The Continental Drift
Continental drift is the suggestion that over long periods of geological time, the continents of the Earth have moved apart from each other and had once formed almost one single land mass known as Pangaea. Whilst the theory that the continents had moved over time had been speculated upon before, the theory wasn’t properly developed until 1912.
Wegner's Theory and Evidence:
The theory of Continental Drift was first proposed in 1912 by the German geologist Alfred Wegner in which he suggested the continents were moving slowly across the Earth’s surface. He suggested that all the continents had once formed a single land mass or super continent which he called Pangaea; the ancient Greek meaning for ‘All Earth’. He supported his theory with geological evidence.
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Evidence in Support of the Continental Drift - QS Study
Wegner’s first piece of evidence was that the continents looked like they fitted together like pieces of a puzzle. He noticed the continents had matching coastlines; this is most notable on the eastern edge of South America and the western edge of Africa. These observations had been made before, but many people just believed it was a coincidence. Another piece of evidence is that fossils of plants, animals and invertebrates have been found on separate continents. Fossils of Mesosoaurs (freshwater aquatic reptiles) have been found on the continents of Africa and South America along with fossils of plant life providing more evidence that the continents were once joined and moved apart over time. The Mesosaurs would not have been able to swim across the ocean due to the fact they are freshwater creatures (and the vast distance between continents), so the mesosaurs at some point must have been able to walk across the continents. Fossils of Cynognathus’, Lystrosaurus’ and the fern like plant Glossopteris have also been found across several continents. Wegner also noticed that rocks and mountain ranges across continents looked similar and were similar ages, the belt of ancient rocks of 2,000 million years ago from the Brazilian coast matches with those from western Africa he also noticed that there were similar geological sequences in eastern Canada and Scotland. Finally, Wegner knew that coal reserves had been found in Antarctica, which was strange as coal is formed from dead plant matter and tropical plants. Which means Antarctica must have had a much warmer climate at one point, supporting Wegner’s theory of Continental drift.
After Pangaea split up, Wegner believed it formed the continents of Gondwanaland and Laurasia about 200,000 years ago before continuing to split apart to form the continents we know today.
It was hard for Wegner to back his theory as he was unable to explain how the continents moved. He was limited by the technology of his time and more evidence to support his claim wasn’t available till scientific technology improved and evidence was found of plate tectonics.
Tectonic Plates:
It was later discovered that the crust of the Earth is comprised of tectonic plates in the Lithosphere (the outside layer formed on the crust or on the top layer of the mantle). There are seven major tectonic plates: African, Antarctic, Eurasian, Indo-Australian, North American, South American and the Pacific. There are also several smaller plates such as the Scotia, Nazca and Cocos. Below the lithosphere is the Asthenosphere which is a viscous area with low resistance in the upper mantle often described as ‘plastic rock’. It is much hotter than the lithosphere. There are two varieties of tectonic plates: Oceanic plates and Continental plates. Oceanic plates are thinner and found under the ocean, they are denser (3.0kgm³) than continental plates and consist of basaltic rock. Continental plates are slightly less dense (2.6-2.7kgm³) and are found under continents and large land masses; they largely consist of granitic rock. Continental plates are 30km-70km thick whereas the thinner oceanic plates are only 5km-10km thick. Temperature and pressure both increase with depth.
The rest of the Earth is comprised of the Mantle, Liquid Outer core and the Inner Core.
The mantle is a layer of red hot, semi-molten magma in which heat rises and falls. It has a density of 5.6kgm³ at the core and a density of 3.3kgm³ at the Moho (the boundary between the mantle and the crust). It also extends to a depth of 2900km. Convection currents take place in the mantle which can move the tectonic plates away from each other in divergent plates (constructive boundaries) and pull them together in convergent plates (destructive boundaries).
Divergent and Convergent Plate Boundaries:
At divergent plate boundaries, the two plates are pulling apart and magma rises through the gap in the Earth forming volcanoes. As magma rises it forces the crust to rise at the ridge. As the crust rises, gravity forces it away from the margin, driving the crust. This is called Ridge Push.
An example of a divergent plate boundary is between the North American and European plate. As the plates pulled apart from each other millions of years ago it caused earthquakes which led to magma rising from the mantle and erupting through the crust forming the Mid Atlantic Ridge as it solidified, this is called sea floor spreading as it is an example of two oceanic plates pulling away from each other. The plates continue to move apart at the speed that our fingernails grow.
