8. GEOLOGIC TIME LAST NAME (ALL IN CAPS): FIRST NAME: Instructions

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1 LAST NAME (ALL IN CAPS): FIRST NAME: Instructions 8. GEOLOGIC TIME Refer to Exercise 6 in your Lab Manual on pages to answer the questions in this work sheet. Your work will be graded on the basis of its accuracy, completion, clarity, neatness, legibility, and correct spelling of scientific terms. When you are done with your lab work, clean your desk and leave all materials you worked with in the same way you found them! INTRODUCTION Two Kinds of Geologic Ages (dates): Numerical and Relative dates Relative age: Order of geologic events are known, but not dates Radiometric (Absolute, Numerical) age: Dates geologic events happened are known Geologic events include: the formation of rocks, faulting, tilting, folding, metamorphism, erosion etc. 1. RELATIVE AGE DATING The order of geologic events can be correctly placed on the basis of the following geologic principles: A) Principle of Original Horizontality States that most sediments and lava flows are deposited in horizontal layers. B) Principle of Superposition States that in any sequence of undeformed sedimentary rocks (or lava flows), the oldest rock is always at the bottom, and the youngest is at the top. C) Principle of Cross-cutting relationships When a fault, or an intrusive igneous rock, cuts through an existing rock unit or feature, it is younger than that rock or feature. D) Principle of Inclusion Inclusions are rock fragments of one rock unit that have been enclosed within another. The rock mass adjacent to the one containing an inclusion must have been there first in order to provide the fragment. Therefore, the rock mass that contains the inclusion is the younger of the two. E) Principle of Unconformities Layers of rock that have been deposited essentially without interruption are called conformable. A complete set of conformable strata representing all of geologic time has not been found anywhere on Earth. Throughout history, the deposition of sediment has been interrupted over and over again. These breaks in the rock record are called unconformities. An unconformity represents a long period during which deposition ceased, erosion removed previously formed rocks, and deposition resumed. Three types of unconformities are recognized: Angular Unconformity: Consists of tilted sedimentary rocks that are overlain by younger, more flat-lying strata. Disconformity: A surface of erosion in which sedimentary layers above and below the unconformity are horizontal. Nonconformity: Consists of younger sedimentary rocks that overlie older igneous or metamorphic rocks. Page 1 of 6

2 2. RADIOMETRIC (NUMERICAL, ABSOLUTE) DATING Makes use of radiometric techniques to provide the actual # of years since rocks (minerals), faults, folds, etc. formed. It is based on the principle of radioactivity. Radioactivity: Spontaneous nuclear transformations that change the number of protons & neutrons in a parent nucleus. The nuclear transformation is due to unusually weak forces binding protons & neutrons of an atom. As a result, the nucleus becomes unstable and spontaneously transforms into another atom (isotope). By measuring the amount of parent (original) isotope and daughter (new) isotope in a mineral or rock sample using a mass spectrometer, it is possible to calculate the actual age of that sample, in years. There are several radiometric age dating methods such as U-Pb, K-Ar, C-N, etc. Half-Life: The time it takes for half of the parent isotope to decay into daughter isotopes. e.g. C N 5730 years; U Pb:4.5 billion years etc. Half-life % of original isotope remaining AGE = # half-lives elapsed X half-life of isotope QUESTIONS A. RELATIVE DATING Figure 9.8 is a geologic cross section of a hypothetical area. Use it to answer Q1-5. Q1.The igneous intrusion, dike E, is than the rock layers A-D? A) Older B) Younger Q2. Fault H is than the rock layers A-D? A) Older B) Younger Q3. Fault H is than the sedimentary layers F and G? A) Older B) Younger Q4. Fault H occurred dike E? A) Before B) After Q5. What evidence supports the conclusion that the igneous feature labeled Sill B is more recent than the rock layers A and C? Page 2 of 6

