Terrestrial continents are one of the unique characteristics of our planet. In contrast to the oceanic crust, which is only 200 million years old due to its regular recycling, the continental crust is much older and allows us to go far back in geological history. However, due to plate tectonics and erosion, continental rocks often change so that their source is difficult to determine. Marion Boye, a CNRS researcher at the University of Clermont-Auvergne, has got them talking and proposes an entirely new model for the formation of continental masses for 3.7 billion years.
To do this, the geochemist has compiled data on the available continental sedimentary rocks from the 1980s until today. These rocks consist of erosive debris of continental rocks. Marion Boye was interested in the nearly stable isotopes of two chemical elements, samarium (Sm) and neodymium (Nd). “The sedimentary processes do not modify the composition of the rocks for these two isotopes,” she explains, so we can estimate that the isotopic composition of the sediments is identical to that of the continental rocks from which they originate.
The first parameter studied by Marion Bowie is the composition of the continental crust over time, which can be determined using the isotopic ratio. 147poison /144Nd: its value is inversely proportional to the level of silica. According to the results obtained, the continental rocks are rich in silica (SiO2) for at least 3.7 billion years, and rates are still above 60% (66.6% today).
This contrasts with previous models, which estimated that the continents were long ago poor in silica and gradually got richer over time. “My results do not call into question the fact that at the beginning of Earth’s history the crust was poor in silica, Marion Boye identifies, but that was more than 3.7 billion years ago. There was certainly a major shift with greater crust differentiation to become more continents.” Rich in silica, but this is not visible in sediments that are 3.7 billion years old to this day.”
Then the geologist evaluated the growth of the Earth’s masses using the isotopic ratio 143second abbreviation/144second abbreviation. “Crust formation is a mixture of newly produced juvenile crust and ancient crust,” she says. Thanks to this method, we can determine the percentage of juvenile crust over time. By comparing proportions measured in rocks over time with values expected in a continuous growth model of continental crust, I showed that the formation of new continental crust is not a continuous but episodic process, with minus six production peaks for 3.7 billion years, or every 500 to 700 million years.
For Marion Bowie, these discoveries raise questions about models that have so far suggested a major revolution in plate tectonics between 2.5 and 3 billion years ago: “We see no difference in the chemical composition of the rocks. Nor in the episodic mechanisms. Formation. No ruptures. As large as many studies previously suggested.Even if we were to be careful, “there is therefore no reason to believe that the mechanisms of plate tectonics have changed over the past 3.7 billion years.”
The silica content in the continental crust is relatively stable over time (top figure). The orange asterisk indicates the current formation, while the green represents the oceanic crust formation, which is the poorest in silica. Continental growth is not a continuous process and appears to follow a fairly regular cycle (At the bottom).
© M. boy
It now remains to determine the mechanisms underlying the episodes of intense formation of new continental rocks. Typically, these form in subduction zones – areas where oceanic tectonic plates sink into the mantle beneath continental plates. Thus, the rate of formation of new continental crust depends on plate tectonics, because “we observe differences in plate velocity throughout Earth’s history. When plates are moving quickly, we have a lot of subduction and we create a lot of continental crust, and we produce less when they are moving slowly.”
Marion Boye estimates that this discontinuity in the growth of the continents could be related to the cycles of formation and disintegration of supercontinents, which also have a periodicity of close to 500 million years. “With the elements we have today, this is the most likely mechanism. But, she says, my study is about 200 million years accurate, and studies of supercontinental cycles are very imprecise from a temporal point of view. So further studies will be necessary to explore the relationship Causality between these cycles and the formation of a new continental crust.
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