Plate Tectonics1

Plate tectonics are a relatively new theory that has revolutionized the way geologists think about the Earth. According to the theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The hypothesis of continental drift was largely developed by the German Alfred Wegener The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth\'s surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges. Plate Boundaries are located around the boarder of the plate. It is where the plates collide. There are different types of Boundaries, Divergent Plates transform boundary Subduction boundary and collisional boundary. A divergent boundary occurs where two plates move apart, allowing magma, or molten rock, to rise from the Earth\'s interior to fill in the gap. The two plates move away from each other like two conveyor belts moving in opposite directions. Plate area is increased as the plates move apart. Plate movement takes place laterally away from the plate boundary, which is normally marked by a rise or a ridge. The ridge or rise may be offset by a transform fault. Presently, most divergent margins occur along the central zone of the world\'s major ocean basins. The process by which the plates move apart is referred to as sea floor spreading. The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this type of plate margin. Also known as spreading boundary. The rate at which each plate moves apart from a divergent margin varies from less than 50 mm per year to over 90 mm per year and can be determined from the pattern of magnetic anomalies either side of a spreading ridge. Either side of a spreading center, weak magnetic anomalies 5-50 km wide and hundreds of kilometers long can be identified. Molten rock cools between diverging plates the magnetic minerals present align themselves with the orientation of the Earth\'s magnetic field at that time. The polarity of the Earth has changed at regular intervals throughout geological time. Magnetic north has alternated between the Arctic (normal polarity) and the Antarctic (reversed polarity). These long linear strips of magnetic anomalies form a symmetrical pattern either side of a spreading center. A record of the changes in the Earth\'s magnetic polarity has been established and dated for the Cenozoic and is the basis for magnetostratigraphy. This record, in conjunction with the magnetic stripes found either side of a spreading ridge, allows the rate and pattern of sea floor spreading to be examined. At a convergent boundary two plates are in relative motion towards each other. One of the two plates slides down below the other at an angle of around 45 degrees and is incorporated into the Earth\'s mantle along a subduction zone. The path of this descending plate can be found from analysis of deep earthquakes and the initial point of descent is marked on the surface by a deep ocean trench . Plate area is reduced along the subduction zone. When two plates of oceanic crust collide a volcanic island arc may form. As one of the plates is subducted beneath the other it begins to melt at a depth of between 90 and 150 km and the resulting magma rises to the surface above the subduction zone to form a chain or arc of volcanoes. The edge of the plate, which is not descending, is therefore marked by a chain of volcanic islands. A transform boundary occurs where two plates slide against each other. But rather than sliding smoothly, the plates build up tension, then release the tension with a spurt of movement. This movement is felt as an earthquake. The San Andreas fault system is the most famous example of this type of boundary. Here two plates move laterally past each other and oceanic crust is neither created nor destroyed. A collisional boundary occurs where two land masses on plates are pushed