1. 9.4 and 9.5 notes

2. Review/Preview (True or False?) 1. The main criticism of Wegener’s hypothesis was his lack of a mechanism. 2. Scientists now know the mechanism of plate motion. 3. The mantle consists of liquid magma. 4. Scientists cannot effectively study the mantle because they cannot experiment with it directly.

3. Theory of Plate Tectonics 1968: updated version on Continental Drift based on new evidence Contients and seafloor move New Evidence for Plate Tectonics:

4. 9.4 The Evidence for Plate Tectonics Strongest new evidence came from ocean drilling (new technology) in the 1960’s Geochronology = seafloor age New seafloor at crust gets older on both sides as you move away shows plate movement. No oceanic crust older than 180 million years shows it gets recycled during subduction.

5. Paleomagnetism Preserved in Lava Flows

6. Paleomagnetism Iron-rich minerals act as tiny magnets As they cool, they line up parallel to earth’s magnetic field Earth’s magnetic field periodically reverses (N becomes S)

7. Evidence for PT: strips of alternating polarity in a mirror image across the ocean ridge

8. Hot Spot – mantle plume rises to surface -- not at a plate boundary -- shows that plates move Ex. Hawaiian Islands

9. 9.5 Mechanisms of Plate Motion Driving force for plate movement is the unequal distribution of heat in earth. Convection – method of heat transfer in fluids where hot material rises and cold material sinks Movement is due to differences in density.

10. But isn’t the mantle made of solid rock? “On a human time scale, rocks are solid, but if you wait for a long time – millions of years – rocks slowly change their shape. They flow.” -- Peter Bunge Asst. Professor of Geophysics Princeton University

11. Mantle Convection Models Heat sources: 1. radioactive decay of elements in mantle 2. residual heat from formation

12. How do scientists study mantle convection? Geophysicists use computer modeling to simulate mantle movement.

13. Burkavici http://www.columbia.edu/itc/ldeo/mutter/jcm/Topic3/Topic3.html

14. Mantle convection model animations http://homepage.usask.ca/~sab248/Mtemp28.WMV http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html http://homepage.usask.ca/~sab248/Mtemp28.WMV http://www.gps.caltech.edu/~gurnis/Movies/movies-more.html

15. Proposed Mechanisms : 1. Ridge push 2. Slab pull At present, none is able to explain all facets of plate motion. There are still many unanswered questions about the details of Plate Tectonics.

16. Pangea Ultima http://www.nytimes.com/packages/html/science/20 070109_PALEO_GRAPHIC/ http://www.nytimes.com/packages/html/science/20 070109_PALEO_GRAPHIC/