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7.1.1. Wegener continental drift

Continental drift hypothesis

Although he was not the first, Wegener, in 1912, formulated a complete mobilistic hypothesis about the displacements of the continents. He claimed that the continents had been part of a great macrocontinent that fragmented, which he called Pangea. Later, the continents moved over the only ocean, Pantalasa, until they reached their current position.

Movimiento de los continentes desde Pangea a la actualidad

By Kious, Jacquelyne; Tilling, Robert I.; Kiger, Martha, Russel, Jane [Public domain], via Wikimedia Commons

For the formulation of this hypothesis, it was based on the observation of numerous phenomena in the field of paleontologypetrology, paleoclimatology and geography.

Evidence of the Continental Drift Theory

Wegener justified his Theory of Continental Drift based on several types of evidence:

Geographical evidence

Wegener observed that continents, such as Africa and South America, could have been united in the past, as the shapes of their coasts seem to fit together. If the continents were part of Pangea, it is logical that their fragments fit together. The coincidence is greater if instead of the coasts, we try to coincide the continental shelves.

Geological evidence

Wegener observed that some geological formations continued on both sides of the Atlantic. They were the same age and types of rocks, so he believed they would have been attached. For example, the existence of diamonds in Brazil and South Africa.

Paleontological evidence

Wegener discovered that fossils of the same species were found on continents that are now separated. Even some of them were terrestrial like reptiles and plants, so it was not possible that they could have crossed the ocean. This is yet another proof that the continents were united in Pangea millions of years ago.
Pruebas paleontológicas de la deriva continental
By Osvaldocangaspadilla (Own work) [Public domain or Public domain], via Wikimedia Commons

Paleoclimatic tests

Wegener used some sedimentary rocks as indicators of the climates in which they originate, such as tillites in a glacial climate, gypsum and halite in an arid climate, or coal in a tropical climate. He drew a map of ancient climates and observed that they could not have occurred on the continents at current positions.

Geomagnetic tests

When a magnetic mineral crystallizes , it is oriented toward the north pole, just like a compass needle does. Magnetic minerals that make up rocks of the same age should point to the north pole but indicate different directions, so it seems clear that they have moved. If we move the continents to their original position, these minerals would point towards a single pole.

La Tierra se comporta como un imán, con el polo norte magnético cerca del polo norte geográfico y el polo sur magnético cerca del polo norte geográfico.

By Users Heron, Kevin Saff on en.wikipedia [Public domain], via Wikimedia Commons

Test: Continental drift.

Game: Pangea puzzle.

Shortcomings of the Continental Drift Theory

Despite the fact that Wegener presented enough evidence to support his hypothesis about continental drift, he was unable to convince the scientific community. The weak point of his theory was that he could not explain the mechanism with which the continents move.

Wegener proposed that the centrifugal force caused by the Earth's rotation was the one that dragged the continents. In turn, the continents were pushing the marine sediments that were in their way forming the mountain ranges. But the centrifugal force did not appear to be strong enough to drag the continents away, nor did it explain why there were mountain ranges elsewhere.

When in the 1960s, with geophysical advances, it was possible to explain the mechanism by which the continents moved, Wegener was given the scientific recognition that was denied when he proposed his hypothesis.

Mantle convection currents

When, in the 1940s, it was verified that the geothermal gradient was maximum in the oceanic ridges and minimum in the sea trenches, the possibility of convection currents in the mantle was suggested.

At present, it is believed that convective currents affect the entire control, rising from the D" level of the lower mantle, in contact with the nucleus. The lithosphere is part of the convective cells, and would descend, by gravity, in the areas subduction, up to level D".