What is evidence of continental drift?
Wegener’s original idea of continental drift is now more than 100 years old. However, despite the very strong evidence still valid today, it would take 60 years for this theory to be openly accepted.
It was in 1912 that Wegener first introduced the idea of continental drift, which would lead to the development of plate tectonics a few years later. At the time, the prevailing theory to explain land masses, landforms, and oceans was that the Earth shrank as it cooled, and that it was like a fruit that shriveled and swelled as it dried.
But Wegener is not convinced. For him, many observations show that the continental masses were previously grouped together as a single continent before breaking up into several parts and then collapsing into their present state. Today, this theory has been refined and made more complex, but the arguments put forward by Wegener at the beginning of the 20th centurye century is still valid.
The shape of the coasts allows us to reconstruct a single supercontinent
The first observation that Wegener would present in support of his hypothesis is, of course, the complementarity of forms between the coasts of different continents, particularly Africa and South America. It’s really easy to see that these two continents fit together almost perfectly. They are not alone either.
In 1915, Wegener thus presented a reconstruction of the world at the end of what was then called the Protoperiod (about -270 million years). According to him, all the continents are then collected in one block, which he called Pangea. This supercontinent is surrounded by a huge ocean, Pantalassa. Current continental movement models show that Wegener’s paleogeographic reconstruction is surprisingly accurate. Because the scientist was not only based on the shape of the ribs.
In his opinion, he used another topographical argument based on the statistical analysis of the earth’s reliefs. Thus, it is based on the Trabert curve, which shows that the frequency distribution of landforms is expressed around two privileged levels of height and depth. One is about 300 meters above sea level, and the other is -4800 meters deep. Although there are higher peaks or deeper ocean trenches, they are extremely rare.
This bimodal form was one of the first elements to suggest the existence of two crusts of different nature, one at the level of the oceans and the other at the level of the continents. But these two shells are in isostatic equilibrium. This observation strongly opposes the idea of thermal contraction of the Earth, because in this case, firstly, the reliefs should be evenly distributed on the surface of the earth, and secondly, the distribution curve should be only a simple Gaussian with a uniform mean. the level is between -2000 and -3000 meters.
Geological and paleontological compatibility
But Wegener goes into more detail. This shows that there are geological correspondences between the continents now separated by huge oceans. Thus, the cratons (regions of very old rocks) of Africa and South America fit together perfectly when the two continental masses are brought together. The similarity of the ages of these cratons further suggests that they were originally only a single formation. Wegener makes the same observation for other major geological units, such as the ancient Caledonian and Hersian mountain ranges. These observations indicate that certain structures were formed long before the separation of the different continents.
Always with the same logic, Wegener studies the distribution and evolution of past fauna and flora in different parts of the sphere. Thus, he notes that until the beginning of the Mesozoic (about 250 million years ago), terrestrial organisms showed great similarities, regardless of the continent under consideration (South and North America, Eurasia, India, Africa, Australia, etc.). On the other hand, the fauna and flora gradually diverged after that, which marks the beginning of isolation in each of the continents.
These paleontological observations would allow Wegener to refine the idea of continental drift. His data indeed highlight that the supercontinent Pangea first split into two large units during the Triassic: Laurasia (located in the north) and Gondwana (further south). Thus, the present continents would be formed as a result of the respective breakup of these two large continental masses during the Cretaceous period.
Strong arguments, but weak on the engine of the movement
Finally, Wegener advances paleoclimatic arguments. Thus, for the Carboniferous period, it indicates a climatic connection between North America, Northern Europe and Northeast Asia. At that time, these various continents were under equatorial climates, while South America and Australia show glacial deposits. For Wegener, these two climate types do not correspond to the current positions of the continents. Further evidence that these were in radically different places 300 million years ago.
Despite many arguments, Wegener’s theory failed to convince all scientists. Thus, until the early 1960s, there was strong opposition. Opponents of the theory of continental drift, even if they were relatively few in the end, relied mainly on the fundamental weakness of Wegener’s theory. To explain the motion of the plates, he appealed to tidal forces associated with the Earth’s rotation. A misconception that several geophysicists have refuted, as well as rejecting the entire continental drift hypothesis. A complete model of plate tectonics was finally established permanently at the end of the 20th century, combining different theories rather than excluding them.e century
