1. Seafloor Spreading What evidence do scientists have to support the fact that the Earth’s crust is continuously moving?

2. Mapping the Ocean Floor Until the mid-1900s people thought that the ocean floor was flat

3. Sonar A device used to map ocean floor Sound waves reflected back from ocean floor Time measured and used to calculate distance to ocean floor

4. Seafloor Spreading Hypothesis that new ocean crust is formed at mid-ocean ridges and destroyed at deep sea trenches Occurs in a continuous cycle

5. Seafloor Spreading Mid-Ocean Ridges ◦ Underwater Mountain Chain ◦ Divergent Plate boundaries ◦ Create New Ocean Floor

6. Seafloor Spreading Deep-Sea Trenches ◦ Narrow Elongated depression in seafloor with very steep sides ◦ Convergent Plate boundaries

7. Seafloor Spreading Process 1. Magma (hotter and less dense) forced up and fills gap in ridge 2. When it hardens, becomes new ocean floor 3. As spreading occurs, more magma is forced upward and the crust moves away from ridge 4. Crust is destroyed by subduction at trenches

8. Seafloor Spreading Process

9. Rock Ages Ridges – Younger Trenches – Older Ocean Crust – 180 Million Years Old Continental Crust – 3.8 Billion Years Old

11. Magnetism Paleomagnetism ◦ Study of Earth’s magnetic field ◦ Basalt is iron rich which produces magnetic readings; acts as a compass Magnetic Reversal ◦ Periodic change in Earth’s magnetic field ◦ Reflected in oceanic crust

13. What causes the plates to move? Convection Currents of magma in the mantle Ridge Trench

14. Wegener’s Flaws Seafloor Spreading answers Wegener’s Flaws! ◦ Convection Currents explain HOW the continents “drifted” ◦ Continents not pushed through crust rather they are passengers as ocean crust moves away from ridges