How they were formed continents on which we walk today? For decades, geology has debated whether the primitive Earth was dominated by “vertical tectonics” – that is, plumes of heat rising from the mantle without lateral movement – or by plate tectonics similar to modern ones. A new study published in Science Advances provides convincing evidence: the first cries of the continents are linked to subduction processes occurred 3.5 billion years ago, in the Archean eonwhen our planet was surrounded by a huge global ocean and the earth’s crust was mainly composed of basalts.
The secret held in the Pilbara Craton: research data
The research team, led by Maarten de Wit and Michael T. Roberts, analyzed the igneous sequences of the Pilbara Cratonin Western Australia. Here, the focus was on Panorama formation and on Member of Strelley Poolrocks that have preserved a chemical signature dating back toMiddle Archean.
The fundamental technical data concerns the relationship between trace elements such as Th/Yb (Thorium/Ytterbium) e Nb/Yb (Niobium/Ytterbium). In the study, the researchers observed a systematic enrichment of Thorium compared to Niobium. This imbalance is an unequivocal indicator of the presence of fluids derived from the dehydration of a sinking plaque: in practice, it is the “signature” of subduction. While i basalts of the ocean floors (MORB) show constant Nb/Yb ratios, the Pilbara samples exhibit deviations indicating significant crustal input into the underlying mantle as early as 3.5 billion years ago. One of the historical objections to early tectonics was the mantle temperaturewhich in the Archean was about 200-300 °C higher than the current one. This heat was thought to make the plaques too “soft” to sink. However, de Wit and Roberts’ study introduces the concept of shallow subduction (shallow subduction). Geochronological data indicate that the crust did not sink vertically for hundreds of kilometers, but slid at a very shallow angle. This allowed the crust to remain relatively cold and to interact chemically with the overlying mantle wedge. In the study, Roberts and de Wit clearly explain the scope of their data, stating:
Our data suggest that plate interaction and recycling of surface crust into the mantle were already operational during the Middle Archean. This process provided the mechanism needed to generate the dense, stable continental crust that allowed the first continents to emerge.
According to scholars, the evidence of fluids derived from subduction and the chemical composition of igneous rocks indicate that the Earth had already found a way to “cool down” and differentiate itself efficiently as early as a billion years after its birth.
The enrichment of large ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs) observed in our samples are only compatible with magmatic arc processes. This suggests that plate interaction and the recycling of surface crust into the mantle were already operational 3.5 billion years ago.
This process provided the necessary mechanism to generate the TTG-type continental crust (Tonalite-Trondhjemite-Granodiorite), the source rocks that make up the heart of our current continents.
A revolutionary discovery
This research is not just a geological curiosity. There early subduction implies that the Earth began to recycle water and carbon between the atmosphere and the mantle very soon. This “global thermostat” stabilized the climate and created the chemical niches necessary for the evolution of complex life. In conclusion, the Pilbara data tells us that the “Earth machine” has turned on its tectonic engines almost a billion years earlier than many previous models suggested, laying the chemical and physical foundations for the world we know.
