Determining Geologic Ages Name ______

Part 1 Period ______Date ______1. In the space provided in Figure 6.3, list the order, first (oldest) to last (youngest), that the cards were laid down. 2. Were you able to place all of the cards in sequence? If not, which one(s) could not be “relative” dated and why? ______Figure 6.4 illustrates a geologic cross section, a side view, of the rocks beneath the surface of a hypothetical region. Use this illustrations to answer questions 3 and 4. 3. Which of the two sequences of rocks, A-D and E-G, was disturbed by crustal movements after its deposition? ______Which law or principle did you apply to arrive at your answer? ______4. Apply the law of superposition to determine the relative ages of the undisturbed sequence of sedimentary rocks. List the letter of the oldest rock layer first. Oldest ______Youngest

Figure 6.3 Figure 6.4

Refer to Figure 6.5 to answer questions 5 and 6. The sedimentary layer B is a sandstone. Letter C is the sedimentary rock, shale. 5. Identify and label the inclusion. 6. Of the two rocks, B and C, which rock is older? ______7. Identify and label an example of an angular unconformity and a disconformity in Figure 6.4 (shown under question 4). ______

Refer to Figure 6.8 to answer questions 8-11. 8. Is the igneous intrusion C older or younger than the sedimentary rocks B and D? ______9. Is fault H older or younger than the sedimentary beds A-E? Figure 6.5 ______10. Is the relative age of fault H older or younger than the sedimentary layer F. ______11. Did the fault occur before or after the igneous intrusion? ______Explain how you arrived at your answer. ______12. Refer to Figure 6.4. Is the igneous intrusion H older or younger than rock layer E? ______Layer D? ______Figure 6.8 13. Refer to Figure 6.4. What evidence supports the conclusion that the igneous intrusive feature called a sill, C, is more recent than both the rock layers B and D and older than igneous intrusion H? ______

______

Part 2 1. What are the two conditions that favor the preservation of an organism as a fossil? a. ______b. ______2. Refer to Figure 6.9. Select the photo A, B, C, D, that best illustrates each of the following methods of fossilization or fossil evidence. a. Petrification: The small internal cavities and pores of the original organism are filled with precipitated mineral matter. Photo: ______b. Cast: The space once occupied by a dissolved shell or other structure is subsequently filled with mineral matter. Photo: ______c. Impression: A replica of a former fossil left in fine- grained sediment after the fossilizing material, often carbon, is removed. Photo: ______d. Indirect Evidence: Traces of prehistoric life, but not the organisms itself. Photo: ______

Figure 6.10 shows a sequence of undeformed sedimentary rocks. Each layer of rock contains the fossils illustrated within it. The three rocks, Rocks 1, 2, and 3, illustrated below the layered sequence were found nearby and each rock contains the fossils indicated. 3. Applying the principle of fossil succession, indicate the proper position of each o the three rocks relative to the rock layers by writing the words Rock 1, Rock 2, and Rock 3 at the appropriate position in the sequence.

Figure 6.11 is a geologic cross section of a hypothetical area. Letters K and L are igneous rocks. Letter M is a fault. All the remaining letters represent sedimentary rocks. Using Figure 6.11 to complete the following questions will provide insight into how the relative geologic history of an area is determined. 4. Identify and label the unconformities indicated in the cross section. 5. Is rock layer 1 older or younger than layer J? ______What law or principle have you Figure 6.10 applied to determine your answer? ______6. Is the fault older or younger than rock layer I. ______What law or principle have you applied to determine you answer? ______7. Is the igneous intrusion K older or younger than layers A and B? ______What two laws or principles have you applied to determine your answer? ______and ______Figure 6.11 8. Is the age of the igneous intrusion L older or younger than layers J, I, H, G, and F. ______9. List the entire sequence of events, in order from oldest to youngest. Oldest ______Youngest 10. Explain why it was difficult to place the fault, letter M, in a specific position among the sequence of events in Figure 6.11. ______

Part 3 Figure 6.12 graphically represents how the ratio of a parent isotope to its stable daughter product continually changes with time. Use Figure 6.12 to help answer the following questions. 1. What fraction of the original parent isotopes still exists after each of the following half-lives has elapsed? Fraction of Parent Isotope Remaining One half-life: ______Two half-lives: ______Three half lives: ______Four half lives: ______

2. Assume you begin with 10.0g of a radioactive parent isotope. How many grams of parent isotope will be present in the sample after each of the following half-lives? Remaining Parent Isotope Figure 6.12 One half-life: ______grams Four half-lives: ______grams 3. If a radioactive isotope has a half-life of 400 million years, how long will it take for 50% of the material to change to the daughter product? ______years 4. A sample is brought to the laboratory and the chemist Work #2: determines that the fraction of the parent isotope remaining is 1/8 of the total amount that was originally present. If the half- life of the material is 600 million years, how old is the sample? ______years old 5. Determine the numerical ages of rock samples that contain a Work #4: parent isotope with a half-life of 100 million years and have the following fractions of parent isotope and daughter product. 1/2 parent and 1/2 daughter: Age = ______1/8 parent and 7/8 daughter: Age = ______1/32 parent and 31/32 daughter: Age = ______Work #5: Previously in the exercise you determined the geologic history of the area represented in Figure 6.11 using relative dating techniques. Assume that the rock layers H and I in Figure 6.11 each contain radioactive materials with known half-lives. 6. An analysis of a sample of rock from layer H in Figure 6.11 indicates an equal proportion of parent isotope and daughter produced from the parent. The half-life of the parent is known to be 425 million years. a. How much of the original parent has decayed to the daughter product? ______b. How many half-lives of the parent isotope have elapsed since rock H formed? ______c. What is the numerical age of rock layer H? Write your answer below and at rock layer H on Figure 6.11. ______years 7. The analysis of sample of rock from layer I in Figure 6.11 indicates its age to be 400 million years. Write the numerical age of layer I on Figure 6.11.

Refer to the relative and numerical ages you determined for the following rocks in Figure 6.11 to answer the following questions. 8. How many years long is the interval of time represented by the unconformity that separates rock layer H from rock layer I? ______Explain how you arrived at your answer. ______9. Is the age of fault M older or younger than 400 million years. ______Explain how you arrived at your answer. ______10. What is the approximate maximum numerical age of the igneous intrusion L? ______11. Complete the following general statement describing the numerical ages of rock layers G, F, and E. All of the rock layers are ( younger / older ) than ______million years.

Part 4 Use the geologic time scale diagram in your book to answer the following questions. 1. What fraction or percent of geologic time is represented by the Precambrian eon? ______2. Explain why approximately 540 million years ago was selected to mark the end of the Precambrian eon and the beginning of the Phanerozoic eon? ______3. Suggest a reason(s) as to why the periods of the Cenozoic era have been further subdivided into several epochs with reasonably reliable accuracy. ______4. How many times longer is the whole of geologic time than the time represented by recorded history, about 5000 years? ______5. For what fraction or percent of geologic time have land plants been present on Earth? ______6. Why do we not know what happened during the Precambrian Era? ______