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