3 Q6. Identify the types of unconformities in Figure 9.11 A, B. A: B: Use Figure 9.12 to answer Q7-12. Q7.Identify and label all unconformities in Figure Q8.Is rock layer I (older or younger) than layer H? What relative dating principle did you apply to determine your answer? Rock layer I is Relative dating principle: Q9. Is fault L (older or younger) than rock layer D? What principle did you apply to determine your answer? Fault L is Principle: Q10. Is igneous intrusion J (older or younger) than layers A and B? What two relative dating principles did you apply to determine your answer? Intrusion J is Principles: and Q11. Is the igneous intrusion labelled dike K (older or younger) than layers C-F? Intrusion (dike) K is Q12. List the entire sequence of events, in order from oldest to youngest, by writing the appropriate letters in the spaces provided on the figure below. Figure 9.12 Page 3 of 6

4 Q13. Refer to Figure Which photo(s), A through I best illustrate(s) the methods of fossilization or fossil evidence listed below? Photos/letters may be used more than once. PETRIFICATION: The small internal cavities and pores of an original organism that are filled with precipitated mineral matter. Photo(s): CAST: The space once occupied by a dissolved shell or other structure is subsequently filled with mineral matter. Photo(s): CARBONIZATION: Preservation that occurs when fine sediment encases delicate plant or animal forms and leaves a residue of carbon. Photo(s): IMPRESSION: A replica of an organism, such as leaf, left in fine-grained sedimentary rock. Photo(s): AMBER: Hardened resin of ancient trees that preserved delicate organisms such as insects. Photo(s): INDIRECT EVIDENCE: Traces of prehistoric life but not the organism itself. Photo(s): Use Figure 9.14 to answer Q14-19 Q14.What is the geologic age range of plants that belong to the group Ginkgo? Q15. What is the geologic range of Lepidodendron, an extinct coal producing plant? Q16. Imagine that you have discovered an outcrop of sedimentary rock that contains fossils of shark teeth and fossils of Archimedes. In which time periods might this rock have formed? Q17. What is the geologic age range of the fossil shown in Figure 9.15? Q18. What is the geologic age range of the fossil shown in Figure 9.16? Q19. Imagine that you found both of the fossils identified in Q17 and Q18 together in one rock. What is the geologic range of this rock? Page 4 of 6

5 Q20. Figure 9.17 illustrates two different sequences of sedimentary rock strata located some distance apart. (I) Determine the geologic age range of each rock layer by its fossil content and write your answer on the figure. (II) Draw lines connecting the rock layers of the same age in outcrop 1 to those in outcrop 2. (III) What term is used for the process of matching one rock unit with another of the same age? (IV) Based on the ages of the rock layers in outcrop 1, identify an unconformity. Draw a wavy line on the figure to represent the unconformity. Figure 9.17 B. RADIOMETRIC DATING Q21. What % of the original parent isotope remains after each of the following half-lives has elapsed? One half-life: Two half-lives: Three half-lives: Four half-lives: % OF PARENT ISOTOPE REMAINING Page 5 of 6

6 Q22. Geologists know that K-40 decays to Ar-40 with a half-life of 1.3 billion years. Analysis of a hypothetical sample of granite reveals that 75% of the K-40 atoms have decayed to form Ar-40. What is the age of the granite sample? Show your work. Q23. Determine the numerical ages of rock samples that contain a parent isotope with a half-life of 100 million years and have the following percentages of original parent isotope. 50%: Age: 25%: Age: 6.25%: Age: Earlier in this exercise (Q7-12) you determined the geologic history of a hypothetical region, shown in Figure 9.12, using relative dating techniques. Use Figure 9.12 to answer Q Q24. An analysis of a mineral from dike K indicates that 25% of the parent isotope is present in the sample. (I) How many half-lives have elapsed since dike K formed? _ (II) If the half-life of the parent isotope is 50 million years, what is the numerical age of dike K? Write your answer below and next to dike K in Figure Dike K is million years old. Q25. An analysis of a mineral sample of intrusion J indicates its age to be 400 million years. Write the numerical age of rock J on Figure Show your answer to your instructor in class! Q26. Are rock layers H and I younger or older than 100 million years? Explain. Q27. What is the possible age range of rock layer E? Layer E is between and million years old. Q28. Determine the age of rock layer A. Rock layer A is greater than million years. Q29. By referring to Figure 9.23, list all four eras from youngest to oldest. Page 6 of 6