Defining Continental Drift
Continental drift is the gradual movement of the continents over time. The top layer of the earth’s crust is broken up into large slabs called plates that sit on a fluid level of molten rock. The movement of this lower molten layer causes the plates to shift. This movement, also called plate tectonics, is still happening today. Scientists estimate that the continents move anywhere from one centimeter to several inches per year.
Continental Drift Theory
Although there had been previous speculation, it wasn’t until 1912 that Alfred Wegener proposed the first complete continental drift theory. Wegener, a German geophysicist, and meteorologist theorized that all of the earth’s continents had once existed as a single land mass, which he called Pangaea. Then at some point between 275 and 175 million years ago Pangaea began to separate. The slow process of fragmentation and drifting continued until the continents eventually reached their current position.
The continental drift theory was based on several factors. One of the arguments, which had been pointed out by others before Wegener, was that some of the continents simply looked like they fit together. Perhaps the most obvious example of this can be seen in the way that the eastern coast of South America corresponds to the western coast of Africa. Beyond the visual clues, however, Wegener also suggested other geological and fossil evidence to support his theory.
Evidence of Continental Drift
Wegener’s theory was extremely controversial and was not widely accepted by the scientific community for many years. However, during the course of their research other scientists began to find corroborating evidence to support Wegener, and his continental drift theory began gaining acceptance.
The main reason that Wegener’s theory was initially dismissed was a question of mechanics: no one could offer an acceptable theory of how the movement was possible. Wegener’s suggestion that it had something to do with the earth’s rotation was generally dismissed. It was only when an understanding of plate tectonics came into the picture that more scientists began to seriously consider the other evidence supporting Wegener’s theory.
One of the main pieces of supporting evidence was the discovery of geological structures on different continents having closely related formation patterns. Additionally, research has also uncovered nearly identical plant and animal fossils on continents that are now separated by large bodies of water. The supposition being, of course, that those plants and animals could not have evolved identically unless they had lived in close proximity to one another.
Wegener’s continental drift theory finally became generally accepted in the 1950s and 1960s through the evidence provided by the study of paleomagnetism. Studying the magnetism of rocks from varying geological periods revealed the orientation of earth’s magnetic field at the time of the rocks’ formation. By tracing the shifting magnetic orientations, scientists have been able to see the paths that the shifting continents have taken.