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Continental Drift Seafloor Spreading and Magnetic Reversals • Predecessor to modern plate tectonics • Shape and “fit” of the continents was the initial evidence – Snider-Pelligrini (1858) – Taylor (1908) – Wegener (1915) Brad Hubeny 1 Wegener (1915): Continental Drift • Proposed that all of the continents were once part of a large supercontinent - Pangaea • Based on: • Similarities in shorelines • Terrestrial Geologic Evidence • Distinctive fossil groups found in Africa & South America 2 Terrestrial Geologic Evidence Only occur in rocks > 145 Ma BP! 3 4 Paleontological evidence •No mechanism to transport across ocean •Divergence of species following break-up 5 Seafloor Spreading • Wegner proposed a mechanism for drift – Less dense silicic rocks plowed through more dense ocean floor – Earth’s rotation was driving force • Although supporting evidence existed, the theory was not widely accepted • Lack of testable hypothesis!! 6 1 Geology of the Ocean Floor Development of Plate Tectonic Theory • Original evidence for continental drift was from continental rocks • Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor • New data provided intriguing new information! • Doc Ewing does NOT find granite when he dredges the ocean bottom!! 7 8 Geology of the Ocean Floor Bathymetry Reveals Oceanic Rift System • Bathymetry of the ocean basins – Sonar revealed Ridge System in ocean basins – Ridge system is continuous around the entire globe – Central rift valley within the ridge 9 10 Harry Hess’ “Essay in Geopoetry” Earthquakes and Volcanoes • • • • 11 Seafloor spreading proposed in 1960 paper Considered new data on ocean floor Proposed NEW more robust mechanisms Great HYPOTHESIS, but needed to be tested 12 2 Earth’s Magnetic Field Enter Paleomagnetics! • Magnetic reversals Geodynamo System! – Earth’s magnetic field polarity has reversed through time • Normal polarity – N magnetic = N geographic • Reversed polarity - Nmagnetic = S geographic • Iron-rich basalts of ocean crust preserve this record of the magnetic field! 13 Geologic Magnetic Recorders 02_10.jpg 14 Polarity Reversals 15 16 Reversals & the Geologic Record 02_23.jpg Geomagnetic Time Scale 17 18 3 Oceanic Magnetometers Development of Plate Tectonic Theory • Vine & Matthews (1963) tested Hess’s hypothesis using magnetism – Magnetic polarity reversals recorded in ocean floor basalt • Magma cools forming new crust • Polarity at time of cooling preserved • Old crust pushed aside • Became a Scientific Revolution! 19 20 Seafloor Ages Seafloor Magnetic Anomalies 21 22 4