Physics final exam question: "resetting" solidification age of sample

Physics 205B Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

A rock sample with an extremely old solidification age (as determined by radioactive dating) is heated until it is molten, and then cooled back down to a solid. Discuss what happens to its solidification age (as determined by radioactive dating), and explain why this happens. Explain your reasoning using properties of radioactive decay.

Solution and grading rubric:
  • p:
    Correct. Discusses:
    1. the solidification age of a rock is determined by comparing the amount of gaseous decay products to the amount of unstable radioactive isotopes in the sample; a higher ratio of gaseous products to unstable isotopes corresponds to a sample that had solidified a long time ago; and
    2. heating the rock until molten would release the gaseous decay products, such that when it cools back down to a solid, it would have nothing to compare to the unstable isotopes that remain in the sample, effectively giving it a zero solidification age.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. One of the two points (1)-(2) correct, other is problematic/incomplete.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Only one of the two points (1)-(2) correct, other is missing, or both are problematic.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying properties of radioactive decay.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of applying properties of radioactive decay.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
Grading distribution:
Sections 30882, 30883
Exam code: finalLd0c
p: 11 students
r: 13 students
t: 6 students
v: 10 students
x: 0 students
y: 1 student
z: 2 students

A sample "p" response (from student 3019):

Another sample "p" response (from student 5080):

A sample "v" response (from student 1107):

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