Time and Geology Chapter 8

Transcription

1 Time and Geology Chapter 8 Where would you hike to find the oldest rocks in this area? (hint : you would use the principle of superposition) Tasks 1. Read about relative ages on pages (skip the sections titled Correlation by fossils on page 189). Note that we will be learning about numerical ages from the special text that I have added to this lecture. 2. Go to the discussion board to ask and answer a question. 3. In preparation for quiz 8, go to the content section of D2L for this chapter and click on Preparation for quiz to answer questions related to chapter 8. You will find the answers to these questions in material presented in this file and in the text book. 4. Prepare yourself a series of note cards with sketches to study with. Use these to help you get to know all the material without looking at your notes. 5. Take the quiz. You may prepare for the quiz by doing items 1-4. You should know the material without looking at your notes. See class calendar for quiz date. 1 1

2 How do we assess the ages of rocks? In this chapter we will be concerned with how geologists assess the ages of rocks. There are two types of ages, relative and numerical, and they are assessed in different ways: Relative age A relative age is an age relative to something else. A relative age assessment involves the ordering of events or objects, from oldest to youngest. This requires knowledge of several geologic concepts and principles. Numerical age A numerical age is an age that is expressed as a number or numbers (e.g., 10 million years old or 3.6 billion years old). A numerical age is assessed by isotopic dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred) Relative Age Determination Because only a few rock types are amenable to isotopic dating methods, geologists often assess the relative ages of rocks. Assessment is relatively simple once you gain knowledge and understanding of the principles and concepts listed below. Much of this is geologic common sense, as you will see. Principles Used to Determine Relative Age original horizontality superposition lateral continuity cross-cutting relationships Other time relationships inclusions Metamorphosed (baked) contacts 2 2

3 My guess is that, even before we discuss the concepts and principles mentioned on the previous slide, you have already learned enough in this class to begin to assess relative ages. For example, the photo below shows three rock bodies: plutonic rock (marked pluton) and two dikes (dike x and dike y ). Can you list the three rock bodies in terms of their relative ages? Let 1 = oldest and 3 = youngest. A quarter is shown for scale. pluton 3= pluton Dike Y 2= 1= pluton Dike Y pluton Principles Used to Determine Relative Ages Contacts - surfaces separating successive rock layers (beds) Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock units 3 3

4 Contacts are surfaces separating two different rock types or ages of rocks. We will consider two types of contacts: depositional and intrusive. A depositional contact is a contact between sedimentary formations (which are groups of beds), beds, or extrusive volcanic rocks. The bottom line is that a depositional contact is a contact between rocks that have been deposited on other rocks. An intrusive contact is a contact between an intrusive igneous body (pluton, dike, sill) and the country rock. This sketch shows strata (group of beds) overlain by a basalt flow. All of the contacts between the different rock types are depositional contacts. The red arrows highlight all the contacts. Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers or beds get younger from bottom to top Lateral continuity an originally horizontal bed extends laterally until it tapers or thins at its edges Question List the sedimentary formations shown in the diagram above from oldest (#1) to youngest (#4). 4= 3= 2= 1= 4 4

5 Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption Intrusions and faults are younger than the rocks they cut through Questions On the basis of the principles and concepts we have discussed, list the rocks in terms of their relative ages from oldest (#1) to youngest (#5). (you may assume the sedimentary rocks predate the pluton) 5= 4= 3= 2= 1= What principle did you use to determine the relative ages of # s 1-4 on your list? What principle did you use to determine the relative age of #5 on your list? Exercise Study the block diagram and then list the rock formations (including the pluton) from oldest (#1) to youngest (#10). Hint: there are two basic groups of rocks: a pluton and strata. The sedimentary rocks are divided into formations that are labeled with red letters. To figure out the relative ages of the sedimentary formations you will need to visualize them un-tilted and then use the principle of superposition to assess their ages. 10 = 9= l m n a b c z y x 8= 7= 6= 5= This block diagram shows tilted strata intruded by a granite pluton and associated dike and sills. The contact between the intrusive rock and the country rock is an intrusive contact. 4= 3= 2= 1= 5 5

