Ancient large areas of continental crust known as cratons stabilized Earth’s continents for billions of years through land displacement, mountain formation, and ocean development. Scientists from Pennsylvania State University have shed light on a long-standing question in Earth’s geological history by proposing a new mechanism that could explain the formation of cratons approximately 3 billion years ago.

Scientists reported in the journal NatureContinents may not have emerged from Earth’s oceans as stable landmasses characterized by a granite-rich upper crust. Instead, about 3 billion years ago, the impact of wind and rain on fresh rocks triggered a series of geological processes that eventually stabilized the Earth’s crust, allowing the crust to survive intact or rebuilding for billions of years. According to scientists, the data obtained may provide new information about how potentially habitable Earth-like planets evolve.

Implications for planetary evolution

“To create a planet like Earth, you need to create continental crust and stabilize that crust,” said Jesse Reimink, an associate professor of earth sciences at the University of Pennsylvania and author of the study. “Scientists thought it was the same; continents became stationary and then rose above sea level. But we say these processes are different.”

Cratons extend more than 150 kilometers (93 miles) from the Earth’s surface to the upper mantle, where they act like the keel of a boat, holding continents at or near sea level throughout geological time, scientists said.

Weathering can ultimately lead to a concentration of heat-producing elements such as uranium, thorium, and potassium in the shallow crust, causing the deeper crust to cool and harden. Scientists say this mechanism creates a thick, solid layer of rock that could later protect the continent’s floor from deformation. This is a characteristic feature of cratons.

Geological processes and heat production

“The recipe for creating and stabilizing continental crust involves concentrating very close to the surface these heat-producing elements, which can be thought of as small heat engines,” said Andrew Smee, an author and associate professor of earth sciences at the University of Pennsylvania. Of work. “You have to do this because whenever a uranium, thorium or potassium atom decays, it releases heat that can increase the temperature of the Earth’s crust. Hot crust is unstable; it is prone to deformation and does not stick.”

As wind, rain and chemical reactions eroded the rocks on the first continents, sediments and clay minerals were washed into streams and rivers and carried to the sea, where sedimentary deposits such as shale containing high levels of uranium, thorium and potassium were formed. aforementioned.

The collision between tectonic plates buried these sedimentary rocks deep into the earth’s crust; Radiogenic heat released by the shale caused the lower crust to melt. The melts floated and rose into the upper crust, trapping heat-producing elements in rocks like granite and allowing the lower crust to cool and solidify.

Cratons are thought to have formed between 3 and 2.5 billion years ago, at a time when radioactive elements such as uranium were decaying about twice as fast and releasing twice as much heat as today. Reimink said the study highlights that the timing when cratons formed in early Middle Earth was unique in terms of the processes that could cause them to become stable.

“We can think of this as a matter of planetary evolution,” Reimink said. “One of the key ingredients to create a planet like Earth would be for continents to emerge relatively early. Because you would create a very hot radioactive fallout that would create a really stable part of the continental crust right near sea level that would be a great environment for life to reproduce.”

The researchers analyzed uranium, thorium and potassium concentrations in hundreds of rock samples from the Archean period, when cratons formed, to estimate radiogenic heat production based on the actual composition of the rocks. They used these values ​​to create thermal models of craton formation.

“In the past, people have looked and looked at the effects of radiogenic heat production changing over time,” Smay said. said. “But our work links heat production in the rock to the formation of continents, the formation of sediments, and the differentiation of continental crust.”

Cratons, often found in the interior of continents, contain some of the oldest rocks on Earth but remain difficult to study. In tectonically active areas, the formation of a mountain belt can bring to the surface rocks that were once buried deep underground.

But the origins of cratons remain deep underground and inaccessible. Future work will involve taking samples from ancient craton interiors and possibly drilling into core samples to test their models, the scientists said.

“These metamorphosed sedimentary rocks that melt to form granites that concentrate uranium and thorium are like black box loggers recording pressure and temperature,” Smay said. “And if we can unlock this archive, we can test our model’s predictions about the flight path of continental crust.”