Sea Floor Spreading

1. Sea Floor Spreading p. 331-337

2. Mid-Ocean Ridge • Longest chain of mountains in the world (50,000 km long!) • Mostly underwater • Iceland part of Mid-Ocean Ridge • We use sonar to map the ocean floor

4. How was the Ridge Made? • Harry Hess, studied the mid-ocean ridge • 1960 he suggested ocean floor like conveyor belts, carrying continents with them • Movement begins at mid-ocean ridge

6. At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. What Happens?

7. Sea-Floor Spreading • Sea-floor spreading: the process that continually adds new material to the ocean floor. • Evidence for Sea-floor spreading: • Molten material • Magnetic stripes • Drilling samples

8. Evidence: Molten Material • Rocks shaped like pillows found – only forms when molten material hardens quickly Scientists inside Alvin 4 km deep

9. Evidence: Magnetic Stripes • Magnetic field around Earth • Poles have reversed themselves (last time 780,000 years ago) • Magnetized “stripes” on ocean floor hold record of reversed poles

10. Evidence: Drilling Samples • Rocks drilled from ocean floor • Age of rocks determined • Further away from ridge, older the rock • Youngest rock found in the center of the ridge

11. Subduction at Deep-Ocean Trenches • How can the ocean floor keep getting wider and wider? • Floor plunges into deep underwater canyons called deep-ocean trenches • These occur where oceanic crust bends downward

12. Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.