1. The three pillars of Earth history Stratigraphy Isotope geochemistry Paleomagnetism & plate tectonics

2. The last 200 million years of earth history revealed by paleomagnetism and sea floor spreading* *Link to Quicktime movie

3. Alfred Wegener, a great SES scientist! (1880-1930) - Thermodynamics of the Atmosphere (1911) - The Origin of Continents and Oceans (1915,1929) - Climate and Geological Pre-history (1924)

4. quantitative fitting of the shapes of W. Africa and E. South America

5. The geology matches very closely Proterozoic orogenic belts >2000 Ma cratons

6. North America, Greenland, and Africa can be tied together

7. And the southern hemisphere Gondwana continents India Australia Antarctica South America Africa Madagascar

8. Late Paleozoic Gondwana glaciation

9. Paleozoic and Mesozoic mountain belts

10. Fossil distributions

11. The evidence for continental drift was compelling and well established during the first half of the 20 th Century but it was not accepted by many geologists (almost all U.S. geologists) until the advent of plate tectonics in the late 1960’s-early 1970’s! Why??

12. Wegener was aggressively attacked and then ignored - because of his lack of geological credentials - his geophysical theories for the dynamics of continental drift were off track

13. Why?? The focus of geologists was on complex continental geology at local to regional scales, not global scales.

14. Why?? The obvious importance of vertical motions in continental geology, leading to the "fixest" (versus "mobilist") synthesis that was the reigning theory for much of the first half of the last century.

15. Why?? the ocean basins were largely “terra incognita” before WWII

16. WWII-Cold War explosion of technology and observations  Marine geophysics  Isotope geochemistry  Global seismological networks  Digital computers Plate tectonics Revolution (1962 – 1972)

17. Simplified view of Earth’s tectonic plates

18. divergent boundary convergent boundary lithospheric plate asthenosphere Plate boundary types

19. divergent boundaries Transform fault boundary lithospheric plate asthenosphere Plate boundary types

20. Relative motions across modern plate boundaries cm/year

21. Continental rifting: formation of an ocean

22. Continental rifting in the Middle East: Red Sea and Gulf of Aden

23. Convergent boundary: subduction zones South American Andes island arc

24. Plate Tectonics revolution The key breakthroughs: Paleomagnetism Seafloor Spreading Contrasting oceanic and continental crustal structure Seismicity of plate boundaries

25. Plate Tectonics revolution The key breakthroughs: Paleomagnetism Seafloor Spreading Contrasting oceanic and continental crustal structure Seismicity of plate boundaries Geomagnetism: major key to Earth’s history

26. seafloor age, Ma Age of ocean floor recorded by Seafloor Spreading “tape recorder”