Around 2.5 billion years ago, something fundamental changed in the way the Earth built its crust, and researchers at Curtin University say they have found the chemical evidence of it: a fingerprint that appears in ancient rocks around the world and then disappears. The finding provides the clearest geologic moment yet for when the processes that shape Earth’s surface began to resemble the plate tectonics seen today, the authors say. A difficult question to answer Plate tectonics shapes continents and oceans and, in deep time, helps regulate Earth’s climate and magnetic field. But scientists have struggled to say when it started, because the process destroys much of its own evidence by recycling crust deep within the planet. The start date is still debated, and other studies of ancient mineral crystals have proposed earlier beginnings for some form of tectonic movement. Searching for a chemical signature The team looked for a signature left when new crust forms by melting older rocks. They looked for it in thousands of rocks around the world, spanning nearly four billion years of Earth’s history, said lead author Janne Liebmann, a researcher in Curtin’s Mineral Systems Time Scales Group. “We find it consistently in rocks over 2.5 billion years old,” he said, showing that they come from a pool of ancient crust that formed about four billion years ago. After 2.5 billion years ago, the fingerprint disappears. “Despite examining thousands of rocks, we never see it again,” Liebmann said. This, he said, points to a basic change in how the Earth’s crust was formed and transformed, towards the dynamic system seen today. From on-site reprocessing to large-scale recycling. Co-author Chris Kirkland, a professor at Curtin’s School of Earth and Planetary Sciences, described the change as a move from a primitive Earth, where ancient crust could be melted and reworked again and again in place, to one capable of moving and recycling crust on a much larger scale. Seeing the old fingerprint disappear at such a specific moment, he said, is what makes the result remarkable. What sets Earth apart The result also helps explain when Earth acquired a feature that distinguishes it from tectonically inactive planets like Venus and Mars. Plate tectonics, Kirkland said, is fundamental to Earth’s evolution, constantly reshaping continents and oceans and influencing the atmosphere and climate. He called the find the clearest geological evidence yet of when the modern Earth’s “tectonic engine” “turned on.” The statement does not say what chemical signature the team used or how many rocks were in the final data set, beyond “thousands.” Liebmann said the fingerprint offers a rare glimpse into how Earth’s first crust evolved. The paper, “Loss of ancient crustal reservoirs marks a shift in Earth’s geodynamic regime,” was published in Proceedings of the Royal Society A.
