1. Earth Science Historical Introduction Sea Floor Spreading & the Earth’s Magnetic Field

2. During WWII ► Maps of ocean floor using  Sonar  Magnetometers Harry Hess, a Captain during WWII, gathered data while cruising from battle to battle

3. 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)

4. Dark blue – deepest areas Yellow – shallowest areas

5. Surprising Data ► Symmetrical patterns of magnetic reversals centered on ocean ridges

6. The Earth’s Magnetic Field Reverses! ► Reversals are “recorded” as iron- rich lava cools ► Reversals occur every ~200,000 years  We haven’t had one for 700,000 years!

7. Reversals recorded in lava flow

8. Age Evidence ► Ocean crust is much younger than continental crust ► Youngest rocks are found closest to ocean ridges

9. 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)

11. Process is driven by convection currents within asthenosphere