New view surfaces of ancient Atlantic.

New view surfaces of ancient Atlantic

Most geologists blieve an ocean similar in size to the North Atlantic occupied that same general area about 500 million years ago. A British geologiest now suggests this ocean looked not like the open body of water that today separates Europe from North America, but instead like a smaller version of the island-dotted sea between Southeast Asia and Australia.

This cluttered sea would have flanked a larger ocean to the south, says Roger Mason of University College in London. Although other geologists accept Mason's geographic analogy, few appear to agree with the small size he proposes for the North Atlantic's ancient predecessor.

Mason's evidence for a smaller northern sea rests on Scandinavian rocks previously identified as ocean-floor fragments. These rocks derive from the bases of volcanic islands formed atop oceanic crust and not from the bottom of a wide ocean, he says. According to plate-tectonic theory the oceanic crustal plate carrying the islands would have disappeared gradually beneath thicker continental crust, eventually rafting the islands into collision with the continent and depositing the once-submerged rocks on what is now the west coast of Norway. In the September GEOLOGY, Mason contends the Norwegian rocks do not possess the full sequence of minerals and mineral-grain sizes associated with deposits in large oceans.

Paleomagnetic data -- which indicate where rocks form in relation to the Earth's magnetic poles -- constitute another pillar of Mason's argument. He admits models based on these data can be risky, but, his report asserts, sediment and fauna patterns along the North Sea and the English Channel also point to the past existence of a smaller sea north of a larger ocean.

Mason's conclusions contradict many models holding that the North Atlantic precursor, often called lapetus, was similar in size to its southern neighbor. These interpretations draw on much the same evidence Mason does, and some explanations include estimated positions of rocks prior to their displacement by interactions between the Earth's drifting crustal plates. Researchers establishing these estimates examine fragments found in a variety of places and conditions, determine the strength and direction of the forces that moved them and create maps locating their probable origins.

One such restoration based on Newfoundland formations shows lapetus covered at least 1,000 miles from east to west, according to Harold Williams of Memorial University of Newfoundland in ST. John's. Another indicator of a wide Iapetus is the continuous coast that stretched from Georgia to Newfoundland from about 570 million until about 480 million years ago, says Dougtas W. Rankin of the U.S. Geological Survey in Reston, Va. But Mason claims this ancient East Coast would have bordered the larger sea to the south, not lapetus.

Rankin and colleague Avery A. Drake agree with Williams that lapetus must have covered more area than Mason suggests. But while Williams says "the modern Atlantic provides a good model for lapetus," Rankin and Drake support Mason's idea that 500 million years ago Iapetus looked more like the island-strewn area now north of Australia. Geologists believe lapetus closed up about 400 million years ago -- 200 million years before the modern Atlantic began to open.

Mason's model indicates that pieces of land now in western Europe migrated into lapetus from the area of ancient Africa, in the same way Asiatic chunks have traveled atop oceanic crust to form much of western North America. This accretion from primeval Africa might help explain inconsistencies in the fauna records of England and northwest France, Mason says.