Geology 101 Lab Worksheet: Geologic Time

Transcription

1 Geology 101 Lab Worksheet: Geologic Time Name: Refer to the Geologic Time Lab for the information you need to complete this worksheet ( All calculations and graphs that you perform in this class are expected to meet the basic scientific standards explained in the Calculations and Graphs Rubric ( In this lab you will complete tables, construct a graph, and calculate ages to learn the principles that underlie absolute dating of geologic materials; examine and label cross-sections and block diagrams to gain experience working with relative dating methods; and apply the principles of relative dating to interpret the geologic history of the Grand Canyon. Materials Needed This worksheet Scientific calculator Colored pencils There are three parts to this lab: Part 1: Determine absolute ages from ratios of radioactive isotopes and daughter products. Part 2: Practice using principles of relative geologic age to interpret block diagrams and cross-sections. Part 3: Apply the principles of geologic time to produce a geologic history of the Grand Canyon. Due by the time the next lab begins next week: 1. This work sheet, completed. PART 1: ABSOLUTE AGES FROM RATIOS OF RADIOACTIVE ISOTOPES AND DAUGHTER PRODUCTS An isotope system may be assumed to be a closed system with regard to the parent and daughter they remain within the system and do not leave it, and at the same time no isotopes of the parent or daughter type enter the system from outside. (In reality, rocks, minerals, or other geologic materials can be checked to see if the isotope system remained closed, rather than assuming so.) At time zero, 100% of the isotopes are the parent isotopes. By the definition of a half-life, the amount of parent isotope at each the end of half-life is half of what it was when the half-life began. As the amount of parent isotope decreases by radioactive decay, the amount of the daughter isotope increases commensurately. Therefore, at any time, the total of parent isotope fraction and daughter isotope fraction add up to 1. Similarly, the percentage of parent isotope percentage and daughter isotope percentage add up to 100% at all times. See the section on parent and daughter isotopes on the Geologic Time Basics page Geology 101 Lab Worksheet: Geologic Time page 1

2 Table 1. Radioactive Isotope Decay in Half-Lives: Using the definition of half-life, fill in the blank boxes below with the decay, half-life by half-life, of a radioactive isotope, and the accumulation of the daughter product isotope that the parent changes into once it decays. Three significant figures are adequate for each percentage. For example the parent fraction after 10 half-lives is 1/1024. To convert this to a percentage, divide 1 by 1,024, round the result to the first three significant figures and multiply the result by 100, as follows: = = %. For another example, the daughter fraction after 6 half-lives is 63/64. To convert this to a percentage, divide 63 by 64, round the result to the first three significant figures and multiply the result by 100, as follows: = = 98.4 %. Half-lives elapsed Parent isotope fraction Daughter isotope fraction Parent isotope percent Daughter isotope percent % 0% 1 1/ Geology 101 Lab Worksheet: Geologic Time page 2

3 After completing Table 1, use the data it contains to complete the graph below, showing the curve of radioactive parent isotope decay and stable daughter product accumulation. Graph two curves one for the parent isotope and one for the daughter isotope. Use a different color for each curve. Plot each data point that corresponds to a half-life elapsed (horizontal axis) and a corresponding % of isotope (vertical axis). After you have finished plotting all the points for either the parent or daughter isotope, make a smooth curve to connect the data. Do not use straight lines to connect data points. Add a key to the graph explaining which color indicates the parent isotope and which color indicates the daughter product. Answer the following questions: 1. Draw a straight line with a ruler to find the value on the graph. If 75% of the parent isotope remains, how many half-lives have passed to the nearest 0.1 half-life? (note: the answer is not 0.5 half-lives) 2. Draw a straight line with a ruler to find the value on the graph. If 1.5 half-lives have passed how much of the parent isotope remains (to the nearest 1%)? Geology 101 Lab Worksheet: Geologic Time page 3

