2. Wegener’s principle of continental drift was often dismissed because he failed to produce a mechanism by which the continents moved

3. In the late 1950’s, a geologist named Harry Hess suggested a new hypothesis Proposed that the valley at the center of a mid-ocean ridge was a crack, or rift. At this rift, magma deep within earth rose to fill the crack As the ocean floor moves away from the ridge, rising magma cooled and solidified to form new crust

4. Known as sea-floor spreading Hess suggested that if the ocean floor is moving, the continents might be moving too

5. The evidence for sea- floor spreading is found in paleomagnetism The study of the magnetic properties of rocks

6. Earth acts as a giant magnet due to the activity of the inner and outer core As magma solidifies to form rock, iron-rich minerals in the magma align with Earth’s magnetic field in the same way that a compass needle does When the rock hardens the magnetic orientation of the minerals becomes permanent

7. Magnetic Reversals Geologic evidence shows that Earth’s magnetic field has not always pointed north as it does now

8. Scientists have dated rocks from different points in Earth’s history which show differing magnetic properties Classified by these properties Help to determine how quickly the crust is moving, and for how long Some rocks show normal polarity, meaning they formed when magnetic north was “normal”, and some rocks show reversed polarity, meaning they formed when magnetic north actually pointed south Information is used to create a pattern known as the geomagnetic reversal time scale

10. Magnetic Symmetry Scientists use the geomagnetic reversal time scale to study the ocean floor They observed that the patterns that occurred on one side of the ridge were mirrored on the other side Information gathered from this symmetry is used to assign ages to the sea-floor rocks The rocks that were located closer to the ridge were younger than rocks that were located further way from the ridge Supports Hess’s claim of sea-floor spreading

12. Mantle Convection Convection is the movement of heated material due to differences in density that is caused by differences in temperature Energy generated by Earth’s core and radioactivity within the mantle heat the mantle material Results in the rising of hot materials and the sinking of cool materials As the mantle material moves, the overlying tectonic plates move along with it. There are two forces that result from mantle convection: ridge push & slab pull

13. Ridge Push Newly formed rock at a mid-ocean ridge is warmer and less dense than older rock nearby As the new rock cools, it becomes denser and sinks down away from the ridge As it slides down it exerts a force on the rest of the plate Known as ridge push Pushes the plate away from the ridge

14. Slab Pull Where plates pull away from each other at a mid- ocean ridge, magma from the asthenosphere rises to the surface As the new lithosphere moves away from the mid-ocean ridge, the lithosphere cools and becomes denser When the lithosphere becomes dense enough, it begins to subduct into the asthenosphere As the leading end of the plate sinks, it pulls the rest of the plate along behind it Known as slab pull