How did Earth's continents form? The leading theory may now be in doubt
The crust of Earth, the planet's outer shell, is divided into two rough categories: The older, thicker continental crust; and the younger, denser oceanic crust. New continental crust forms when its building blocks are passed to Earth's surface from continental arc volcanoes. These are found in parts of the globe where oceanic plates sink beneath continental plates, regions called subduction zones.
The low levels of iron found in continental crust has been hypothesized to be the result of the crystallization of garnet in the magmas beneath these continental arc volcanoes. This process removes non-oxidized iron from the terrestrial plates, while also depleting iron from molten magma thus leaving it more oxidized as it forms continental crust.
"You need high pressures to make garnet stable, and you find this low-iron magma at places where the crust isn't that thick and so the pressure isn't super high," Cottrell said in a, adding that the team was skeptical of the crystallization of garnet as an explanation for the buoyancy of continental crust.
In a series of 13 different lab tests performed by the team, Cottrell and Holycross grew samples of garnet from molten rock under pressures and temperatures mimicking conditions inside magma chambers deep in Earth's crust.These lab-grown garnets were analyzed using X-ray absorption spectroscopy which can reveal the composition of objects based on how they absorb X-rays. The results were compared to garnets with known concentrations of oxidized and unoxidized iron.
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