Chapter 12 Questions Answer key - Chapter 12 Questions 1 a 5 b 9 a 2 b 6 c 10 d 3 d 7 b 11 d 4 d 8 b 13 These are the only rocks that are laid down

Chapter 12 Questions Answer key - Chapter 12 Questions 1 a...

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Unformatted text preview: Chapter 12 Questions 1. a 5. b 9. a 2. b 6. c 10. d 3. d 7. b 11. d 4. d 8. b 13. These are the only rocks that are laid down horizontally, although lava flows and ash volcanic ash layers are also often horizontal. 14. The rock that is cut or broken by an intrusion or fault is older than the rock it breaks or cuts across. 15. Unconformities form when surfaces are eroded, thus leaving a gap in the rock record. 20. A half- life is the amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope. 21. Radiometric dating makes use of the known decay rates of radioactive isotopes. 23. Relative dating correlates rock layers based on their positions and only puts them in order by age. Radioactive decay dating involves calculations and gives an absolute age. Relative dating is done in the field and radioactive dating occurs in the laboratory. 24. Carbon- 14 has a half life of 5700 years, which is too short for material older than 75,000 years. There is so little left after that it’s hard to determine how many half- lives have passed. 28. Fault A is older because it does not cut across the to 4 layers. Fault B reaches the top so it formed after those last 4 layers were deposited. 29. Intrusion B is older because it doesn’t cut across the top 4 layers. Intrusion A cuts across to the top so it formed after those top 4 layers. 30. Two half- lives have passed (you know this because 50 cut in half twice is 12.5, and the sample is 2.6 billion years old (2 x 1.3). 34. We would need to wait 6 half- lives, which is 27 billion years. 2. A 3. D 4. C 5. Sill C was intruded after layer D formed. You know this because there are included fragments of layer D inside Sill C, so C must be younger. Because layers A, B C and D are all tilted and eroded at the top, you know C formed before the unconformity and before layer E. 6. Radioactive decay rates do not change over time and depend only on what isotope you are using for radiometric dating. Therefore they can be used in confidence. ...
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