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plate tectonics.docx

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Earth Sciences
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Maureen Padden

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Plate Tectonics Pangaea Wegener definition for the supercontinent that assembled at the end of the Paleozoic era and then fragmented into separate continents into where they are found today Continental drift continents have moved and are still moving across the Earths surfaceSeafloor spreading Hess and Dietz suggested that continents moved toward each other when old ocean floor between them sinks back down into Earths interior this process is now called subduction Theory of plate tectonics the theory that the lithosphere consists of separate plates that move with respect to one another Glaciers are rivers or sheets of ice that flow across the land surface As a glacier flows it carries sediments of grains of all sizes Grains protruding from the base of the moving ice carves scratches called striations in the substrate When the ice melts it leaves the sediment in deposit called till and buries striations The occurrence of till and striations at a location serves as evidence that the region was covered by a glacier at a point in time By studying the age of glacial till deposits geologists have determined that large areas of the land were covered by glaciers during the ice age326267 Ma end of Paleozoic era Wegener said that once all the continents of Australia Antarctica South America South Africa and South India were all adjacent to each other on the map of Pangaea Magnetic Dipole imaginary arrow that points from the north magnetic pole to the south magnetic pole of a magnetic field Magnetic pole ends of magnetic dipole all magnetic dipole have a north pole and south pole At present the magnetic poles lie hundreds of km away from geographic poles so the magnetic dipole tilts at 11 degree relatively to the Earths spin axis Magnetic declination angle between the direction a compass needle points at a given location and the direction of a true northMagnetic inclination angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earths surfacePaleomagnetism record of ancient magnetism preserved in rocks When measuring the magnetic field of a rock the orientation of this field by the imaginary bar magnet does not point to the present day north magnetic pole and the inclination in not 0 degree The reason for the difference is that the magnetic field of ancient rocks indicate the orientation of the magnetic field at the time the rock formed Paleopole refer to the supposed position of the Earths magnetic north pole in the past and geologists went to track the position of the paleopoles over time They measured the paleomagnetism of rocks of different ages from the same general location on the continent and plotted the position of the associated succession of paleopoles on a map
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