9. RELATIVE AND RADIOMETRIC AGES

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1 LAST NAME (IN CAPS): FIRST NAME: Instructions: 9. RELATIVE AND RADIOMETRIC AGES Your work will be graded on the basis of its accuracy, completion, clarity, neatness, legibility, and correct spelling of scientific terms. For questions that require you to show your work, you must present your work in a step-by-step manner and with clarity. INTRODUCTION Two Kinds of Ages: Relative age: Order of geologic events are known, but not dates Absolute (Radiometric, Numerical) age: Dates geologic events happened are known Geologic events include: the formation of rocks, faulting, tilting, folding, metamorphism, erosion etc RELATIVE AGE DATING The order of geologic events can be correctly placed on the basis of the following geologic laws (principles): LAW OF ORIGINAL HORIZONTALITY: Water-laid sediments are deposited in horizontal strata (Figure 1). Figure 1. Horizontally layered sedimentary rocks LAW OF SUPERPOSITION: In any undisturbed sequence of strata, each stratum is younger than the stratum below it and older than the stratum above (Figure 2). Figure 2. Sedimentary layers with oldest at the bottom and youngest at the top Page 1 of 8

2 LAW OF CROSS-CUTTING RELATIONSHIPS: A rock is always older than any feature (e.g. fault, dike etc.) that cuts it (Figure 3). Figure 3. Sedimentary layers cut by a dike (left); On the right, layers A, B, C, and dike D are cut by a fault (labelled E). LAW OF INCLUSION: If a rock unit contains pieces of another rock (inclusions), then it must be younger than the pieces it engulfed (Figure 4). Figure 4. Tilted sedimentary layers intruded by a magma that ultimately turned to a granite. Broken pieces of the sedimentary rocks fall into the magma. The trapped sedimentary rocks (inclusions) were already rocks before the magma turned into granite. Therefore, the inclusions are older than the granite. Page 2 of 8

3 LAW OF UNCONFORMITIES: Unconformities are depositional gaps in the rock record. They indicate a time when uplift and erosion have occurred such that layers deposited at an earlier time have been stripped away by erosion (Figure 5). Figure 5. Three types of unconformities. Disconformity: A surface of erosion in which sedimentary layers above and below the unconformity are horizontal (parallel). Angular Unconformity: A surface of erosion between two groups of sedimentary rocks in which the orientation of older strata, below, is at an angle to younger strata, above. Nonconformity: A surface of erosion that separates younger sedimentary strata above from older igneous or metamorphic rocks below. RADIOMETRIC (ABSOLUTE) DATING Makes use of radiometric techniques to provide the actual number of years since rocks (minerals), faults, folds, etc formed. Radioactivity refers to the 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 atoms to decay into daughter atoms (Figure 6). Figure 6. Half-life values of select radioactive decay pairs Page 3 of 8

4 Take a starting amount of parent isotope (at time=zero): 100 % Half-life 1 2 % of parent isotope (element) in sample Suppose the half-life value of a radioactive element is 100 m.y. How many years will it take for 50% of the parent isotope to change (decay) to the daughter product? 3 4 If the % of parent isotope found in a sample is 12.5%, how old is the sample? QUESTIONS RELATIVE AGES OF ROCKS Use the geologic cross section of a hypothetical area, Figure 7, 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? Figure 7. Page 4 of 8

5 Use Figure 8 to answer Q6-10. Q6.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 Based on the relative dating Law Q7. Is fault L (older or younger) than rock layer D? What principle did you apply to determine your answer? Fault L is Law: Q8. 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 Laws: and Q9. Is the igneous intrusion labelled dike K (older or younger) than layers C-F? Intrusion (dike) K is Q10. List the entire sequence of events, in order from oldest to youngest, by writing the appropriate letters in the spaces provided to the right of Figure 9 above. Figure 8. Page 5 of 8

6 Figure 9. Some select index fossils and their age ranges. Page 6 of 8

7 Use Figure 9 to answer Q11-16 Q11.What is the geologic age range of plants that belong to the group Phacops? From Period through the Period Q12. What is the geologic range of the extinct plant Neuropteris fern? From Period through the Period Q13. Imagine that you have discovered an outcrop of sedimentary rock that contains fossils of shark teeth and fossils of Neuropteris fern. In which time periods might this rock have formed (relative age)? Q14. What is the relative age range of the fossil shown in Figure 10? Q15. I) What index fossils from Figure 9 are present in Figure 11? II) Based on the overlap of range zones for these index fossils, what is the relative age (name of one or more periods of time) of the rock? Figure 10. III) What is the radiometric (absolute) age range (in m.y.) of the rock? Q16. Identify the two types of index fossils in Figure 12. Based on the overlap of range zones for these index fossils, and using Figure 9, I) What is the relative age range of the rock? II) What is the radiometric (absolute) age range of the rock? Figure 11 Figure 12 Page 7 of 8

8 RADIOMETRIC AGES OF ROCKS Q17. A rock was submitted for analysis to a mass spectrometric lab. The result shows a parent to daughter ratio of 1:1. How many half-lives have elapsed since that rock was formed? Explain. Q18. The half-life of C-N (C-14, or Radiocarbon) is 5,730 years. How old is a fossil that contains 6.25% of the original (parent) C-14? Show your work. Q19. K-40 decays to Ar-40 with a half-life of 1.3 billion years (Figure 6). Analysis of a hypothetical sample of a granite reveals that 75% of the K-40 atoms have decayed and formed Ar-40. What is the age of the granite sample? Show your work. Earlier in this worksheet (Q6-10) you determined the geologic history of a hypothetical region (Figure 8) using the relative age dating techniques. It is reported that a mineral sample of intrusion J has an age of 400 million years Once again refer to Figure 8 to answer Q Q20. 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 radiometric age of dike K? Write your answer below and next to dike K in Figure 8. Dike K is million years old. Q21. Are rock layers H and I younger or older than 100 million years? Explain. Q22. What is the possible age range of rock layer E? Layer E is between and million years old. Q23. Determine the age of rock layer A. Rock layer A is greater than million years. Page 8 of 8