The formation of continental crust on Earth could be linked to the movements of the Solar System in the Milky Way

How were the first continents formed? In a previous article (read below), Futura described the debate 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 some of the oldest continental rocks that exist on Earthresearchers 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 cratons of Greenland and Pilbara (in Australia), the researchers discovered that the period of formation of the first continents was spread out from -2.8 to -3.8 billion years. But the growth of the continents would have taken place intermittently and cyclically. A production peak of continental crust is indeed observed approximately every 200 million years. Digging into the data side orbitalsthe researchers found that this recurrence strangely corresponds to the phases of transit from Solar system across the four primary spiral arms of our Galaxy. This transit could be like a galactic year, in a way. Her duration is estimated to be 225-250 million years old.

Indeed, our Solar System as well as the spiral arms of the Milky Way revolve around the center of the galaxy in a huge movement circular, but at gears different! As the spiral arms orbit at a speed of 210 km/second, the Sun and his cohort 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, this one is surrounded by a cometary cloud, called Oort Cloud. For scientists, when the Solar System enters a new galactic arm, it causes a disruption 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 in mass 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 energy higher 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 liquid magmatic, by brutal decompression of the coat. These molten rocks, enriched in light elements such as silicaI’aluminumthe sodium and the potassiumwould then rise to the surface, forming a kind of proto-crust floating on the surface of the ocean from magma primitive. These embryos of crust would be at the origin of the first continental masses.

For’astrophysicist Phil Sutton, co-author of the study whose results appeared in the journal Geology, 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.

Giant asteroids at the origin of continents?

The question of the formation of the first continents is still and always strongly debated. In question, the difficulty of finding elements dating back more than 4 billion years. However, a new study has brought up to date the hypothesis of an origin linked to major meteorite impacts.

Article of Morgane Gillard published on August 13, 2022

If today the amount of continental crust remains relatively stable and represents about 30% of the earth’s surface, it has not always been so. Originally, our planet was made up of just one huge ocean of magma. It’s from this ocean of rock in merger that the first continental crust formed, giving rise to the first continents. If we know today that continental growth is mainly associated with the volcanism of the zones of subductionthe mechanisms involved in the formation of the first continental masses are still not clearly determined, the tectonic plates in which the subductions take part being non-existent before 3.8 billion years ago. However, we found some minerals typical of continental crust, zircons, which display a much higher age: more than 4 billion years.

Several theories to explain the formation of the first continental crust

There are thus several theories to explain the formation of the first continents from the ocean of primitive magma. Some scientists suggest that it all starts with training of a proto-crust of composition very different from that of our current continents, but which could have served as a “base” for the generation of the first continental crust. Others involve giant meteorite impacts.

Although proposed for several decades, this second hypothesis had never been clearly supported by elements solid. In new study published in Naturea team of scientists from the University of Curtin (Western Australia) is however bringing this theory up to date by providing new elements.

The first continental rocks formed under the heat of meteorite impacts?

As with other studies of the origin of the first continents, Tim Johnson and his colleagues based themselves on the study of zircons from the Pilbara Craton in Western Australia. Cratons are indeed the oldest regions of the Earth and the most likely to contain traces, although extremely tenuous, of the origin of the first continental crust. After analyzing the chemical composition of zircons, and in particular the proportions of the different isotopes oxygen, the researchers suggest that the first continental rocks would have formed from an episode of surface melting that progressed to the depth, and not the reverse. However, this discovery is in agreement with the effect produced by a major meteorite impact.

The formation of the first continental rocks could therefore have been initiated in the regions impacted by meteorites giants, like the one that caused, billions of years later, the disappearance of the dinosaurs. This type of catastrophic event was far from rare 4 billion years ago. The Earth was then still very intensely bombarded.

The scientists now wish to strengthen their theory by analyzing zircons from other regions of the globe, in order to show that it is indeed a global mechanism and not a local specificity.