6 Other time relationships Inclusions are rock fragments embedded in host rock that are older than the host rock d v k a Inclusions are another thing that we can use to determine relative ages of rocks. Two important types of inclusions are xenoliths and pebbles of rock derived by erosion of an underlying formation. Study the adjacent cross section and then answer, solely on the basis of inclusions, the following questions. What do the xenoliths tell you about the relative ages of the tilted strata vs. the granite? What do the pebbles of the granite in formation k tell you about whether the pluton intruded before or after the deposition of formation k? Metamorphosed (baked )contacts refers to a contact between an igneous intrusion and surrounding rocks, wherein the surrounding rocks have experienced contact metamorphism If we see an intrusive igneous body, how do we know whether the contact between the intrusion and the surrounding rock is intrusive or depositional? One important way is to see if the surrounding rocks are metamorphosed in a contact aureole; if they are then you are looking at an intrusive contact. If the rocks are not metamorphosed then you are looking at a depositional contact. The adjacent cross section shows a pluton where all contacts are intrusive except for the part that I have highlighted with a red dashed line. We know this contact is not intrusive because the rocks above it are not metamorphosed. Please remember (for the remainder of this class), and understand,the following definition of a cross section. A cross section shows what the rocks would look like beneath the Earth s surface (as if you made a vertical cut through the crust and removed the rock on one side of the cut so that you could see what is below the surface). 6 6

7 Exercise Determine the relative ages of rocks shown in the block diagram below. You will need to use your knowledge of the principles and concepts that we have just covered. There are ten different rock bodies. You may check your answer in the book. Unconformities An unconformity is a contact that represents a gap in the geologic record. Generally, there is a substantial amount of time not represented by rocks across an unconformity. Unconformities are a special type of depositional contact. Unconformities are useful for assessing the geologic history of an area There are three types of unconformities: disconformity, angular unconformity, and a non-conformity 7 7

8 Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other. The sketches below illustrate how a disconformity commonly forms. g f e d c b a Deposition of a-g Erosion of d-g Deposition of h and then i c b a i h c b a Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folded layers Exercise The adjacent diagram shows an example of how an angular unconformity may form. Describe, in your own words, the sequence of events necessary to form an angular unconformity. 8 8

9 Nonconformity - an unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock Plutonic and metamorphic rocks are exposed by large amounts of erosion Typically represents a large gap in the geologic record Exercise The dashed red line denotes the approximate location of a famous nonconformity in the Grand Canyon. The layering you see above the nonconformity represents bedding. The metamorphic rocks below the nonconformity do not appear layered. Describe, in your own words, the sequence of events necessary to form this nonconformity. Question R T X A G This is a photo of the entrance to Dunbar Cave. The layers of rock that you see are limestone beds. Please determine which of the beds that I have labeled with white letters is the oldest and which is the youngest? 9 9

10 x y z Exercise Determine the relative ages of the beds at points x, y, and z. Exercise A cross section of the Grand Canyon area is shown below. List the Navajo Sandstone, Vishnu Schist, Coconino Sandstone and Bright Angel Shale in chronologic order from oldest to youngest. Also, indicate the type of unconformity that the black arrow is pointing to. 4= 3= Bright Angel Shale 2= 1= 10 10

11 The Standard Geologic Time Scale Geologic time is divided into the Precambrian and the three eras: Paleozoic, Mesozoic and Cenozoic. The three eras are further subdivided into periods. Precambrian is a generalized term that denotes the vast amount of time preceding the Paleozoic era and it is not divided into eras or periods. Precambrian This timescale subdivides geologic time based on fossil assemblages. Sedimentary rocks from each period are defined by unique fossils of organisms that evolved and went extinct in that particular period. Consequently, if you find the unique fossils in the rocks you can assign a relative age to the rocks. This timescale expresses relative (not numerical) geologic time Questions Disconformities are generally found by recognizing that strata of a certain period are missing. Can you find the disconformities in the cross sections showing sedimentary formations below? Tertiary Cretaceous Jurassic Mississippian Devonian Silurian Cambrian Quaternary Triassic Permian Pennsylvanian Mississippian Devonian Precambrian What geologic era are we living in today? What geologic period are we living in today? 11 11

