Because the Earth is constantly being squeezed by plate tectonics, most of the rocks on the surface are geologically young. At the centers of continents, however, are thick and highly resistant blocks of crust called cratons that have persisted for billions of years. How these cratons formed has long been a mystery.
The prevailing theory links craton formation to a change in plate tectonics: at some point, the planet shifted from one plate to multiple collisions. As the plates moved, the old theory goes, some crustal regions were squeezed and thickened, becoming stronger and more stable.
The authors of a new study propose an entirely different mechanism.
In the study, published NatureThe researchers suggest that the cratons are linked to the weathering of the first landmasses that emerged from the oceans during the Archean Eon between 2.8 and 2.5 billion years ago.
„This watery world with no continents above sea level is Archaean,” he said Jesse Reiming, one of the study’s co-authors. When Reiming and his colleagues wondered what other worlds could exist besides continents, they came up with an answer: “It [wouldn’t] creates sediments,” Reimingk said.
Hot and cold
The researchers proposed a surprising link between surface processes and the formation of stable continental crust. When the first continents formed, rain, wind, and chemical reactions broke down the rocks and formed sediments that formed sediments such as shales that accumulated significant amounts of radioactive elements.
When these sedimentary layers were later buried during subduction, the heat from the radiation melted large volumes of rock into magma. As that buoyant magma rose, it carried the radioactive elements with it and redeposited them in the upper crust.
This process depleted the deep crust of these heat-generating elements, making the rocks there cooler, harder, and more resistant to erosion. According to researchers, these cratonic roots persist as the stable cores of the continents today.
„Melting is really important.”
To test their idea, the scientists assessed the heat-generating capacity of rocks from the Archean. They found that these „hot rocks” melt many rocks remarkably quickly—within tens of thousands of years.
„Melting is really important,” Reiming said. „Once you put the sediments down, it overwhelms the melting very quickly.”
If the theory is correct, rocks from the deep crust would have recorded the pattern of temperature and pressure they experienced over time, which differs from other theories of craton formation. The presence of residual sediment in the deep crust would further support the theory, Reiming added.
„What’s really exciting is that there’s this link between surface processes like weathering and the deep Earth,” Reiming said. This previously unappreciated link highlights how small chemical reactions occurring at Earth’s surface can significantly affect the composition of the deep crust over geologic time.
Colin McDonnellThe Dartmouth College geologist, who was not involved in the new study, said it neatly explains a variety of information in the geologic record about how cratonic crust became tectonically stable continents.
„It’s a good crustal evolution model and certainly a significant piece of work for understanding craton science.”
Other previous hypotheses, incl Heating by mantle plumes, is not necessarily well reflected in the geologic record, McDonnell added. In contrast, this new stabilization theory explains the necessity of crustal heating and partial melting without invoking other, more complex mechanisms. „It’s kind of an Occam’s Razor scenario,” McDonnell said.
„It’s a good crustal evolution model and certainly a significant piece of work for understanding craton science,” he said Jyotirmai Pal, a geophysicist at the University of Oslo, was not part of the new study. However, Ball said, it’s not entirely clear whether this process stabilizes the entire lithosphere or the crust. „If the lithosphere is destroyed or the lithosphere is unstable, we can’t call it a craton,” he added.
The early appearance of terranes may be particularly important to this process because planets contain finite amounts of radioactive elements that decay over time. Reiming pointed out that these elements were more abundant billions of years ago, making the process more effective in the early stages of a planet’s development.
-Javier Barbuzano (@javibarbuzano), science writer