Plate Tectonics

3. a magnetic orientation the same as that of Earth’s current magnetic field
6. a relatively small rising column of mantle rock that is hotter than surrounding rock
8. the hypothesis proposed by Alfred Wegener that Earth’s continents were once joined together and later split and drifted apart
9. the portion of the upper mantle just beneath the lithosphere, extending from about 100–350 kilometers below the surface and consisting of weak, plastic rock where magma may form
10. a change in Earth’s magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa, which has occurred on average every 500,000 years over the past 65 million years

1. the hypothesis that segments of oceanic crust are separating at the Mid-Ocean Ridge
2. an undersea mountain chain that forms at the boundary between divergent tectonic plates within oceanic crust
4. the process in which two lithospheric plates of different densities converge and the denser one sinks into the mantle beneath the other
5. a theory stating that the lithosphere is segmented into several plates that move about relative to one another by floating on and sliding over the upper mantle
7. the concept of balance between gravity and buoyancy that causes the lithosphere to float on the asthenosphere at different elevations
8. the upward and downward flow of fluid material in response to density changes produced by heating and cooling, which occurs slowly in Earth’s mantle and much more quickly in the oceans and atmosphere