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

The rocks on Earth are not all of the same age, and in fact, most of them are much younger than the planet itself.

It is believed that the oldest sections of the oceanic crust, which are 200 million years old, correspond to the billionth age of the planet.

Maud Boyai, a geochemist at the Auvergne University of Clermont in France, explains these differences by saying, “Earth is an active planet, and this distinguishes it from other planets in our solar system, as well as from the Moon.”

In practice, this means that the constant movement of tectonic plates, according to the plate tectonics of our planet, is constantly processing rocks.

When an oceanic plate meets a continental plate, it slides underneath it into the mantle (Earth’s core) – a process called induction, in which ancient rock breaks down and dissolves. Newer rocks then form from the molten mantle magma.

Some very old rock layers have been discovered, such as the billion-year-old Nuvvuagittuk greenstone belt in Canada’s Hudson Bay, as well as similar ancient outcrops in Australia, China, Greenland, and South Africa. But even this very ancient rock has a complicated history.

“Exposure to high temperatures during impact in the past can change the chemical composition. This disrupts the isotopic system that we use to determine the age of rocks, ”Pouyai notes.

Therefore, determining the exact age of the Earth was a difficult task. And the original rocks that were present at the early stages of its creation no longer exist. Before the creation of our planet, we had to look beyond our world.

“There are no plate tectonics on our moon. We can say with certainty that about 80% of the Moon’s surface is very old – at least 3 billion years old,” Pouyai explains.

Radiometric dating is used to confirm the age of rocks from the ratio of two different isotopes. The radioisotope decays over a predictable period of time, allowing geologists to determine the age of the sample.

So lunar rock dating has given us a clearer picture of the age of our solar system. Another key component is the radiometric dating of meteorites formed in the first tens of millions of years of the Solar System’s formation.

And all of this data—from Earth and beyond—allowed scientists to estimate the age of the Earth at about 4.5 billion years.

However, the absence of 4.5 billion-year-old rocks means that scientists still don’t know for sure what the Earth really was like when clouds of gas and dust condensed to form our planet.

This is important because to accurately depict the evolution of the Earth, we need to know what happened during the first few million years.

Puyai aimed to solve this problem with their latest ISOREE project. In particular, she analyzed the composition of the chemical element neodymium in primitive meteorites.

“Our conclusion is that the Earth was enriched in neodymium as a result of multiple collisions during the first million years of the existence of the solar system, which destroyed up to 20% of the mass of the Earth. We were able to shed light on the role of collisions in the formation of planets and the impact on their formation.”

This study has helped us better understand how the Earth and solar system formed. From now on, Puei is interested in the possibility of sampling rocks deep underground, under large volcanoes such as Hawaii. “Perhaps we can find here reservoirs of rocks that were formed earlier and have not been mixed in all this time. Measuring small isotopic differences in these regions can give us more information about the early development of the Earth.”

Source: phys.org.