What Is The Continental Drift Theory And How Is It Related To Plate Tectonics?

Continental drift, proposed by Alfred Wegener in 1912, is the idea that the continents are not fixed but slowly drift across Earth’s surface. He argued they were once joined in a single supercontinent, Pangaea, that broke apart. Wegener lacked a workable mechanism, but his idea later became the foundation of the theory of plate tectonics.

The Earth has seven continents: North America, South America, Africa, Asia, Europe, Australia and Antarctica. These continents constitute about 29% of the Earth’s surface, while the remaining 71% is made up of water. However, did you know that once upon a time, all the continents were actually one large landmass? Believe it or not, it’s true!

Continental Drift Theory Definition

Alfred Wegener was a German scientist (a meteorologist and geophysicist by training). He first presented his theory of continental drift in lectures in 1912, then laid it out in full in his 1915 book The Origin of Continents and Oceans. In it, Wegener said that the continents were not fixed in place. He said that all the continents had initially been one big landmass, which he called Pangaea. This landmass eventually broke up and split into the smaller landmasses that are now the continents we know today. These continents are in a state of constant motion, although it is imperceptible on a daily basis. However, Wegener must have had some reason to come to this conclusion. One doesn’t just concoct such a story… so what was the basis of his theory?

Evidence Of Continental Drift

Wegener based his theory on 2 basic facts. 

First was the geological fit. Wegener noticed that the coastlines of all the continents seemed to fit together, like a jigsaw puzzle. Even now, if you look at a world map or a globe, you will notice that the west coast of Africa fits into the east coast of South America. This is only one of many examples.

The second proof was the study of fossils. Wegener studied papers based on fossil findings and realized that fossils of various extinct animals were found in two or more disconnected regions. These animals could not have crossed an entire ocean to reach both places. One such example is the Mesosaurus, a small aquatic reptile (about 1 meter, or 3.3 feet, long) that lived in freshwater lakes and ponds roughly 280 million years ago. Because it could only survive in fresh water, it had no way to swim across the salty open ocean. Yet its fossils turn up in just two places on Earth: eastern South America and southern Africa.

If you look at a globe, those two regions are separated by the Atlantic Ocean, which a freshwater reptile like the Mesosaurus could never have crossed. So there is no way it could have ended up in both regions unless they were once joined by land. Additionally, certain fossils were found in places that wouldn’t have been conducive to the growth of that particular plant or animal. Even the existence of  certain types of rocks supported this piece of evidence.

Looking at these two main factors, Wegener built his continental drift theory. He pictured the lighter continents slowly plowing through the denser rock of the ocean floor, like icebergs drifting through water. For the force behind that motion, he proposed two candidates: a pole-fleeing force linked to the centrifugal force of Earth’s rotation, and the tidal pull of the Sun and Moon. (Notably, Wegener never invoked convection currents deep in the mantle. That mechanism was suggested later, in 1929, by the British geologist Arthur Holmes, and it eventually became part of modern plate tectonics.)

Rejection Of Wegener’s Theory

Wegener’s theory was not accepted, as those two factors were not considered good enough proof to support his claim. He was also unable to convincingly explain what actually moved the continents. Physicists ran the numbers and showed that the pole-fleeing and tidal forces he proposed were nowhere near strong enough to shove continents through solid ocean crust. This missing mechanism, more than anything else, led to the rejection of his hypothesis. However, history has proven that most theories that are groundbreaking revelations seem outlandish in their initial stages!

Plate Tectonics

Wegener’s theory did not gain the popular vote of the scientific community until after his death. Advancements in technology led to the collection of new data, which corroborated certain aspects of Wegener’s theory. As his theory was based on the data available to him then, it was proven wrong on a number of points, but it still provided the foundation for one of the most important theories in geology: the theory of plate tectonics. Plate tectonics did not come from a single person. It was pieced together by several scientists during the 1960s, building on the discovery of seafloor spreading. One key contribution came in 1965 from the Canadian geophysicist J. Tuzo Wilson, who described transform faults (a third kind of plate boundary) and is credited with coining the word "plate" itself.

In this theory, the continents and the ocean crust together form the rigid outer shell of the Earth, called the lithosphere. The lithosphere is broken into large plates that ride on top of a deeper layer called the asthenosphere. The rock in the asthenosphere is not actually molten, but it is so hot and under such enormous pressure that it behaves like a thick, slowly flowing solid (think of how cold honey or putty oozes). The plates of the lithosphere drift on this slowly churning layer and are in constant, very gradual motion. It is this movement that broke Pangaea apart into the continents we have today.

The plate tectonics theory was essential, as it could also explain a number of irregularities in other theories. For instance, the formation of mountains. Earlier, it was believed that the Earth was initially a ball of molten mass. When this started to cool, it led to the surface cracking up in some places and folding on itself in others. This is how mountains were believed to have been created. However, according to this theory, all mountains should have been formed at approximately the same time, which is not the case.  Plate tectonics provides another, more plausible explanation for this. Due to the constant movement and interaction of the tectonic plates with each other, various geological structures were formed. In places where a plate faced resistance to its movement, it would fold upward and create mountains. 

The theory of Plate Tectonics is now widely accepted. There is sufficient proof to support it, and it is an important aspect of geology, oceanography, geophysics and even paleontology. Wegener’s theory may not have been entirely accurate, but with the data he had available to him at that time, it was still quite a feat.