Sea-Floor Spreading

1. Sea-Floor Spreading

2. Marie Tharp • Oceanographer • First to map details of the ocean floor on a global scale • Her observations became crucial to the eventual acceptance of the theories of plate tectonics and continental drift

3. Marie Tharp's "World Ocean Floor Map” 1977

4. Seafloor Spreading • In the 1960’s, a scientist named Harry Hess made a discovery that would vindicate Wegener • Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges • Harry Hess proposed the sea-floor spreading theory

5. Echo Sounding Device Used By Hess

6. Seafloor Spreading • Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges • Then, it flows sideways, carrying the seafloor away from the ridge in both directions

7. Seafloor Spreading • As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created • The older seafloor moves away from the ridge in opposite directions • This helped explain how the crust could move —something that the continental drift hypothesis could not do

8. Sea Floor Spreading

9. Seafloor Spreading - The process by which molten material adds new oceanic crust to the ocean floor

10. Evidence from Molten Material • Strange rocks shaped like pillows or like toothpaste squeezed from a tube have been found • These rocks can form only when molten material hardens quickly after erupting under water • These rocks showed that molten material has erupted again and again from cracks along the central valley of the mid-ocean ridge Mid Atlantic Ridge – Molten Material

11. Evidence from Magnetic Stripes • Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes” • 780,000 years ago, magnetic poles reversed themselves • If they reversed today, the needle in a compass would point south instead of north • The rock in the ocean is made of iron, which began as molten material Magnetic Reversals

12. Evidence from Magnetic Stripes • Plates move apart • Magma rises and cools forming new sea-floor • youngest rock is near the ridge, it gets older as you move away • The earth’s magnetic field is recorded in the rock as it forms • We have a record of the earth’s magnetic reversals on either side of the ridge 100 Greatest Discoveries: Magnetic Reversals

13. Polarity Reversal • Scientists later discovered that the earth’s magnetic field changes polarity every few thousand years

14. Magnetic Reversals in the Seafloor As the rock cools, it records these magnetic reversals in the seafloor Magnetization of the sea floor

16. Evidence of Sea Floor Spreading

18. Key Idea As distance from the ridge increases, the age of the sea-floor increases

19. Evidence from Drilling Samples • When scientists sampled the rocks, they found that the further away from the ridge the rocks were the older they were • The younger rocks were always in the center of the ridges

20. Important Things to Remember • Theory of sea floor spreading - hot, less dense material below the Earth’s crust rises toward the surface at the mid-ocean ridges. Then it flows sideways, carrying the seafloor away from the ridge in both directions • As distance from the ridge increases, the age of the sea-floor increases

21. Important Things to Remember • Henry Hess proposed the seafloor spreading theory • He proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions

22. Quick Check Turn and tell your neighbor the three pieces of evidence that support the theory of seafloor spreading Sea Floor Spreading Hydrothermal Vents Ocean Exploration