Asteroid impacts built the continents. That is the argument from Tim Johnson and colleagues at Curtin University in Perth, Australia. Earth is the only known planet with buoyant, silica-rich continents, and geologists have spent decades unable to explain why they began forming roughly four billion years ago, half a billion years after the planet itself coalesced.
The evidence for how this happened is nearly gone. The oldest confirmed continental-type rocks crystallized around 4.03 billion years ago. A few basaltic rocks push that to 4.2 billion years, and rare zircon crystals reach 4.4 billion. Beyond that, the geological record is essentially empty. Johnson's team argues that sustained, intense asteroid bombardment during that missing window kept early crust hot and thin, creating the precise conditions needed to generate the lighter, buoyant material that became continents.
The mechanism matters as much as the conclusion. The full article details how the team models that bombardment process, why existing theories like plate tectonics and mantle plumes fall short as standalone explanations, and what the Hadean eon's rock scarcity tells us about a period that shaped every landmass humans have ever stood on.
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