According to the researchers, the continents first sank into the mantle and then “rebelled”.
A new study of the oldest rocks discovered on Earth showed , that the first continents of our planet were unstable. They plunged into the mantle before resurfacing and stabilizing, Science Alert reports.
This may explain some of the most puzzling characteristics of cratons, extremely old and stable parts of the lithosphere. They survived continental changes and contain the “chronicle” of the Earth's ancient history.
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According to scientists from Monash University in Australia, the age of cratons, which are inside the continents, exceeds three billion years. They formed on the early Earth and contain data about how our planet changed over time.
Scientists do not have accurate data about how the Earth's continents formed. There is nothing like it on any other planet. Researchers were able to find evidence that the continents were formed inside the mantle from cratonic cores. But there is no consensus on the formation of cratons.
At the moment, 35 cratons are known, they differ in rigidity and buoyancy compared to other parts of the lithosphere, which gives them stability. But their composition differs from the later parts of the lithosphere, which consist of a set of materials and minerals of different ages. Such heterogeneity implies processing and reconstruction.
In the course of the study, scientists conducted simulations that showed how the evolution of the Earth took place during the first billion years of its existence. It showed that the first continental blocks on the planet were unstable and sinking back into the mantle. There they melted and mixed with the material until they dissolved completely.
Some fragments remained in the mantle for a long time, accumulating layers that gave them buoyancy. This can explain the heterogeneity of the composition of the craton: older rocks from different places are combined with younger rocks. Some of these fragments may still be in the mantle, waiting for the right moment.
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The team of scientists called this mechanism “massive regional relamination” (MRR). According to researchers, it could become a key component in the formation of continents on the early Earth.
