1. REVIEW: Seismic Waves give us information on:
1) Plate boundary locations
2) Types of plate boundaries
3) Internal Structure of the Earth
4) How Plates can move – Brittle Lithosphere “floating” on hot plastic Asthenosphere.

2. We can use gravity to examine the Earth’s Interior

3. ISOSTACY

5. Topography of the Sea-floor
Development of the fathometer (echo –sounding) for World War II

6. Harry Hess and Sea-floor Spreading
Hess had no real evidence that this was occurring.
He called is own theory “Geopoetry”
Evidence would be provided by new data from
The Earth’s Magnetic Field

7. The Earth has a magnetic field – a Geodynamo

8. A B

10. The Earth’s Magnetic Field has
reversed itself many times during the geologic past.

11. Polar Wandering?
Or……… Continental Drift?

15. So, Earth’s Magnetism Explains:
Apparent polar wandering
Magnetic reversals recorded in rocks
Seafloor spreading

16. Final Proof of Sea-floor spreading came from dating the Ocean floor

19. MAP OF PLATE BOUNDARIES

20. 1) Divergent Boundaries