Lab 2_ Geologic Time_FA22

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© Michelle Stoklosa 2020 1 Laboratory Exercise 2: Geologic Time Introduction to Geology II: Earth’s Surface Processes Name____________________ Score: _______/25 INTRODUCTION In the reading and discussion, you have a been introduced to the two different categories of dating methods in geology: relative and numeric. In this exercise we will practice applying these methods to geologic problems. Part A: Relative Dating Methods Relative dating methods are those that help us to put geologic events in a relative order to each other (this came first, then this, and so on…). Review the following principles using your textbook and/or the short lesson page in our module for this week, and make a few notes beside each so that you remember what they are: • Principle of Original Horizontality • Principle of Lateral Continuity • Principle of Superposition • Principle of Cross-cutting Relationships • Principle of Inclusions • Principle of Fossil Succession

© Michelle Stoklosa 2020 2 Task A1: Applying the relative time principles For this part of the exercise, you will need to apply the relative time principles above in order to put the geologic units in the following diagram in order. Note that units B & D are igneous rocks, C and E are sedimentary, and the brown and blue wavy unit is a metamorphic rock. A and F refer to faults. Figure for Task A1, from Moser (2017). Questions: (3 points) 1. Which of the geologic units in this diagram is older, D or C? 2. Which relative time principle did you use to determine your answer to question 1? 3. Which of the geologic units is the YOUNGEST on this diagram? (list the letter) 4. What is the name of the irregular/wavy surface in the middle of the diagram just below unit C?

© Michelle Stoklosa 2020 3 Task A2: Putting geologic units in order Use the relative time principles that you reviewed to put the rock units and features in the cross section below into an order. Be sure to put the oldest rock units/features at the bottom and the youngest ones at the top. Note that some letters refer to faults or unconformities. All rocks are sedimentary except A and E which are igneous (see the key for specific rock types). (7 points) Figure for Task A2, from AGI & NAGT (2015). Youngest = F Oldest = E

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© Michelle Stoklosa 2020 4 Part B: Numeric dating methods As you have already learned, there are various ways to determine a numeric age for a feature on Earth, but we more often use radiometric dating in geology. Review the concepts of isotopes, half-life and radiometric decay in your textbook and the short lesson provided for this week, then complete the tasks in this section. Task B1: Half-lives The half-life of an unstable, radiometric isotope is the time it takes for half of the original, unstable parent material to decay into daughter atoms. Review Figure (not table) 10.12 in your textbook and the table below ( From Ludman and Marshak, 2015). Questions: (4 points) 1. If you have 50% radioactive parent material in a mineral crystal and 50% daughter atoms, how many half-lives have elapsed since the mineral formed (use Table 17.2 on the page before)? 2. If you have 6.2% parent atoms remaining in a mineral crystal, how many half-lives have elapsed? (Use Table 17.2 again.) 3. If you have 100 atoms of parent material, and one half-life goes by, about how much parent material (in terms of a percentage) do you have remaining?

© Michelle Stoklosa 2020 5 4. If you have 100 atoms of parent material and TWO half-lives go by, a. How much parent material do you have remaining (in terms of percentage)? b. How much daughter material (%) do you now have? Task B2: Applying radiometric ages to a geologic problem Examine the geologic diagram below (from Ludman & Marshak, 2015), and put its units in order from oldest to youngest in the table below it. The oldest and youngest are done for you. (4 points) Figure for Task B2, from Ludman & Marshak (2015). Now let’s use the diagram, the relative time principles and radiometric dating concepts to put some numeric ages on these rocks. C B

© Michelle Stoklosa 2020 6 Questions: (8 points) 1. Unit “e” (in the diagram from Task B2) has been found to contain minerals with the parent isotope Uranium-235 (Uranium is the name of the element, and 235 is the atomic mass for that particular isotope of Uranium). Uranium-235 (also written as 235 U) decays over time into the daughter isotope Lead-207, (also written as 207 Pb). If the minerals in unit e have been found to have a parent: daughter ratio (also written as 235 U: 207 Pb) of 11.05, how many half-lives have elapsed since it first formed? (You need to use Table 17.2, under Task B1 for find this). 2. To get an age for this unit, you will need to multiply the number of half-lives that you found in question 1 above by the value (or duration) of the half-life for that particular isotope pair, 235 U: 207 Pb. To determine this value, see Table 10.1 in your textbook. Show your work below, then put your answer in the blank following. Age of unit e = ______________million years WAIT! Did you notice that the units above are in million years? This means that 1,000,000 years is the same at 1 million years, right? Be sure to adjust your answer, if needed (or ask for clarification). 3. If unit “ a ” (same diagram for Task B2) has a mineral with a 238 U: 206 Pb ratio of 22.81, what is its age? (again, you will need to consult Table 17.2 in this lab exercise to determine how many half-lives have passed and Table 10.1 in your textbook to determine the value of this half-life, then multiply. Note that a different isotope pair was used for this unit. Show all of your steps below, and put your answer in the blank. Age of unit a = ___________________million years 4. Now apply the steps that you used for both units “ a ” and “ e ” to find the age for one more rock unit, “f”. Unit “f” has a mineral 84.1% of the parent isotope 40 K (also known as Potassium-40). Show your work below and put your final answer in the blank.

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© Michelle Stoklosa 2020 7 Age of unit f = _______________________million years 5. Units a, e, and f are igneous rocks, so are more easily dated using radiometric dating. B, H, K and J, however, are all sedimentary rocks, so cannot be easily dated using this method. Instead we will need to use the ages found for units a, e and f and the relative time principles in order to speculate on the age range of these units. a. Would you say that Unit K is older or younger than units a, e and f? (choose one of these options) b. If you decided that K was younger, then K would be less than the age of those units, right? If you decided that K was older, then K would be older than the age you determined for those units. So which is it? How old is Unit K, in millions of years? ( Give a range, such as older than xx million years or younger than xx million years, rather than a specific age .) 6. Using the same reasoning that you used for question 5, speculate how old Unit D is, in millions of years. Explain your reasoning, and give a range, not a specific value. 7. Now use the Geologic Time Scale in your textbook (Figure 10.16 on page 370) to determine the names of the periods (not eras, eons or epochs) that Unit D could have formed during, according to the age range you determined in number 6. 8. If a paleontologist found a dinosaur bone in Unit D and speculated that this dinosaur lived during the Jurassic Period of the Geologic Time Scale, would your data support the paleontologist’s hypothesis? Why or why not? 9. BONUS Reflection Question: In a sentence or two, explain why it is easier to use igneous rocks, rather than sedimentary rocks, for radiometric dating.

© Michelle Stoklosa 2020 8 References AGI and NAGT (American Geosciences Institute and the National Association of Geoscience Teachers). (2015). Laboratory Manual in Physical Geology (10 th ed.). Upper Saddle River, NJ: Pearson Education. Ludman, A. & Marshak, S. (2015). Laboratory Manual for Introductory Geology , 3 rd ed. New York and London: W.W. Norton & Company. Moser, C. (2017). Block diagram to apply relative dating principles. [Online image]. Retrieved from https://opengeology.org/textbook/7- geologic-time/ .

Lab 2_ Geologic Time_FA22

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