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| A new study of the chemical components of rocks led by researchers at Penn State and Columbia University provides the clearest evidence yet for how Earth's continents became and remained so stable — and the key ingredient is heat. Photo Credit: Jaydyn Isiminger / Penn State (CC BY-NC-ND 4.0) |
The new discovery has implications beyond geologic history, such as the search for critical minerals and habitable planets beyond Earth
For billions of years, Earth’s continents have remained remarkably stable, forming the foundation for mountains, ecosystems and civilizations. But the secret to their stability has mystified scientists for more than a century. Now, a new study by researchers at Penn State and Columbia University provides the clearest evidence yet for how the landforms became and remained so stable — and the key ingredient is heat.
In a paper published today (Oct. 13) in the journal Nature Geoscience, the researchers demonstrated that the formation of stable continental crust — the kind that lasts billions of years — required temperatures exceeding 900 degrees Celsius in the planet’s lower continental crust. Such high temperatures, they said, were essential for redistributing radioactive elements like uranium and thorium. The elements generate heat as they decay, so as they moved from the bottom to the top of the crust, they carried heat out with them and allowed the deep crust to cool and strengthen.
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| The researchers analyzed whole-rock chemical data from hundreds of samples of metasedimentary and metaigneous rocks — the types of rocks that make up much of the lower crust — and then categorized the samples by their peak metamorphic temperatures, when rocks undergo physical and chemical changes while remaining mostly solid. Andrew Smye, left, associate professor of geosciences, is pictured analyzing a rock sample with his student research team. Photo Credit: Jaydyn Isiminger / Penn State (CC BY-NC-ND 4.0) |
The implications of the discovery go beyond geology, the researchers said, to open avenues for modern applications such as exploration for critical minerals — which are essential for modern technologies like smartphones, electric vehicles and renewable energy systems — and the search for habitable planets.
The processes that stabilized Earth’s crust also mobilized rare earth elements — lithium, tin and tungsten — providing new clues for where to find them. Those same processes that promoted stability of continental crust are likely to operate on other Earth-like planets, the researchers said, offering planetary scientists new signs to look for life in other worlds.
“Stable continents are a prerequisite for habitability, but in order for them to gain that stability, they have to cool down," said Andrew Smye, associate professor of geosciences at Penn State and lead author on the paper. “In order to cool down, they have to move all these elements that produce heat — uranium, thorium and potassium — towards the surface, because if these elements stay deep, they create heat and melt the crust.”
Continental crust as we know it emerged on Earth around 3 billion years ago, he said. Before this time, the crust had a distinctly different composition than the silicon-rich composition of today’s modern crust. Scientists have long thought that melting of pre-existing crust is an important ingredient of the recipe that produces the stable continental plates that support life. However, before this study, it was not recognized that the crust must reach extreme temperatures to become stable.
“We basically found a new recipe for how to make continents: they need to get much hotter than was previously thought, 200 degrees or so hotter,” Smye said.
Think of forging steel, he said.
“The metal is heated up until it becomes just soft enough so that it can be shaped mechanically by hammer blows,” Smye said. “This process of deforming the metal under extreme temperatures realigns the structure of the metal and removes impurities — both of which strengthen the metal, culminating in the material toughness that defines forged steel. In the same way, tectonic forces applied during the creation of mountain belts forge the continents. We showed that this forging of the crust requires a furnace capable of ultra-high temperatures.”
To make their conclusions, the team sampled rocks from the Alps in Europe and the southwestern United States, as well as examined published data from the scientific literature. They analyzed whole-rock chemical data from hundreds of samples of metasedimentary and metaigneous rocks — the types of rocks that make up much of the lower crust — and then categorized the samples by their peak metamorphic temperatures, when rocks undergo physical and chemical changes while remaining mostly solid.
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| To make their conclusions, the team sampled rocks from the Alps in Europe and the southwestern United States, as well as examined published data from the scientific literature. Here is a chemical analysis performed in Smye's lab at Penn State. Photo Credit: Jaydyn Isiminger / Penn State (CC BY-NC-ND 4.0) |
The researchers distinguished between high-temperature (HT) and ultrahigh-temperature (UHT) conditions. Smye and his co-author, Peter Kelemen, professor of earth and environmental sciences at Columbia University, noticed a striking consistency to the compositions of rocks that had melted at temperatures above 900 C: they had significantly lower concentrations of uranium and thorium compared to those in rocks that had undergone melting at lower temperatures.
“It's rare to see a consistent signal in rocks from so many different places,” he said. “It's one of those eureka moments that you think ‘nature is trying to tell us something here.’”
He explained that melting in most rock types occurs when the temperature gets above 650 C or a little over six times as hot as boiling water. Typically, the further into the crust you go, the temperature increases by about 20 C for every kilometer of depth. Since the base of most stable continental plates is about 30 to 40 kilometers thick, temperatures of 900 C are not typical and required them to rethink the temperature structure.
Smye explained that earlier in Earth’s history, the amount of heat produced from the radioactive elements that made up the crust — uranium, thorium and potassium — was about double what it is today.
“There was more heat available in the system,” he said. “Today, we wouldn't expect as much stable crust to be produced because there's less heat available to forge it.”
He added that understanding how these ultra-high temperature reactions can mobilize elements in the Earth’s crust has wider implications for understanding the distribution and concentration of critical minerals, a highly sought-after group of metals that have proved challenging to mine and locate. If scientists can understand the reactions that first redistributed the valuable elements, theoretically they could better locate new deposits of the materials today.
“If you destabilize the minerals that host uranium, thorium and potassium, you're also releasing a lot of rare earth elements,” he said.
Published in journal: Nature Geoscience
Title: Ultra-hot origins of stable continents
Authors: Andrew J. Smye, and Peter B. Kelemen
Source/Credit: Pennsylvania State University | Marina Naumova
Reference Number: es101325_04
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