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2. Before people understood plate tectonics, what evidence led some scientists to believe
in continental drift?
* match of coastlines across the Atlantic Ocean
* match of rock formations across the Atlantic Ocean
* match of ages of continental rocks across the Atlantic Ocean
3. If the coastlines across the Atlantic Ocean are spreading apart, why isn’t the Atlantic
Ocean deepest in its center?
 New ocean floor wells up and forms under the mid-Atlantic Ridge
4. What evidence confirmed seafloor spreading? Be brief but to the point
 Reversals of the Earth’s magnetic field are preserved in ocean-floor basalt, as
matching patterns across the mid-oceanic ridge. The youngest ocean-floor basalt is at
the ridge.
5. Distinguish between Earth’s crust, lithosphere, asthenosphere, and mantle.
 Earth’s crust overlies mantle – distinguished by composition: oceanic crust is
basaltic composition and continental crust is “granitic” composition.
 Earth’s lithosphere overlies asthenosphere – distinguished by rock
properties/behavior/strength: lithosphere is more-rigid; asthenosphere is moreplastic/easily deformed.
6. What are the main types of lithospheric plate boundaries in terms of relative motions?
Provide a real example of each (by name or location).
* rift (or spreading) zones: Mid-Atlantic ridge
* subduction zones: Cascadia subduction zone
* continental collision zones: India against Asia (Himalayas)
* transform faults: San Andreas Fault
7. What does oceanic lithosphere consist of and how thick is it?
* Earth’s crust and uppermost mantle (basalt over peridotite)
* about 100 km
8. Along which type of lithospheric plate boundary are earthquakes common?
* convergent, divergent, and transform
9. Along which type(s) of lithospheric plate boundary are andesite stratovolcanoes
common? Provide an example.
* continent – ocean plate collision (Cascadia subduction zone/ Cascade volcanoes)
10. Why does oceanic lithosphere almost always sink beneath continental lithosphere at
convergent plate boundaries?
* oceanic lithosphere is denser/heavier
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