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2/10/16 EVIDENCE AFTER WEGENER: SEAFLOOR SPREADING AND PALEOMAGNETISM Objective: I will elaborate on the concepts of seafloor spreading and magnetic striping, used as evidence for the Theory of Plate Tectonics. Page 61 Mid-Ocean Ridges • prior to 1940, scientists thought the seafloor was flat • after WWII, new submarine technology used to map the ocean floor • they discovered a worldwide underwater mountain range • called it the Mid-Ocean Ridge • this led to new curiosity about Earth’s crust Mid-Ocean Ridges Seafloor Spreading • Harry Hess developed the idea of seafloor spreading (1962) by looking at the ages of oceanic crust • oldest continental crust ~4 billion years old, oldest oceanic crust only ~175 million years old Seafloor Spreading • crust was younger by mid-ocean ridges and older further away from the ridge • Hess proposed that new oceanic crust was being made at cracks in the Mid-Ocean ridge called rifts Seafloor Spreading Paleomagnetism • Earth’s magnetic field has flipped many times in history • when new crust is formed, it “locks in” the current magnetic field direction of Earth • normal polarity- magnetic fields that point north • reverse polarity- magnetic fields that point south Paleomagnetism • scientists noticed an alternating pattern of normal and reverse polarity in oceanic rocks • this proved that oceanic crust is created at different times, supporting seafloor spreading Magnetic Striping and Symmetry