Geology Reference
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have on rivers? Is there any evidence from the continen-
tal shelves that might bear on this? If so, what?
12. The most distant part of a 30-million-year-old aseismic
ridge is 1000 km from an oceanic ridge. How fast, on
average, did the plate with this ridge move in centime-
ters per year?
13. How do mountains of mid-oceanic ridges differ from
mountain ranges on land?
14. Although seafl oor fractures may extend for hundreds of
kilometers, earthquakes occur only on those parts of the
fractures between offset ridge segments. Why?
15. How did geologists determine the nature of the upper
mantle and oceanic crust even before they observed
mantle and crust rocks in the ocean basins?
16. What kinds of evidence would you need to demonstrate
that an atoll evolved from a fringing reef to a barrier reef
and, fi nally, to an atoll?
17. What are calcareous ooze and pelagic clay, and where are
they found?
18. Why are abyssal plains common around the margins of
the Atlantic, but rare in the Pacifi c Ocean basin?
19. Identify the types of continental margins in Figure 9.7
and list the characteristics of each.
20. Explain how an aseismic ridge forms.
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