1. Evidence for Continental Drift Wegener (1915) proposed Pangaea – one large continent existed 200 million years ago Wegener (1915) proposed Pangaea – one large continent existed 200 million years ago Panthalassa – one large ocean Panthalassa – one large ocean Noted puzzle-like fit of modern continents Noted puzzle-like fit of modern continents

2. Evidence for Continental Drift Matching sequences of rocks and mountain chains Matching sequences of rocks and mountain chains Similar rocks on different continents Similar rocks on different continents

3. Evidence for Continental Drift Glacial ages and other climate evidence Glacial ages and other climate evidence Evidence of glaciation in now tropical regions Evidence of glaciation in now tropical regions Direction of glacial flow and rock scouring Direction of glacial flow and rock scouring Plant and animal fossils indicate different climate than today Plant and animal fossils indicate different climate than today

4. Evidence for Continental Drift Distribution of organisms Distribution of organisms Same fossils found on continents that today are widely separated Same fossils found on continents that today are widely separated Modern organisms with similar ancestries Modern organisms with similar ancestries

5. Objections to Early Continental Drift Model Hostile criticism and open ridicule; Wegner was professional toast Hostile criticism and open ridicule; Wegner was professional toast Continents cannot plow through ocean basins Continents cannot plow through ocean basins Tidal gravitational attractions too small to cause actual crustal movement Tidal gravitational attractions too small to cause actual crustal movement

6. Evidence for Plate Tectonics Earth’s magnetic field and paleomagnetism Earth’s magnetic field and paleomagnetism Earth has magnetic polarity Earth has magnetic polarity North and South polarities North and South polarities Magnetic polarity recorded in igneous rocks Magnetic polarity recorded in igneous rocks –Magnetite in basalt

7. Magnetic Polarity Reversals  Earth’s magnetic polarity reverses periodically  Recorded in ancient rocks

8. Paleomagnetism and the Ocean Floor 1955 – deep water rock mapping 1955 – deep water rock mapping Magnetic anomalies – regular pattern of north-south magnetism “stripes” Magnetic anomalies – regular pattern of north-south magnetism “stripes” Stripes were symmetrical about the Mid Atlantic ridge suggesting that an age sequence of the rocks was also symmetrical Stripes were symmetrical about the Mid Atlantic ridge suggesting that an age sequence of the rocks was also symmetrical

9. Sea Floor Spreading Harry Hess Harry Hess Depth recordings show sea floor features Depth recordings show sea floor features History of Ocean Basins History of Ocean Basins Sea-floor spreadingSea-floor spreading Mantle convection cells as driving mechanismMantle convection cells as driving mechanism

10. Plate Tectonic Processes: New Rocks form at the spreading zone

11. Sea Floor Spreading Evidence Frederick Vine and Drummond Matthews (1963) – everyone now believes in Wegner’s idea … Frederick Vine and Drummond Matthews (1963) – everyone now believes in Wegner’s idea … Sea floor stripes record Earth’s magnetic polarity Sea floor stripes record Earth’s magnetic polarity Two halves of ocean floor are mirror images Two halves of ocean floor are mirror images

12. Age of Ocean Floor Late 1960s deep-sea drilling Late 1960s deep-sea drilling Radiometric dating of ocean rocks Radiometric dating of ocean rocks Symmetric pattern of age distribution about mid-ocean ridges Symmetric pattern of age distribution about mid-ocean ridges Oldest ocean floor only 180 million years old Oldest ocean floor only 180 million years old