Astronomy midterm question: solidification age reset

Astronomy 10 Midterm 2, spring semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q4.2

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, and explain why this happens.

Solution and grading rubric:

• p:
  Correct. 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. 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:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Recognizes that during melting, the amount of decay products in the sample somehow goes to zero, or is transmuted, thus "resetting" the solidification age to zero when it cools and solidifies.
  
• t:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Solidification age is reset, but due to loss/gain/renewal of unstable isotopes.
  
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distributions:
Section 4160
p: 2 students
r: 2 students
t: 6 students
v: 26 students
x: 2 students
y: 2 students
z: 0 students

Section 5166
p: 5 students
r: 15 students
t: 13 students
v: 28 students
x: 2 students
y: 0 students
z: 1 student

A sample "p" response (from student 2501):
A sample "x" response (from student 3635), with credit given for effort, rather than merit.

Previous post: Astronomy quiz question: bubble, bubble, toil and trouble