Short answer questions: 1. When did the direction of the Pacific plate change, and what direction was it moving before this change? * changed at 43 million years. * Before this change, it was moving north-northwest. 2. If the ocean floor is getting wider, why is the Earth not becoming larger? * old ocean floor sinks at subduction zones (trenches) 3. Distinguish between Earthâs crust and mantle. * Crust overlies mantle. It is basalt composition under the ocean basins, granitic composition in the continents 4. Roughly how many tectonic plates move around on the surface of the Earth? * 10 or 12 5. What characteristics of tectonic plates distinguish them from deeper Earth materials? * They are rigid. 6. Do tectonic plates consist of crust, mantle, or some combination of crust and mantle, and if so, what part of parts of each? * all of the crust and part of the upper mantle. 7. What keeps the Appalachians standing as a mountain range even though they have been continuously eroding since they formed hundreds of millions of years ago? * Isostacy. As material is eroded off the range, it floats higher, just as removing ice from the top of an iceberg causes it to float higher. 8. What is the basis for the âscientific method?â * tentative hypotheses are tested using new observations and experiments 9. Why do many oceanic volcanoes occur as long lines of volcanoes that are active at only one end? How does the process work? * They are hotspot volcanoes. A hot plume of magma rising through the mantle burns through the overlying lithosphere to erupt as a volcano. Since the lithosphere moves over the deeper mantle, new volcanoes form progressively in a line over the deep plume. 10. Distinguish between Earthâs lithosphere and asthenosphere in terms of both location and properties. ïª The lithosphere overlies the asthenosphere. It is rigid and the asthenosphere deforms plastically.