Wandering Continents

Article excerpt

Wandering continents

ANYONE who has looked at a world map must have noticed how snugly the coastlines of Africa and the Americas could be made to fit together, if the intervening ocean were removed. Modern geophysics has established that all of the Earth's landmasses were indeed joined together in one supercontinent, Pangaea, hundreds of millions of years ago, and that this supercontinent was broken apart, with the land masses drifting to their present positions on the globe.

This idea took many years to become established. Speculations about the fit of the continents go back to Francis Bacon (1561-1626), but the acknowledged "father' of the idea of continental drift was the German astronomer and meteorologist Alfred Wegener, who published the first comprehensive statement of the theory in 1912. Wegener thought that the continents might move through the thinner crust of the ocean floor, like icebergs ploughing through the sea, and he gathered a wealth of evidence showing how well the continents could be fitted together like some global jigsaw puzzle. But the idea of continents moving through the rocks of the sea floor did not seem feasible, and found little favour until the 1950s, when the development of new geological techniques provided conclusive evidence that the continents do move.

The key evidence came from magnetic studies of the ocean floors. These showed that the crust of the Atlantic Ocean floor is arranged symmetrically on either side of a great ridge of volcanic activity which runs roughly down the centre of the ocean bed. The interpretation of this discovery is that new oceanic crust is being created at the mid-ocean ridge, where it wells up through a crack in the Earth's crust and is pushing out on either side, steadily widening the Atlantic.

In other parts of the world the reverse happens. The North Pacific, for example, has no oceanic ridge, but there is a deep trench running down the west of the ocean floor, next to the Eurasian landmass. There the thin crust of the ocean floor is being pushed under the continent, back down into the mantle where it melts and is ultimately recycled. The net effect is that there is no change in the surface area of the Earth--spreading in the Atlantic and at other sites is balanced by contraction of the Pacific. …