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Transcript
Earth Science Historical Introduction Sea Floor Spreading & the Earth’s Magnetic Field During WWII ►Maps of ocean floor using Sonar Magnetometers Harry Hess, a Captain during WWII, gathered data while cruising from battle to battle Surprising Data ►Deepest areas (trenches) were close to the continents ►Underwater mountain ranges ran through every ocean Each mid-ocean range had a central valley running down the middle (rift valley) Dark blue – deepest areas Yellow – shallowest areas Surprising Data ►Symmetrical patterns of magnetic reversals centered on ocean ridges The Earth’s Magnetic Field Reverses! ►Reversals are “recorded” as ironrich lava cools ►Reversals occur every ~200,000 years We haven’t had one for 700,000 years! Reversals recorded in lava flow Age Evidence crust is much younger than continental crust ►Youngest rocks are found closest to ocean ridges ►Ocean Data provides support for “Sea Floor Spreading” ►Lava erupts at mid-ocean ridges and cools, creating new sea floor ►Sea floor dives back under continental crust at trenches (Subduction Zones) Process is driven by convection currents within asthenosphere