Craton Enigma: Scientists Propose New Theory of Continental Formation

Fault lines of planet Earth Tectonic plates

A new study by Penn State researchers suggests that cratons, the ancient structures that stabilize Earth’s continents, formed about 3 billion years ago through processes initiated by atmospheric weathering of rocks, not just the formation of stable landmasses. This challenges traditional views and has implications for understanding planetary evolution and the conditions leading to life.

Ancient, vast stretches of continental crust known as cratons have stabilized Earth’s continents for billions of years through land shifts, mountain formations, and ocean development. Penn State researchers 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 geologic history.

Scientists reported about it in the journal Nature that continents may not have emerged from Earth’s oceans as stable landmasses characterized by a granite-enriched upper crust. Rather, the exposure of fresh rock to wind and rain about 3 billion years ago triggered a series of geologic processes that eventually stabilized the crust—allowing the crust to survive for billions of years without being destroyed or reset.

The findings may represent a new understanding of how potentially habitable Earth-like planets develop, the researchers said.

Implications for planetary evolution

“To make a planet like Earth, you have to make continental crust, and you have to stabilize that crust,” said Jesse Reimink, an assistant professor of geosciences at Penn State and an author of the study. “Scientists thought it was the same thing – the continents settled and then rose above sea level.” But we say these processes are separate.”

Cratons extend more than 150 kilometers, or 93 miles, from Earth’s surface to the upper mantle — where they act like the keel of a ship, keeping the continents at or near sea level over geologic time, the researchers said.

Weathering may have eventually concentrated heat-producing elements such as uranium, thorium, and potassium in the shallow crust, allowing the deeper crust to cool and harden. This mechanism created a thick, hard layer of rock that could have protected the continents’ floors from later deformation — a characteristic of cratons, the researchers said.

Geological processes and heat production

“The recipe for producing and stabilizing continental crust involves concentrating these heat-producing elements — which can be thought of as small heat engines — very close to the surface,” said Andrew Smye, associate professor of geosciences at Penn State and author of the book. studies. “You have to do it because every time atom Uranium, thorium, or potassium decays, releasing heat that can raise the temperature of the crust. Hot crust is unstable—it’s prone to warping and won’t stick.”

As wind, rain, and chemical reactions broke up rocks on the early continents, sediments and clay minerals were washed into streams and rivers and carried out to sea, where they formed sedimentary deposits such as shale with high concentrations of uranium, thorium, and potassium. the researchers said.

Ancient metamorphic rocks called gneisses

Found on the Arctic coast, these ancient metamorphic rocks called gneisses represent the roots of the continents now exposed on the surface. Sedimentary rocks interspersed within these rock types would have provided the heat engine to stabilize the continents, the researchers said. Credit: Jesse Reimink

Collisions between tectonic plates buried these sedimentary rocks deep in the Earth’s crust, where radiogenic heat released by the shale triggered melting of the lower crust. The melts floated and rose back up to the upper crust, trapping heat-producing elements in the rocks, such as 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 like uranium would have decayed about twice as fast and released twice as much heat as today.

The work highlights that the time when cratons formed on the early Middle Earth was uniquely suited for processes that could have led to their stabilization, Reimink said.

“We can think of this as a question of planetary evolution,” Reimink said. “One of the key ingredients you need to create a planet like Earth may be the formation of continents relatively early in its life. Because you create radioactive sediments that are very hot and that produce a really stable part of the continental crust that lives right around sea level and is a great environment for life to spread.”

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

“Previously, people have looked at and considered 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 continental crust.”

Cratons, usually found in the interior of continents, contain some of the oldest rocks on Earth, but are still challenging 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 origin of the cratons remains deep underground and is inaccessible. The researchers said future work will include sampling the ancient interiors of the cratons and possibly drilling core samples to test their model.

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

Reference: “Subaerial weather drove continental stabilization” by Jesse R. Reimink and Andrew J. Smye, 08 May 2024, Nature.
DOI: 10.1038/s41586-024-07307-1

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