Laboratory Manual for Introductory Geology 4e

EXERCISE 12.7
Putting It All Together to Decipher Earth History
Name:
Course:
Section:
Date:
(a) First, let’s practice calculating ages using Tables 12.1 and 12.2. How old is a rock if it contains
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a uranium-235:lead-207 ratio of 46.62? years
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a rubidium-87:strontium-87 ratio of 89.9? years
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6.2% of its original carbon-14 atoms? years
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97.9% of its original potassium-40 atoms? years
Now let’s add some numerical ages to the cross sections in Exercise 12.5.
(b) In geologic cross section 3 of Exercise 12.5, Dike e contains zircon with a uranium-235:lead-207 ratio of 11.05; Dike
a contains zircon with a uranium-235:lead-206 ratio of 22.81; and Pluton f contains hornblende with 84.1% of its
parent potassium-40.
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How old is Dike e? Dike a? Pluton f?
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How old (in years and using period names) are Units B, H, K, and J?
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When were these layers folded?
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When did the unconformity separating Unit I from the underlying rocks form?
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How old are Units I and D?
(c) In geologic cross section 4 of Exercise 12.5, Unit H has a thin volcanic ash bed at its base and zircon with a
uranium-235:lead-207 ratio of 46.62; Dike d contains zircon with a uranium-235:lead-207 ratio of 2.413; and
Dike f has hornblende with 50% of its parent potassium-40.
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How old is Unit H? Dike d? Dike f?
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When (in years and using period names) were Units C, G, and J folded?
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How large a gap in the geologic record is represented by the unconformity below Unit B?
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How old is Unit I?
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If Unit I contains the brachiopods Neospirifer, Chonetes, and Schizophoria, how does this help narrow its possible age?
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In that case, how large a gap in the geologic record is represented by the unconformity below Unit H?
318 CHAPTER 12 INTERPRETING GEOLOGIC HISTORY