We might be wrong about where the continents came from, study suggests


Andrew Griffin
Fri, 5 May 2023 

(Getty Images)

The Earth’s continents are part of the reason the Earth is habitable, and what makes it unique among the planets within our solar system. But they still remain largely mysterious, and scientists do not know why the Earth’s surface is divided into those important chunks.

Those continents were able to stand above sea level – itself necessary for terrestrial life – because the continental crust is lower in iron and more oxidised compared with that under the ocean. That means that the continents are less dense and more buoyant, which means they sit higher up.

One story suggests that happens because the crystallisation of the mineral garnet, which was proposed in 2018 and became a popular explanation of the formation of the earth as we know it. In that explanation, garnet crystallises in the magma beneath continental arc volcanos, where an oceanic plate goes beneath a continental one, and that process removes non-oxidised iron.

But the new study eliminates that hypothesis, as well as providing a better understanding of the continents. The team behind it looked for a way to test that hypothesis about the crystallisation of garnet, by replicating a version of that heat and pressure in the lab.

They did so by using “piston-cylinder presses”, which can apply huge amount of forces to tiny samples, as well as using a heating assembly that heats them up at the same time. In 13 different experiments, researchers grew samples of garnet from molten rock in conditions that were similar to those inside Earth’s crust.

Then they gathered samples of garnet from around the world. Those had been analysed so that their makeup was known, and researchers knew how much oxidised and unoxidised iron they had.

The researchers then compared the two, with X-ray beams that were used to understand their composition. They found that the garnets did not take enough unoxidised iron to account for the make-up present in the Earth.

“These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron depleted,” said Elizabeth Cottrell, one of the researchers on the new paper, in a statement. “It’s more likely that conditions in Earth’s mantle below continental crust are setting these oxidized conditions.”

That means that one of the leading theories can now be dismissed. But it is still unknown what exactly is happening, and researchers hope to work to conduct further research to understand what is going on in this mysterious process.

An article describing the findings, ‘Garnet crystallization does not drive oxidation at arcs’, is published in the journal Science.