12 Isotopic Dating Isotopic dating utilizes radioactive isotopes to place numerical ages on rocks. It is mostly used for dating igneous and metamorphic rocks that contain minerals that are amenable to isotopic dating. The methods of isotopic dating are somewhat complicated, but our goal here is to show you how it works in principle. Your task is to read the following discussion about isotopic dating and then answer the questions at the end. However, before we talk about dating, you need to review some chemistry that we discussed in chapter two on minerals. Please retrieve your notes from Chapter two and answer the following questions: What defines an element? What is an isotope of an element? Isotopes are defined by mass number. For example, there is an isotope called U238 where 238 is the mass number. What is the mass number of an isotope? Minerals that contain radioactive (unstable) isotopes may be used to date the time a rock formed. Most isotopes are stable, but radioactive isotopes are unstable because they lose or gain protons overtime and will ultimately decay into a stable but different element. A radioactive isotope is often called the parent and the stable isotope that it decays into is called the daughter product. How can we use minerals that contain a radioactive isotope to date a rock? The best way to explain this is to proceed by example using the uranium (U238)-lead (Pb206) method, which is commonly used for dating igneous rocks. The radioactive isotope U238 will decay into a stable isotope of lead (Pb) called Pb 206. First, we will explore some of the basics about the conversion of U238 to Pb206 and then we will discuss how this phenomenon can be used to date a rock. The isotope U238 has 92 protons and 146 neutrons ( = 238 = mass number) and is radioactive (Figure 1). Overtime an atom of U238 will lose 10 protons and 22 neutrons and will turn into Pb 206, which has 82 protons and 24 neutrons ( = 206 = mass number). Because this decay process involves changing the atomic number from 92 to 82, the element changes from U to Pb. The isotope Pb206 is unique because it only forms from the decay of U238. Figure 1. Diagram illustrating the decay of U238 to Pb 206. For igneous rocks, the U-Pb method is typically used to date a mineral called zircon that grew at the time the rock crystallized. Zircon occurs in trace amounts in igneous rocks (Figure 2) and it contains the radioactive isotope U238. When zircon forms it contains some U238 but no Pb 206. Zircon does not incorporate the element lead during growth because Figure 2. Example of tiny zircon grains that a geologist has mechanically separated from granite (the penny is for scale). 12

13 it is not part of its chemical structure. However, after the mineral is formed Pb206 will be produced by decay of U238, and this Pb will be locked inside the zircon. Consequently, as time goes on the amount of U238 decreases and the amount of Pb206 correspondingly increases. The decay of U to Pb can be used to date the rock because we can measure the amount of U remaining and the amount of Pb produced (this is done by a device called a mass spectrometer), and we know the rate of decay, i.e., the rate at which U turns into lead. In summary, these basic assumptions and techniques are used in dating: (1) No Pb is assumed to exist in the mineral when it formed (2) The amount of U238 decreases over time and the amount of Pb206 increases (3) We know the rate of decay and we can measure the amount of U and Pb in the mineral using a device called a mass spectrometer We will now discuss the basic idea behind producing an isotopic age. The most important thing to understand is the rate of decay, which is measured in terms of half-life. A half life is the time it takes for a given amount of radioactive isotope to be reduced in half. The graph in Figure 3 illustrates the concept of half life. The percent of original radioactive isotope is on the vertical axis and time in terms of half life is on the horizontal axis. The black line shows the amount of radioactive isotope remaining and the red dashed line shows the growing percentage of stable daughter product. When the mineral first forms there is 100 % parent and 0% daughter. After one half life there is 50% parent remaining and 50 % has decayed to the daughter product. After two half lives there is 25% parent remaining and 75 % has decayed to the Figure 3. daughter product. From the graph you can see that a sample whose age is equal to one half life would have a ratio of parent to daughter of 1:1. When this sample is two half-lives old the ratio of parent to daughter would be 1:3. Because the half life of a radioactive isotope is known, all we need to do to place an isotopic age on a mineral or rock is to measure the amount of parent and daughter and look at their ratios. To better visualize this concept lets imagine that we have a zircon crystal with 1000 U238 atoms and see what happens to the amount of U238 over time (Figure 4). U238 has a half life of Age Amount of parent present Amount of daughter present Figure 4 Time when mineral formed 0 b.y. 4.5 b.y. 9.0 b.y U ~4.5 billion years (b.y.). When the mineral first forms there are 1000 U atoms and no Pb atoms. After 4.5 b.y. there are 500 U atoms present and 500 U atoms have converted to Pb, hence the ratio of U to Pb is 1:1. After another half-life has elapsed and the rock is 9.0 b.y. old, there are 250 U atoms remaining and 750 Pb atoms have been produced by decay of U. Thus after two half-lives the ratio of parent to daughter is 1:3. Although we have only discussed a few ratios of parent to daughter, it is possible to 0 Pb Time after 1 half life has elapsed 500 U 500 Pb Time after 2 half lives have elapsed 250 U 750 Pb 13

