The Fit of the Coastlines
Look at a map of the Atlantic and a coincidence is hard to miss: the eastern bulge of South America seems made to nest into the western hollow of Africa, like two pieces of a single torn sheet. Others had remarked on the resemblance before him, but in 1912 the German meteorologist Alfred Wegener turned the observation into a theory. The continents, he argued, had once been joined in a single great landmass, which he called Pangaea, and had since drifted slowly apart. He named the idea continental drift.
Wegener did not rest his case on the shape of the coastlines alone. He gathered evidence of a kind that was difficult to explain any other way. Identical fossils of land-dwelling creatures, such as the reptile Mesosaurus and the fern Glossopteris, turned up on continents now separated by thousands of miles of ocean that the animals could not have crossed. Bands of rock and ranges of mountains ended abruptly at one coast and resumed, matching, on another. Glacial deposits lay in regions that are now tropical, and coal, which forms in warm swamps, lay beneath the polar ice. Each oddity dissolved if the continents had once sat together and drifted to their present places.
The objection that sank the theory was not about this evidence, which few disputed, but about cause. Wegener could not say what force moved a continent. The mechanisms he proposed, a drift driven by the tides and a 'pole-fleeing' force, were calculated by physicists to be orders of magnitude too weak to push a continent through the solid rock of the ocean floor. To the geophysicists of the day, the prominent Harold Jeffreys among them, a process with no plausible engine was no process at all, however suggestive the fossils. The evidence was real; the explanation was missing.
So the theory was set aside for the better part of fifty years. It did not help that Wegener was an outsider to geology, a meteorologist proposing to overturn the earth sciences from beyond their walls, and his idea was treated in many quarters with open ridicule. He died in 1930 on a research expedition across the Greenland ice, his theory still in disrepute. The matching fossils and coastlines did not go away in those decades; they were simply filed under puzzles to be explained some other way, because the one explanation on offer had no mechanism behind it.
The break came not from new fossils but from the floor of the sea. As naval surveys after mid-century mapped the ocean bed, they revealed a continuous chain of submarine mountains, the mid-ocean ridges, running down the middle of the oceans. In the early 1960s the American geologist Harry Hess proposed what these ridges were doing. New ocean crust, he suggested, is created at the ridges from molten rock rising from the mantle, and spreads slowly outward to either side, a process he called seafloor spreading. The continents did not have to plough through the ocean floor after all; they rode upon it as it moved. The decades-old objection, the want of a mechanism, suddenly had its answer. The notion of a churning, convecting mantle that Arthur Holmes had floated years earlier now had a place to act.
Proof of the spreading came from an unexpected quarter: magnetism. Iron-bearing rock, as it cools, locks in the direction of the Earth's magnetic field at that moment, and the field is known to reverse its polarity from time to time. If the seafloor was spreading, each reversal would be frozen into the new crust as it formed, producing matched, symmetrical stripes of normal and reversed magnetism running parallel to a ridge on both sides. When surveys found exactly that pattern, mirror-image bands flanking the ridges, the seafloor was shown to be a slow tape recording of its own creation, and spreading was no longer a hypothesis but a measurement.
By the end of the 1960s these strands had been drawn together into the theory of plate tectonics, which holds that the Earth's surface is broken into rigid plates that move, carrying continents with them. Wegener, it turned out, had been right about the drift and wrong, or simply silent, about its cause, and the gap had cost his idea two generations. What had been missing was never more evidence; it was a mechanism, and the patience to wait for the tools that would reveal it. The continents do fit together, as the map suggests. It just took the floor of the ocean to explain why.