Forged in fire: The 900°C heat that built Earth’s stable continents

In a new study published in Nature Geoscience, the team found that creating long-lasting continental crust required extreme temperatures -- over 900 degrees Celsius -- in the planet's lower crust. These intense conditions allowed radioactive elements such as uranium and thorium to move upward. As these elements decayed, they produced heat, and by migrating from the deep crust to higher levels, they carried heat away. This process helped the lower crust cool and solidify, ultimately strengthening it.

According to the researchers, the findings extend beyond understanding Earth's geology. They could also aid modern efforts to locate valuable critical minerals, which are essential for technologies like smartphones, electric vehicles, and renewable energy systems, as well as guide the search for potentially habitable planets elsewhere.

The same processes that stabilized Earth's crust also redistributed rare earth elements such as lithium, tin, and tungsten, revealing new clues about where these minerals may be found today. Similar heat-driven mechanisms could occur on other rocky planets, offering planetary scientists additional signs to identify worlds capable of supporting life.

"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."

Smye explained that Earth's continental crust, as it exists today, began forming about 3 billion years ago. Before that, the planet's crust was very different -- lacking the silicon-rich composition of modern continents. Scientists had long suspected that the melting of older crust played an important role in forming stable continental plates, but this study shows that the process required far higher temperatures than previously realized.

"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.

He compared the process to forging steel.

"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 reach their conclusions, the researchers analyzed rock samples from the Alps in Europe and the southwestern United States, along with data from previous scientific studies. They examined chemical information from hundreds of samples of metasedimentary and metaigneous rocks, which form much of the lower crust, and organized them based on their peak metamorphic temperatures -- the highest temperatures reached while the rocks remained mostly solid but underwent physical and chemical changes.

The team compared rocks formed under high-temperature (HT) and ultrahigh-temperature (UHT) conditions. Smye and his co-author, Peter Kelemen, professor of earth and environmental sciences at Columbia University, discovered that rocks that had melted at temperatures above 900 °C consistently contained much lower amounts of uranium and thorium than those formed at cooler conditions.

"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.

The U.S. National Science Foundation funded this research.