14 mathematically assess an age for any ratio if you have been given the half life and the ratio of parent to daughter. Although we have focused on the U-Pb method of dating, there are many other radioactive isotopes that can be used for dating. These other methods are based on principles that are similar to what we have described for U-Pb. One example of a common method of dating is the K-Ar method. This involves the decay of a radioactive isotope of potassium (K) to a stable isotope of argon (Ar). Minerals containing K are used for this method. For example, it is commonly used for orthoclase and muscovite (see Figure 5) in igneous rocks. Figure 5. Photographs of muscovite and orthoclase, which are two K bearing minerals used for K Ar dating. Do you know which picture is of muscovite and which is of orthoclase? Questions (please answer the following questions based on what you have just read) (1) What is the difference between a stable and a radioactive isotope? (2) What is the meaning of the terms parent and daughter product? (3) Explain what happens to an atom of the radioactive isotope U238 over time. (4) If a granite sample has 5000 atoms of U238, how many are remaining after 1 half life? How many are remaining after 2 half lives? (5) If a granite sample has 400 atoms of U238, how many Pb206 atoms have been produced after 1 half life? How many have been produced after 2 half lives? (6) Explain in your own words the reasons why zircon can be used for isotopic dating. (7) Imagine you have a sample with 500 atoms of radioactive isotope X. Isotope X decays to stable isotope Y with a half life of 2 million years. How much time will it take to produce 375 atoms of isotope Y? Hint: to solve this problem you might want to construct a diagram similar to that shown in Figure 4. (8) Imagine you have a sample with 36 atoms of radioactive isotope W. Isotope W decays into stable isotope Z with a half life of 1 million years how much time will it take for 27 atoms of W to decay to Z? Hint: to solve this problem you might want to construct a diagram similar to that shown in Figure 4. (9) Name an example of a mineral that is commonly used for the U-Pb method of dating and a mineral that is commonly used for the K-Ar method of dating. 14

15 Relative vs. Numerical ages Things we have covered Concepts and principles used for relative age determination: contacts, original horizontality, superposition, lateral continuity, cross-cutting relationships, baked contacts, inclusions, unconformities (angular unconformities, nonconformities, disconformities) How to view rocks and cross sections and determine the relative ages of rocks and to delineate the various types of unconformities The eras and periods of the geologic timescale and what the geologic time scale is based on Atomic number vs. mass number and stable vs. unstable isotopes The principles behind isotopic dating (decay, half-life etc.) utilizing the U-Pb method as an example Next up: Now that you have reviewed this lecture file and read the chapter, please go to the content section of D2L for this chapter and click on Preparation for quiz to answer questions related to this chapter. You will find the answers to these questions in material presented in this file and in the text book