Research Shows U.S., Antarctica Were Neighbors
SAN FRANCISCO — Scientists piecing together the geological history of the Earth have added another important piece to the mosaic: More than half a billion years ago, North America’s Pacific Southwest bordered the east coast of Antarctica.
This represents the first real evidence of the origins of the North American continent, according to scientists who presented their findings Tuesday to the Geological Society of America and the Seismological Society of America. Their evidence indicates that North America was tucked between Antarctica and South America before the supercontinent known as Gondwanaland broke up into the continents we see today.
Scientists have generally agreed for several years that all the continents were clustered around the South Pole until about 550 million years ago. But they have been puzzled over how the pieces fit together, and the evidence presented here could help resolve that complex riddle.
The conclusion is based primarily on the analysis of a billion-year-old rock formation that begins near Quebec in Canada and runs through the Adirondack Mountains, down through Texas and southern Arizona and across northern Mexico. Identical rocks have been found in eastern Antarctica, indicating that they were formed at the same time as those in North America, apparently as great chunks of the Earth’s crust ground together in that tight community of continents.
The rock formation, known as the Grenville Front, later broke apart in a process that launched North America on its northward pilgrimage. Some of the rocks remained in North America, and others in Antarctica, enabling the scientists to conclude that the two continents were once united along the Grenville Front.
Using a computer to reconstruct the world as it existed then, Ian W. D. Dalziel of the University of Texas pushed the continents back together “and lo and behold the Grenville Front forms a continuous feature,†he said. Dalziel, who has made at least 20 expeditions to Antarctica, said the rock formation is the best evidence yet of the ancient history of North America.
“We now have a framework†in which to study the origin of the modern world, he said.
The findings are the latest chapter in a particularly rancorous debate that has raged through the world of geology and geophysics for the past half-century. When proposed, the theory of “continental drift†was considered so unscientific that it drove a bitter wedge into the scientific community.
Over the years, so much evidence has emerged in support of the theory that virtually no expert seriously doubts it today, although it is known now as plate tectonics instead of continental drift.
Scientists generally believe that the modern era began when the giant land mass at the South Pole mysteriously broke up, sending its pieces drifting off across the globe, although not all at the same time. India waited until about 90 million years ago to begin drifting away from Antarctica, and it collided with Asia about 50 million years ago.
The movement of the continents for the past 175 million years can be documented relatively precisely by clues from rocks taken from the ocean floor, according to Dalziel’s partner, Eldridge M. Moores, a geology professor at UC Davis.
“It provides a road map†of the movement of the continents because of such things as sea floor spreading and rift zones that pushed continents apart, Moores said.
But the ocean floor is younger than the continents, and because no rocks from the seabed are more than 175 million years old, the record runs out at that point.
“But 96% of the Earth’s history is prior to that,†Moores said in an interview.
Thus, the best way to reconstruct the ancient history is through what has come to be known as the “ripped newspaper†technique. If you rip a newspaper apart, you can reconstruct it by placing the parts together so that the type can be read from one ripped section to another.
Similarly, scientists look for unusual geological structures that are common to different areas of the globe. An ancient rock formation found on the coast of West Africa several decades ago was matched to a similar structure on a beach in eastern Brazil, convincing scientists that the two continents had once been joined.
Other evidence of moving continents consists of plant and animal life found in regions so widely separated that they could not have migrated, yet so similar that they must have had common ancestors.
Because geologists look to rocks for their evidence, there was much support shown here for two scientists who believe that the marble-like rocks of the Grenville Front reveal much about the orientation of the continents half a billion years ago.
“It all fits together,†Dalziel said.
Dalziel said he does not have to look far from his home in Austin, Tex., to find other examples.
He believes the gray granite rocks used to build the Texas state Capitol were formed near the South Pole long ago as part of the Grenville Front.
The two scientists are planning other trips to Antarctica to bolster their findings.
A CLUSTER OF CONTINENTS
Scientists believe all the world’s continents were clustered together around the South Pole 500 million years ago. New evidence indicates that the Southwestern corner of what is now the United States rested along the eastern edge of Antarctica. The Grenville Front is a billion-year old rock formation found both in that region of Antarctica and across North America, indicating that the rocks were formed when the continents were aligned like this.
The view from the South Pole perspective today.
The current layout of the continents.
Source: University of California, Davis and University of Texas, Austin
