1. To discover how Plate Tectonics works we must study the Earth using several fundamental physical principles & properties: 1)Seismic Waves 2)Gravity 3)Isostacy 4)Magnetism

2. 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.

3. To discover how Plate Tectonics works we must study the Earth using several fundamental physical principles & properties: 1)Seismic Waves 2)Gravity 3)Isostacy 4)Magnetism

4. Gravity: One of the 4 Fundamental forces in the universe 1)Gravity is a force of attraction between any two objects that have mass. 2) The strength of the force is proportional to the mass of the objects and inversely proportional to the distance between them. 3) Differences in the force of gravity can be measured by how a weight stretches a spring at different locations – this is how a scale works.

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

6. Why a positive Gravity Anomaly in the upper Mid-West? Why a negative anomaly across the Rocky Mts.?

7. Ancient Rift Zone Our Continent began to pull apart over a billion years ago but stopped. Dense basaltic magma flowed upward to surface, hence the positive gravity anomaly.

8. Why a negative anomaly across the Rocky Mts.? We need to understand property #3 - Isostacy

9. 1) Seismic Waves 2) Gravity 3) Isostacy 4) Magnetism

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

14. 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

15. 1) Seismic Waves 2) Gravity 3) Isostacy 4) Magnetism

16. The Geodynamo

23. Polar Wandering?

24. Or……. Continental Drift?