Earth is the only planet we know of with silica-rich continents floating on its surface. But geologists still can’t agree on how they got there.
“The continents started appearing around four billion years ago,” says Tim Johnson, a geologist at Curtin University. “The Earth is four and a half billion years old. Why then? We don’t know.”
Johnson and his team have a new theory: asteroid impacts. Lots of them. A sustained barrage kept the early crust hot and thin enough for buoyant continents to form. The land under your feet exists because space rocks pummeled the planet.
The problem with studying this is most of the evidence is gone. The oldest continental rocks are 4.03 billion years old. A few zircons push back to 4.4 billion. Before that — nothing. Scientists have been guessing.
Two main ideas existed before: plate tectonics was already running, or mantle plumes (like wax in a lava lamp) built the crust. Neither worked with Earth’s heat budget. It was too cold.
“Nobody could make it fit,” Johnson says. “Because we didn’t consider energy from outside Earth.”
That energy came from asteroids. The team used lunar data — the Moon’s crust is covered in craters — to estimate the impact rate on early Earth. Scaling it up: thousands of impactors larger than 10 kilometers across. And they delivered a massive amount of heat.
The models show impact energy was the dominant heat source for most of the Hadean eon. It was ten times stronger than internal heat. The crust was less than 5 kilometers thick, with widespread melting starting just 2-3 kilometers down. Plate tectonics couldn’t function.
As the impact rate dropped between 3.9 and 3.5 billion years ago, the crust cooled and thickened. It reached about 30 kilometers by the early Archean. That’s when the first continental rocks appear.
Johnson acknowledges it’s modeling, not rock samples. But with no rocks to study, it’s what they’ve got. A team in Canada has dated a 4.2 billion-year-old rock recently. More could come. “Hopefully you’ll read about it in the next couple of months,” he says.
