READING QUESTIONS: Plate Tectonics
NAME________________________
GEOL 131
DUE: Tuesday, November 24th
Fall 2015
58 pts
All answers should be in your own words.
Continental Drift: An Idea Before Its Time (p. 281-284)
1. What was the first piece of evidence that led scientists to suspect that many continents were once
connected? (2 pts)
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2.
Explain why the discovery of fossils of Mesosaurus in South America and Africa, but nowhere else,
supports the continental drift hypothesis. (3 pts)
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3. In the early 20th century, what were the three prevailing explanations for how land animals migrated
across large oceans? (3 pts)
a. ___________________________________________________________________________
b. ___________________________________________________________________________
c. ___________________________________________________________________________
4. How did Wegener explain the presence of glaciers in southern continents at a time when areas in
North America, Europe, and Asia supported tropical swamps? (2 pts)
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The Great Debate (p. 284-285)
5. What mechanism did Wegener propose for continental drift, and what objection did physicist Harold
Jeffreys have to this mechanism? (4 pts)
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6. Fill in the blanks: Wegener incorrectly suggested that the larger, sturdier continents
________________________ the thinner oceanic crust, much as ____________________________
cut through ice. No evidence existed to suggest that the ocean floor was _____________ enough to
permit passage of the continents without the continents being appreciably _____________________
in the process. (2 pts)
The Theory of Plate Tectonics (p. 285-287)
7. What major ocean floor feature did oceanographers discover the extent of after World War II?
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8. Next to each characteristic below, indicate whether it applies to continental lithosphere, oceanic
lithosphere, both kinds of lithosphere, or the asthenosphere. Indicate your choice by writing “C”,
“O”, “CO”, or “A” next to each characteristic. (3 pts)
a. Cool
_____
b. Warm
_____
c. Flows
_____
d. Bends or breaks
_____
e. Mafic in composition
_____
f. Felsic in composition
_____
9. List any six of the seven largest lithospheric plates. (3 pts)
a. _______________________________
b. _______________________________
c. _______________________________
d. _______________________________
e. _______________________________
f. _______________________________
10. Match the name of each type of plate boundary with the arrows that show the kind of motion that
occurs there. (3 pts)
a. Transform ___
A.
b. Divergent ___
B.
c. Convergent___
C.
Divergent Plate Boundaries and Seafloor Spreading (p. 288-290)
11. What is the average rate of seafloor spreading in modern oceans? _______ cm/year
12. EXTRA CREDIT: All of Earth’s modern ocean floors were created at divergent boundaries within
the last 200 million years. Although this seems like a long time, it is only ______ % of Earth history.
13. Fill in the blanks: The primary reason for the ___________________ position of the oceanic ridge is
that newly created lithosphere is ______, which means it is ________________ than cooler rocks
found away from the ridge axis. (3 pts)
14. Fill in the blanks: Continental rifting occurs where plate motions produce ________________
forces that ________ the lithosphere and promote upwelling in the mantle. Stretching causes the
brittle crust to ____________ into large ____________ that sink, generating a ________________.
Continued spreading generates a long, narrow _______ similar to the present-day Red Sea.
Eventually, an expansive _____________________ containing a centrally located
_________________ is formed by continued seafloor spreading. (4 pts)
Convergent Plate Boundaries and Subduction (p. 290-293)
15. Why does the global rate of lithosphere creation roughly balance with the rate of lithosphere
destruction? (2 pts)
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16. Fill in the blanks: The angle at which an oceanic plate subducts depends largely on its _____ and
therefore its ________________. (2 pts)
17. List the three possible combinations of lithosphere types that can occur at a convergent boundary. (3
pts)
a. _______________________________
b. _______________________________
c. _______________________________
18. EXTRA CREDIT: List one geographic example of a modern volcanic island arc, and one example
of a continental volcanic arc. (2 pts)
a. Volcanic island arc: _______________________________
b. Continental volcanic arc: ______________________________
19. On page 290, the textbook refers to deep-ocean trenches as “surface manifestations” of subduction
zones. Why does the book use the word “surface” even though these trenches are located in the
deepest parts of the oceans? (2 pts)
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20. Oceanic lithosphere subducts because it is ____________________ than the asthenosphere beneath
it.
21. Give a brief description of how the Himalayas formed, with reference to the buoyancy of continental
lithosphere. (3 pts)
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Transform Plate Boundaries (p. 294-295)
22. On this diagram of a fracture zone along an oceanic ridge, label the segment of the zone that is an
active transform boundary by writing “transform” on one of the three blanks.
23. EXTRA CREDIT: List two examples of transform boundaries that cut through continental crust
instead of through ocean basins. (2 pts)
a. ________________________________________
b. ________________________________________
Testing the Plate Tectonics Model (p. 298-303)
24. What is the age of the oldest sediments recovered by deep-sea drilling? ______________________
25. Continental rocks have been discovered that are older than _____________________ years.
26. Fill in the blanks: Drill cores have revealed that sediments are almost entirely ___________ on the
crests of oceanic ____________, and that sediment thickness ______________ with increasing
distance from the ridge. This suggests that the age of the ocean crust _______________ the further
it is from an oceanic ridge. (2 pts)
27. Examine figure 31 on page 302. The white stripes indicate oceanic crust exhibiting normal magnetic
polarity, and the red stripes indicate reversed polarity. How many time intervals of reversed polarity
are represented by the oceanic crust in this figure? (Remember that, during each time interval of
reversed or normal polarity, two stripes of crust with that polarity are created along divergent
boundaries.)
Number of time intervals of reversed polarity: ________
What Drives Plate Motion? (p. 305-307)
28. Give short descriptions of slab pull and ridge push. (4 pts)
Slab pull: _____________________________________________________________
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Ridge push: ___________________________________________________________
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29. Which mechanism (slab pull or ridge push) is thought to contribute more to tectonic plate motion?
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