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continental-drift04

January 6, 1912: Wegener Presents Continental Drift

On this day in 1912 German geophysicist Alfred Wegener first presents his theory of continental drift to the German Geological Society. Even in todays “enlightened” age it is still hard to imagine that the continents were once attached. Imagine the reaction from the world in 1912.

389px-Alfred_Wegener_1910Wegener was the first to use the phrase “continental drift” and formally publish the hypothesis that the continents had somehow “drifted” apart. The complementary arrangement of the facing sides of South America and Africa is obvious, it was this temporary feature which inspired Wegener to study what he defined as continental drift.

Although he presented much evidence for continental drift, he was unable to provide a convincing explanation for the physical processes which might have caused this drift. In other words, what was the driving force? His suggestion that the continents had been pulled apart by the centrifugal pseudo-force of the Earth’s rotation or by a small component of astronomical precession was rejected as calculations showed that the force was not sufficient.

Mesosaurus

Mesosaurus

Mesosaurus

Mesosaurus

Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around different continent shores, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, was found both in Brazil and South Africa.

Lystrosaurus

Lystrosaurus

Lystrosaurus

Lystrosaurus

The discovery of fossils of the land reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. There is also living evidence of the same animals being found on two continents. For instance some earthworm families (e.g.: Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa.580px-Snider-Pellegrini_Wegener_fossil_map_svg

The theory of continental drift was not accepted for many years. At the time it was thought that maybe huge land bridges spanning the Atlantic and Indian oceans accounted for the similarities of fauna and flora on different continents. This failed to account for glaciation in India, Australia and South Africa.

One problem was that a plausible driving force was missing. And it did not help that Wegener was not a geologist. It is now accepted that the plates carrying the continents do move across the Earth’s surface; ironically one of the chief outstanding questions is the one Wegener failed to resolve: what is the nature of the forces propelling the plates?

800px-Antonio_Snider-Pellegrini_Opening_of_the_Atlantic

David Attenborough, who attended university in the second half of the 1940s, recounted an incident illustrating its lack of acceptance then: “I once asked one of my lecturers why he was not talking to us about continental drift and I was told, sneeringly, that if I could prove there was a force that could move continents, then he might think about it. The idea was moonshine, I was informed.”

Jack Oliver

Jack Oliver

Geophysicist Jack Oliver is credited with providing seismologic evidence supporting plate tectonics which encompassed and superseded continental drift with the article “Seismology and the New Global Tectonics”, published in 1968, using data collected from seismologic stations, including those he set up in the South Pacific.

nutshell

It is now known that there are two kinds of crust, continental crust and oceanic crust. Continental crust is inherently lighter and of a different composition to oceanic crust, but both kinds reside above a much deeper “plastic” mantle. Oceanic crust is created at spreading centers, and this, along with subduction, drives the system of plates in a chaotic manner, resulting in continuous orogeny and areas of isostatic imbalance. The theory of plate tectonics explains all this, including the movement of the continents, better than Wegener’s theory.

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