Apollo 8 pilot Bill Anders captured this iconic photo of Earth from orbit around the Moon on Christmas Eve, December 24, 1968. Earth’s continents–unique in the solar system–are visible, rising above the ocean. Credit: NASA
The new experiments raise questions about the common explanation for the properties that give rise to dry land.
Although a crucial factor in making Earth a more hospitable place for life compared to other planets in the solar system, the unique origins and characteristics of the continents, and massive chunks of the planet’s crust, remain largely a mystery.
A recent study by Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian National Museum of Natural History, and Megan Hollickross, Peter Buck Fellow and National Science Foundation Fellow at the museum and now an assistant professor at Cornell University, has advanced our knowledge of the Earth’s crust by testing and disproving a widely held theory related to The lower iron content and higher redox levels of the continental crust compared to the oceanic crust.
The poor composition of iron in the continental crust is a major reason why vast portions of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.
The study recently published in the journal Sciencesuses laboratory experiments to show that the iron-depleting oxidative chemistry typical of Earth’s continental crust likely did not come from the crystallization of the mineral chalcedony, common interpretation proposed in 2018.
The building blocks of new continental crust erupt from deep within the Earth in what are known as continental arc volcanoes, which are found at subduction zones where an oceanic plate sinks beneath a continental plate. In garnet’s explanation of the depleted and oxidized state of iron in the continental crust, the crystallization of garnet in magma beneath these continental volcanoes removes unoxidized iron (reduced or ferric, as it is known among scientists) from the Earth’s plates, depleting iron at the same time. The magma molten iron leaving it more oxidized.
Micrographs from an experiment conducted for this study. The image contains glass (brown), large agate (pink), and other small mineral crystals. The field of view is 410 μm wide, about the size of a sugar crystal. Credit: J. MacPherson and E. Cottrell, Smithsonian
A major consequence of the decrease in iron content in Earth’s continental crust relative to oceanic crust is that it makes the continents less dense and more buoyant, causing continental plates to rise above the planet’s mantle from oceanic plates. This discrepancy in density and buoyancy is a major reason why continents have dry land while oceanic crusts are underwater, and why continental plates always appear on top when they meet oceanic plates at subduction zones.
Garnet’s explanation for iron depletion and oxidation in the magma continental arc was convincing, but Cottrell said one aspect of it just didn’t fit her.
“You need high pressures to make the agate stable, and you find these low-iron magmas in places where the crust is not as thick, so the pressure is not very high,” she said.
In 2018, Cottrell and her colleagues set out to find a way to test whether crystallization of garnets at depth beneath these arc volcanoes was indeed necessary for the formation process of continental crust as understood. To achieve this, Cottrell and Holicros had to find ways to replicate the extreme heat and pressure of the Earth’s crust in the laboratory, then develop techniques sensitive enough to not only measure how much iron is present, but to distinguish between oxidation of that iron.
To recreate the enormous pressure and heat found beneath continental arc volcanoes, the team used so-called piston cylinder presses at the museum’s high-pressure laboratory and at Cornell. The piston of a hydraulic piston cylinder is about the size of a mini-fridge and is made mostly of incredibly thick and strong steel and tungsten carbide. The force applied by a large hydraulic piston results in very high pressures on small rock samples, about a cubic millimeter in size. The assembly consists of electrical and thermal insulators surrounding the rock sample, as well as a cylindrical furnace. The combination of a piston-cylinder press and heating set allows experiments that can reach the very high pressure levels and temperatures found under volcanoes.
Elizabeth Cottrell, research geologist and curator of rocks at the Smithsonian Institution’s National Museum of Natural History, uploads an experiment into her museum lab. Credit: Jennifer Renteria, Smithsonian
In 13 different experiments, Cottrell and Holicros grew garnet samples of molten rock inside a piston cylinder press under pressures and temperatures designed to simulate conditions inside magma chambers deep in the Earth’s crust. The pressures used in the experiments ranged from 1.5 to 3 gigapascals—roughly 15,000 to 30,000 Earth pressures, or 8,000 times greater than the pressure inside a soda can. The temperatures ranged from 950 to 1230 degrees[{” attribute=””>Celsius, which is hot enough to melt rock.
Next, the team collected garnets from Smithsonian’s National Rock Collection and from other researchers around the world. Crucially, this group of garnets had already been analyzed so their concentrations of oxidized and unoxidized iron were known.
Finally, the study authors took the materials from their experiments and those gathered from collections to the Advanced Photon Source at the U.S. Department of Energy’s Argonne National Laboratory in Illinois. There the team used high-energy X-ray beams to conduct X-ray absorption spectroscopy, a technique that can tell scientists about the structure and composition of materials based on how they absorb X-rays. In this case, the researchers were looking into the concentrations of oxidized and unoxidized iron.
The samples with known ratios of oxidized and unoxidized iron provided a way to check and calibrate the team’s X-ray absorption spectroscopy measurements and facilitated a comparison with the materials from their experiments.
The results of these tests revealed that the garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron depletion and oxidation present in the magmas that are the building blocks of Earth’s continental crust.
“These results make the garnet crystallization model an extremely 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.”
Like so many results in science, the findings lead to more questions: “What is doing the oxidizing or iron depleting?” Cottrell asked. “If it’s not garnet crystallization in the crust and it’s something about how the magmas arrive from the mantle, then what is happening in the mantle? How did their compositions get modified?”
Cottrell said that these questions are hard to answer but that now the leading theory is that oxidized sulfur could be oxidizing the iron, something a current Peter Buck Fellow is investigating under her mentorship at the museum.
Reference: “Garnet crystallization does not drive oxidation at arcs” by Megan Holycross and Elizabeth Cottrell, 4 May 2023, Science.
DOI: 10.1126/science.ade3418
This study is an example of the kind of research that museum scientists will tackle under the museum’s new Our Unique Planet initiative, a public–private partnership, which supports research into some of the most enduring and significant questions about what makes Earth special. Other research will investigate the source of Earth’s liquid oceans and how minerals may have served as templates for life.
The study was funded by the Smithsonian, the National Science Foundation, the Department of Energy, and the Lyda Hill Foundation.
