Hallam, Ch. 6: CONTINENTAL DRIFT

What shape are the plates?
"They can't curl down;
They must curl up
To form a kind of dish
To stop the oceans spilling out
And losing all the fish."

B.C. and G.C.P. King, Nature, 232: 37.

Take your time with this chapter! It is longer than others, and information-rich. As you read, continually reflect on what stance you might have adopted at any time given the evidence then available.

Terms

As you read this chapter, watch for and define the following terms.
Note: Conceptually-related terms may not be given together; rather, they are listed in the order in which they appear in the chapter.

 

 

1. Isostasy, Isostatic adjustment

    

2. Geosyncline

    

3. Sial

    

4. Sima

    

5. Gondwana

    

6. Eustasy

    

7. Overthrust

    

8. Lateral compression

    

9. Polar wandering

    

10. Pangaea

     

11. Geodetic observations

     

12. Glossopteris flora

     

13. Tillites

     

14. Laurasia

     

15. Tethys

     

16. Magnetic axis

     

17. Continental Drift

     

18. Magnetic reversals

     

19. Mid-Atlantic ridge

     

20. Project Mohole

     

21. Asthenosphere

     

22. Lithosphere

     

23. Subduction

Focus Questions

 

 

1. Describe the mountain-building theory of Elie de Beaumont. How did his view of the earth differ from that of Charles Lyell?

    

2. Describe the mountain-building theory of Edward Suess. How is his theory illustrated by a baked apple as it dries and cools?

    

3. Describe Osmond Fisher's theory for the origin of the moon, continents, and ocean basins.

    

4. In addition to the "jig-saw fit" of continental coastlines, explain and critique six arguments which Alfred Wegener used to support his theory of continental drift.
    
   1. Two topographic levels.

       

   2. Isostasy theory.

       

   3. Geodetic observations.

       

   4. Orogenic and other geological continuities.

       

   5. Palaeontological continuities.

       

   6. Palaeoclimatic patterns.

    

5. What criticisms of continental drift were made by Charles Schuchert?

    

6. What post-world-war-II exploration was necessary for adequate assessment of continental drift?

    

7. Why did Wegener argue that geology is an integrative and synthetic discipline? What does this mean? Is it an accurate assessment of the character of geology?

    

8. Hallam writes that opinion "hardened against the notion of drift," and asks: "why did not such an exciting new idea stimulate active research into testing it, rather than promoting such a vehement adverse reaction on the part of so many?" Discuss why this was so.

    

9. How is it that some rocks, such as basalts or red sandstones, can serve as fossil compasses?

    

10. "The oceans are not primordial features of the crust." Defend this statement, and contrast the composition of ocean basins with the materials of the continents.

     

11. What was the evidence for Harry Hess's "sea-floor spreading" hypothesis?

     

12. Describe the idea of crustal plates as proposed by Tuzo Wilson and Jason Morgan. What happens where crustal plates meet?

     

13. Where does subduction occur? If there were no subduction, what might happen?

Further Reading

 

 

• Mott T. Greene. Geology in the Nineteenth Century. Ithaca: Cornell University Press, 1985. Excellent analysis of the background to Wegener. Highly recommended.

   

• Alfred Wegener. The Origin of Continents and Oceans. Translated from the 4th ed. (1929) by John Biram. New York: Dover, 1966. Go to the source!

   

• Homer E. Le Grand. Drifting Continents and Shifting Theories. Cambridge: Cambridge University Press, 1988. An interesting overview of the historical development of plate tectonics with an assessment of its ramifications for the philosophy of science and the practice of geology.

   

• Continents Adrift. Edited by J. Tuzo Wilson, reprinted by San Francisco: W. H. Freeman and Company, 1952-1972. A superb collection of classic Scientific American articles chronicling the development and acceptance of plate tectonics in the late 60's and early 70's.