How were the first continents formed? In a previous article, Futura described the debate that is currently animating the scientific community about this question. While several theories exist, the one involving meteorite impacts resurfaced with the publication a few weeks ago of an article in the journal Nature.

Based on the analysis of continental rocks among the oldest existing on Earth, researchers have indeed proposed that the production of continental crust was initiated under the effect of major meteorite impacts on the young Earth then occupied by a huge ocean of magma.

A cycle of continental growth modeled on the duration of a galactic year

A new study completes this hypothesis. By studying the isotopic composition of zircons found in the Greenland and Pilbara cratons (in Australia), the researchers discovered that the period of formation of the first continents ranged from -2.8 to -3.8 billion years ago. . But the growth of the continents would have been intermittent and cyclical. A peak in the production of continental crust is indeed observed approximately every 200 million years. Digging around for orbital data, the researchers discovered that this recurrence oddly matches the transit phases of the Solar System through the four primary spiral arms of our Galaxy. This transit could be like a galactic year, in a way. Its duration is estimated at 225-250 million years.

Indeed, our Solar System as well as the spiral arms of the Milky Way revolve around the center of the galaxy in a huge circular motion, but at different speeds! While the spiral arms orbit at a speed of 210 km/second, the Sun and its cohort of planets, including the Earth, transit at a higher speed of 240 km/second. This shift has a major consequence: the Solar System thus crosses the four galactic arms at regular intervals. However, it is surrounded by a cometary cloud, called the Oort Cloud. For scientists, when the Solar System enters a new galactic arm, it causes a disturbance of the Oort Cloud.

An influx of comets at the time of entry into a new arm of the galaxy

On this occasion, frozen cometary bodies would then be dispersed en masse in the Solar System, leading to an increased risk of impacts with the planets and in particular the Earth. These comets would also arrive with a higher energy than those of the bodies coming from the asteroid belt. On the young Earth, these powerful impacts would thus have had the capacity to produce a greater quantity of magmatic liquid, by brutal decompression of the mantle. These molten rocks, enriched in light elements such as silica, aluminum, sodium and potassium, would then have risen to the surface, forming a kind of proto-crust floating on the surface of the ocean of primitive magma. These embryos of crust would be at the origin of the first continental masses.

This hypothesis deserves further investigation. “We want to make that connection and start the conversation to look at the geological processes that are happening beyond Earth, beyond the Solar System, and what might be driving them. We didn’t just form in isolation. »

The hypothesis shows how much forces external to our planet, and even our solar system, could have influenced the landscape of the earth. However, the idea seems difficult to prove and the number of arguments remains very tenuous for the moment.

Continents also formed because Earth passed through the spiral arms of the Milky Way.

The formation of continents does not depend solely on “terrestrial” phenomena?

This new study strengthens the conceptual basis of a theory concerning the formation of the Earth’s continents. According to this theory, the formation of the continents is not exclusively due to geological phenomena or other terrestrial phenomena, but also to “exogenous” phenomena, ie not exclusively concerning our planet.

The study

The team of researchers, led by Professor Kirkland of the Curtin School of Earth and Planetary Sciences, analyzed various samples of minerals taken from the earth’s crust. The researchers found that the rate of production of the Earth’s crust itself appears to match that of the trajectory of the solar system in areas of the Milky Way.

The passage of the solar system through the spiral arms of the Milky Way

The solar system orbits the center of the Milky Way and passes through its component spiral arms every 200 million years or so, says Kirkland. By analyzing various characteristics of mineral samples taken from a site in Western Australia and another in the North Atlantic, Greenland, the researchers discovered a similar rhythm between the production of the earth’s crust and the periods during which the Earth itself and the entire solar system passed through the most densely populated areas of the stars.

The impacts of comets and asteroids melted the mantle and favored the formation of continents.

Crossing more densely populated areas means increasing the likelihood of body impacts on the surface of our planet, including comets and asteroids. It is precisely the increase in these impacts that would have caused greater melting of the mantle due to decompression, which would have facilitated the formation of continents. Linking the formation of the Earth’s continents to such an “external” factor as the trajectory of the Earth and the solar system in the galaxy upsets previous theories regarding the formation of the continents themselves on our planet, explains the researcher.

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