Scientists have proposed a new continental formation theory

A new study by Penn State researchers shows that cratons, the ancient structures that anchor Earth’s continents, formed about 3 billion years ago through processes initiated by atmospheric weathering of rock, not just From the emergence of stable terrain. This challenges conventional views and has implications for understanding planetary evolution and the conditions favorable for life.

Ancient, massive blocks of continental crust known as cratons have stabilized Earth’s continents for billions of years through land changes, mountain building, and ocean growth. Penn State scientists have suggested 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.

Scientists gave this information in the journal Nature Continents would not have emerged from Earth’s oceans as stable landmasses characterized by granite-rich upper crust. Rather, about 3 billion years ago the exposure of fresh rocks to wind and rain 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. Could stay.

Scientists said these findings could represent a new understanding of how potentially habitable, Earth-like planets develop.

Implications for planetary evolution

“To form an Earth-like planet you need to form continental crust, and you need to stabilize that crust,” said study author Jesse Reimink, assistant professor of geology at Penn State. “Scientists have thought of these as the same thing – continents settled and then emerged above sea level. But what we are saying is that those processes are different.

Scientists said cratons extend more than 150 kilometers, or 93 miles, from the Earth’s surface into the upper mantle — where they act like the keel of a boat, keeping continents floating at or near the ocean floor throughout geological time. Live.

Weathering can eventually cause heat-producing elements such as uranium, thorium and potassium to be concentrated in the shallow layer, causing the deeper layer to become cold and hard. Scientists said this mechanism created a thick, rigid layer of rock that would have protected the continents’ bottoms from deforming later on – a distinctive feature of cratons.

Geological processes and heat production

“The method of forming and stabilizing continental crust involves concentrating these heat-producing elements – which can be thought of as tiny heat engines – at the surface,” said study author Andrew Smee, associate professor of geology at Penn State. so close to.” Study. “You have to do that because every time a atom The decay of uranium, thorium or potassium releases heat which can increase the temperature of the crust. Hot crust is unstable – it’s prone to deforming and won’t stick.

As wind, rain and chemical reactions broke down the rocks on the early continents, sediments and clay minerals were swept into rivers and streams and carried to the ocean where they formed sedimentary deposits such as shales that contained high concentrations of uranium, thorium and potassium. Were. Scientists said.

ancient metamorphic rocks called gneisses

These ancient metamorphic rocks, called gneisses, found on the Arctic coast represent the roots of the continents now exposed on the surface. Scientists said the sedimentary rocks underlying these types of rocks would provide a heat engine to stabilize the continents. Credit: Jesse Reimink

The collision between tectonic plates buried these sedimentary rocks deep into the Earth’s crust, where radiogenic heat released by the shale caused the lower layer to melt. The melting was rapid and rushed back up the upper layer, trapping heat-producing elements in rocks like granite and allowing the lower layer to cool and harden.

The craton is thought to have formed between 3 and 2.5 billion years ago – a time when radioactive elements such as uranium decayed at about twice the rate and released twice as much heat as today.

Reimink said the work highlights that the time when cratons formed on early Middle Earth was uniquely favorable for the processes that caused them to become stable.

“We can think of this as a planetary evolution question,” Reimink said. “One of the key ingredients needed to form an Earth-like planet may be the emergence of continents relatively early in its lifetime. Because you’re going to create radioactive sediments that are very hot and that generate a really stable tract of continental crust that sits right around the ocean floor and is a great environment for life to proliferate.

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

“People have looked at and considered the effects of radiogenic heat output changing over time before,” Smee said. “But our study links rock-based heat production to the emergence of continents, the origin of sediments and the differentiation of continental crust.”

Cratons, usually found in the interiors of continents, contain some of the Earth’s oldest rocks, but they 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 craton’s origins remain deep underground and inaccessible. The scientists said future work will include sampling the ancient interiors of Craton and, perhaps, drilling core samples to test their models.

“These metamorphic sedimentary rocks that have melted and produced granite that concentrate uranium and thorium are like black box flight recorders that record pressure and temperature,” Smee said. “And if we can unlock that archive, we can test our model’s predictions for the flight path of the continental crust.”

Reference: Jesse R. Reimink and Andrew J. “Subaerial weathering drove stabilization of continents” by Smee, 8 May 2024, Nature,
DOI: 10.1038/s41586-024-07307-1

Penn State and the US National Science Foundation provided funding for this work.



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