Why are most of the rocks on Earth so much younger than the planet itself?

The rocks on Earth are not all the same age. In fact, most are significantly younger than the planet itself. The oldest sections of the oceanic crust are thought to be 200 million years old, a blink of an eye in the billion years of the planet’s life. What’s going on here?

“Earth is an active planet,” explains Boyet, a geochemist at the University of Clermont Auvergne in France. “This makes it different from other planets in our Solar systemas well as our moon”.

What this means in practice is that our planet’s constantly shifting tectonic plates are constantly recycling rocks. When a oceanic plate meets a continental plate, slides under it into the mantle, a process called subduction, where the ancient stone It is destroyed. Newer rocks are then formed from the melted mantle.

Some very old vein rocks have been discovered, such as the billion-year-old Nuvvuagittuq greenstone belt in Hudson Bay, Canada, as well as similar ancient outcrops in Australia, China, Greenland and South Africa. But even this very old rock has had a complex history. “Exposure to high temperatures during a past collision can change the chemistry,” Boyet notes. “This disturbs the isotopic system we use to date rocks.”

Extraterrestrial help in dating Earth.

Therefore, accurately dating the age of the Earth has been quite a challenge. The original rocks that existed in the early stages of its creation are simply not there anymore. Until the date of the creation of our planet, we have had to look beyond our own world.

“Our moon has no tectonic plates“, Boyet explains. “We can say with certainty that about 80% of the moon’s surface is very old, at least 3 billion years old.”

Radiometric dating is used to confirm the age of rocks by looking at the ratio of two different isotopes. Radioactive isotopes decay in a predictable time, allowing geologists to determine the age of a sample.

Therefore, the dating of lunar rock has given us a clearer idea of ​​the age of our solar system. Another key element has been the radiometric dating of meteorites, which formed in the first tens of millions of years of the formation of the solar system. All of this data, from Earth and beyond, has allowed scientists to estimate the age of Earth to be around 4.5 billion years.

Discovering the evolution of our planet

Still, the lack of 4.5-billion-year-old rocks means scientists still don’t know for sure what Earth was really like, when those clouds of gas and dust condensed to form our planet. This is important, since in order to accurately trace the evolution of the Earth, we need to know what happened during the first few million years.

Boyet sought to address this challenge through the recent ISOREE project. In particular, he analyzed the composition of the chemical element neodymium in primitive meteorites.

“Our conclusion is that the Earth was enriched with neodymium through repeated collisions in the first million years of the solar system, which destroyed up to 20% of the Earth’s mass,” he adds. “We were able to highlight the role of collisions in shaping planets and affecting its composition”.

This research has helped increase our understanding of how Earth and the solar system formed. In the future, Boyet is interested in the possibility of taking rock samples deep within the Earth, beneath large volcanoes such as in Hawaii.

“Maybe we can find rock deposits here that formed early and haven’t mixed all this time,” she says. “Measuring small isotopic variations from these points could give us more information about Earth’s early evolution.”