Laboratory Manual for Introductory Geology 4e

EXERCISE 12.6
Name:
Course:
Dating Rocks by Overlapping Fossil Ranges (continued)
Section:
Date:
(b) Now apply these overlaps to cross section 1 in Exercise 12.5.
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If Neospirifer is found in Unit D, Platystrophia in F, and Strophomena in A, suggest an age for C. Explain your reasoning.
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What is the length of the gap in geologic time represented by the angular unconformity below Unit D?
(c) In cross section 2 of Exercise 12.5, Strophomena is found in Unit E, Platystrophia and Petrocrania in F, and
Petrocrania, Chonetes, and Neospirifer in C.
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When were Units E, A, F, and I tilted? Explain your reasoning.
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What is the length of the gap in the geologic record represented by the contact between Unit C and the tilted rocks
beneath it? Explain your reasoning.
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When in geologic time did the fault cutting Units K, C, I, F, A, and E occur? Explain your reasoning.
12.4 Determining Numerical Ages of Rocks
The geologic time scale (see Fig. 12.8) was used to describe the relative ages of rocks
for about 100 years before numerical ages (in years) could be added. The principal
method of determining numerical ages, called radiometric dating, is based on the
fact that the nuclei of some atoms of elements found in minerals (parent elements)
decay to form atoms of new elements (daughter elements) at a fixed rate regardless
of conditions. This decay is called radioactivity, and the atoms that decay are radioactive
isotopes.
12.4 DETERMINING NUMERICAL AGES OF ROCKS
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