Supercontinent Pangaea Pushed, Not Sucked, Into Place
Richard A. Lovett
for National Geographic News
|September 5, 2008|
Supercontinents can form when a huge plume of hot rock from deep inside Earth wells up between the continental plates, pushing them apart until all Earth's landmasses collide.
This is the finding from a new study that suggests—contrary to accepted theory—that such a process formed the supercontinent Pangaea 300 million years ago. Today's continents are thought to have formed from Pangaea's gradual breakup.
Earth's shifting plates have been forming and breaking up supercontinents for billions of years, scientists believe, and traditionally they thought that suction is the driving force. (See an interactive map of Earth's tectonic plates.)
In seismically active places such as the Ring of Fire in the Pacific region, slabs of Earth's crust descend into the interior in a process called subduction.
This creates a downward current that sucks the continents into collision above it, like soapsuds being drawn together as water flows down a drain.
But in the new paper, J. Brendan Murphy of St. Francis Xavier University in Nova Scotia, Canada, and colleagues suggest that a plume of subducting crust in the middle of the ancestral Pacific Ocean descended so deeply it hit Earth's core.
Superheated, it then rebounded like a bubble in a boiling pot. That produced a superplume strong enough to push, not suck, the ancient continents back together and form Pangaea.
Peter Cawood, director of the Tectonics Special Research Centre at the University of Western Australia, called the finding "excellent and stimulating."
"Although provocative, I believe it is probably right," Cawood said by email.
Matching up rock types, examining magnetic signatures, and cataloging fossils have allowed scientists to trace the cycle of supercontinents forming and breaking apart back at least a billion years. Earth is believed to be about 4.5 billion years old.
The data show there were two supercontinents before Pangaea called Rodinia and Gondwana that formed and broke up hundreds of millions of years ago.
(The older supercontinent called Gondwana is different from the southern landmass known as Gondwanaland that formed 200 million years ago as Pangaea split apart.)
Wouter Bleeker, of the Geological Survey of Canada, has dubbed this cycle "the pulse of the Earth."
Murphy agrees. "Most people believe that for at least the last two and a half billion years, the Earth's history has been dominated by the amalgamation, breakup, and reforming of supercontinents," he said.
"It really is an underpinning of the evolution of the planet."
But according to Murphy's new study, published this month in the journal Geology, there's something wrong with the suction-driven model in the case of Pangaea.
The problem, he said, is that it's very clear from the geologic record that the formation of Pangaea from the fragments of Gondwana occurred in two stages.
First Gondwana split, producing a steadily widening "young" ocean in its heart, much like the Atlantic Ocean that now separates the fragments of Pangaea.
Then something shifted. Rather than continuing to widen, as it would if its motion was being driven by suction, the new ocean started to shrink. The continents reversed course and slammed back into each other to form Pangaea.
This accordionlike action, dubbed the Wilson Cycle, has been recognized for more than 40 years, but the forces responsible for it are unknown.
Moreover, if current models thought to be responsible for these movements were applied to a 500-million-year-old Earth, they would not produce Pangaea in the right configuration.
Why this reversal happened is unclear, and that's disconcerting, Murphy said, because even though Pangaea is the best studied of the supercontinents, "something happened that we don't understand."
(Related: "Ancient Imbalances Sent Earth's Continents 'Wandering'" [April 7, 2008].)
The new theory of a superplume interfering with the suction process could put the pieces of what occurred into place, although more data would be needed to cement the idea.
"It's speculative," Murphy said. "What we'd like people to get out of the article is that there's a fundamental problem in understanding how Pangaea formed."
Murphy added that his theory could have implications for the long-term future of the planet.
Right now the continents are converging on the mid-Pacific Ocean, where, if present motions continue, they will collide into a new supercontinent in about 75 to 80 million years.
But if Murphy's study is right, the process could reverse. North America could be driven back toward Europe, as happened in the formation of Pangaea.
Such a scenario, once thought unlikely, makes Earth's future a lot more fun to study, Murphy added.
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