Bryn Mawr geologist’s new theory looks to explain the breakup of the Pangaea supercontinent

Posted September 25th, 2008 at 10:15 am.

This summer Associate Professor of Geology Arlo Weil and his colleagues published a groundbreaking report offering a new explanation of the process by which the supercontinent Pangaea broke apart and ultimately gave shape to many of today’s mountain ranges and other major geographic features.

“Pangaea was this nice stable supercontinent. The question is, ‘Why did it break up into a lot of pieces?’” says Weil.

A map of Pangaea, illustrating the Paleo-Tethys sea (Image: Wikimedia Commons/Kieff)

A map of Pangaea, illustrating the Paleo-Tethys sea (Image: Wikimedia Commons/Kieff)

The key concept is something called “self-subduction.”

As Weil explains it, geologists are pretty much in agreement about how the earth’s geography has changed over the last 200 million years, but consensus starts to break down once you go further back in time.

“The modern sea floor records the drift of continents through time for the past 200 million years, but beyond that it becomes less clear. We lose the oceanic record due to its consumption back into the earth’s mantle at active plate margins,” says Weil.

When talking about the shifting of tectonic plates, geologists usually envision plate boundaries where active crust formation takes place. Where two separate plates meet, one plate is pushed under the other and subducted into the Earth’s mantle in a never-ending cycle.

These processes of crust formation and subduction create volcanic and seisimic activity and drive current plate movement.

“That’s why there’s so much volcanism along the mid-oceanic ridges and in the Andes, the Cascades, Alaska, Japan and down into Southeast Asia,” explains Weil. “Two hundred million years into the future, North America and South America are going to collide with Asia. Australia is moving northward and will sweep up all of Southeast Asia and collide with China and India.”

However, Weil and his colleagues have hypothesized that a somewhat different process—self-subduction—was the primary mechanism that led to the breakup of Pangaea.

Before breaking up some 300 million years ago, Pangaea was a supercontinent made up of most of today’s existing continents. In the center of the supercontinent was a large triangular ocean. At the top of this ocean was Eurasia; at the bottom, India. When self-subduction occurred the ocean began to close up like a Pac-Man closing its mouth. This kind of plate cannibalism ultimately caused plate failure and the formation of new internal plate boundaries.

In addition to creating two plates where there was once only one, self-subduction caused dramatic geologic changes to occur all across the Pangaea supercontinent.

“Think of Pangaea like a pie with a slice taken out of it,” says Weil. “However, when you cut out that piece, all the others move around the center to fill in the gap, and you get compression around the center and extension around the outside.

“The idea for self-subduction was inspired by a lack of consistent and reasonable explanations for a number of geological observations that a lot of my colleagues and I were working on in the core of what was formerly Pangaea,” he adds.

Before Weil and his colleagues proposed this hypothesis, researchers believed the Pangaea supercontinent either collapsed under its own thickness or broke apart due to intense thermal anomalies from the mantle. But these ideas did not explain many of the contemporaneous geologic events that were occurring throughout the globe at the time of Pangaea breakup. None of the preexisting ideas explained all the available geologic data.

Weil joined this research team because of his own interest in trying to uncover the answer to a question few probably ever think to ask: Why are so many mountain ranges curved?

“If you look at the earth today, almost all mountain ranges have some degree of curvature in their map view. The Appalachians bend around; the Himalyas are certainly very bendy; the Alps kind of swing through Europe.”

In trying to unravel the mystery of bending mountain ranges, Weil has spent much of the last 13 years in the Cantabrian-Asturian mountains of northern Spain, which would have been at the core of Pangaea when self-subduction occurred.

“Prior to self-subduction there was a much more linear belt there, but when that plate started scrunching in on itself it caused the belt to bend around and become curved like it is today. This is one of the fundamental manifestations of the self-subduction model,” Weil says.

“What’s more, we can now look at many geological features on the periphery of Pangaea that previously geologists associated with more localized ad hoc phenomena, and put them into a unifying global-scale hypothesis supported by extensive geologic data. It’s somewhat of a fantastical idea but it seems to work” he adds.

Weil and his colleagues are in the process of trying to raise more funds to allow them to continue to test their hypothesis, and he’s planning to return to Spain to continue his research in the summer.

“As with any scientific endeavor, you try to answer one question and you get a million more,” says Weil.

Sixteen Bryn Mawr students and two other faculty members will get a firsthand view of Weil’s active and ongoing research program when he leads a Geology Department field trip to northern Spain over the fall break.

A Google Earth tour of the upcoming trip is available on the Department of Geology’s Web site.

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