Home Work 4: Interpreting the Geologic Record: Putting it all together This assignment is due on Wednesday, October 7 This is a series of exercises meant to help you synthesize some of the ideas about rocks, the rock cycle, and the sedimentary column. (Based on an exercise in Tarbuck, Lutgens, Pinzke. 2006. Applications and Investigations in Earth Science, 5th edition. Prentice Hall, Upper Saddle River, NJ.)
Just as the present is key to understanding the past, the past is key to predicting the future. If we can understand how the Earth systems have responded to change in the past, it may be possible to predict how they will respond to change in the future. There is a dynamic tension or equilibrium among Earth systems such that when one system changes or is altered, those changes affect all other Earth systems. Many of the changes in the hydrosphere, atmosphere, and biosphere, as well as the geosphere, are recorded in the geological record. It is, in part, for this reason that interpretation of the geologic column is of such great interest. In your reading assignments and in class, you’ve learned about the principles and techniques geologists use to decipher the Earth’s history by placing geological events in their proper sequence and establishing how long ago the event occurred. The exercises below ask you to use the information you’ve learned to interpret hypothetical portions of the geologic column. For the 2nd exam you should be able to (along with other concepts and definitions): 1. List and explain some of the principles that are used to determine relative ages of geological events. 2. Determine the sequence of geological events that have occurred in an area by applying the techniques and procedures for relative dating. 3. Explain how radioactive decay of certain elements can be used to determine the age of the Earth. 4. Apply the techniques of radioactive dating to determine the numerical age of a rock. Terms you should be able to define and concepts you should understand: Relative dating (see lecture notes) Uniformitarinism (see lecture notes) Principle of Horizontality (see lecture notes) Principle of Superposition (see lecture notes) Inclusion (see below) Unconformity (see text, section 4-4) Cross-cutting (see below) Radiometric dating (see lecture notes) Half-life (see lecture notes)
Name ______________________________ Figure one illustrates a geological cross section, a side view, of the rocks beneath the surface of a hypothetical region. Use this figure to answer the two questions below the figure.
Figure 1. Geological block diagram of a hypothetical region showing igneous intrusive features and sedimentary rocks.
1. Of the two sequences of rocks, A-D and E-G, which were disturbed by crustal movement after its deposition, A-D or E-G? What principal did you apply to arrive at your conclusion? 2. Apply the principle of superposition to determine the relative ages of the undisturbed sequence of sedimentary rocks. List the oldest rock first. Oldest _________________________________________Youngest
Inclusions Inclusions are pieces of one rock unit that are contained within another unit (Figure 2). The rock mass adjacent to the one containing the inclusions must have been there first in order to provide the rock garments. Therefore, the rock containing the inclusions is the younger of the two. Refer to Figure 2, below, to answer the next two questions. 3. Identify and label the inclusions in the figure 4. Of the two rocks, B and C, which rock is older?
Figure 2. Geologic block diagram showing sedimentary rocks.
Unconformities As long as continuous sedimentation occurs at a particular place, there will be an uninterrupted record of the material and fossils. However, if the sedimentation process is suspended by an emergence of the area from below sea level, then no sediment will be deposited and an erosion surface will develop. The result is that no rock record will exist for a part of geological time. Such a gap in the rock record is termed an unconformity. An unconformity is typically shown on a cross-sectional diagram by a wavy line. Several types of unconformities are illustrated in your text book in section 4.4. 5. After studying section 4-4 in the text, identify and label an example of an angular unconformity in Figure 1.
Principle of Cross-Cutting Relationships Whenever a fault or intrusive igneous rock cuts through an existing feature, it is younger than the structure it cuts. For example, if a granite dike cuts through a sandstone layer, the sandstone had to be there first and, therefore, is older than the dike. Figure 3 (see next page) illustrates a geological cross section showing sedimentary rocks (A,B,D,E,F, and G), an igneous intrusion feature called a dike (C), and a fault (H). Use this figure to answer the following 4 questions. 6. 7. 8. 9.
Is the igneous intrusion, C, younger or older than sedimentary rocks B and D? Is fault H younger or older than the sedimentary beds A-E? Is the relative age of fault H younger or older than the sedimentary layer F? Did the fault occur before or after the igneous intrusion? Explain how you arrived at your answer.
Figure 3. Geological block diagram of a hypothetical area showing an igneous intrusion (C), a fault (H), and sedimentary rocks.
Refer to Figure 1. 10. Is the igneous intrusion H younger or older than rock layer E and younger or older than stratum D? 11. What evidence supports the conclusion that the igneous intrusive feature called a sill, C, is more recent than both of the rock layers B and D and older than the igneous intrusion H?
Applying Relative Dating Techniques Geologists often apply several of the techniques of relative dating when investigating the geologic history of an area. Use Figure 4 to complete the questions below. 12. Which of the structures or layers are obviously igneous rocks? 13. Which structure is a fault? 14. Identify the unconformities in the figure. 15. Is rock layer I older or younger than layer J? What principle did you apply to determine your answer? 16. Is the fault older or younger than rock layer I? What principle did you apply to determine your answer? 17. Is the stratum K older or younger than layers A and B. What two principles did you apply to determine your answer? 18. Is the age of intrusion L older or younger than layers J, I, H, G, and F? 19. List the entire sequence of events, in order from oldest to youngest, by writing the appropriate letter in the space provided on the figure.
20. Explain why it was difficult to place the fault in a specific position among the sequence of events.
Figure 4. Geological block diagram of a hypothetical area showing igneous intrusive features, a fault, and sedimentary rocks.
Applying Radiometric Dates When used in conjunction with relative dates, radiometric dates help geologists refine their interpretations of the geological history of an area. Previously, you completed a homework assignment applying radiometric dating to determine the age of a rock. Apply the same concepts to answer the questions below. In Figure 4, assume that the rock layers H and I can be dated with radioactive minerals of known half-lives (Why is this a poor assumption?). 21. An analysis of a sample of rock from layer H 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. What percent 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 the rock layer H? The analysis of a sample of rock from layer I indicates its age to be 400 million years. 22. How many years long is the interval of time represented by the unconformity that separates rock layer H from layer I? Explain how you arrived at your answer. 23. Is the fault older or younger than 400 million years? How did you arrive at your answer? 24. What is the approximate maximum numerical age of the intrusion L? Complete the following general statement describing the numerical ages of rock layers G, F, and E. 25. All of the rock layers are (younger/older) than ________ million years.