READING QUESTIONS: Geologic Time
NAME________________________
GEOL 131
DUE: Thursday, November 19th
Fall 2015
42 pts
All answers should be in your own words.
Creating a Time Scale: Relative Dating principles (p. 254-259)
1. What is the difference between relative dating and absolute dating? (2 pts)
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2. Place the features in the diagram below in their correct relative age sequence, with youngest on the left
and oldest on the right, using relative dating principles. (6 pts)
Youngest ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ ____ Oldest
3. What does an unconformity represent? (3 pts) _____________________________________________
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4. Label each unconformity in the figures below with the correct name. (3 pts)
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Igneous rock
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Correlation of Rock Layers (p. 262-264)
5. What is the goal of correlation of rock layers? (2 pts)
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6. State the principle of fossil succession. (2 pts) ___________________________________________
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7. The figure below consists of two parts. On the left is a “reference” time scale showing the fossil ranges
of a number of different species. On the right are four rock layers of different ages showing the fossils
species found within them (fossil assemblages). Using the fossil ranges on the reference rock section
and the fossil assemblages in the rock layers, indicate in which time period each of the rock layers
formed. List only one time period for each layer. (“Time period names” refers to the names in the
reference time scale: Cambrian, Ordovician, etc.). (4 points)
8. Give one example of how fossils can be used as environmental indicators (clues to past Earth-surface
conditions). (3 pts)
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Dating With Radioactivity (p. 265-268)
9. Define radioactive decay (radioactivity). (2 pts) __________________________________________
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10. Why is radiometric dating of rocks reliable?
a. Because radioactive decay rates do not vary
b. Because radioactive decay produces isotopes
c. Because radioactive decay rates have been precisely measured
d. Because the number of half-lives present in an isotope never varies
e. (A) and (B)
f. (B) and (C)
g. (C) and (D)
h. (A) and (C)
11. Define half-life. (2 pts) ______________________________________________________________
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12. If there are 25 atoms of the radioactive isotope potassium-40 (K-40) in a mineral sample and 75 atoms
of stable argon-40 (Ar-40), how many half-lives of K-40 have elapsed since the mineral formed? (See
figure 21 on page 266.)
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13. How old is the mineral from the previous question, given how many K-40 half-lives have elapsed since
its formation?
___________________ years
14. Obtaining a correct radiometric date for a sample is not possible unless _________________________
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The Geologic Time Scale (p. 269-271)
15. List the four basic units of the geologic time scale in order, from longest to shortest. (2 pts)
Longest
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Shortest
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16. Why are all three eras of the Phanerozoic Eon named with words that end in –zoic? (2 pts)
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17. Give two reasons why the Precambrian is not subdivided into as many smaller time units as the
Phanerozoic Eon. (4 pts)
a. ______________________________________________________________________________
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b. ______________________________________________________________________________
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