Geology: Piecing Continents Together

Were all continents once snuggled together in a mammoth land mass surrounded by a single shimmering sea? Did the continents begin to drift apart some 200 million years ago? Some scientists believe so, and many recent findings support them. This month still more compelling evidence of continental drift was reported by U.S. and Brazilian geologists. Their principal finding was that two highly distinctive adjacent geological areas on the Atlantic coast of Africa match perfectly with a pair of rock regions located along Brazil’s northeast coast.

Doubting & Digging. “Actually, we set out to disprove the theory when we started,” said M.I.T. Geology Professor Patrick M. Hurley, 55, adding that “Harvard and M.I.T. have been hotbeds of geological conservatism for years.” Hurley and his colleagues became interested in the theory at a 1964 scientific meeting in London. There, Cambridge Geophysicist Sir Edward Bullard disclosed that a computer study of shorelines on both sides of the Atlantic —at a depth of 500 fathoms, to allow for coastal idiosyncrasies—showed that they would still match if they were set side by side. “The results were rather amazing,” said Hurley. “The study went right down the whole Atlantic and fitted together everything, including Greenland and all the other islands with less than one degree error in the fit.”

Still leary of the theory, Hurley returned to the U.S. and organized a joint group of U.S. and Brazilian scientists to compare radioactively dated rock samples from two African regions with others from South American areas.

The African regions are divided neatly by a boundary running northeast through Ghana, Togo, Dahomey, Upper Volta, Niger, Mali, and into Algeria (see map). To the east of the boundary lies the Pan-African region, dated as 550 million years old. West of it is the 2-billion-year-old Eburnean area. According to Bullard, if the South American bulge had once fitted under the bulge of Africa, the continuance of the delineation between the two rock regions would be found running southwest through Brazil from a point near the city of Sao Luis 2,070 miles north of Rio.

“There’s never really a eureka moment on any of these projects,” said Hurley, “but when I began to plot these samples, the correlation was astounding. They all fitted exactly.” In addition to the identical ages of the regions, Hurley, his M.I.T. associates and their collaborators at the University of Sao Paulo found the boundary line between the 550-million-and 2-billion-year-old areas in northeast Brazil exactly where they had predicted it would be.

The discovery provides important support for the continental-drift theory. Among other recent evidence is the finding that the ocean floor is patterned with belts of rock magnetized in opposite directions. Recent studies indicate that the earth’s magnetic field has reversed at least nine times in the past 3,600,000 years. Thus the belts provide a dependable time map that shows the effects of the reversing magnetic fields of the earth as the ocean floor expanded. This study also shows that the ocean floor is spreading at about the rate of two centimeters a year—which would just about account for the present distance between the continents if they began drifting apart, as estimated, 200 million years ago.

“To us, this evidence is quite conclusive,” says Hurley. “It’s very difficult to argue against it. It looks as though opposition to the continental-drift theory is dying.”