Researchers from Penn State and Columbia University have discovered that Earth's stable continents formed through extreme heat exceeding 900 degrees Celsius in the lower crust. This process involved radioactive elements migrating upward to cool and strengthen the landmasses. The findings also offer insights into mineral distribution and planetary habitability.
For billions of years, Earth's continents have provided a stable foundation for life, but scientists have long puzzled over their endurance. A new study published in Nature Geoscience explains that forming long-lasting continental crust required temperatures over 900 degrees Celsius in the planet's lower crust. At these ultra-high temperatures, radioactive elements such as uranium, thorium, and potassium shifted upward. As they decayed, they generated heat but also carried it away from deeper layers, allowing the lower crust to cool, solidify, and gain strength.
The continental crust as it exists today began forming about 3 billion years ago, evolving from an earlier, less silicon-rich version. The research shows this process demanded temperatures around 200 degrees Celsius hotter than previously estimated. Lead author Andrew Smye, associate professor of geosciences at Penn State, compared it to forging steel: "The metal is heated up until it becomes just soft enough so that it can be shaped mechanically by hammer blows. 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."
To reach these conclusions, the team analyzed hundreds of rock samples from the Alps in Europe and the southwestern United States, focusing on metasedimentary and metaigneous rocks. They compared samples from high-temperature and ultrahigh-temperature conditions, finding that rocks melted above 900°C contained significantly lower amounts of uranium and thorium. Smye noted, "It's rare to see a consistent signal in rocks from so many different places. It's one of those eureka moments that you think 'nature is trying to tell us something here.'"
Earlier in Earth's history, radioactive heat production was about double today's levels, enabling such intense forging. Today, with less heat, stable crust formation would be less likely. Beyond geology, the study aids in locating critical minerals like lithium, tin, and tungsten, redistributed by these processes, which are vital for technologies including smartphones and electric vehicles. Smye explained, "Stable continents are a prerequisite for habitability, but in order for them to gain that stability, they have to cool down." This could also help identify habitable exoplanets with similar heat-driven mechanisms.