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Unformatted text preview: Geol 105 Lab #3 Sea Floor Spreading & Plate Tectonics Week of Jan. 30 Feb. 3, 2012 Introduction New Discoveries in the 20 th century: Sea Floor Spreading During World War II there was a tremendous amount of submarine travel thus making information about the topography (or bathymetry) of the ocean floor invaluable. Harry Hess was a submarine commander during this war. The use of sonar (sound waves, which will bounce off surfaces and return to the sender) to record topography or bathymetry was in use. Hess and others used sonar data to construct an image of the ocean floor. The results were spectacular (see National Geographic maps) and give us a view of what the earths surface would look like if we drained all the water out of the oceans. Four prominent features were discovered: 1) large mountain ranges called mid-oceanic ridges (sometimes the are in the middle of the oceans, such as in the Atlantic, but not always); 2) trenches where the ocean became very deep; 3) long, linear belts of volcanoes called island chains; and 4) large faults, often cutting through the mid-oceanic ridges, called transform faults. Figure caption: Harry Hess (1906-1969) Proposed idea of sea floor spreading in 1962. In 1962 Harry Hess proposed his idea of sea floor spreading (which would shortly after be combined with continental drift to form the Plate Tectonics paradigm). Hess suggested that the mid- oceanic ridges were locations where hot magma came to the surface of the earth to form oceanic floor or crustal plates, which then split and moved apart. Trenches were locations where one oceanic plates slide under another and returned to the mantle. Continents sit on plates and move with them thus explaining the evidence for continental drift. Harry Hess used Holmes idea of mantle convection to drive movement of these plates. Note that Plate tectonics is different from continental drift since oceanic plates move and continents just go along for the ride rather than continents trying to plow through the oceanic plates. Sea- floor spreading also implies that the ocean basins were no longer old since they are constantly getting subducted down trenches and returned to the mantle. In Arthur Holmes 1965 revised book he summarized four new data sets that now convinced him of plate tectonics (and thus continental drift). 1) Clear evidence of large displacement of the continents on great transform faults; 2) evidence from paleomagnetic studies (i.e., studies of magnetic minerals in rocks which record information about the orientation and magnitude of past magnetic fields) had established that polar wandering did occur (by movement of the continents, not the poles) and that magnetic stripes were preserved in ocean floor rocks and were symmetrical on either side of the mid oceanic ridge; 3) advances in geochronology had better established the ages of orogenies and of the ocean floor: specifically the ocean floor turned out to be quite young compared to the age of the earth (the oldest...
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