Drifting Continents

The first step in the development of modern plate tectonic theory can be traced back to the early days of the twentieth century and an idea that became known as continental drift. Simply stated, continental drift is the hypothesis that all of the landmasses on Earth were joined together into one giant "supercontinent" at some time in the distant past. Over millions of years, this single landmass slowly broke apart and the continents gradually "drifted" into the positions that we see them in today.

As you might expect, when this idea was first proposed, most scientists were dead set against it. After all, the idea that the continents had somehow moved over time was directly opposed to the idea that Earth was a solid, stable planet. Was there any evidence to suggest that such a radical concept may have been possible? Yes—all that is needed is a map of the world.

By looking at a map of the world, even a nonscientist can see a very striking pattern. For example, in the southern Atlantic Ocean, the east coast of South America and the west coast of Africa appear to be near-perfect matches for each other. Moving up to the North Atlantic, the coasts of Europe and North America also seem to fit. Moving Greenland south a bit would slide it into a wedge-shaped space there. It is almost like these continents were pieces in some global jigsaw puzzle.

The truth is, long before the idea of continental drift was proposed, some people had noted these similarities and were raising

Pangaea Puzzle Africa
Figure 2.1 By examining a map of the world, one can see how the coasts of North America and South America seem to fit together with the coasts of Europe and Africa, as if the continents were once part of a big jigsaw puzzle.

questions about them. Soon after Columbus made his historic voyages back in the late 1400s, mapmakers began drawing what they thought the Atlantic Ocean looked like. As more details became available and the maps got better, people started to wonder if the continents might have once been connected together.

In his work Thesaurus Geographicus, published in 1596, the Dutch mapmaker Abraham Ortelius suggested that Europe and Africa might have been torn apart by "earthquakes and floods." Later, in 1666, a French naturalist named Francois Paget suggested that the reason the coastlines looked as if they fit together was due to the sinking of a large section of land down into the Earth, after which the seas rushed into the gap to form the Atlantic Ocean.

For most geologists working in the seventeenth and eighteenth centuries, however, the shape of the continents was thought to be

Figure 2.2 This antique map of the world shows how scientists of the eighteenth century thought the continents were configured. Not all parts of the world had been extensively mapped by Europeans yet, which is represented by blank spaces such as the one covering what we now know to be the northwest corner of North America.

Figure 2.2 This antique map of the world shows how scientists of the eighteenth century thought the continents were configured. Not all parts of the world had been extensively mapped by Europeans yet, which is represented by blank spaces such as the one covering what we now know to be the northwest corner of North America.

a simple coincidence rather than something that needed further investigation. Once the ideas of Hutton and Lyell on uniformi-tarianism gained acceptance, though, a growing number of scientists became interested in the question again. As you might suspect, some of their theories seem pretty far fetched by today's standards.

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