1. Honors 1360 Planet Earth Last time: Leading Up to Plate Tectonics… Obs : Mountain mass balanced by deep buoyancy Hyp : “Solid” rock flows on long time scales Obs : “Jigsaw Puzzle” fit of continental boundaries Obs : Paleoclimate: Fossils, glacial striae at wrong latitudes Obs : Similar mountain ranges, rock formations, fossils on either side of the Atlantic Hyp : Continental Drift: Continents once were one large supercontinent (Pangea) & drifted to current locations Obs : Ocean Bathymetry Hyp : ??? Today: Plate Tectonics Cont’d: A Paradigm Shift 15 September 2008

2. Earth in the News… Unusually long “earthquakes” are actually “icequakes” in actively moving glaciers… Greenland icequakes show significantly increasing frequency in the past three summers. September 2008 “Physics Today”

3. Then (ca. 1950’s) came the Cold War… Measured Ocean Bathymetry to facilitate submarine warfare; discovered mid-ocean ridges, trenches, and fracture zones

4. As part of an ultimately fruitless attempt to use magnetism to search for subs, measured Seafloor Magnetic Lineations These were found to be Symmetric Across Ridges! Now Here’s a Puzzle…

5. To understand, needed two pieces of information from continental studies of rock magnetism: (1) Rocks acquire the ambient magnetic field direction as they cool through the Curie temperature (called remanant magnetization) (2) Magnetic dipole direction occasional Reverses

6. Magnetic Lineations (Symmetric Across Ridges!) Magnetite in basalt records field direction when solidified Field direction changes! Ocean floor slowly moves away from ridges “Seafloor Spreading” Testable Predictions?

7. Age of the Ocean Floor Drilled/dredged rocks from seafloor and age-dated them!

8. Question: Why Are There Ridges Where The Ocean Is Youngest?

9. Wikipedia:

10. WRONG! Never trust a Wiki…. Ocean crust is ~ constant 6 km thickness! Always remember foundations of CRITICAL THINKING :  Use multiple sources of information  Evaluate the reliability of sources  Think about what makes sense!

11. Recall thermal expansion & thermal contraction? Hot Rock Cool Rock fast seismic velocity slow seismic velocity