04/04/2023
By Brooke Coupal

Earth as we know it today looked drastically different several billion years ago.

According to a paper recently published in the Proceedings of the National Academy of Sciences (PNAS), the composition of the continents resembled the ocean floor, consisting of a dense crust that sits lower on the Earth’s very hot and soft mantle. But about 3 billion years ago, the continental crust transitioned, making the planet more hospitable to life.

“The Earth transitioned to this distinct rock type that gives rise to the continents, so you get a lot more land above sea level, making Earth more habitable to advanced life,” says Environmental, Earth and Atmospheric Sciences Assoc. Prof. Richard Gaschnig, who co-authored the research paper.

The researchers’ findings on the evolution of the Earth’s crust could lead to a better understanding of the formation of exoplanets – planets outside of our solar system. According to NASA, more than 5,000 exoplanets have been discovered in the Milky Way Galaxy since the 1990s.

“We are looking at the Earth back then to figure out how planets operate,” Gaschnig says. “As researchers keep discovering extrasolar planets, we’re trying to find what the process of planetary evolution is going to look like and how that might apply to whether life can develop or not.”

Research Supports Transition of Earth’s Crust

Scientists have debated whether the Earth’s crust went through a major change, and the research behind the PNAS publication shows that it did, says Gaschnig, whose work was funded by a $226,000 National Science Foundation grant.

To find how the continents’ composition changed through time, Gaschnig collected samples of ancient glacial deposits from around the world, including in China, Namibia, South Africa, Bolivia, Canada and the United States. The deposits are the result of ice ages in which massive ice sheets ground up rock material before dumping them in areas later discovered by scientists. Like geological time capsules, the deposits offer a glimpse into the past.

“If you analyze these vast mixtures of sediments that get left behind, you get a general idea of what the composition of Earth’s continental crust was like at that time,” Gaschnig says. “If you do this for a bunch of different periods in its history, you have a time scale to go along with this.”

The researchers measured isotopes of the chemical element vanadium within the samples to determine if the composition resembled the ocean crust or the current continental crust. Their results showed that the continents used to have a crust similar to the ocean’s, known as a mafic crust, before transitioning to a less dense crust, known as a felsic crust, about three billion years ago. The change in Earth’s crust may have been caused by the onset of plate tectonics.

“Today’s crust is broken up into slabs that move around and bump into each other, causing mountains to form, earthquakes and volcanoes,” Gaschnig says. “We’re on the North American plate, and it’s drifting across to the west at about an inch or two a year, but that process might not have been happening at all back then. The Earth might have just been a single-plate planet.”

Gaschnig hopes his future research will continue to uncover the mysteries of the Earth.

“Science is very cumulative,” he says. “You’re always building on your research and everyone else’s work.”