240PreLab01PlateTectonics

240PreLab01PlateTectonics - PRELAB EXERCISE #1 revised...

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PRELAB EXERCISE #1 revised sp’05 GEOL 240 Week of Sept. 7-11, 2009 PLATE TECTONICS AND PLATE BOUNDARIES Materials Needed: scientific calculator pencil Introduction The single most important concept in earth sciences today is the theory of plate tectonics . Plate tectonics is the idea that the outermost part of earth, or the lithosphere , is made up of twelve large plates that move past each other (Fig. 1). The lithosphere is the strong, rigid outer shell of earth that contains the crust and the uppermost part of the mantle. Color Figure 1. The major lithospheric plates on earth’s surface and their related plate boundaries. The numbers represent the present-day rate or plate motion and the arrows indicate the direction of plate motion. Numbers are centimeters per year based on space-based geodesy measurements (NASA). (from Busch, ed., 2003 AGI/NAGT Laboratory Manual in Physical Geology). Each of these plates acts as a separate entity that moves over the asthenosphere , the weak, partially molten part of earth in the upper mantle (Fig. 2). The mantle is the region of earth between the crust and the core and includes the lower lithosphere and asthenosphere. The plate tectonics theory explains every major tectonic feature on earth’s surface as well as the occurrence, location, and origin of almost all earthquakes. For this reason, we need to have a basic understanding of these concepts in order to study and understand earthquakes. 1 of 5
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Color Figure 2. Block diagram showing the locations of the crust, lithosphere, and asthenosphere in a divergent, mid-ocean ridge plate boundary (from Busch, ed., 2003, AGI/NAGT Laboratory Manual in Physical Geology). Plate Boundaries and Fault Types The outermost part of earth is composed of continental crust and oceanic crust . The average thickness of continental crust is 40 km and the average thickness of oceanic crust is 7 km. For comparison, a marathon is 42 km (26.2 miles). The rigid lithosphere is typically about 100 km thick (Fig. 2). Beneath the lithosphere is the weaker asthenosphere, over which the lithospheric plates move. The boundaries between the lithospheric plates delineate surfaces along which the plates are moving past each other. The majority of these boundaries are faults . A fault is a fracture or discrete surface separating two bodies of rock which have moved relative
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240PreLab01PlateTectonics - PRELAB EXERCISE #1 revised...

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