Earth is a unique planet in more ways than one. One of its peculiarities is the presence of a crustal shell that is divided into two large geological units: oceanic crust and continental crust. Continental formation was also a crucial link in the evolution of life, providing a variety of ecological niches, both in the shallow marine domain bordering coastlines and in the terrestrial domain.
This continental crust differs from oceanic crust in its very rich composition MetalsMetals Silicates. But the processes that led to their emergence from a primordial magma ocean are still a matter of debate. But we know, thanks to the study of cratons, these continental regions that are the oldest on Earth, that this continental crust began to form between 3 and 2.5 billion years ago.
Cratons, these continental cores that allowed the formation of continents
In addition to containing the oldest rocks on Earth, cratons have other characteristics compared to the rest of the continental crust. In these areas, LithosphereLithosphere (cortex and coatcoat upper) is very thick, 150 km or more, while the average value of classical continental lithosphere is 100 km. The granite crust is also rich in certain elements such as gold, PlatinumPlatinum And the LithiumLithiumBut alsoUraniumUranium (sh), ThoriumThorium (w) f PotassiumPotassium (k) It has three components IsotopesIsotopes Radioactive materials whose abundance and activity are sufficient to produce a heatheat Significant radioactivity.
However, we note that these three elements are particularly present in the upper crust and in a much smaller percentage in the lower crust. This differentiation means that the deeper levels of the cratons are rather cold, which has the effect of strengthening the cratonal lithosphere, making it very resistant to tectonic deformation. In fact, cratons appear to be very stable “cores” that have undergone relatively little deformation over geological time.
However, it was this stabilization of the first continents that allowed the growth of continental crust. This stage of crustal differentiation, i.e. the accumulation of elements that produce radiant heat in the upper part, appears to have been a key moment in the formation of the first continents, allowing for their stabilization. How this happened remains poorly understood until now.
What are the mechanisms behind cortex differentiation?
Several hypotheses have been proposed in previous studies to explain the production of this GraniteGranite Neoarchean is rich in uranium, thorium and potassium, from Partial fusionPartial fusion From pre-existing crust. These three elements are actually so-called incompatible elements, that is, they preferentially accumulate in the phase liquidliquid During an episode of partial fusion. Thus, most models relied on heating the base of the crust following different mechanisms (mantle columnmantle columnConvective instability in the mantle…). The creation of this granite significantly preceded the emergence of the first continents. Scenarios that, although they respond to many observations, pose a problem, because they would have led to thermal erosion of the Earth. Cortical rootCortical rootWhich leads to its destruction. In this case, we are far from the expected stabilization effect.
Is rock weathering and plate tectonics the origin of cratons?
In a new article published in review natureJesse Remink and Andrew Smay of the University of Pennsylvania tell a different story. They showed that rock alteration, through the exposure of pre-existing crust, can actually concentrate these heat-producing elements in the upper crust, allowing the formation of a thick, cool root that is very crust-resistant.
From the unstable primary crust, windswindsrain, and Chemical reactionsChemical reactions With ingredientsAtmosphereAtmosphere Hence it was produced SedimentsSediments And the clayclay Which will then be transported by rivers to the sea, where they will have formed sedimentary deposits rich in U, Th and K. MovementsMovements Tectonics then did the rest of the work, burying these “hot” sediments within the crust, especially during periods of collision.
This concentration of radioactive elements may then heat the lower crust, causing partial melting of the rocks. Thus, the produced magma, which is less dense than the surrounding area and rich in the incompatible elements U, Th, and K, has begun to rise toward the upper crust, thus transporting these elements from the base to the top of the crust and working gradually over millions of years, this critical stage of cratonian differentiation. .
Cratons formed about 3 billion years ago, so this new model suggests the emergence of “protocontinents” very early in Earth's history. The researchers also explain that one of the keys to Earth's habitability can also be found in the production and deposition of these deposits rich in radioactive elements that release heat. These processes certainly created environments conducive to the development of life at the edge of these proto-continents.
