Describe the main three types of the Earth’s tectonic plate boundaries with examples of where they occur and explain the main cause of plate movement
There have been many theories about plate movement, one of the first theories was by geophysicist Alfred Wegener. Wegener proposed the idea that the continents used to fit together like a jigsaw puzzle, this was named Pangaea (Smithson et al, 2008). This process was called continental drift which was proposed in 1912. Wegener collected a lot of evidence to support his theory for example: the continental margins of Africa and South America fit together perfectly. Another piece of evidence was from palaeontology: fossils of dinosaurs such as the Mesosaurus were found on either side of the southern Atlantic Ocean (Mcknight et al, 2008). Wegener later published his theories in his ground-breaking book, Die Entstehung der Kontinente and Ozeane (The Origin of Continents and Oceans), in 1915. Another theory regarding plate movement is sea floor spreading by an American geophysicist Harry Hess in 1960. Seafloor spreading is a geological course in which tectonic plates move apart from each other. This is a result of convection currents (Evers, 2011). Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is made by volcanic action and then moves from the ridge. To conclude, one part of Wegener’s theory was wrong: the continents are rooted in the lithospheric plates which move by the action of seafloor spreading, therefore it is not just the continents that are drifting (Mcknight et al, 2008). By 1968 the theory of plate tectonics was being widely accepted by scientists.
There are three different types of plate boundaries. The first being divergent plate boundaries whereby the plates are moving away from each other. This happens above rising convection currents. This rising convection current pushes upwards on the bottoms of the lithosphere which causes it to lift and flow horizontally below it (King, 2015). The horizontal flow causes the plate material above to be pulled in the direction of flow. This is then followed by the plate on top being stretched thin, breaking then being pulled apart. Magma from the asthenosphere starts to rise up in the gap between the plates. This upwards movement of magma produces a line of volcanoes (Mcknight et all, 2008). Divergent plate boundaries usually produce a mid-ocean ridge, an example of this is the Mid-Atlantic Ridge. The formation of a mid-ocean ridge is also seen through figure 1. The oceanic rift which runs along the spine of the mid-ocean ridge is typical of what is known as an oceanic spreading centre. Spreading centres are connected with earthquakes that have ruptures about “75 kilometres of the surface” (Mcknight et al, 2008). According to Mcknight et al. (2008) Divergent plate boundaries can also result in a continental rift valley by developing within a continent. An example of this is the East African Rift Valley.
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