Activity 8.4 Numerical Dating of Rocks and Fossils Name: Course/Section Date: A A solidified lava low containing zircon mineral crystals is present in a sequarnce of rock layers that are exposed in a billside A mass spectrometer analysis was used to count the atoms of uraniam-235 and lead-207 isotopes in zircon samples from the lava flow. The analysis revealed that 71% of the atoms were uranium-235, and 2% of the atoms were lead-297. Refer to Fig. 8.14 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 numerical age of the lava flow based on its zircon crystals? Explain how you arrived at your answet What is the age of the rock layers above the lava flowh 3. 4. What is the age of the rock 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 bolh uranium-238 and lead-206. Based on Fig. 8.14, what is Earth's approximate age? Explain your reasoning- C The radioactive isotope carbon-14 (C14) is continuously replenished in organisms while they are alive. When an organism dies, it is no longer able to take in new C14, and so the amount of C-14 decreases as it decays to its stable daughter product: nitrogen-14 (N-14). 1. The carbon in a buried peat bed has about 6% of the C-14 of modern shells. When the plants that now form the buried peat were alive, they absorbedC14 and probably had about the same amount of C-14 as modern shells, so about 94 % of the peat's original C-14 has decayed. What is a reasonable initial estimate of 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 as it cools into igneous rock. It is also useful for numerical dating (Fig. 8.14). 1. If you walk on a modern New Jersey beach, then you will walk on some zircon sand grains. Yet if you determine the numerical age of the zircons, it does not indicate a modern age (zero years) for the beach. Whv? 2. Suggest a rule that geologists should follow when they date rocks based on the radiometric ages of mineral grains inside the rocks REFLECT & DISCUSS An "authentic dinosaur bone" is being offered for sale on the Internet. The seller claims that he had it analvzed 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 seller's claims with a partner or in a small group. Should you be suspicious of this bone's authenticity? Explain. (See Figs. 8.13 and 8.14.)