Screen Shot 2023-11-15 at 2

Activity 8.4 Numerical Dating of Rocks and Fossils Name: Course/Section: Date: Learmng GOAL You will analyze the ratios of parent and daughter isotopes and calculate the age of a geologic sample. You will also explain why samples must be carefully chosen to avoid errors. 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 1o 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 Fig. 8.14 10 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 answer. 3. What can you say about the age of the rock layers above the lava flow? 4. What can you say about 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 othgasoeky materials in our solar system, including the oldest meteorites. The oldest meteorites ever found on Earth contain nearlyiqual amounts of both uranium-238 and lead- 206, Based on Fig. 8.14, what is Earth’s approximate age? Explain your reasoning, € The radioactive isotope carbon-14 (C-14) is continuously replenished in organisms while they are alive. When an organism dies, it is no longer able to take in new C-14, 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% @f the C-14 of modem shells. When the plants that now form the buried peat were alive, they absorbed C- 14 and probably had about the same amount of C- 14 as modern shells, so abeut 4% of the peat’s original C-14 has decayed. WhagdS a réasonable initial estimate of the age of the peat bed? Explain 2. In sampling the peat bed, you must be careful 1o avoid any young plant roots or old limestone. Why? D Zircon (ZrSi0,) fomms.in magma as it cools into igneous rock. It is also useful for numerical dating (Fig. 8.14). 1. If you walkken 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 modem age (zeto years) for the beach. Why? [ . Suggest a rule that geologists should follow when they date rocks based on the radiometric ages of mineral grains 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 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) 226 Printed by: ryan9693 @mylaurier.ca. Printing is for personal, private use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted.