Text settings Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only Learn more Minimize to nav Earth is the only planet we know of with buoyant, silica-rich continents. But, despite decades of research, geologists still don’t agree on how they formed. “The continents started appearing around about four billion years ago—that’s the oldest continental rock we know about,” said Tim Johnson, a geologist at Curtin University in Perth, Australia. “The Earth is four and a half billion years old, so why they started appearing then is unknown, as is the mechanism to make that continental crust.”
Johnson and his colleagues are now arguing that the formation of continents on Earth was caused largely by an intense, sustained barrage of asteroid impacts that kept the early crust hot and thin enough to make buoyant continents possible. In short, the lands we live on are here because of ancient bombardment from space.
The problem with studying the formation of continents is that the geological evidence of this process is almost gone. The oldest known continental-type rocks crystallized around 4.03 billion years ago, right at the end of the Hadean eon (the earliest era in Earth’s history, spanning the first 500 million years of its existence). Rare basaltic rocks date back about 4.2 billion years, and a handful of the oldest zircon crystals push the record back to 4.4 billion years. Beyond that, there’s hardly anything else. So, scientists looking into the origin of continents had to rely largely on educated guesses. “There are huge debates about what was going on in the early Earth, because the data is so scarce,” Johnson said.
One dominant idea holds that plate tectonics, much like today’s, was already running in the Hadean, with continental crust forming above subduction zones—areas where tectonic plates collide. The other claims that early Earth was too hot for rigid plates, and that crust instead formed above mantle plumes rising from deep within the planet, a phenomenon comparable, Johnson said, to the wax blobs rising inside a lava lamp.
The issue with both these ideas, though, was that Earth, based on most models, appeared too cold for all this to happen. “People have tried to understand Earth’s heat budget through time, and nobody could make it fit,” Johnson said. “Nobody could make it fit because we did not consider the energy coming from outside of Earth.” This energy, he argues, came from asteroid and meteorite impacts that were far more frequent back when the solar system was young. Adding these impacts to the early Earth’s heat budget, though, proved rather challenging because Earth has a peculiar way of healing its scars.
The reason we don’t really know what was happening on Earth four billion years ago is that plate tectonics effectively recycles the surface of the planet back into the mantle. “One place where we do know what was going on back then is the Moon,” Johnson said. “We have sent people there. We have collected sample from there. We have immense amounts of high-quality data from the Moon.” Because the Moon does not have plate tectonics, its crust is a single, solid, continuous shell. And this shell, Johnson’s team noted, is peppered with impact craters.
Calibrated against dated lunar samples, crater counts on the Moon let Johnson’s team estimate how frequently large bodies were hitting our closest celestial neighbor shortly after the Earth had formed. “Scaling that flux up to Earth’s larger size and stronger gravity makes it clear the planet must have been hit by thousands of impactors that were greater than 10 kilometers in diameter,” Johnson said. When his team determined the most probable frequency of impacts and the size of impactors, they could calculate how much energy this immense bombardment delivered to Earth and, consequently, how much heat it produced.
Most prior modeling of early Earth’s heat budget focused on internal sources like heat left over from accretion and core formation plus the ongoing decay of radioactive isotopes—we thought these were absolutely dominant. Johnson’s space bombardment model showed they were not.
The team focused on modeling how the kinetic energy of each impact would ultimately end up as heat. The physics, Johnson said, is straightforward even if the details are complex. “It really is as simple as converting the size and the velocity of the impactor into energy,” he explains. When a large body hits, some of the impact energy goes into vaporizing or melting rock right at the impact site. But, especially when an impactor is big, most of it propagates into the mantle below. “This energy basically heats up the entire upper mantle,” Johnson said.
This heat drives more melting and more basaltic volcanism, a process that plays out not just in the minutes-to-hours timescale of the actual collision, but in tens or even hundreds of millions of years afterward. When Johnson and his colleagues added up these contributions, impact heating exceeded radiogenic and core heat for most of the Hadean by roughly an order of magnitude.
Feeding this reworked heat budget into geodynamic simulations led the team to the conclusion that the Earth’s crust in the Hadean was thin and largely molten underneath. The models suggest it was less than 5 kilometers thick, with widespread partial melting starting just 2 to 3 kilometers below the surface. At around 5 kilometers depth, melt fractions exceeded 30 percent by volume—well past the point where rock can hold together as a coherent slab.
The key takeaway was that plate tectonics could not work in such conditions. “Subduction and plate tectonics require that your lithosphere is rigid and it can jostle around and subduct,” Johnson said. “That’s just not possible if our calculations are anywhere close to the mark.”
The simulations that captured the localized effects of individual large impacts also produced wholesale recycling of crust back into the mantle, with material dripping down to depths of at least 600 kilometers. Johnson thinks this recycling explains why so little Hadean crust survived to the present. It also explains, he argues, the near-total absence of shock-deformed Hadean zircons in the geological record. The researchers suggest that with so much melt present at shallow depths, it would have absorbed and scattered shock waves before they left lasting deformation in surviving crystals.
The impact flux didn’t stay high forever; it declined more or less exponentially. Between 3.9 and 3.5 billion years ago, it had dropped enough that internal heat sources took over as the dominant influence on the crust. As impact heating faded, the upper mantle cooled, and the once-thin basaltic crust thickened.
The team’s modeling suggests crustal thickness reached around 30 kilometers by the early Archean, the era that came after the Hadean. This thicker, cooler, more rigid crust was also finally able to support plate tectonics, and it’s around this same time that the first continental rocks show up in the geological record. “As soon as you can create thick crust and you can create a mantle lithosphere underneath, you can start building continents,” Johnson said.
The team admits much of the argument rests on physics-based modeling rather than rock samples. In the absence of geological evidence, though, Johnson thinks reliance on modeling is justified. “We need to start taking seriously the outputs of these models rather than just say, well, we can’t find any rocks, so let’s give up,” he said. But ancient rocks, as hard to find as they are, may also pop up in near future—the Earth is extremely good at covering the tracks of its history, but it’s not perfect.
“In Nuvvuagittuq Greenstone Belt in Canada, a team of North American researchers has recently dated a dark, mafic rock as 4.2 billion years old,” Johnson said. “I also know another group has found a rock which is possibly even older. Hopefully you will be able to read about it in the next couple of months.”
Science, 2026. DOI: 10.1126/science.aeb5402
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