Scientists have eliminated one possible origin for the Earth’s continents.
Despite the importance of Earth’s continents, and the huge pieces of planet crust that divide its oceans, little is known about what gave rise to these large land masses that make our planet unique in the solar system and play a major role in allowing it to host life.
For years, scientists assumed that the crystallization of opal in magma beneath volcanoes was responsible for removing iron from the Earth’s crust, allowing the crust to stay afloat in the planet’s seas. Now, new research challenges that theory, forcing geologists and planetary scientists to rethink how to remove this iron from the materials that will go on to form the continents we see today on Earth.
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The Earth’s crust, the planet’s outer shell, falls into two rough categories: the older and thicker continental crust; and the younger and denser oceanic crust. New continental crust is formed when its building blocks are passed to the Earth’s surface from continental arc volcanoes. These are found in parts of the globe where oceanic plates sink below continental plates, areas called subduction zones.
The difference between dry continental crusts and oceanic deep sea crusts is the lack of iron in the continental crust. This means that continental crusts are buoyant and rise above sea level, forming the dry land masses that make terrestrial life possible.
The low levels of iron found in the continental crust are hypothesized to result from the crystallization of garnets in the magma beneath these arc volcanoes. This process removes unoxidized iron from the Earth’s plates, while also depleting the iron from the molten magma causing it to become more oxidized as it forms continental crust.
A team of researchers led by Cornell University assistant professor Megan Holy Cross and geoscientist at the Smithsonian National Museum of Natural History Elizabeth Cottrell has improved understanding of continents by setting out to test and debunk this hypothesis first formulated in 2018.
Cottrell said in the book launch (Opens in a new tab)adding that the team was skeptical about crystallization of garnet as an explanation for the buoyancy of continental crust.
Create harsh conditions from the ground in the laboratory
To test the garnet theory, the team recreated the enormous pressure and heat found beneath continental arc volcanoes using piston-cylinder rakes housed in the Smithsonian Museum. High pressure lab (Opens in a new tab) And at Cornell University. Composed of steel and tungsten carbide, these compact presses can apply enormous pressures to tiny rock samples while being simultaneously heated by a surrounding cylindrical furnace.
The pressures generated were 15,000 to 30,000 times those of Earth’s atmosphere, and the temperatures generated were between 1,740 and 2,250 degrees Fahrenheit (950 to 1,230 degrees Celsius), hot enough to melt rock.
In a series of 13 different lab tests conducted by the team, Cottrell and Holicros grew garnet samples of molten rock under pressures and temperatures that simulated conditions inside magma chambers deep in the Earth’s crust.
These lab-grown garnets were analyzed using X-ray absorption spectroscopy which can reveal the composition of the bodies based on how they absorb the X-rays. The results were compared with garnet with known concentrations of oxidized and unoxidized iron.
This revealed that chalcedony growing from rock in subsurface-like conditions did not take up enough unoxidized iron to explain the levels of iron depletion and oxidation seen in the magma that makes up the continental crust.
“These results make the garnet crystal model a very unlikely explanation for why magma from continental volcanoes is oxidized and iron is depleted,” Cottrell said. “It is likely that conditions in the Earth’s mantle below the continental crust create these oxidative conditions.”
The geologist added that what the team’s results cannot currently do is provide an alternative hypothesis to explain the formation of the continental crust, which means that the results ultimately raise more questions than they answer.
“What is the action of an oxidant or depleted iron?” Cottrell asked. “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? How have their compositions been modified?”
These questions are difficult to answer, but Cottrell is currently guiding researchers at the Smithsonian Institution who are studying the idea that oxidized sulfur causes oxidation of iron below the Earth’s surface.
The team’s research was published Thursday (May 4) in the journal Sciences. (Opens in a new tab)