Download Evidence after wegener: seafloor spreading and

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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
• 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
• 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