Page 250 - Laboratory Manual in Physical Geology

ACTIVIT Y 8.4 Absolute Dating of Rocks and Fossils

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. A solidified lava flow containing zircon mineral crystals is present in a sequence of rock layers that are exposed in a hillside.
A mass spectrometer analysis was used to count the atoms of uranium-235 and lead-207 isotopes in zircon samples from the
lava flow. The analysis revealed that 71% of the atoms were uranium-235, and 29% of the atoms were lead-207. Refer to
FIGURE 8.11 to help you answer the following questions.

1. About how many half-lives of the uranium-235
to lead-207 decay pair have elapsed in the zircon crystals? ______________

2. What is the absolute age of the lava flow based on
its zircon crystals? Show your calculations.

3. What is the age of the rock 4. What is the age of the rock
layers above the lava flow? _______________ layers beneath the lava flow? _______________

B. Astronomers think that Earth probably formed at the same time as all of the other rocky materials in our solar system,
including the oldest meteorites. The oldest meteorites ever found on Earth contain nearly equal amounts of both

uranium-238 and lead-206. Based on FIGURE 8.11 , what is Earth’s age? Explain your reasoning.

C. If you assume that the global amount of radiocarbon (formed by cosmic-ray bombardment of atoms in the upper atmosphere

and then dissolved in rain and seawater) is constant, then decaying carbon-14 is continuously replaced in organisms while
they are alive. However, when an organism dies, the amount of its carbon-14 decreases as it decays to nitrogen-14.

1. The carbon in a buried peat bed has about 6% of the carbon-14 of modern shells. What is the age of the peat bed?
Explain.

2. In sampling the peat bed, you must be careful to avoid any young plant roots or old limestone. Why?

D. Zircon (ZrSiO ) forms in magma and lava as it cools into igneous rock. It is also useful for absolute age dating ( FIGURE 8.11 ).

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1. If you walk on a modern New Jersey beach, then you will walk on some zircon sand grains. Yet if you determine the
absolute age of the zircons, it does not indicate a modern age (zero years) for the beach. Why?

2. Suggest a rule that geologists should follow when they date rocks based on the radiometric ages of crystals inside the rocks.

E. REFLECT & DISCUSS An “authentic dinosaur bone” is being offered for sale on the Internet. The seller claims that he had
it analyzed by scientists who confirmed that it is a dinosaur bone and used carbon dating to determine that it is 400 million
years old. Discuss the sellers claims with a partner or in a small group. Should you be suspicious of this bone’s authenticity?
Explain. (See FIGURES 8.10 , 8.11 ).

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