4 There is another, more precise way to determine the values of half-lives or isotope percentages that occur during radioactive decay. You can calculate the numbers directly using the equation for the decay of radioactive isotopes. The curve you drew on the graph for the radioactive decay of the parent isotope represents this equation. Use the following form of the equation to calculate geologic ages from measurements of radioactive isotopes and their daughter products in rocks or minerals. In this equation: age = (half-life) ln (D/P + 1) age is the absolute age, we will use years as the time unit ln is the natural logarithm (you need to use a scientific calculator) D/P is the ratio of daughter product to parent isotope in the rock or mineral The half-life is a fixed number that depends on which isotope system is being used To calculate the equation for completing Table 2, add 1 to the ratio of the daughter product to parent isotope (D/P), take the natural log of the result, divide by , and multiply by the half-life of the isotope system. See the Absolute Dating Section of the Geologic Time Basics page for more information on the isotope systems commonly used for measuring the ages of geologic materials Table 2. Geologic Age Calculation: Sample # Isotope system: Parent - Daughter Half-Life (years) Ratio of daughter product to parent isotope (D/P) 14 C 14 N 5, C 14 N 5, K 40 Ar 1,300 million K 40 Ar 1,300 million U 206 Pb 4,500 million U 206 Pb 4,500 million Calculated geologic age (years) Be sure your answers are rounded to the number of significant figures that can be justified by your input data. Geology 101 Lab Worksheet: Geologic Time page 4

5 PART 2: DETERMINING RELATIVE AGES Use the principles of relative dating to answer the questions for each of the following 7 diagrams. Refer to the principles of relative geologic age determination on the Geologic Time Basics page ( ) DIAGRAM 1 BLOCK DIAGRAM (2 QUESTIONS) 1. Label the strata in Diagram 1 in age sequence, starting with 1 for the oldest stratum. Label the strata on all faces of the block diagram: both of the cross-sections (sides) and the map (top). 2. Which principle of relative geologic age did you use to determine the age sequence of the strata? DIAGRAM 2 BLOCK DIAGRAM (2 QUESTIONS) 1. Label the strata in Diagram 2 in age sequence, starting with 1 for the oldest stratum. Label the strata on all faces of the block diagram: both of the cross-sections (sides) and the map (top). Assume that the layers were not so tilted as to be completely overturned which can actually happen in real rocks. 2. Which principle of relative geologic age tells us that the strata have been tilted? Geology 101 Lab Worksheet: Geologic Time page 5

6 DIAGRAM 3 BLOCK DIAGRAM (2 QUESTIONS) 1. Label the strata in Diagram 3 in age sequence, starting with 1 for the oldest stratum. Label the strata on all faces of the block diagram: both of the cross-sections (sides) and the map (top). 2. Which principle of relative geologic age tells us that the strata have been folded? DIAGRAM 4 CROSS-SECTION (6 QUESTIONS) Diagram 4 shows three geologic strata, a layer of sandstone on top of a formation of conglomerate on top of a granite batholith. The batholith is made of the same type of granite that is in the pebble clasts of the conglomerate. The contact between the granite and the conglomerate is irregular, similar to the way the present-day surface of the land is irregular. In answering the questions below, you may wish to refer to the unconformities section in the Geologic Time Basics page ( ) 1. What is the age order of the three types of rock? List them by name, in sequence from oldest to youngest. 2. Which principle of relative geologic age did you use to determine the age sequence of the conglomerate and sandstone? Geology 101 Lab Worksheet: Geologic Time page 6

7 3. Which principle of relative geologic age did you use to determine the age sequence of the granite batholith and the conglomerate. 4. Color the unconformity red. It is the line between the granite and the conglomerate. 5. What type of unconformity is the contact between the granite and the conglomerate? 6. What must have happened in the time between when the granite intruded and solidified, and the conglomeratic sediment was deposited? DIAGRAM 5 CROSS-SECTION (5 QUESTIONS) Diagram 5 shows four types of rock in relationship to each other: granite, shale, sandstone and, along the contact of the shale with the granite, hornfels. 1. What is the age relationship of the different bodies of rock in the cross-section? List them by rock type name, in order from oldest to youngest. 2. Which principle of relative geologic age allows you to deduce whether the sandstone is older or younger than the shale? 3. Which two principles of relative geologic age allow you to deduce whether the granite is older or younger than the shale? Geology 101 Lab Worksheet: Geologic Time page 7

