Slow-Motion Ripples in Earth’s Mantle Built Mysterious and Stunning Highland Landscapes, Study Finds

Considering how long humans have been on Earth, it’s remarkable how much we’re still learning about how it works. For instance, the theory of plate tectonics—that the planet’s surface is composed of rocky plates that move around over geologic time—wasn’t widely accepted until the mid-1960s.

Now, a new study builds on that theory, suggesting the break-up of an ancient continent sent ripples through Earth’s interior that reverberated for millions of years. These waves created dramatic plateau landscapes in surprising places, like South Africa, Brazil and India, according to the research published last week in the journal Nature.

The edges of tectonic plates are known for the spectacular geologic events that unfold there—the crushing, grinding and drifting forces at these seams create volcanoes, generate earthquakes and form mountains or trenches. Interior regions of the massive plates, meanwhile, are thought to be more stable—but around the world, stunning highlands have formed in these areas.

“Scientists have long suspected that steep kilometer-high topographic features called Great Escarpments—like the classic example encircling South Africa—are formed when continents rift and eventually split apart,” lead author Thomas Gernon, a geologist at the University of Southampton in England, says in a statement. “However, explaining why the inner parts of continents, far from such escarpments, rise and become eroded has proven much more challenging.”

Researchers have now unearthed an explanation for these formations—and the story starts with the break-up of an ancient supercontinent. This rifting warps the continental crust, initiating motion in the Earth’s mantle beneath it.

“It’s like stretching a piece of toffee,” Gernon says to BBC Science Focus’ Tom Howarth. “You get this deformation in the middle where the crust gets thinner. This causes an upwelling of hot material from below.”

After that material rises to the base of the crust, it cools and begins to sink. This convective motion sends out waves through the hot, rocky mantle, which sits below Earth’s brittle top layer. These waves propagate outward in extremely slow motion, at rates of 15 to 20 kilometers per million years.

As they move, the hot mantle rocks strip off the dense bottom layers of the continents, leaving behind lighter material. The newly buoyant continents then rise more than a kilometer above their previous elevation. Over time, erosion from wind and water carves more rock off the highland, making it even lighter and able to rise slightly more.

To reach these conclusions, researchers used a computer model to simulate the mantle waves generated by the break-up of the ancient supercontinent Gondwana roughly 140 million years ago. They then tested the model by looking at topographic maps of highlands around the world to confirm that the locations of inland plateaus matched the model’s predictions.

They also analyzed published geologic data from more than 60 sites across highlands in South Africa and Brazil, which would have been connected to each other before the supercontinent split apart. These data tracked the how the rocks cooled over millions of years. The moment of fastest cooling—corresponding to maximal uplift of the highlands—paralleled the model’s projected movement of the mantle waves.

“When I come up with a model, I like to try to kill the model,” Gernon tells Science’s Hannah Richter. This model, though, is “still alive.”

Future research might study break-ups of other land masses to see if the findings apply more widely, he adds.

Steve Jones, a geologist at the University of Birmingham in England and a co-author of the study, says these mantle waves could be important in the development of many more parts of Earth’s history than just scenic highlands. “What we have here is a compelling argument that rifting can, in certain circumstances, directly generate long-lived continental scale upper mantle convection cells,” he says in the statement, “and these rift-initiated convective systems have a profound effect on Earth’s surface topography, erosion, sedimentation and the distribution of natural resources.”

Gernon adds that “destabilizing the cores of the continents must have impacted ancient climates too.”

Other geologists are excited by the findings. Scott King, a geophysicist at Virginia Tech who wasn’t involved in the new research, tells Science that researchers sometimes overstate the importance of their results. But not this team, he says. “I think in this case, they may not have made as big a to-do … as they could have.”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.