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Anchoring the Continental Drift Theory

Geologists find a wealth of data in the rocks and fossils of Rhode Island, once part of a chain of volcanic islands in the Southern Hemisphere.

March 16, 2003|Richard Lewis | Associated Press Writer

JAMESTOWN, R.I. — For years, scientists have come to this island finger to study how the Earth's continents were formed hundreds of millions of years ago.

Standing just beyond the spray of waves, Dan Murray and other geologists examine the tissued layers of rocks at Beavertail Point and hunt for fossils in their petrified folds.

Their findings have helped prove how North America, Europe and Africa were once joined, and how their last split created what is now the Atlantic Ocean.

"It's a nice story [of Earth's history]," said Murray, a professor at the University of Rhode Island. "It's like reading the Bible. But where's the meat? The meat is in the rocks."

The folds of the mostly dark-gray sedimentary rocks at Beavertail Point resemble a slipshod pile of flapjacks. Scientists say they were formed by the cataclysmic collision of North America, parts of Europe and northwest Africa about 300 million years ago.

It has taken decades for geologists to establish the link, done by a laborious assembling of rocks' ages, characteristics and fossils, and matching them with similar finds elsewhere.

"You get all these pieces of information," Murray said, "and it says, 'What is the most reasonable, most compelling [explanation]?' and that is you had a collision of continental masses 300 million years ago, and the continents drifted apart."

Fossils found at Beavertail Point show an even older transcontinental link.

"It's like a mini-lab," said James Skehan, founder of Boston College's geology department who has studied rock formations worldwide for more than 50 years.

In fact, the continents are still on the move. North America is pulling away from Europe by about an inch a year, widening the Atlantic Ocean. India is moving under Asia at a pace of 1 to 6 inches per year.

"It's no longer that we're debating continents' movement, but trying to figure out the effects of those movements on the history of life," said Bruce Wardlaw, chief paleontologist at the U.S. Geological Survey.

The oldest rocks at Beavertail Point are about 525 million years old. Back then, scientists believe, Rhode Island was part of a chain of volcanic islands in the Southern Hemisphere.

Pieces of the nation's smallest state were grouped with eastern Canada, New England, South Carolina and patches of Europe when it drew close or became attached to present-day Africa. The island group eventually moved westward and joined North America, a process that took hundreds of millions of years.

Scientists had circumstantial evidence of this chain of events, but the breakthrough came in 1975 when Murray's students from Amherst College in Massachusetts found rocks with the skeletal remains of trilobites, ancient horseshoe crabs that trolled coastal waters.

They confirmed the find with Peter Palmer, a trilobite expert formerly with the U.S. Geological Society of America.

"It was clearly un-American and this just confirmed that," Palmer said. "It was quite a discovery."

Geologists then discovered that the fossils generally matched the species of trilobites embedded in rocks in Spain, Portugal and Morocco.

Scientists knew trilobites were not swimmers. So, finding them in Africa, North America and Europe helped clinch the theory that the continents were at least close enough for the crabs to travel from one to the other. "It was undeniable evidence, a smoking-gun kind of evidence" of a transcontinental link, Murray said.

Scientists now know how North America, Europe and Africa scraped against each other in a series of encounters during the Paleozoic Era about 300 million years ago. The supercontinent that stretched from pole to pole was called Pangaea.

The king of the collisions came during what is called the Alleghenian orogeny, about 275 million years ago. It was so violent it compressed the North American continent like an accordion, creating the Appalachians.

The North American lands heading the collision included much of the United States' Atlantic Coast and Canada's Maritime Provinces. Rhode Island was the "bumper" in the crash, Murray said, its bedrock either shoved into the belly or slid on top of an ancient African mountain range with a height rivaling today's Himalayas.

Scientists believe the ancient continental divide is in the hills of central New England, just east of Springfield, Mass., said Skehan, who's writing a series of books about the rocks of New England.

Geologists have ample evidence of the continental crash at Beavertail Point. The folds of rocks show how crusts of the Earth slid over each other, much like the shuffling of a deck of cards. Similar deposits have been found in New England, eastern Canada, parts of western Europe and western Africa.

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