Continental Drift

Tap for animated GIF

Public Domain

Alfred Wegener had a lot of evidence for continental drift, but he didn’t have an explanation for how they did it. He had the curious way South America and Africa looked as if they should fit together. This was noticed almost as soon as good maps were available, but it was largely dismissed as coincidence. After all, it would imply that the two land masses had moved apart, and everyone knew that couldn’t happen. The idea was ridiculous.

He also had a geological connection. The rocks of South America and Africa matched up where they would have been joined had they once been a single land mass. It is the same two billion year old rock on the two separate continents.

In a similar example, there is an old mountain range — over 400 million years old — that today has its remnants in the widely separated areas of Canada, Greenland, Ireland, Scotland, England and Scandinavia. When these areas are put together, the so-called Caledonian mountain belt re-emerges.

Old glacial deposits put down during the Permo-Carboniferous glaciation 300 million years ago are found in the present day Antarctica, Africa, Australia, India and South America. The most economical explanation for this is that these continents were gathered around the south pole at the time.

Finally there is the fossil evidence. Often the same type of fossil is found on continents that are separated today, while being found nowhere else. Either this is because the continents drifted apart after the fossils were laid down, or something more improbable happened, such as breeding pairs swimming together to another continent and establishing the species there.

Public Domain

Alfred Wegener was born in 1880 and died in 1930, but his continental drift theory, first put forward in 1912, didn’t achieve wide acceptance until the 1950s. The expanding theory was developed in the four editions of his book, The Origins of Continents and Oceans, accumulating increasingly impressive evidence as it went. The theory had a few supporters, such as Milutin Milankovich, but since Wegener couldn’t come up with a convincing mechanism for how the continents moved, most scientists were sceptical. One even argued that the continents simply couldn’t “plow through” the oceanic crust. They also found fault with the imperfect fit of the jigsaw coastlines, not realizing that he was matching them at their continental shelves, where it is a much better fit.

Paleomagnetism, a new science in the 1950s, produced much evidence to support Wegener. The ancient magnetic field was imprinted in the rocks and can be read today. India is in the northern hemisphere today, but its paleomagnetic signature shows that it was in the southern hemisphere in the past, as predicted by Wegener. As the evidence quickly mounted, and with additional evidence of seafloor spreading, scientists came to accept the theory. Eventually the theory of plate tectonics brought it all together.

Original upload by en:User:Tbower – USGS animation A08, Public Domain, Link

Today we can directly measure the movement of the continents with the Global Positioning System (GPS.) Alfred Wegener could have used that when he was exploring Greenland, the continent that eventually killed him.

rjb