Hallam, Ch. 7: MASS EXTINCTIONS

Terms

As you read this chapter, watch for and define the following terms.

 
1. Background extinctions

    

2. Mass extinctions

    

3. Marine transgressions and regressions

    

4. K-T boundary

    

5. Chondritic meteorites

    

6. Tektites

    

7. Paraconformity

    

8. Boundary clay

    

9. Nannoplankton

    

10. Deccan Traps

     

11. Magnetic Zone 29R

     

12. Galactic Plane hypothesis

     

13. Nemesis

     

14. Periodic mass extinctions

     

15. Episodic mass extinctions

Focus Questions

 
1. Identify the geological periods of the "Big Five"; i.e., the five most significant mass extinctions.

    

2. What explanations for mass extinction events were suggested by Otto Schindewolf, Digby McLaren, and Harold Urey?

    

3. When did Luis Alvarez, Walter Alvarez, Frank Asaro and Helen Michel first propose their asteroid impact hypothesis? Explain their scenario for the Cretaceous-Tertiary mass extinction, and identify the evidence upon which it was based.

    

4. What is the iridium anomaly?

    

5. Describe the evidence used by many paleontologists to argue for the gradual nature of the K-T mass extinction.

    

6. Explain and assess the argument from Osmium isotopes.

    

7. Explain and assess the argument from shocked quartz.

    

8. Explain and assess the argument from soot.

    

9. Describe the critique of bolide impact hypotheses made by Charles Officer and Charles Drake.

    

10. Explain the argument of William Clemens against an asteroid-impact explanation for the mass extinction of the dinosaurs.

     

11. Reflect on the attitude of Luis Alvarez toward the investigative methods of Clemens and other paleontologists.

     

12. What evidence has compelled impact-advocates more recently to move toward multiple-impact scenarios?

     

13. Hallam notes that "The normal method of reporting the temporal ranges of fossils artificially concentrates the last occurrence at stratigraphical boundaries." With this in mind, reflect on the hazards and complexities attending methods of statistical analysis for interpreting the fossil record, and comment on the difficulties of identifying mass extinction events.

     

14. Assess the extinction periodicity theories of Raup and Sepkoski.

     

15. Assess Hallam's current evaluation of the impact hypothesis. Why does he regard it as a case of "too many theoreticians . . . chasing too few facts"? What is Hallam's preferred explanation for the K-T extinctions?

     

16. Discuss the role of the media in scientific controversies. Has the media been fair and responsible in their coverage of the impact controversies?

     

17. Right or wrong, is there a place for outrageous hypotheses in geology? Identify and discuss some of the benefits and some of the harmful effects of impact hypotheses on the recent practice of geology.

Further Reading

 
• David Raup. The Nemesis Affair. New York: Norton, 1986. A fascinating account of the early years of the impact hypothesis, told by one of its principal participants and advocates.

   

• Charles Officer and Jake Page. The Dinosaur Extinction Controversies. A forceful presentation of the case against impact hypotheses, written in an accessible style.