December 19, 2000
SAN FRANCISCO—Researchers at the University of Michigan and the Geological Survey of Norway say they have solved a longstanding and controversial puzzle over the position of Pangea, the ancient supercontinent that began breaking up some 200 million years ago to form today's continents. They presented their findings Dec. 19 at a meeting of the American Geophysical Union here.
Scientists have long known that the continents are not fixed in place on Earth's surface, but gradually change positions over millions of years. Based on geological evidence, researchers have come up with several models that show how the continents might have fit together when they were tightly clustered. One widely accepted model, dubbed Pangea A and reproduced in countless textbooks, shows what is now South America nestled against the southern edge of North America, with Africa just east of South America, adjacent to the Atlantic coast of North America and southwest of Europe.
But geologists who study paleomagnetic datarecords of Earth's magnetic field captured in rocks over eonshave been troubled by data that just don't fit the Pangea A model. Paleomagnetic data reveal the latitude at which rocks were located when the magnetization was recorded. That information, in turn, provides clues to the positions of the continents.
The problem is that, according to the paleomagnetic data, "the southern continents should be a little bit farther north" than they are in the Pangea A model, explains Rob Van der Voo, a professor of geological sciences at U-M. That dilemma has led to alternative models that place northwestern South America along the east coast of North America or push it even farther east to lie just south of Europe. While the revised models may satisfy researchers who specialize in paleomagnetism, they gall other geologists who find no evidence in fossils or mountain chains to suggest that the continents have ever been in those positions.
Now, Van der Voo and colleague Trond Torsvik of the Geological Survey of Norway have found a way to reconcile the paleomagnetic data with the classical Pangea A model. The key, they say, lies in assumptions about Earth's magnetic field. Scientists generally have assumed the field is like that of a dipole, an object such as a bar magnet, with north and south magnetic poles. That view is not exactly correctthe field does have some non-dipole components todaybut because those components vary from century to century, they have been presumed to cancel out over long spans of time.
But suppose, says Van der Voo, "that the main magnetic field wasn't what we have always assumed as perfectly dipolarthat there was a longstanding non-dipolar field that did not get averaged out." If that were true, positions indicated by paleomagnetic data would be slightly different from those that assume a purely dipolar field. Sure enough, when Van der Voo and Torsvik performed an analysis they found long-term non-dipole fields, and inclusion of these fields produced a near perfect continental fit with the Pangea A model.
"The broader implications of this study," says Van der Voo, "are that paleomagnetic results for other times and other continental configurations must now be re-evaluated with the new geomagnetic field model that should include some 10 percent non-dipole fields, and this will keep us busy for decades."
Contact: Nancy Ross-Flanigan
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