1. Plate Tectonics A Unifying Theory

2. Early Theory Contraction Theory – Young earth was made up of molten rock – Contracted (got smaller) as it cooled forming mountain ranges.

3. Early Theory Realized that the earth was heated by the breakdown of radioactive elements – Earth might be getting warmer not cooler and expanding

4. Alfred Wegener Wasn’t buying it! In 1910, Started lecturing and writing about his new hypothesis

5. The Continental Drift Hypothesis Continents were all one supercontinent called Pangaea. – Pangaea fragmented and pieces drifted to the current location of the continents

6. Problems with Continental Drift Scientist could not explain what caused continents to move Didn’t believe the evidence was conclusive

7. Evidence for Continental Drift Matching Coastlines – South America and Africa Matching Geology – Similar age of rocks on the coast – Similar geologic features

8. Evidence for Continental Drift Fossil Evidence – Glossopteris fossils in South Africa, South America, Antarctica and India

9. Evidence for Continental Drift Glacial Deposits – Grooves from glacial sheet movement match up

10. Evidence for Continental Drift Paleomagnetism- The magnetic poles of the earth have changed over time – Igneous rocks magnetize based the location of the poles while they are cooling. (mineral grains will point to magnetic north)

11. The Missing Piece Paleomagnetism in the sea floor – Sea floor was made up of bands of igneous basalt with alternating polarities – Bands were symmetric across the mid-Atlantic ridge

12. Sea Floor Spreading Rocks on either side of the ridge were moving away from each other New molten rock was filling the gap as the floor spread. Older rocks are farther from the ridge and new rocks are closer

13. How fast do the plates move?