Answers to the Review Questions

1. Alfred Wegener is credited with developing the continental drift hypothesis.

 2. The puzzle-like fit of the continents, especially Africa and South America.

 3. Pangaea, the super continent named by Alfred Wegener, is thought to have begun to break apart about 200 million years ago.

 4. Wegener and his associates found that the fit of the continents, fossil evidence, paleoclimatic evidence,

     and similarities in rock type and structural features all seemed to bridge together the now-separated continental landmasses.

 5.  If Mesosaurus was able to swim well enough to cross the vast ocean currently separating Africa and South America, its remains should also be found on other continents. Since this is not the case, we conclude that South America and Africa were joined during the time period that these animals existed.

 6.  Early in this century, migration of land animals was thought to have occurred via land bridges. According to this view, these land bridges were recently submerged as sea level rose following the last glacial event.

 7.  First, it is difficult to explain, by assuming that a general cooling trend occurred, why late Paleozoic glaciation was essentially confined to the Southern Hemisphere. This and other paleoclimatic data can best be reconciled by assuming that the continents were located at latitudes different from where they are today. Second, glacial striations in some locations indicate that the ice movement was from areas presently occupied by water. We know, however, that thick accumulations of ice can form only on land.

 8.  The first approximations of plate boundaries were made on the basis of earthquake and volcanic activity.

 9.  Divergent boundaries—where plates are moving apart

      Convergent boundaries—where plates are moving together

      Transform boundaries—where plates slide past one another along faults

10. Sea-floor spreading refers to the creation of new sea floor at the oceanic ridges along with its conveyor belt movement away from the ridge crests. Sea-floor spreading occurs today along the oceanic ridges.

11. Subduction zones occur in the deep-ocean trenches where slabs of oceanic crust are descending into the mantle. Subduction zones are associated with convergent plate boundaries.

12. Lithosphere is being consumed at convergent boundaries where a slab of oceanic crust plunges into the asthenosphere. The production and destruction of the lithosphere must take place at about the same rate because Earth is neither growing nor shrinking in size.

13. The Himalaya Mountains formed as a result of a collision between the Indian landmass and the Asian continent.

14. Transform fault boundaries, such as the San Andreas fault, represent areas where plates slip past one another. Lithosphere is neither produced (as along divergent boundaries) nor destroyed (as along convergent boundaries) along transform faults.

15. If the movement along the San Andreas fault continues in its present manner, the part of California west of the fault will indeed slide out to sea, eventually becoming an island off the west coast of the United States and Canada. This, however, will take millions of years.

16.  Paleomagnetism is the natural magnetism in rock bodies that was acquired from Earth's magnetic field at the time when the rock formed.

17. Evidence from studies of paleomagnetism indicate that Earth's magnetic poles have migrated during the past 500 million years. This apparent migration can also be explained if the magnetic poles remained stationary while the landmasses moved. Thus, continental drift does in fact account for the apparent wandering of Earth's magnetic poles.

18. The age of the oldest sediments recovered by deep-ocean drilling is about 160 million years. Some continental crust has been dated at 3.9 billion years.

19. Hot spots are relatively stationary plumes of molten rock rising from Earth's mantle. According to the plate tectonics theory, as a plate moves over a hot spot, magma often penetrates the surface, thereby generating a volcanic structure. In the case of the Hawaiian Islands, as the Pacific plate moved over a hot spot, the associated igneous activity produced a chain of five major volcanoes. The oldest of the Hawaiian Islands is Kauai. The youngest and only active volcanic island in the chain is the island of Hawaii.

20. Three mechanisms that have proposed to explain plate motion include slab pull, ridge push, and slab suction. Slab pull refers to older slabs of oceanic lithosphere that are more dense sink into the asthenosphere and “pull” the trailing plate along. Ridge push is a gravity-driven mechanism that results from the elevated position of the oceanic ridge, which causes slabs of lithosphere to move down the flanks of the ridge. Slab suction arises from the drag of a subducting slab on the adjacent mantle. An induced mantle circulation pulls the subducting and overriding plates toward the trench or “suck” in nearby plates.

21. Himalayas—These mountains have formed along a convergent, continent–continent, collisional boundary between the Indian subcontinent and Eurasia.

      Aleutian Islands—These islands are the oceanward part of a volcanic island arc situated on the northwestern margin of the North American plate; the volcanoes lie above the subducting Pacific plate.

      Red Sea—The Red Sea occupies a major rift zone and very young seafloor spreading center that has opened between Africa and the Arabian block.

      Andes Mountains—The Andes are a volcanic and plutonic arc resting on the western margin of the South American plate; they lie above subducting, oceanic lithosphere of the Nazca and Antarctic plates.

      San Andreas fault—This is a transform fault that forms the boundary between the North American and Pacific plates. The crustal sliver composed of westernmost California and the Baja California peninsula on the eastern edge of the Pacific plate is moving northwestward with respect to North America.

      Iceland—Iceland and nearby smaller islands comprise a major zone of basaltic volcanism that probably overlies a mantle hot spot located directly beneath the Mid-Atlantic Ridge, the divergent boundary between the Eurasian and North American plates.

      Japan—The Japanese Islands lie on the eastern margin of the Eurasian plate, above subducting parts of the Pacific and Philippine oceanic plates.

      Mount St. Helens—This is a very young stratovolcano in the state of Washington; it is part of the Cascade Range, a continental–margin, volcanic arc extending from the Canadian border to northern California.