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Plate tectonics.

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Plate Tectonics Liana Herzig Geology 1/23/04 The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which "float" on the fluid-like asthenosphere. Due to convective currents in the asthenosphere, the tectonic plates move in different directions. The point where one plate meets another is known as a plate boundary; these areas are commonly associated with geological events and features such as earthquakes, mountains, volcanoes, and oceanic trenches. Plate boundaries are home to most of the world's active volcanoes. Tectonic plates are broadly divisible into two groups of plates: continental and oceanic. The distinction is based on the density of their constituent materials; oceanic plates are denser than continental plates due to their greater silicate mineral content. As a result, the oceanic plates are generally below sea level, the continental plates above. The left or right lateral motion of one plate against another along transform or strike slip faults can cause highly visible surface effects. ...read more.


(Mantovani, E. et all, 2001) The birth of divergent boundaries is sometimes thought to be associated with phenomena known as hotspots. Giant convective cells bring large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be enough to break apart the lithosphere. It is believed that there is a hot spot located in the Mid-Atlantic Ridge system, which currently is under Iceland and widening at a rate of a few centimetres per year. Divergent boundariesare shown in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge, and in the continentental lithosphere by rift valleys such as the famous East-African Rift. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not parallel, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are fracture zones, a major source of submarine earthquakes. (Tilling, 1985) Scientists found one of the most important pieces of evidence at the mid-ocearn ridges, forcing acceptance of the sea-floor-spreading hypothesis. ...read more.


When two oceanic plates converge they form an island arc as one oceanic plate is subducted below the other. Japan is a good example of this. (Noson, Qamar, and Thorsen, 1988) The Continental Drift was first proposed in 1912 by Alfred Wegener, who noticed the similarity in the shape of the coasts of Africa and South America. His controversal and radical ideas were not taken seriously by geologists of the time, who pointed out that there was no visible or possible mechanism for continental drift. This changed drastically in the 1960s, when Wegener's theory was verified by a number of discoveries, most notably the Mid-Atlantic ridge. With plate tectonic evidence quickly falling into place, the answer became clear. Collisions of converging plates had the force to lift the sea floor into thin atmospheres. The cause of marine trenches, strangly placed just off island arcs or continents and their associated volcanoes, became clear when the processes of subduction at converging plates were understood. Within a matter of only a few years, geophysics and geology were revolutionized. Figure 1 Subduction Boundaries (www.tahomascience.com/geology/ notes.contdrift.html) Figure 2 Divergent Boundaries (www.tahomascience.com/geology/ notes.contdrift.html) Figure 3 Transform Boundary (cronopio.geo.lsa.umich.edu/ Lec3/lec3. ...read more.

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