1. Seafloor Spreading Fill in your student handout

2. Review of continental drift HypothesisEvidence Wegener: Continents had once been joined to form a single supercontinent (Pangaea) Continental puzzles Matching fossils different landmasses Tropical fossils found in Arctic Rock Types Mountains separated by ocean Coal fields Ancient Climates Glacial deposits in present mild climates Rejection:Could NOT explain what caused the landmasses to move or how they moved (mechanism).

3. Seafloor Spreading

4. Mapping the Ocean floor New technology (~1950s) New technology (~1950s) Sonar -uses sound waves to calculate the distances to an object Sonar -uses sound waves to calculate the distances to an object DISCOVERY –mid-ocean ridges stretch along the center of much of the Earth’s ocean floor –Deep-ocean trenches: along the edge of some ocean basins (deepest parts; Mariana Trench is 11 km deep)

5. longest feature on Earth, winds more than 70,000 km through all the major ocean basins 2-3 km above ocean basins 1,000 to 4,000 km wide Mid-Ocean Ridges

6. Iceland: mid-ocean above the surface

7. Process of Seafloor spreading at mid-ocean ridge Hypothesis of seafloor spreading: Hess Hypothesis of seafloor spreading: Hess 1. Rift or split in the crust 2. Molten rock forced up into the rift (its density is less than surrounding rock) 3. Flows sideways, carrying seafloor away from the ridge in both directions

8. 4. Magma cools and becomes solid – new seafloor 5. New sea floor moves away from the ridge *cools, contracts and becomes denser 6. denser, colder seafloor sinks helping to form the ridge 7. Subduction: old ocean floor sinks beneath the trench and returns to the mantle

9. Evidence for seafloor spreading

10. 1. Mid-ocean ridges 2. Age of the seafloor drilling rig drilled into the seafloor to obtain rock samples (1968 Glomar Challenger) drilling rig drilled into the seafloor to obtain rock samples (1968 Glomar Challenger) DISCOVERY: DISCOVERY: –youngest rocks located at the mid-ocean ridges –ages of rocks become increasing older farther from the ridges

11. Age of sea floor as measured by fossils - Older as one moves away from ridges - Youngest rock is next to the ridge

12. 3. Magnetic field reversals Using a magnetometer (towed behind a boat) Using a magnetometer (towed behind a boat) –detects magnetic fields in iron-bearing minerals found in the rocks of the seafloor DISCOVERY: many periods of magnetic reversals in strips parallel to the mid- ocean ridges. DISCOVERY: many periods of magnetic reversals in strips parallel to the mid- ocean ridges.

13. Magnetic field reversal Proves the Earth magnetic field reverses itself every 27000 years Proves the Earth magnetic field reverses itself every 27000 years

14. Why are magnetic reversals important? showed that new rock was being formed at the mid-ocean ridges showed that new rock was being formed at the mid-ocean ridges explained how the crust move explained how the crust move

15. Missing link Seafloor spreading was the missing link Wegener needed to complete his model of continental drift. Seafloor spreading was the missing link Wegener needed to complete his model of continental drift. How do the continents move? How do the continents move? –SEAFLOOR SPREADING (we will soon learn what causes the continents to move)

16. Pillow Lava rocks