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

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. Ocean Ridges
Underwater Mountain Chain
Divergent Plate boundaries
Deep Sea Trenches
Narrow Elongated depression in seafloor with very steep sides
Convergent Plate boundaries

6. Magma (hotter and less dense) forced up and fills gap in ridge
When hardens adds new ocean floor
As spreading occurs, more magma is forced upward and the crust moves away from ridge
Crust is destroyed by subduction at trenches

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

10. Magnetism Paleomagnetism Magnetic Reversal
Study of Earth’s magnetic field
Basalt is iron rich which produces magnetic readings; acts as a compass
Magnetic Reversal

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

13. 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