If the history of the Earth is told to us by the geological archives contained in the continents, there is, however, a gray zone, which corresponds to the moment of the birth of the first continental masses, more than 3 billion years ago. A new study sheds light on the processes that would have been in play at the time.
The continents are an essential element to our understanding of the Earth system. Their durability and their relative stability over time have allowed them to record almost all the episodes that mark the evolution of the Earth: meteorite falls, volcanism, the appearance and evolution of life and, of course, tectonics and plate movement. From this point of view, the continents differ greatly from the oceanic crust, the other component of the earth’s crust. Being rapidly recycled in the mantle, the latter is not perennial and therefore cannot store geological events of more than 200 million years.
The continents, archives of the history of the Earth since its formation… or almost
The continents they are, therefore, the only archives that go through the history of the Earth in detail and that allow us to go back to the formation of our Planet, or almost. Because there is indeed a gray area, which is located at the time of the birth of the first continental crust. If we know today that the first continents appeared relatively soon after the formation of the Earth, the witnesses of this birth are extremely rare and tenuous, leaving room for many uncertainties, either in terms of dating or process. Therefore, today there are several theories to explain when and how the first continents were built. of the primitive magma ocean and how they evolved during the early stages of their existence.
Today, the oldest continental regions found on Earth are called “cratons.” The cratons, however, do not represent intact samples of the first continents, but rather contain fragments, in the form of tiny grains: zircons. The study of these grains teaches us that the first continental rocks formed less than a billion years after the formation of the Earth.
Of the first continents unable to float and sinking into the mantle
The geochemical signature of the mantle located directly under the cratons (we speak of the continental lithospheric mantle) also gives some clues as to how the first continents were formed and shows that the mantle rocks have played a large part in the construction and evolution of the Earth. Cortex. It also seems that the root of the mantle of the cratons is complex: it would be a very heterogeneous mixture of materials of different ages, compositions and origins. These observations suggest that the first continents would have been rapidly remodeled and partially recycled.
In a new study published in PNAS, a group of scientists has managed to detail the series of events that occurred to give rise to the first continents. This day, the low density of continental rocks in relation to the underlying mantle it allows the continental masses to “float” on the surface. But the results of the study, obtained through numerical models, suggest that the first continental blocks, resulting from partial mantle melting, were particularly unstable and would have quickly sunk into the mantle. There, they would have been recast, the products of this fusion then mixing with the mantle material until they disappeared, but not completely. Because it seems that certain fragments of continental blocks could have remained for a certain period of time before arising.
Formation of a rigid sole that allows the construction of new continents
These fragments would thus have accumulated under the new continental lithosphere in formation, ensuring its buoyancy and rigidity. Around these stable “nuclei” the definitive continents would then have been built. This mechanism, called “massive regional relamination”, would also explain well the strong heterogeneity in age and composition of the current cratonic roots. It would have participated and would still participate today, although to a lesser extent, in the stratification of the mantle and in the formation of the superficial mantle.