Earth’s continents may owe their existence to bombardment from outer space

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-old formative years. A particular problem is the crucial biographical detail of how the first continents came to be. Studies of ancient rocks suggest that fragments of solid crust existed as far back as 4bn years ago, when was mostly covered in water, but their genesis remains disputed. 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.