6_GeologicTime_StudyGuide_GEOL101

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101

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Geology

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May 6, 2024

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GEOL101 Study Questions for Lesson 2: Geologic Time Use these questions to evaluate your knowledge of the material to better target concepts you should study further. They can provide a personalized road map of what concepts you need to focus on. The following questions will help you review the MAJOR concepts and relationships you should understand and may not comprehensively cover every question on the exam for this unit. Many of the questions require synthesis and integrate several concepts; they will take you some time to complete. Therefore, you work through these questions as we cover them in class to allow enough time for you to identify areas you need to study in more detail. Trying to complete this study guide hours before the exam will be overwhelming and is not the best approach for concept mastery. You do not have to submit this for a grade. In addition to these questions, you should review all assigned readings and videos, assignments, quizzes, and handouts. Geologic Time: Relative Time 1. How old is Earth? 4.543 billion years 2. What is the difference between absolute and relative age? Give an example of each. How do they each play a role in the construction of the geologic time scale? Relative age is the age of a rock layer compared to other layers, and can be determined by looking at the postion of rock layers. Absolute age is the numeric age of a layer of rocks or fossils 3. Discuss the relevance of Hutton’s observations at Siccar Point, Scotland to the development of the principle of uniformitarianism. Known as the birthplace of modern geology, were unconformities there, which led to the principle of uniformitarianism. This principle says the present is key to the past, and that the processes that are acting today were operating in the past at the same rate. 4. You will need to know the definitions of the following as well as have a working knowledge of how to apply them to decipher a geologic history from a cross-section (consult the practice cross-sections done in lecture, the Geologic Time Worksheet, the Group Evaluation, and the examples at the end of this guide): The principle of original horizontality States that layers of sediment are originally deposited horizontally under the action of gravity – sloping of layers must have occurred after layers were formed “ “ of superposition The oldest layer is at the base and the layers are progressively younger with ascending order in sequence “ “ cross-cutting relations (for igneous intrusions and faults) Geologic feature which cuts another is younger of the two features “ “ fossil succession is a technique used to define the relative age of the fossil . It identifies rock layers where the fossils are found and can place them in a chronological order according to rock strata. “ “ of baked contacts states that the heat of an intrusion will bake (metamorphose) the rocks in close proximity to the intrusion . Hence the presence of a baked contact indicates the intrusion is younger than the rocks around it.
5. Describe how unconformities form. List 3 types of unconformities and what defines each of them. Draw each of them. You should also know how to identify them to decipher a geologic history from a cross- section (consult the in-class assignment and the example at the end of this study guide). Unconformities form by weathering and erosion which happens with a rise and fall in sea levels. Nonconformities(sedimentary over non sedimentary), Disconformities(sed over sed), and angular unconformities(Sed over sed but rocks below are at an angle) 6. How did geologists reconstruct Earth’s history if there is not a single stratigraphic section that represents it? How do fossils and lithology aid in this? Incorporate the terms marker bed and index fossil into your discussion. 7. What characteristics distinguish an index fossil from a typical fossil? every fossil tells us something about the age of the rock its found in but index fossils are those that are used to define periods of geologic time 8. Describe how the geologic column was created and how it later became the geologic time scale . You should incorporate relative and absolute time concepts in your answer. Absolute Time 9. Explain how relative and absolute dates are used in tandem to discern a geologic history of an area. They used relative dating to divide Earth's past in several chunks of time when similar organisms were on Earth. Later, scientists used absolute dating to determine the actual number of years ago that events happened. The geologic time scale is divided into eons, eras, periods, and epochs. 10. Explain in your own words how isotopes allow geologists to calculate the numeric age of a mineral/rock. Incorporate the following terms correctly: parent isotope, daughter product, radioactive decay, half-life. Isotopes are important to geologists because   each radioactive element decays at a constant rate, which is unique to that element . These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed. Half-lives are how long it takes for half of a group of parent atoms to decay 11. Apply your understanding of radioactive decay and half-lives to plot the decay curves for parent and daughter isotopes on the following graph.
0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 90 100 Parent and Daughter Isotopes in Radioactive Decay Parent Daughter Number of Half Lives % of Isotope 12. Examine the plot from the previous question and answer the following questions: why is radioactive decay exponential? Does this apply to both the parent and daughter isotopes? Why or why not? Each atom has the same chance of decaying in a fixed time interval and this leads to an exponential decay 13. Discuss how a numeric age could be artificially too old or too young. 14. Describe how an isotopic system’s closure temperature can reflect the age of the mineral/rock. Discuss how multiple closure temperatures of different minerals and different isotopic systems in a single rock can yield information about the rock’s thermal evolution. Consult the assigned readings in our textbook for support. The Anthropocene 15. What is the Anthropocene? A new proposed division in geologic time scale 16. How do geologists (stratigraphers) define a new division on the geologic time scale? A new time scale must reflect global long term change and be marked by a identifiable boundary 17. What are some options for defining the start of the Anthropocene based on the assigned readings? Which of these options has been agreed upon by the Anthropocene Working Group and is currently being searched for to designate the Global boundary Stratotype Section and Point (GSSP) a.k.a. “the golden spike”? Does it make sense to define it at the 1945 event when we've been altering earth’s system many years before that Issues  Finding an appropriate marker preserved  Scientific process  The effects of humans on the earth system prior to 1945  18. Discuss the issue social scientists have with the scientific use of “the Anthropocene” and how it’s at odds with how geologists (stratigraphers) propose to scientifically define “the Anthropocene”.
Practice 19. Use the relative and absolute age principles to determine the geologic history of the cross section below. Remember periods of weathering and erosion should be included as an event. There are 16 events. Start with the oldest event and work forward. Use the clues below: All units are sedimentary rocks except for J which is a granite and K which is a mafic dike (gabbro). Unit B has fossils that are ~600 Ma. Units C-G are Permian in age based on trilobite and brachiopod fossils. Unit H has bivalves that are no older than 56 Ma (million years). _____ ____ ____ _____ _____ _____ ____ _____ _____ _____ _____ _____ ____ _____ _____ _____ 20. Outline any angular unconformities in the above block diagram in red, any disconformities in blue, and any nonconformities in green. 21. What principle did you use to determine if unit J or the succession of units A-G came first? 22. Was unit E deposited before K? What principle did you apply to determine this? 23. Do units A-G obey the rules of original horizontality and superposition? What happened to make them look the way they do now? 24. Based only on relative age relationships, what are the age constraints of K? 25. You isolate zircons from unit K (gabbro) to run 235 U- 207 Pb analysis. In one zircon you measure 25 grams of 235 U and 75 grams of 207 Pb. Assuming no isotopes were lost or added to the zircon, how old is K? (Hints: consult Figure 10.12 and Table 10.1 in the textbook; if this is still difficult, first complete the #3 Practice Question below and check your answer in the answer key.) Does this age make sense with your geologic history, the relative age relationships, and the fossil ages? Why or why not? 26. Shade the areas that have baked contacts.
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