150Lec5-1

150Lec5-1 - Beginning of Material for Exam #2: Lecture #5:...

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Beginning of Material for Exam #2: Lecture #5: Plate Tectonics (Abbott, Chapter 3 and p. 211) The Theory of Plate Tectonics The theory of plate tectonics states that the Earth’s lithosphere (which includes the rigid upper mantle and both the continental and oceanic crusts) is broken up into several plates that slide past, into or away from each other over time (Figure 3.3, p. 52). Some plates, such as the Nazca Plate, consist entirely of oceanic crust. Other plates, for example, the North American plate, have significant continental crust. In plate tectonics, the rigid lithospheric plates slowly move on the plastic portion of the mantle, the asthenosphere (Figure 3.2, p. 51; Figure 2.11, p. 30). The top of the asthenosphere is usually located at depths of about 100 kilometers into the Earth and the layer is about 250 kilometers thick (p. 32; Figure 2.10, p. 29). Earth's Interior Figure 2.10 (p. 29) shows a diagram of the Earth's interior. The continental crust is 30-70 kilometers thick and generally has a felsic (Lecture #3) composition (Figure 2.11, p. 30). The continental crust is thickest in high mountainous regions. The mafic oceanic crust is relatively thin, only about 7 kilometers thick (Figure 2.11, p. 30). The boundary between the crusts and the underlying mantle is called the Mohoroviçic Discontinuity or Moho . The upper most mantle probably consists of rigid peridotite , which is an olivine-rich (Lecture #3) rock that has even more iron and magnesium and less silica than a gabbro. Peridotite is often called an ultramafic rock even though it's metamorphic rather than igneous . The Movement of Tectonic Plates Tectonic plates primarily move because of the generation of heat within the Earth from the decay of naturally occurring radioactive elements, such as uranium. The exact mechanism(s) that cause the movement of tectonic plates is (are) unknown. Most of the hypotheses of plate movement involve convection currents . Convection Currents
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In the convection currents that probably move tectonic plates, the hot, plastic, and partially molten materials in the asthenosphere rise towards the crust. The hot materials are less dense than the surrounding cooler rocks. The pressure on the hot rising materials also decreases as the materials rise towards the Earth's surface, simply because there are less overlying rocks exerting pressure on the rising materials. Under lower pressure, the rising materials largely melt to produce mafic magmas that may eventually erupt as lavas and produce basalts at spreading zones (Figure 8.6, p. 189; Figure 3.2, p. 51; Figure 3.8, p. 56). The erupting lavas form new crust at the spreading zones (Figure 3.12, p. 59). Any upper mantle melts that remain within the Earth and that do not erupt will eventually cool. When they cool, they become denser, and may sink back into the deeper mantle. In the deeper mantle, they reheat and rise again. The cycle of rising hot and sinking cooler materials creates the convection currents, which may move the overlying tectonic plates,
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150Lec5-1 - Beginning of Material for Exam #2: Lecture #5:...

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