Simulations that captured the localized effects of large individual impacts also produced complete recycling of the crust into the mantle, with material dripping to depths of at least 600 kilometers. Johnson believes this recycling explains why so little Hadean crust has survived to the present. It also explains, he maintains, the almost complete 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 dispersed the shock waves before they left lasting deformation in the surviving crystals.
A turning point
The impact flow did not stay high forever; decreased more or less exponentially. Between 3.9 and 3.5 billion years ago, it had diminished enough for internal heat sources to assume 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 that crustal thickness reached about 30 kilometers in the Early Archean, the era that came after the Hadean. This thicker, colder, stiffer crust was eventually able to support plate tectonics as well, and it is around the same time that the first continental rocks appear in the geological record. “As soon as you can create a thick crust and a lithospheric mantle underneath, you can start building continents,” Johnson said.
The team admits that much of the argument is based on physics-based models rather than rock samples. However, in the absence of geological evidence, Johnson believes he is justified in relying on the models. “We need to start taking the results of these models seriously instead of just saying, well, we can’t find any rocks, so let’s give up,” he said. But ancient rocks, however difficult to find, may also appear in the near future: the Earth is extremely good at covering the traces of its history, but it is not perfect.
“In the Nuvvuagittuq Greenstone Belt in Canada, a team of North American researchers recently dated a dark mafic rock to be 4.2 billion years old,” Johnson said. “I also know that another group has found a rock that is possibly even older. I hope you can read about it in the coming months.”
Science, 2026. DOI: 10.1126/science.aeb5402
