Module 2: Absolute Time Handout Questions

Answer the questions on the handout. Be sure to submit your answers in written form, as a single document, which restates the questions.


1) Fossiliferous limestone from two different areas are found to contain similar fossils. What does this say about the relative age of the rocks? (2 pts)

2) If a rock has multiple index fossils in it, which is more useful for relative dating, the fossil with a broad time range, or the fossil with a narrow time range? Why? (2 pts)

3) Two horizontal sedimentary rock layers in contact with one another are found to have absolute ages of 425 Ma, and 260 Ma. Explain how these rock layers came in contact with one another. (2 pts)

4) Are igneous intrusions younger, or older than the rock layers surrounding them? Why or why not? (2 pts)

5) What is the difference between an unconformity and a disconformity? Include a diagram of a disconformity. (2 pts)

For questions 6 and 7 you will need to use this table and graph.

Absolute time.jpg

6) A solid lave flow is found in a sequence of rock layers, and it is rich in zircon crystals. Using a mass spectrometer, the atoms of Uranium-235 and Lead-207 isotopes were counted from zircon samples found in the lava flow. 71% of the atoms were found to be Uranium-235, and the remaining 29% of atoms were Lead-207. Refer to Fig. 5 to help you answer the questions.

a) How many half-lives of the Uranium-235 to Lead-207 decay have occurred within the zircon grains? (2 pts)

b) What is the absolute age of the lava flow? Please show your work. (2 pts)

c) What is the age of the rock layer that is directly beneath the lava flow? (2 pts)

d) What is the age of the rock layer that is directly above the lava flow? (2 pts)

7) A piece of cotton cloth is found at a burial site and needs to be dated:

a) What isotope should be used for dating the cotton cloth? (2 pts)

b) It is found that 16.7% of the parent isotope is remaining. How many half-lives have occurred in the cloth? (2 pts)

c) What is the absolute age of the cloth? (2 pts)

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