What Observation First Led to the Continental Drift Theory?

If you look at a globe it's easy to see that the west coast of Africa and the east coast of South America are shaped in such a way that they could nestle right up against each other. That could have been coincidence, but in fact continents move, and in the distant past the coastlines of Africa and South America were joined. When that idea was first proposed it faced stiff opposition because continents are massive blocks of stone and it was hard to imagine a way they could have moved. Piece-by-piece the evidence fell into place, and now continental drift is an established fact.

German meteorologist Alfred Wegener, born in 1880, was not the first person to notice the similarities in the shape of continental borders, but he was the first to collect different observations and propose a theory of continental displacement. Wegener noticed that the continents matched even better when the edges of the undersea continental shelves were brought together. This wasn't true just for Africa and South America, but for all the continents. Wegener proposed the idea that all the continents had once been connected in a giant mass he called Pangaea, meaning all earth.

If you look at the geological structures in Africa and South America, for example, you will see patterns that match. Imagine the two continents as gigantic connected puzzle pieces, then further imagine you built some clay shapes on top of them. When you disconnect the two puzzle pieces the clay shapes will break at the edges, but they'll still match up with each other as you separate the pieces. So it is that broad valleys, mountain belts and underlying geological structures called "shields" all line up when you connect the continents like puzzle pieces. Even regions with coal deposits and diamond mines match up if the continents are put together.

Wegener also examined evidence related to climates of the distant past, more than 200 million years ago. Rock formations of about that time indicate glaciers were present in disconnected areas of South America, Africa, Australia and India, while at the same time not present in areas of those continents that are colder in today's configuration.

He also looked at fossils from that time and found a remarkable similarity among fossils in Antarctica, South America, India, Africa and Australia about 200 million years ago. With today's geography those regions are separated by thousands of miles, but when the continents are fit together according to Wegener's pattern, those regions match up.

At the time of Wegener's death in 1930 his theory of continental drift was not widely accepted. He had one big problem -- everyone knew the continents were solid rock, and no one could figure out how they could possibly move. In the 1950s and 1960s researchers identified undersea structures between the continents, such as ridges along a line under the Atlantic Ocean, separating Africa from South America and North America from Europe. That provided a hint that the continents were splitting apart, creating a giant crack through which the hot sludgy melted-rock magma could push up. There are still questions about the precise mechanisms of continental drift, but it is now an established fact that the continents move in a process called plate tectonics.