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EESA06H3 (234)
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Chapter 2

Chapter 2 - Plate Tectonics

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Environmental Science
Nick Eyles

CHAPTER 2 - PLATE TECTONICS What is Plate Tectonics? Tectonics is the study of the origin and arrangement of the broad structural features of the Earths surface. (ie folds, faults, mountain ranges, etc..) Plate tectonics describes the Earths surface as being divided into a few large, thick plates that move slowly and change in size. At the boundaries of plates there is intense geologic activity. o Explains several diverse features on the Earth: Earthquakes, origin of mountains, sea-floor topography. o Idea developed in the 1960s; a combination of two theories, continental drift ad sea-floor spreading. o Continental Drift Idea that the Earths CONTINENTS moved freely over the Earths surface, changing their shape and position relative to other continents over time. o Sea-floor spreading - Idea that the Earths sea floor forms at the crests of the mid-oceanic ridges, moves horizontally away from the ridge and eventually sinks at the oceanic trenches; like a conveyer belt. How did the Plate Tectonics Theory Evolve? Alfred Wegner was the first to make a strong case for the continental drift. He noticed: o That the continents all seemed to fit together like a puzzle. o Fossils from several continents had the same Palaeozoic rocks and fossils. Wegner called the super continent Pangaea, which he believed split into Laurasia (Todays Northern Hemisphere continents) and Gondwanaland (had all of todays southern hemisphere continents + India). More Evidence Based on the distribution of the continents today, a huge ice age would have to cover the entire planet to account for the Palaeozoic glacial deposits we have found around the world. However, those deposits would make more sense if we assume the continents were all joined together at one point. THEN, we would only observe glaciations deposits in the southern hemisphere and India. www.notesolution.com When examining sedimentary rocks from the paleoclimatic reconstructions, Wegner found that the distribution of the polar positions were very different in the past compared to now. Hence, he hypothesized that in the past continents had moved. Skepticism about Continental Drift o Wegner believed that the continents ploughed through he oceanic crust, compiling mountain ranges in the front. Knowing the strength of rocks at the time, this was a bad explanation. o Wegner also proposed that based on a combination of two forces, the centrifugal force and gravitational forces that continental drift was possible. HOWEVER, through carful calculations it was shown that these forces are way too small to account for the movements of continents. The study of the sea-floor and geophysical research (rock magnetism) renewed interest in continental drift in the 40s and 50s. Geophysical Research- When rocks are formed they have within them magnetite (cooled basaltic lava flow or other metallic minerals) that point towards the poles and once cooled get frozen in time. This frozen compass needles tell us where the magnetic poles were at the time of rock formation. It was originally believed based on the results, that the poles were moving ver time. Now, however its believed that the continents (the rocks) were the ones moving. o Assuming there was only one pole, and lets take out the atlntic ocean in between North America and Europe....then the polar wandering paths for Europe and North America would be the EXACT same. Hence, this suggests that at one time Europe and NA were both joined together (supercontinent) and that over time the Atlantic ocean formed in between. More Evidence Rock types matches: boulders in South America have been traced to sources in Africa. Rock Matches show continents were at one point together. Pangea split up to 200 million years ago, but evidence shows that the continents have been moving for up to 2 billion (some say 4 billion) years now. What is Sea Floor Spreading? Harry Hess suggested that the sea floor was spreading; totally opposite idea to Wegners (that the sea floor was still while the continents moved through them) www.notesolution.com
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