Simulations that captured the local effects of individual large impacts also provided a wholesale return of the crust to the mantle, dropping material to depths of at least 600 kilometers. Johnson thinks this recycling explains why so many small Hadean shells have survived to this day. This, he argues, explains the almost complete absence of shock-deformed Hadean zircons in the geologic record. The researchers suggest that while there was plenty of melt at shallow depths, it absorbed and scattered the shock waves before leaving permanent deformation in the surviving crystals.
A turning point
The impact flow did not always remain high; decreased more or less exponentially. Between 3.9 and 3.5 billion years ago, it decreased so much that internal heat sources became the dominant influence on the Earth’s crust. As the heating of the impact subsided, the upper mantle cooled and the once thin basaltic crust thickened.
The team’s modeling suggests that the thickness of the Earth’s crust reached about 30 kilometers in the early Archean, which followed the Hadean. This thicker, cooler, harder crust was also eventually able to support plate tectonics, and around the same time that the first continental rocks appeared in the geologic record. “Once you can create a thick crust and create a mantle lithosphere underneath, you can start building continents,” Johnson said.
The team admits that much of the argument is based on physics-based modeling rather than rock samples. In the absence of geological evidence, Johnson thinks the reliance on modeling is justified. “We have to start taking seriously the results of these models, 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 turn up in the near future—Earth is pretty good at covering up traces of its history, but it’s not perfect.
“A team of North American researchers in the Nuvvagittug Greenstone Belt in Canada recently determined that the dark, mafic rock is 4.2 billion years old,” Johnson said. “I also know that another group has found a possibly even older rock. I hope you can read about it in the next few months.”
Science, 2026. DOI: 10.1126/science.aeb5402
