Plate Tectonics Earth’s Operating System G A Davie RockandSky Plate Tectonics Earths Operating System G A Davie [email protected] www.rockandsky.net Copyright 2020 Rock and Sky P a g e | 1 Why Plate Tectonics? Plate Tectonics is the lens through which earth scientists view their world. Life scientists also draw on the theory to explain and predict the evolution and distribution of species. The acceptance of the theory completely revolutionised the study of the Earth Sciences and provided a mechanism for explaining almost everything, which until then had not been available. 1965 was the year the world changed, for geologists at least. Everything is a result of plate motion. The shape of the ocean basins, the high Alps and the high Himalayas, the volcanoes of the Japan and the Andes are due to plate tectonics. The two very destructive tsunamis of the last 20 years are a result of plate tectonics. Earthquakes in California or Nepal are the result of plate tectonics. Uplift, rejuvenation of rivers, and erosion are all the result of plate tectonics. Speciation of animals and humans alike is the result of plate tectonics. Mineral deposits, so important to our modern lifestyles, are the result of plate tectonics. Our very existence is a result of the sequestration of carbon via the Carbon Silicate Cycle – a feedback loop that keeps temperatures under control and by extension makes our planet habitable. Go search the Carbon Silicate Cycle on the Rock and Sky website to find out some more about this vital and fundamental process. Geomorphology is fundamentally controlled by the underlying geology. Geology however is in thrall to plate tectonics and it is impossible to study physical geography without having a broad understanding of geology and plate tectonics. For example, huge granite inselbergs which poke their bald heads through ancient metamorphosed terranes create landscapes that are very different to those underlain by soft mudstones, which in turn are very different to landscapes underlain by hard, resistant sandstones. All landforms are to some extent controlled by the underlying rock types. Sure, there are other processes involved but fundamentally it boils down to geology. P a g e | 2 The study of physical geography should therefore fall under the umbrella of plate tectonics – it should be the first port of call in terms of framing the syllabus, for all the reasons given above. Rock and Sky P a g e | 3 Chapter 1 The History The story behind the development of the theory is a grand and fascinating tale. Some portions of the theory were worked out by independent scientists, some portions were worked out thanks to international collaboration, some scientists were ahead of the game, some were subject to the prejudices and chauvinistic attitudes of their day. Some weren’t even geologists. You can of course skip this chapter, but it is a wonderful story and what’s more, there are some fundamental concepts that this chapter introduces and frames beautifully and which will assist later on when we get to the nut and bolts of the theory. Read on to find out how big egos and big science eventually brought us to where we are now in terms of our understanding of the theory. It was Abraham Ortelius (Figure 1.1), the leading map maker of the16th Century, who noticed the similarities between the shape of the African and South American coastlines. Figure 1.1 Abraham Ortelius Rock and Sky P a g e | 4 In 1855 Antonio Snyder Pellegrini pointed out the similarity between the coastlines of South America and Africa and ventured a reconstruction of the continents based on his observations, shown in Figure 1.2. Figure 1.2 Pellegrini’s 1855 reconstruction Later, HMS Challenger (Figure 1.3), after taking thousands of soundings of the depth of the oceans between 1872 to 1876, found that there was a prominent rise in the seafloor that ran the length of the Atlantic. The Germans confirmed these findings during the 1925-1927 Meteor Expedition, managing to trace the South Atlantic ridge through into the Indian Ocean. The Danes also carried out surveys of the ocean floor; and in fact there were host of nations running oceanographic expeditions at that time, including Sweden, Scotland, France, Holland, USA, Japan and Egypt. Figure 1.3 HMS Challenger Rock and Sky P a g e | 5 Italian geologist Roberto Mantovani (Figure 1.4) was also writing papers between 1889 and 1909, proposing an expanding Earth hypothesis to account for the shape of the ocean basins. American geologist Frank B Taylor, in 1908, also came up with an independent theory for the distribution of the continents but invoked the idea that the gravitational pull of the moon was dragging the continents towards the equator. Figure 1.4 Roberto Mantovani Then an upstart