A new study by Penn State researchers suggests that cratons, ancient structures that stabilize Earth’s continents, were formed about 3 billion years ago by processes initiated by the atmospheric weathering of rocks, and not just by the emergence of stable land masses . This challenges traditional views and has implications for understanding planetary evolution and the conditions conducive to life.
Ancient, vast stretches of continental crust known as cratons have stabilized Earth’s continents for billions of years through shifts in land masses, mountain building and ocean development. Penn State scientists have proposed a new mechanism that could explain the formation of cratons about 3 billion years ago, shedding light on a long-standing question in Earth’s geological history.
The scientists report this in the magazine Nature that the continents may not have emerged from Earth’s oceans as stable landmasses, the hallmark of which is a granite-enriched upper crust. Rather, the exposure of fresh rock to wind and rain about 3 billion years ago triggered a series of geological processes that ultimately stabilized the crust – allowing the crust to survive for billions of years without being destroyed or reset.
The findings could represent a new understanding of how potentially habitable, Earth-like planets evolve, the scientists said.
Implications for planetary evolution
“To make a planet like Earth, you have to make a continental crust, and stabilize that crust,” said Jesse Reimink, assistant professor of geosciences at Penn State and author of the study. “Scientists have viewed this as the same thing: the continents became stable and then rose above sea level. But what we are saying is that these processes are separate.”
Cratons extend more than 150 kilometers, or 93 miles, from the Earth’s surface to the upper mantle – where they act like the keel of a boat, keeping the continents afloat at or near sea level over geological time, according to the scientists.
Weathering may have eventually concentrated heat-producing elements such as uranium, thorium, and potassium in the shallow crust, cooling and hardening the deeper crust. This mechanism created a thick, hard layer of rock that may have protected the bottoms of the continents from subsequent deformation – a characteristic feature of cratons, the scientists said.
Geological processes and heat production
“The recipe for creating and stabilizing the continental crust involves concentrating these heat-producing elements – which can be thought of as tiny heat engines – very close to the surface,” said Andrew Smye, associate professor of geosciences at Penn State and author of the book. study. “You have to do that, because every time there is one atom When uranium, thorium or potassium decays, heat is released that can increase the temperature of the crust. Hot crust is unstable; it is prone to deformation and does not stick.”
As wind, rain and chemical reactions broke down rocks on the early continents, sediments and clay minerals were washed into streams and rivers and transported to the sea, where they created sedimentary deposits such as shale, high in uranium, thorium and potassium. said the scientists.
These ancient metamorphic rocks called gneiss, found on the Arctic coast, represent the roots of the continents that now lie on the surface. The scientists said that sedimentary rock between layers in these types of rocks would provide a heat engine for stabilizing the continents. Credit: Jesse Reimink
Collisions between tectonic plates buried these sedimentary rocks deep in the Earth’s crust, where radiogenic heat released by the shale caused the melting of the lower crust. The melts were buoyant and rose back to the upper crust, trapping the heat-producing elements there in rocks like granite and allowing the lower crust to cool and harden.
Cratons are thought to have formed between 3 and 2.5 billion years ago – a time when radioactive elements such as uranium would have decayed about twice as fast and released twice as much heat as they do today.
The work highlights that the time when the cratons formed in early Middle-Earth was uniquely suited to the processes that may have led to them becoming stable, Reimink said.
“We can think of this as a planetary evolution question,” Reimink said. “One of the key ingredients needed to create a planet like Earth could be the emergence of continents relatively early in its lifespan. Because you’re going to create radioactive sediments that are very hot and that produce a very stable piece of continental crust that lives right around sea level and is a great environment for the propagation of life.”
The researchers analyzed the uranium, thorium and potassium concentrations of hundreds of samples of rocks from the Archean period, when the cratons formed, to assess radiogenic heat productivity based on actual rock compositions. They used these values to create thermal models of craton formation.
“Previously, people have looked at the effects of changing radiogenic heat production over time,” Smye said. “But our study links rock-based heat production to the formation of continents, the formation of sediments and the differentiation of the continental crust.”
Cratons are typically found in the interiors of continents and contain some of the oldest rocks on Earth, but remain challenging to study. In tectonically active areas, the formation of mountain belts can bring to the surface rocks that were once buried deep underground.
But the origin of the cratons remains deep underground and inaccessible. The scientists said future work will include sampling ancient interiors of cratons and perhaps drilling core samples to test their model.
“These metamorphosed sedimentary rocks that have melted and produced granite that concentrates uranium and thorium are like black box flight recorders that record pressure and temperature,” Smye said. “And if we can unlock that archive, we can test our model’s predictions for the flight path of the continental crust.”
Reference: “Subsurface Weathering Stabilized Continents” by Jesse R. Reimink and Andrew J. Smye, May 8, 2024, Nature.
DOI: 10.1038/s41586-024-07307-1
Penn State and the US National Science Foundation funded this work.