The Earth’s hot and largely turbulent interior remains a mystery to scientists. Even at shallow depths, our planet’s crusty outer surface contains many unknowns.
Magma deep in Earth’s mantle that rises to form continental crust Inherently different to the magma seeping from the hills in the middle of the ocean. The magma that erupts from volcanoes on Earth contains less iron, which makes it noticeably lighter. This is the key that allows the Earth to float above the Earth’s mantle, while the oceanic crust forms a lower basin.
However, experts can only speculate as to why this difference exists. Now, a major hypothesis proposing a source for the volcanic magma that formed the continental crusts has come under scrutiny by scientists in the United States.
In 2018, researchers Proposal That the iron-depleted nature of the continental crust stems from the way tectonic plates interact with each other at subduction zones. The authors argued that as one plate slides under the other, pressure crystallizes the silicate material in the rising magma, sucking iron out of the molten rock in the process.
Geologist Elizabeth Cottrell is not convinced.
Very high pressures are needed to remove the iron by crystallization of the silicates, she says, and some parts of the Earth’s crust where iron-deficient magma flows are not that thick.
So the pressure is not too high. Explain Cottrell, who works with the Smithsonian Institution’s National Museum of Natural History.
Simulating the heat and pressure of these subduction zones in the lab, Cottrell and colleague Megan Hollickross, an atmospheric scientist at Cornell University, measured levels of iron and iron oxide in simulations of continental crust.
They used a piston cylinder that could compress small samples of molten rock with extreme pressures and temperatures to match the conditions of magma chambers in the Earth’s crust.
Even with pressure 15,000 to 30,000 times greater than Earth’s atmosphere, crystals of a silicate substance, known as garnet, fail to remove enough iron from rock to account for the unique composition of Earth’s continental crust.
Agate crystal, the authors He says“not likely to be responsible” for the “trend of iron depletion observed in the continental crust”.
“It is likely that conditions in the Earth’s mantle below the continental crust create these oxidative conditions,” he said. He says Cottrell.
“If the agate is not crystallizing in the crust and is something to do with how magma got out of the mantle, what is going on in the mantle?”
Just this year, scientists using seismic waves discovered a new layer of partially molten rock hiding deep in Earth’s interior, which could fundamentally change our understanding of how the mantle belches magma.
One of Cottrell’s colleagues is now investigating whether oxidized sulfur could play a role in removing iron from mantle magma before it erupts.
The puzzle continues.
The study has been published in Sciences.
