A study published in Nature on April 2 upended conventional views about Earth’s earliest geological evolution. The research revealed that Earth’s first crust, formed about 4.5 billion years ago, likely had chemical features similar to those of today’s continental crust. The finding challenged widely held beliefs regarding the formation of continents and the onset of plate tectonics, and it offered evidence that much of the early crust was extracted from a core‐depleted mantle.
“Scientists have long thought that tectonic plates needed to dive beneath each other to create the chemical fingerprint we see in continents,” said Simon Turner, professor emeritus at Macquarie University’s Faculty of Science and Engineering, according to a report by astrobiology.com. “This discovery has major implications for how we think about Earth’s earliest history,” he added, according to a report by astrobiology.com.
The study focused on the protocrust, Earth’s earliest crust, which formed during the Hadean eon, about 4.5 to 4.0 billion years ago. Data from zircon minerals—dated to 4.38 billion years—in Jack Hills metasediments demonstrated that evolved rocks with a chemical profile similar to today’s continental crust existed nearly from Earth’s inception. The evidence suggested that the protocrust naturally developed these chemical characteristics without modern-style plate tectonics.
Mathematical models developed in collaboration with researchers from Australia, the United Kingdom, and France simulated early Earth conditions when the planet’s core was still forming and its surface was covered by an ocean of molten rock. The models indicated that the chemical signature of the protocrust matched that of today’s continental crust and suggested a global crust thickness of about 15 km—equivalent in chemical character to a 35‑km-thick continental crust when scaled to the present surface area.
Under the reducing conditions of Earth’s early magma ocean, elements such as niobium behaved differently. Niobium became siderophilic and sank into Earth’s core, producing the negative niobium anomaly observed in continental rocks. “I realized there might be a connection between early core formation, high siderophile element patterns, and the negative niobium anomaly observed in continental crust,” said Turner.
The research questioned the onset and workings of plate tectonics. Previous models assumed that only subduction—where one tectonic plate dives beneath another—could produce the chemical fingerprint found in continents. The new model demonstrated that Earth’s early crust fractured, with portions thickening into the precursors of continents while the molten magma between these fragments generated crust resembling the basaltic composition of modern ocean floors.
The study noted that major meteor impacts continued until about 3.8 billion years ago. During that period, plate tectonics may have operated intermittently, triggered by impacts that disrupted and recycled the early crust. As the rate of meteor bombardment decreased with the stabilization of solar orbits, a continuous pattern of plate tectonics eventually emerged. “Our research shows that the chemical signatures we see in continental crust were created in Earth’s earliest period—regardless of how the planet’s surface was behaving,” said Turner.
The article was written with the assistance of a news analysis system.