Scientists have eliminated one possible origin for Earth’s continents.
Despite the importance of Earth’s continents, the large pieces of the planet’s crust that divide its oceans, little or no is thought about what gave rise to those large landmasses that make our planet unique within the solar system and play a key role in allowing it to host life.
For years, scientists have theorized that the crystallization of garnet in magma beneath volcanoes was liable for removing iron from Earth’s crust, allowing the crust to stay buoyant within the planet’s seas. Now, recent research is difficult that theory, forcing geologists and planetary scientists to rethink how this iron could have been faraway from the fabric that will go on to form the continents we see today on Earth.
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The crust of Earth, the planet’s outer shell, is split into two rough categories: The older, thicker continental crust; and the younger, denser oceanic crust. Latest continental crust forms when its constructing blocks are passed to Earth’s surface from continental arc volcanoes. These are present in parts of the globe where oceanic plates sink beneath continental plates, regions called subduction zones.
The excellence between dry continental crusts and oceanic deep-sea crusts is the shortage of iron within the continental crust. This implies continental crusts are buoyant and rise above sea level to form the dry land masses that make terrestrial life possible.
The low levels of iron present in continental crust has been hypothesized to be the results of the crystallization of garnet within 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 because it forms continental crust.
A team of researchers led by Cornell University assistant Professor Meghan Holycross and Smithsonian National Museum of Natural History geologist Elizabeth Cottrell improved the understanding of the continents by setting about testing and eventually eliminating this hypothesis first formulated in 2018.
“You would like high pressures to make garnet stable, and you discover this low-iron magma at places where the crust is not that thick and so the pressure is not super high,” Cottrell said in a release (opens in recent tab), adding that the team was skeptical of the crystallization of garnet as a proof for the buoyancy of continental crust.
Creating the extraordinary conditions of Earth’s interior within the lab
To check the garnet theory, the team recreated the large pressure and warmth found below continental arc volcanoes using piston-cylinder presses positioned on the Smithsonian Museum’s High-Pressure Laboratory (opens in recent tab) and at Cornell University. These mini-fridge-sized pistons composed of steel and tungsten carbide can induce massive pressures on tiny rock samples while they’re concurrently heated by a surrounding cylindrical furnace.
The pressures induced were similar to 15,000 to 30,000 times that created by Earth’s atmosphere and temperatures generated were between around 1,740 and a pair of,250 degrees Fahrenheit (950 to 1,230 degrees Celsius), hot enough to melt rock.
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 might reveal the composition of objects based on how they absorb X-rays. The outcomes were in comparison with garnets with known concentrations of oxidized and unoxidized iron.
This revealed that the garnets grown from rocks in conditions resembling the inside of Earth didn’t take up enough unoxidized iron to elucidate the degrees of iron depletion and oxidation seen within the magmas that form continental crust.
“These results make the garnet crystallization model an especially unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron-depleted,” Cottrell said. “It’s more likely that conditions in Earth’s mantle below continental crust are setting these oxidized conditions.”
The geologist added that what the team’s results cannot currently do is provide another hypothesis to elucidate the creation of continental crust, meaning the findings ultimately pose more questions than they answer.
“What’s doing the oxidizing or iron depleting?” Cottrell asked. “If it isn’t garnet crystallization within the crust and it’s something about how the magmas arrive from the mantle, then what is going on within the mantle? How did their compositions get modified?”
These questions are difficult to reply, but Cottrell is currently mentoring researchers on the Smithsonian which might be investigating the concept oxidized sulfur is causing the oxidation of iron beneath the Earth’s surface.
The team’s research was published Thursday (May 4) within the journal Science. (opens in recent tab)