Because Earth is constantly churned by plate tectonics, most rocks on the surface are geologically young. At the cores of continents, however, lie thick and extremely resistant blocks of crust called cratons, which have persisted over billions of years. How these cratons formed is a long-standing mystery.
The prevailing theory links craton formation to a change in plate tectonics: At some point, the planet transitioned from having a single plate to multiple colliding ones. As the plates moved around, the old theory goes, certain crustal areas were squeezed and thickened, becoming stronger and more stable.
The authors of a new study propose a completely different mechanism.
In the study, published in Nature, researchers suggest cratons are linked to weathering of the first landmasses that emerged from the oceans during the Archean eon between 2.8 and 2.5 billion years ago.
“The Archaean was this water world without continents above sea level,” said Jesse Reimink, one of the study coauthors. As Reimink and his colleagues wondered what else—besides continents—would be absent in such a world, they came up with an answer: “It [wouldn’t] produce sediments,” Reimink said.
Hot and Cold
The researchers proposed a surprising link between surface processes and the formation of stable continental crust. As the first continents emerged, rain, wind, and chemical reactions broke down rocks, producing sediments that formed deposits such as shales, which concentrate significant amounts of radioactive elements.
When these sedimentary layers were subsequently buried during subduction, the heat from radioactivity melted large volumes of rock into magma. As that buoyant magma rose, it carried most of the radioactive elements with it, reconcentrating them in the upper crust.
This process depleted the deeper crust of these heat-producing elements, making the rocks there cooler, harder, and more resistant to deformation. According to the researchers, these cratonic roots persist as the stable cores of continents today.
“The melting is really the key.”
To test their idea, the scientists estimated the heat-producing capacity of rocks from the Archean. They found that these “hot rocks” could melt a lot of rock remarkably quickly—within tens of millions of years.
“The melting is really the key,” Reimink said. “As soon as you put sediments down there, it supercharges the melting really quickly.”
If the theory is correct, Reimink said, the rocks from the deep crust would have recorded a pattern of the temperature and pressure they experienced through time that would differ from other theories of craton formation. The presence of residual sediment in the deep crust would further support the theory, Reimink added.
“The really exciting thing is that there’s this link between surface processes like weathering and the deep Earth,” Reimink said. This previously underappreciated link highlights how seemingly minor chemical reactions at Earth’s surface can significantly influence the composition of the deep crust over geologic time.
Kalin McDannell, a geologist at Dartmouth College who wasn’t involved in the new study, said the research elegantly explains various pieces of information in the geologic record related to how cratonic crust became the tectonically stable continents we have today.
“This is a nice crustal evolution model and definitely a significant work to understand the craton science.”
Other previous hypotheses, including heating by mantle plumes, aren’t necessarily well reflected in the geologic record, McDannell added. In contrast, this new stabilization theory explains the need for heating and partial melting of the crust without invoking other, more complex mechanisms. “It’s kind of an Occam’s razor scenario,” McDannell said.
“This is a nice crustal evolution model and definitely a significant work to understand the craton science,” said Jyotirmoy Paul, a geophysicist at the University of Oslo who wasn’t part of the new study. Still, Paul said, it isn’t entirely clear whether this process would stabilize the whole lithosphere or just the crust. “If the lithosphere is destroyed or the lithosphere is not stable, then we cannot call this a craton,” he added.
Early emergence of landmasses might be particularly important to this process because planets have a finite amount of radioactive elements, which decay over time. These elements were much more abundant billions of years ago, Reimink pointed out, potentially making this process more effective in the early stages of a planet’s development.
—Javier Barbuzano (@javibarbuzano), Science Writer
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