Monday, May 20, 2019

Plate Tectonics Theory Essay

value how scurf tectonics theory helps our understanding of the distribution of seismic and volcanic events In 1912, Alfred Wegener published his theory that a single super continent named Pangaea once existed about 300 million years ago. He proposed that Pangaea then later split into two continents of Laurasia in the north and Gondwanaland in the south and that todays continents were the result of further splitting of these two land masses. Where the shells split atomic number 18 kn birth as home house boundaries. Wegeners theory of Continental d interruption was support by both geological and biological attest that these areas were once joined. The geological evidence included the stimulate sequences in Northern Scotland closely agreeing with those undercoat in East Canada, indicating that they were laid experience under the identical conditions in one location as wellspring as the obvious jig saw fitting appearance of todays continents, in particular, the take off of south America fitting into the indent below west Africa.The biological evidence comprised of fossil findings linking diametrical continents. Fossil brachiopods found in Indian limestones were comparable with similar fossils in Australia and the fossil remains of Mesosaurus were found in both South America and southern Africa. It is unlikely that the same reptile could have unquestionable in both areas or that it could have migrated across the Atlantic. Despite the evidence, Wegeners theory was unable to rationalize how continental movement had occurred. However from the 1940s additional evidence accumulated after the discovery of the mid-Atlantic ridgeline and huge naval trenches. Examination of the ocean crust either side of the mid-Atlantic ridge suggested that sea-floor spreading was occurring. Magnetic surveys of the ocean floor in the 1950s, showed regular patterns of paleomagnetic striping surrounding the ridges. It was discovered that when lava erupts on the ocean floor, magnetic domains at bottom iron rich minerals in the lava are aligned with the magnetic compass of the earth. This fixes as the lava cools and records the earths polarity at the time of their cooling.As the polarity of the earth reverses every 400,000 years, bands of normal and change by reversal polarity rocks are mirrored on either side of the mid ocean ridges, suggesting that new-sprung(prenominal) rocks are being added equally on either side. Surveys also established the age of the rocks and found young ages for places on or near the ridges and much older ages for rock nearer to the continental masses, demonstrating that older crust is continually being pushed aside bynew crust. The discovery of sea floor spreading led to the premiss that the earth must be getting bigger until now this was non the case and the discovery of pelagic trenches allowed for the conclusion that plates must be being destroyed at contrary boundaries to accommodate the increase in their size at m id-oceanic ridges. Hot spots around the core of the earth generate thermal convection currents within the mantle which cause magma to rise towards the crust and then spread before cooling and sinking.This circulation of magma is the driving ram down of plate movement. This movement has an effect on all the plates determining their type of boundary all(prenominal) with their own accepts and resulting volcanic and seismic events. Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle resulting in oceanic ridges and rift valleys. Where two oceanic plates are moving apart they produce mid oceanic ridges with their corpse influenced by the rate at which the plates separate. Volcanic activity occurs along the ridge forming submarine volcanoes which sometimes rise above sea direct accommodating fairly gentle sides and frequently gentle eruptions. An example of this is located in Surtsey, to the sout h of Iceland, and Iceland itself. As new crust forms and spreads, transform faults occur at right angles to the plate boundary due to dress pressure. The parts of the spreading plates on either side of these fault lines may move at different rates causing shallow focus earthquakes. Where two continental plates are spreading they produce rift valleys.The brittle crust fractures at sections as it moves apart causing a normal fault where intermission wall locomote down relative to the foot wall due to tensional stress. A feature of a rift valley is known as a graben which forms when a block of rock falls between two faults and creates the valley floor and also a horst which is formed when a block of rock is pushed up between two faults. This area is associated with volcanic activity as the crust is much capillary than in neighbouring areas. Convergent plate boundaries occur when two plates are moving towards each other. Where oceanic and continental crusts meet, the denser oceanic crust is forced under the lighter continental plate known as subduction. The down warping of the oceanic plate forms a very deep ocean trench and the continental plate edge is affected by the reverse fault lines that cause folding of the plate to produce uplift rock that forms Fold Mountains. Asthe oceanic crust descends, the increase in pressure can set forth major earthquakes along the line of the sub ducting plate. As it descends further the surroundings become hotter and additional wake from the friction causes the rock to melt in the benioff zone which begins to rise as plutons of magma. When they reach the surface they form composite explosive volcanoes.Eruptions can also occur offshore producing volcanic islands referred to as island arcs. Where two types of the same plate meet they create collision zones by which the compression of the two plates results in the folding of the plate to form Fold Mountains. As there is little subduction, there is no volcanic activity howeve r the movement of plates can trigger shallow-focus earthquakes. Conservative plate boundaries occur when two crustal plates slide medieval each other and the movement of the plates is parallel to the plate boundary. The movement of the plates creates stresses between the plate edges and as they rub past each other the release of friction triggers shallow focus earthquakes. However as there is no subduction, there is no volcanic activity. The best known example of a conservative boundary is the San Andreas cracking in California, where the pacific and North American plates move parallel to each other.Volcanic activity that does not occur along any plate boundary can be the result of many a fault lines and hot spots beneath the crust. Alfred Wegeners theory allowed us to gain insight into the dominance creation of our tectonic plates and their boundaries. The evidence provided by wegeners theory and the record of paleo-magnetism upon the ocean floor supported the idea that the tect onic plates are moving. Supported by the theory of convection currents, the movement of these plates helps our understanding of the distribution of seismic and volcanic events by allowing us to identify varying plate boundaries that create different features and as a result cause these events. This explains their distribution, as events such as these are located in areas above plate boundaries, apart from the odd one which can occur above many a fault lines or hot spots caused by the movement of plates.

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