Car mechanic

Mar 04, 2024

Dem10/Jennifer Renteria, Smithsonian.

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Earth, our cherished home in the cosmos, boasts a remarkable feature that sets it apart from its planetary peers—the continents, and their height above sea level. These colossal land masses, teeming with life and diversity, contribute to the unique habitability of our planet. Yet, the origins of Earth's continents, along with their distinct properties, have long puzzled scientists.

NASA

Now, a new study by Elizabeth Cottrell, a research geologist and rock curator at the Smithsonian's National Museum of Natural History, and Megan Holycross, an assistant professor at Cornell University, may have brought us closer to resolving this mystery.

That is, their study—published in Science—challenges and debunks a popular hypothesis that sought to explain why continental crust boasts a lower iron content and is more oxidized when compared to its oceanic counterpart. This is important because the lack of iron is critical to why so much of our planet's surface is above sea level, allowing life to thrive on land.

To truly grasp the impact of this research on our understanding of Earth's continental origins, Interesting Engineering (IE) engaged in an enlightening conversation with Elizabeth Cottrell herself.

"Amongst the rocky planets of the inner solar system, continental crust is unique to planet Earth. Importantly, continental crust has lower concentrations of the element iron relative to ocean crust," Cottrell explained to IE.

She described how this causes the continental crust to be less dense and more buoyant than the ocean crust, such that it does not easily recycle back into Earth's interior as Earth's tectonic plates shift.

"As a result, Earth's continents are ancient—exceeding 4 billion years in age in some places—and very stable relative to the ocean crust, which seldom survives more than 200,000 years before being recycled back into Earth's interior due to its high density," she added.

She also explained that the iron in the continental crust is more likely to be in an oxidized chemical state than the iron in the oceanic crust. Significantly, geologists have long sought to understand how Earth's oxidized and iron-depleted continental crust forms and why it does not form on other planets in our solar system.

"In our study, we have come closer to understanding the viable mechanisms that might operate to deplete the continental crust of iron," Cottrell stated.

"One popular hypothesis suggested that crystallization of the mineral garnet [a group of silicate minerals, and January's birthstone] from molten rock deep beneath Earth's surface might work to extract iron."

She described how removing an iron-rich mineral from molten rock would result in the remaining rock having less iron, making it similar to the crust which forms the continents. This hypothesis was appealing because iron is present in a mineral called garnet in two different forms: one with less oxygen (called "reduced") and one with more oxygen (called "oxidized").

Additionally, she highlighted that if garnet selectively removed the type of iron with less oxygen, it would not only result in rocks with less iron but also rocks with more oxygen, meeting two crucial characteristics of continental crust.

"In our lab, we decided to test this theory. We grew crystals of the mineral garnet from molten rock at high pressures and temperatures in a special device called a piston cylinder press," she said. In this way, the researchers attempted to replicate the intense heat and pressure of the Earth's crust in the lab.

Smithsonian

"Piston-cylinder presses are not actually very high-tech—they operate on the same principle as a car jack... We have some of the same skills as a car mechanic in this regard," she added.

"We then cooled the mixture so rapidly that the chemistry "froze in," generating the mineral garnet surrounded by glass that was formerly molten rock."

"After these relatively low-tech experiments, we performed several very cool high-tech analyses. To quantify the chemical state of the iron in our garnets and glasses (to know whether it was reduced or oxidized), we had to travel to Argonne National Laboratory – home to the Advanced Photon Source (APS)."

She explained that the APS accelerates electrons to almost the speed of light. The electrons zoom around a storage ring that is over a kilometer in circumference, and as the electrons round the bends in the track, they emit synchrotron radiation—ultra-bright X-rays that the researchers could focus on their samples.

"We went to the APS many times to X-ray our samples to quantify the electronic state of iron in the garnets and glasses. Every trip to the synchrotron lasts many days, and every trip is very exciting," she revealed.

"It takes enormous teams of engineers and scientists to keep a facility like the APS operational. We are very lucky to have access to such a phenomenal tool."

"We also measured the concentration of iron in the garnets and glass on an instrument called an electron microprobe at the Smithsonian Institution," she added.

Notably, the analyses revealed that the garnets contained smaller amounts of reduced iron than hypothesized. "This means that crystallization of the mineral garnet from molten rock will not generate the oxidized and iron-depleted chemistry of continental crust," Cottrell reasoned.

G. Macpherson and E. Cottrell, Smithsonian.

When asked about the key moment that made her realize the popular garnet explanation didn't align with the formation of continents, her answer was crystal clear:

"It was when we were at Argonne National Laboratory, and we observed the X-ray spectra of the garnets and glasses we had created in the lab," she explained. "At that moment, we knew that garnet couldn't function as hypothesized in the generation of continental crust."

"Our work is limited by the extent to which we can simulate Earth's interior in the laboratory," Cottrell admitted.

She emphasized that a significant obstacle they encounter is the lack of suitable containers in their laboratory that can withstand the molten rock without causing undesired chemical reactions.

In this regard, they continuously strive to innovate and explore new and imaginative methods for designing experiments. Their goal is to better understand Earth's interior while minimizing any interference or distractions caused by equipment limitations.

"Some members of my team remain focused on problems related to continental crust formation," she said. "If garnet cannot lead to iron depletion and oxidation, what is the mechanism?" The team's future steps will involve conducting experiments and analyzing natural rocks to find this out.