes would have more profound origins. “Giant impacts plus water is a really good way of making a continent,” says Dr Johnson.
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