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Unformatted text preview: EAS 1600 Introduction to Environmental Sciences ____________________________ Class 23- Plate Tectonics: Part 3 The Global View ________________________________ In this lecture we see how geophysicists have used all the various aspects of the theory of plate tectonics to piece together to provide a: comprehensive global view , of how our dynamic earth works and has evolved. The Plates and Their Motion Today By combining seismic, geologic, and other data, geophysicists have mapped the worlds plates and their current direction and rate of motion. Here are the so-called major plates: Note the correspondence of the plate boundaries to regions of major orographic and tectonic activity By combining the information on plate boundaries with our knowledge of where the rifts are located, and motion of lithosphere over hotpsots, we can deduce the directions the plates are moving as well as their speed. Taken from Monroe, J.S. and R. Wicander, Physical Geology: Exploring the Earth , West Publishing, St. Paul, MN, 639 pp, 1992 Question: There are some strange looking trajectories for some of the plates. For example, the eastern edge of the North American plate is moving westward (away from the Mid-Atlantic Ridge, but the western edge of the plate along the west coast of US is moving southward. How can that be? Answer: The arrows represent relative motion. In fact, the sum of all arrows must equal 0. Do you know why? The sum of all arrows must equal 0, otherwise earths surface would shrink or expand. Do a plate cycle and confirm that the surface is conserved. For example, consider motion relative to the South American Plate (SAP) Nazca Plate (NP) is moving 10.1 cm/yr to the east relative to SAP Pacific Plate (PP) is moving 17 cm/yr to west relative to NP, so 7 cm/yr west relative to SAP Indian-Australian Plate (IAP) moving 7 cm/yr east relative to PP; 0 cm/yr east relative to SAP African Plate (AP) is moving 4 cm/yr west relative to IAP; 4 cm/yr west relative to SAP...
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- Spring '08