8 4. What is the age relationship of the granite and hornfels? Is the hornfels older, younger, or virtually the same age as the granite? Explain. 5. What caused the hornfels to form? DIAGRAM 6 BLOCK DIAGRAM (2 QUESTIONS) Diagram 6 shows four sedimentary rock formations, a dike of igneous rock, and a fault a total of six geologic events, including the fault. 1. What is the age relationship of the bodies of rock and the fault in the block diagram? To answer the question, number each of the six events on the block diagram to show the relative age sequence from oldest to youngest, with 1 being the oldest. Remember to number the fault and the dike. Number each geologic event on all faces of the block diagram on which it appears. 2. Which principle of relative geologic age allows you to deduce whether the fault is older or younger than the dike? Geology 101 Lab Worksheet: Geologic Time page 8

9 DIAGRAM 7 CROSS-SECTION (3 QUESTIONS) Diagram 7 shows nine rock formations and two unconformities. Beginning with the schist, the diagram represents 12 geological events, including tilting. 1. Color the two unconformities red. 2. Although the younger unconformity is one line on the cross-section, it actually consists of two different types of unconformities, which are named according to what lies below the line. Name each type of unconformity and describe in one or two words what lies beneath it. a. b. 3. What is the age relationship of the bodies of rock, the tilting, and the unconformities in the block diagram? To answer the question, number the events on the block diagram to show the relative age sequence from oldest to youngest, including the unconformities and the tilting, with 1 being the oldest. For the tilting, put the number of where it is in the sequence just outside the crosssection next to the tilted strata, with tilting written after the number. Geology 101 Lab Worksheet: Geologic Time page 9

10 PART 3: APPLYING PRINCIPLES OF GEOLOGIC TIME TO THE GRAND CANYON Use the principles of relative dating and the diagram of the stratigraphy of the Grand Canyon below to complete the fill in the blank and short essay questions. Refer to the Grand Canyon section of the Geologic Time Lab web page ( for more information about the geology of the Grand Canyon. The following questions have a number after the names of the geologic formation(s) referred to in the question. The numbers refer to the numbers on the simplified stratigraphic column above. Write the appropriate term in each blank for the fill-in-the-blank questions. Write complete essay answers for the essay questions. 1. The Zoroaster Granite (2) is than the Vishnu Schist (1) based on the relative geologic age principle of. 2. The specific type of unconformity between the Vishnu Schist (1), and the bottom of the Grand Canyon Supergroup (3 6), is a. 3. The strata of the Grand Canyon Supergroup (3 6) are at high angles from horizontal, therefore, they have been since they formed, according to the relative geologic age principle of. 4. The type of unconformity between the Grand Canyon Supergroup (3 6) and the Tapeats Sandstone (7) is an. Geology 101 Lab Worksheet: Geologic Time page 10

11 5. The Tapeats Sandstone (7), Bright Angel Shale (8), and Muav Limestone (9) are a sequence, formed when sea level gradually relative to the level of the land during the Cambrian Period. 6. The Muav Limestone (9) formed during the Period and the Redwall Limestone (10) formed during the Period. Between these two periods of geologic time, the and periods are missing. Therefore, where the Redwall Limestone (10) is directly on top of the Muav Limestone (9), the specific type of unconformity between them is a, 7. Briefly describe the depositional environments of the Redwall Limestone (10). Along with describing each environment, refer to some of the evidence in the Redwall Limestone that points to that type of environment. 8. Briefly describe some of the depositional environments of the Supai Group (11). Along with describing each environment, refer to some of the evidence in the Supai Group that points to that type of environment. 9. The Supai Group (11) sits right on top of the Redwall Limestone (10). What must have happened to sea level, relative to the level of the land, between the time the Redwall sediments were being deposited and the the time the Supai sediments were being deposited? 10. Where the bottom of the Supai Group (11) is directly on top of the Redwall Limestone (10), the specific type of unconformity between them is a. Geology 101 Lab Worksheet: Geologic Time page 11