meteorologist put forward a more detailed and reasoned argument for continental drift in a book titled “The Origins of Continents and Oceans.” His name was Alfred Wegener (Figure 1.5), and he is now widely acknowledged as the father of the ‘continental drift’ theory. He became interested in the topic in 1911 when he accidently came across a scientific paper describing the distribution of identical plant and animal fossils across what we now know as Gondwana. With his interest piqued, he carried out additional work on the subject, which ultimately led to him publishing his own scientific paper in 1912, and a book called “The Origin of Continents and Oceans” in 1915. It was initially published in German, but when the English translation came out in 1922 (Figure 1.6), the world was changed forever. Rock and Sky P a g e | 6 Figure 1.5 Alfred Wegener However, his ‘mad’ theory was not well received by the earth science community in Britain and America. Perhaps some of the hostility towards his ideas may have been partly due to anti-German feeling thanks to the long, bitter conflict of World War I. Also, as a meteorologist, Wegener was treading on the geologist’s toes – and besides, the idea did seem totally absurd - Wegener was not able to come up with a plausible mechanism to explain how continents could drift across the planet’s surface. Figure 1.6 Wegener’s “The Origin of Continents and Oceans” Rock and Sky P a g e | 7 But Wegener was an exceptionally clever man, who along with his father-in-law Wladimir Köppen, had authored the leading text book on meteorology. Nor was he a man given to sitting around inventing armchair theories, but was actively involved in leading expeditions to Greenland to collect meteorological data and measure the thickness of Arctic ice. He was also well versed in palaeobotany and palaeontology, and was acutely aware that Permian-aged Glossopteris coal fields were to be found in Africa, Australia, South America, Madagascar, India and Antarctica. Figure 1.7 shows a reconstruction of Glossopteris. Figure 1.7 Glossopteris – the tree which makes up the Permian-aged coal fields of Gondwana. Rock and Sky P a g e | 8 Figure 1.8 Lystrosaurus Furthermore, a fossil dicynodont called Lystrosaurus (see Figure 1.8) had also been found in India, Antarctica and South Africa, and a fossil of a freshwater reptile called Mesosaurus (Figure 1.9) was discovered in southern Africa and Brazil. It would have been impossible for Mesosaurus to have made the journey across a salty Atlantic Ocean. Figure 1.9 Mesosaurus skeleton Rock and Sky P a g e | 9 At the time, most geologists believed in the notion of land bridges which allowed the movement of different species between the continents, but this land-bridge theory could not explain how a freshwater reptile could have migrated between South America and Africa. Finally, Cynognathus, a ‘dog jawed’ reptile was also found in South Africa, Argentina and Namibia. The distribution of this Gondwana flora and fauna is shown in Figure 1.10. Figure 1.10 The distribution of flora and fauna across Gondwana Figure 1.11 shows Dwyka Tillite, the Permian aged glacial deposits from South Africa. Figure 1.11 Permian-aged glacial deposits from South Africa Rock and Sky P a g e | 10 Based on his field observations, fossil evidence, discussions with other scientists, and his formidable powers of deduction, Wegener concluded that the continents had at one stage been joined together in a supercontinent, which he named Pangaea. Little did he realise that his big idea would meet with so much hostility. For example, in Britain, a year after the first English translation of Wegener’s book, Philip Lake, who was then professor of Geography at Cambridge, had this to say at a meeting of the Royal Geographical Society: “He is not seeking the truth; he is advocating a cause, and is blind to every fact and argument that tells against it. It is easy to fit the pieces of a puzzle together if you distort their shape, but when you have done so, your success is no proof that you have placed them in their original positions. It is not even a proof that the pieces belong to the same puzzle or that all the pieces are present.” Ouch! “Utter damned rot,” said William Scott, geology professor at Princeton, and Edward Berry, an American palaeobotanist, called Wegener’s theory “a selective search through the literature for corroborative evidence, ignoring most of the facts that are opposed to the idea, and ending in a state of auto-intoxication.” Whatever you might say about Berry, he certainly had a way with words. Wegener tragically died in a Greenland blizzard in 1930, and his theory was orphaned, left to grow up in a hostile world without any real guardians to see her through to adulthood.