Professor Thomas Orf
Chapter 14 Questions
1) The first evidence Wegner was able to find was the remarkable number of close affinities of geologic features on both sides of the Atlantic Ocean. He found the continental margins of the subequatorial portions of Africa and South America fit together with jigsaw-on-like precision. He also determined that the petrologic records on both sides of the Atlantic show many distributions- such as ancient coal deposits-that would be continuous if the ocean did not intervene. 2) Despite the vast number of distributional coincidences, most scientists felt that two difficulties made the theory improbable if not impossible: (1) Earth’s crust was believed to be too rigid to permit such large-scale motions-after all, how could solid rock plow through solid rock? (2) Further, Wegener did not offer a suitable mechanism that could displace such large masses for a long journey. 3) When any rock containing iron is formed-such as iron-rich ocean floor basalt-it is magnetized so that the iron-rich grains become aligned with Earth’s magnetic field. This orientation then becomes a permanent record of the polarity of Earth’s magnetic field at the time the rock solidifies. In 1963, Fred Vine and D.H. Matthews used paleomagnetism to test the theory of seafloor spreading by studying paleomagnetic data from a portion of the midocean ridge system. If the seafloor has spread laterally by the addition of new crust at the oceanic rides, there should be a relatively symmetrical pattern of magnetic orientation-normal polarity, reversed polarity, normal polarity, and so on-on both sides of the ridges. Another confirmation of seafloor spreading is that several thousand ocean floor cores of sea-bottom sediments were analyzed, and it was evident from this work that, almost invariably, sediment thickness and age increase with increasing distance from the midocean ridges, indicating that sediments farthest from the ridges are oldest. 4) The relationship between earthquakes and plate tectonics are that plate tectonics tells us about the outer shell or also known as the lithosphere. the lithosphere is the thinnest part of the earth. 5) Plate tectonics is a coherent theory of massive crustal rearrangement based on the movement of continent-sized lithospheric plates. The driving mechanism for plate tectonics is thought to be convection within Earth’s mantle. A very sluggish thermal convection system appears to be operating within the planet, bringing deep-seated heated rock slowly to the surface. Plates may be “pushed” away from midocean ridges to a certain extent, but it appears that much of the motion is a result of the plates being “pulled” along by the subduction of dense oceanic lithosphere down into the asthenosphere. 6) Divergent plate boundary is a location where two lithospheric plates spread apart. Convergent plate boundary is a location where two lithospheric plates collide. Transform plate boundary is two plates slipping past one another laterally. 7) A divergent boundary is usually represented by a midocean ridge. Most of the midocean ridges of the world are either active or extinct spreading ridges. Such spreading centers are associated with shallowing-focus earthquakes (meaning that ruptures that generate the earthquakes are within about 70 kilometers [45 miles] of the surface), volcanic activity, and hydrothermal metamorphism. Divergent boundaries can also develop with a continent, resulting in a continental rift valley such as the great East African Rift Valley that extends from Ethiopia southward through Mozambique. 8) Oceanic-continental convergence: Because oceanic lithosphere includes dense basaltic crust, it is denser than continental lithosphere, and so oceanic lithosphere always underrides continental lithosphere when the two collide. The dense oceanic plate slowly and inexorably sinks into the asthenosphere in the process of subduction. The...
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