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  • Question: EXERCISE 12.7 Putting It All Together to Decipher Earth History Course: Name: Section: Date: (a) First, let's practice calculating ages using Tables 12.1 and 12.2. How old is a rock if it contains a uranium-235 lead-207 ratio of 46.62? years a rubidium-87:strontium-87 ratio of 89.97 years * 6.2% of its original carbon-14 atoms? - years . 97.9% of its

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    (a)1. Uranium 235 to Lead 207 ratio is 46.62, so we will multiply the half life by 0.031 as per tabl...

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EXERCISE 12.7 Putting It All Together to Decipher Earth History Course: Name: Section: Date: (a) First, let's practice calculating ages using Tables 12.1 and 12.2. How old is a rock if it contains a uranium-235 lead-207 ratio of 46.62? years a rubidium-87:strontium-87 ratio of 89.97 years * 6.2% of its original carbon-14 atoms? - years . 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 contains hornblende with 84.1% of its parent potassium.40. - How old is Dike e? _ Dikea? Pluton ? • How old in years and using period names) are Units B, H, K, and J? . When were these layers folded? • When did the unconformity separating Unit Ifrom the underlying rocks form? • 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; Diked contains zircon with a uranium-235:lead-207 ratio of 2.413; and Dikef has hornblende with 50% of its parent potassium-40. • How old is Unit H? Diked? Dike fa . When (in years and using period names) were Units C, G, and I folded? • How large a gap in the geologic record is represented by the unconformity below Unit B? • How old is Unit 12 • If Uniti contains the brachiopods Neospinifer, Chonetes, and Schizophoria, how does this help narrow its possible age? - In that case, how large a gap in the geologic record is represented by the unconformity below Unith? INTERPRETING GEOLOGIC HISTORY 318 CHAPTER 12 Geologic cross section 3 Youngest Go D D Oldest Geologic cross section 4 Youngest G Oldest