The Earth's crust consists of two types: thin oceanic crust, and thick continental crust, without which there would be no plate tectonics. However, this process occurred so long ago that due to the constantly changing surface of our planet, the vast majority of all traces have disappeared. How thick continental crust begins to form has been discussed by many different theories, but these often require some sort of external influence to generate.
the University of British Columbia The researchers reported that Nature Communications Magazine that they found rocks that prove that continents could have formed from oceanic crust during the first hundreds of millions of years of our planet's life without any outside influence.
In order to understand the continental evolution that occurred during the Archaic Era (4-2.5 billion years ago), it is necessary to identify the components of the Earth's crust that existed at that time. The crust that formed at that time was a very distinctive rock composed of three types of components: tertiary tonalite, trondgemite, and granodiorite, and is referred to by experts as just TTG (such rocks are in the opening image). However, the geological processes that led to its creation are very complex, and many things happened between its first melting and final crystallization.
In the context of previous research, the trace element composition of these rocks was examined, in the hope of obtaining information about the magma needed to form the rocks. Canadian researchers have now examined a specific group of these trace elements, which were unaffected by subsequent metamorphism and faithfully preserved signs of the original magma components. With their help, it was possible to decipher the composition of the original melt and where it could come from. These tests indicated a precursor to gabbro (a deep igneous rock).
Gabbro, a type of deep volcanic rock
Source: Wikimedia Commons
TTG rocks created in the Paleozoic can still be found on the continents, for example in Canada, huge areas of ancient continental remains dominated by TTG and slightly younger granite making up the interior of the country.
“All these rocks, especially their groups, are explained by the model we present,” explained Dr. Matthijs Smit, leader of the research. Based on their model, slow burial, melting and thickening of a region of former oceanic plateau-like crust occurred, he said. “The continental crust was forced to evolve in this way as it gradually became more and more buried, so that the only way for the underlying rocks was to melt. In the process, they created TTG rocks, which was the best recipe for the growth and survival of continents.”
Formation of TTG rocks and with them the steps for the birth of continental crust.
Source: Nature Communications
In some areas, the oceanic crust is thicker than the average 6-7 km, and these are oceanic plateaus. However, the volcanic eruptions that occur here and are associated with rising bottom water in the mantle can cause these areas of the crust to thicken significantly, so that they can reach a thickness of 35-40 kilometers typical of continents. However, the several kilometers thick layers of basalt accumulated by volcanic eruptions also create a burden and push this area downward. As a result, the bottom of the protocontinent is able to be pushed to a depth where melting can begin and the magma that forms TTG rocks can be formed, which then comes to the surface with subsequent volcanism.
This process provides an explanation not only for the formation of TTG rocks, but also for those associated with them. Hence the formation and growth of continental germs could take place quite spontaneously, without the need for influences, for example, as previous theories had supposed. In this way, the researchers also proved that TTG rocks are not witnesses to the first subduction zones, that is, their presence does not indicate the beginning of plate tectonics.
