The continents, the solid blocks of land beneath our feet, weren’t always as strong as they’ve come to be. Now, scientists from Monash University in Australia have devised a new mechanism to explain how the roots of the continents—cratons—came to be. Using numerical models to simulate the conditions of Archean era Earth, the researchers’ findings, published in Nature, show that a strong base for the continents emerged from the melting and stretching of the cratonic lithospheric mantle.
Cratons form the base of continents and hold the title of the oldest existing portion of the lithosphere. They’re extremely thick and began to form up to 3 billion years ago, in the Archean eon. “They’re the secret keepers of the Earth,” said Catherine Cooper, an associate professor of geophysics in the School of the Environment at Washington State University in Pullman. Cooper was not involved in the new research. By studying cratons, scientists might learn how major components of Earth arose and how plate tectonics began. “If you can understand the role of the secret keepers within [Earth], then we can try to answer some of those questions better.”
Scientists can also use this knowledge to study other planets. “Because these processes are the creators of the continents, they are also the processes that create topography, that create an atmosphere,” said Fabio Capitanio, lead author of the new study and an Australian Research Council Future Fellow in Monash University’s School of Earth Atmosphere and Environment. “In principle, they are related to the way we understand life, evolution of planets.”
The Craton Conundrum
The fact that cratons are so thick and enduring poses a problem for scientists. “To make really thick lithosphere requires a good deal of deformation,” said Cooper. “How do we create long-lived, stable features out of material that was once deformable?”
To figure this out, Capitanio and his colleagues turned to numerical models. To simulate the dynamics of the Archean lithosphere, the researchers modeled these layers’ estimated temperatures, pressure, convection, and viscosity, all variables involved in melting rock.
A Surprising Solution
The model revealed a counterintuitive story for craton formation: Parts of the lithospheric mantle became stronger as parts of it were extracted. “The part that is extracted [from the mantle] is essentially melt,” Capitanio said. “Imagine a volcano taking out the lava from the interior of the Earth.” That melt came up through the lithospheric mantle, where it cooled to form crust, leaving behind a portion of mantle devoid of fluids. This process, called dehydration stiffening, left behind a thicker, stronger, and cooling mantle embedded in the lithosphere, forming the roots of the continents.
This residual mantle acts almost like a pin from which the lithosphere stretches laterally, creating new spaces for deformation (melting) and a new zone of stretching. This stretching, or rifting, brings the warmer, deeper material closer to the surface. “In doing so, then you’re having higher temperatures at lower pressures, which then can cause [further] melting to occur,” said Cooper. While the residual portion of the mantle cools, the whole process—dehydration stiffening, rifting, and cooling—repeats in a new section.
“This is a very nice study that unifies many parts of the complicated story of craton formation,” said Lijun Liu, a geodynamicist at the University of Illinois at Urbana-Champaign not involved in the research. “Because it’s a numerical model, it comprehensively brings together many parts [of craton formation] that were hard to reconcile previously.” But, he added, this mechanism doesn’t explain the entire story of cratonic origins.
“It sets the stage for the right material,” said Cooper. But scientists know that cratons are extremely thick, and she said that this mechanism doesn’t fully explain how that happened. “This is a great way to form the material that needs to be thickened later, or further thickened,” said Cooper.
This mechanism aligns with observations of modern cratons. By studying the composition of xenoliths containing pieces of the Archean cratonic lithosphere (brought to Earth’s surface through volcanic activity), scientists can learn about the composition of cratons. The composition also suggests what kinds of conditions might have existed to form that rock, and Capitanio’s mechanism accounts for the pressure and temperature conditions that scientists know are needed to form material from the Archean cratonic lithosphere.
As scientists gain a firmer grasp of the origins of cratons, they’re better able to understand processes that might be happening within other planets as well as the processes that helped form our own. “[Cratons] have kind of gone along for the ride, picking up all of Earth’s secrets for all this time,” said Cooper. “They’re such an intriguing scientific story.”
—Jackie Rocheleau (@JackieRocheleau), Science Writer
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