Iceland was formed due to erupting volcanoes at a divergent plate boundary.
At convergent plates, oceanic and continental plates collide. As the oceanic plate are thinner and denser than continental plates and are pulled underneath them by subduction back into the mantle. The force of subduction can cause volcanoes to erupt and earthquakes to occur around the subduction zone.
An example of a convergent plate boundary is the Pacific Plate and the Eurasian plate. As they collide, the pacific plate is subducted under the continental Eurasian plate. Another term for subduction is slab pull which suggests that as the denser oceanic plate is subducted at a convergent margin, they become denser than the underlying mantle and pulls the rest of the plate behind it.
Convergent boundaries can also occur between two continental plates. As they are the same density, they break when they collide and are forced upwards instead. Where the plates collide they form fold mountains e.g. The Himalayas. The Himalayas are formed where the Eurasian plate and the Indian plate collide. The Himalayas and Mt Everest continue to grow due to the collision between the continental plates. Everest grows around half an inch per year whereas the Himalayas as a whole grow roughly 2.4 inches a year.
Other examples of Fold Mountains are: The Andes, The Alps and The Rockies.
Why does Mount Everest grow each year? (askinglot.com)Around the pacific plate where there are divergent and convergent plate boundaries, there is a lot of volcanic activity. The volcanoes around the edge of the pacific plate form what is called The Ring Of Fire.Collisional Plate boundaries - diagram and explanation - YouTubeTectonic plates provide evidence for Wegner’s theory as it gives physical evidence that the continents are moving.
Magnetic Inclination in Rocks:
Magnetism in rocks can also be used as evidence for the continental drift. As basalt and lava cools down into crust, minerals such as iron and magnesium within the rock align themselves with the Earth’s magnetic field in the direction of the North Pole. Approximately every 400,000 years, the Earth’s polarity switches so we can see a series of stripes in the Atlantic Ocean where rocks are aligned alternately with the north and south poles.
As the divergent plates move apart and new magma rises from the mantle it will record the current alignment of the North Pole. As we travel further from the mid Atlantic ridge we can see the alignment changing which proves the continents have been moving away from each other. We can also see that the basaltic rock gets older the further we travel from the mid Atlantic ridge, showing that new basaltic rock is made as the magma rises from the diverging plates.
Separation of the Continents:
It is believed that Pangaea was formed roughly 300 million years ago and broke apart during the early Jurassic period. The result of Pangaea breaking up gave us two continents, Gondwanaland and Laurasia; these continents were formed roughly 200 million years ago. Gondwanaland contained all the masses in the southern hemisphere such as Africa, Antarctica, South America and Australia. Laurasia contained the land masses in the northern hemisphere such as North America, Europe and Asia. Around 66 million to 30 million years ago it is believed Laurasia fragmented into the continents in the northern hemisphere we know today. Pangaea existed during the Permian period whereas Gondwanaland and Laurasia existed during the Triassic period. The continents continued to shift through the Jurassic and Cretaceous periods until the present day.
Earthguide: Online Classroom - Definition: Pangaea (ucsd.edu)There were also many supercontinents that existed before Pangaea such as Ur which is said to have existed 3 billion years ago, Columbia which existed 2.5 billion years ago and Rodinia which existed 1.3.-1.9 billion years ago. Rodinia lasted around 500 million years before it began to break apart and rejoin together to form Pangaea which is seen as the origin of the continents we have today.
Conclusion:
In conclusion we can see that Wegner developed a theory with plenty of evidence that was later even further developed by the discovery of tectonic plates and other factors such as magnetic inclination in rocks and the effects of divergent and convergent plate boundaries. With the development of modern science and physical proof that the Earth’s continents are moving, the evidence is overwhelming and it is hard to deny Wegner’s theory that the continents once formed a single land mass and drifted apart over a long period of geological time.
Bibliography of Sources:
- Evidence in Support of the Continental Drift - QS Study
- Continental Drift: Alfred Wegener Song by The Amoeba People - YouTubehttps:www.youtube.comwatch?v=_Ugwf6IaE_k
- Why does Mount Everest grow each year? (askinglot.com)Collisional Plate boundaries - diagram and explanation - YouTubeEarthguide: Online Classroom - Definition: Pangaea (ucsd.edu)The Super-Continents Before Pangea - YouTube
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