Chapter 24 Geologic Time. Physical Science II Module 3, Part 4
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1 Chapter 24 Geologic Time Physical Science II Module 3, Part 4
2 Geologic Time Geologists study the rocks/minerals, the structures, and the processes that occur on Earth today. In addition, geologists look back and attempt to interpret the history of the Earth over geologic time. The Earth is very old, about 4.6 billion. (4560 million) Humans have only been around during the last 2 million years Intro
3 Geologic Time In this chapter we will discuss geologic time and how geologists are able to measure it. Our discussion of geologic time should serve to: Heighten our sense of responsibility as present-day custodians of the Earth; Show the enormous complexity of the processes that have resulted in today s biota. We will discuss fossils, relative geologic time, radiometric dating, absolute geologic time, and the geologic time scale Intro
4 Fossils Fossil any remnant or indication of past life that is preserved in rock Paleontology is the study of fossils. The study of fossils is of great interest to both geologists and biologists. Paleontologists combine present-day biologic information with ancient fossil and rock data to make an interpretation of past events and environments Section 24.1
5 Fossil Preservation Fossils are preserved in rocks in a number of ways. In general, the three most important factors that lead to good preservation include: quick burial, lack of oxygen, and the presence of hard material that can be preserved. Under extremely rare circumstances, the soft parts of organisms may be preserved in some manner Section 24.1
6 Fossil Preservation Original Remains Ancient insects and other organisms occasionally became encased in sticky tree resin. The resin hardens into amber and original organism is perfectly preserved. Intact wooly mammoths have been recovered in Alaska and Siberia, encased in ice. Shark s teeth and marine shells may also be found in original condition Section 24.1
7 Fossil Preservation Replaced Remains Fossils are more commonly found composed of replacement minerals. Silica (SiO 2 ), Calcite (CaCO 3 ), and Pyrite (FeS 2 ) are common replacement minerals. The hard parts (bones, shell, etc) of ancient organisms is slowly replace by the circulation of mineralized groundwaters after death/burial. 24 7
8 Mode of Fossil Preservation Replaced Remains Dinosaur bones are commonly composed of silica. (SiO 2 ) Copyright Bobby H. Bammel. All 24 8 Section 24.1
9 Fossil Preservation Carbonization Carbonization will occur when plant remains decay under conditions of very low oxygen or anaerobic conditions. Most elements except carbon are driven off as the plant material decays, leaving behind a carbon residue. In many cases the carbon residue will retain many of the features of the original plant Section 24.1
10 Fossil Preservation Molds and Casts In many cases the entire embedded shell, bone, or piece of wood is completely dissolved, leaving behind a hollow void a mold. New mineral or sediment material may later fill the mold, creating a cast of the original fossil. Molds and casts only show the shape and size of the original organism. Internal details of the organism is not preserved Section 24.1
11 Modes of Fossil Preservation Molds and casts of marine organisms Copyright Bobby H. Bammel. All Section 24.1
12 Fossil Preservation Trace Fossils Trace fossil any type of imprint or trail made by the movement of an ancient animal Ichnology is somewhat broader and includes the study of plant and animal traces. Trace fossils include tracks, burrows, borings, and tooth marks Section 24.1
13 Modes of Fossil Preservation Trace Fossils Fossilized Burrows Copyright Bobby H. Bammel. All Section 24.1
14 Fossil Evidence of Life Fossilized blue-green algae is the earliest fossilized remains of life on Earth 3.5 billion years ago! As time moved forward the fossil record indicates that life became more complex. Fossils serve as exceptional indicators of past environments. A rock layer containing fossil coral indicates that it was deposited in a shallow, warm sea Section 24.1
15 Microfossils Just as life today occurs in all sizes and shapes, ancient life did also. Some rocks contain countless microfossils, so small that they can only be studied with the aid of powerful microscopes. Microfossils are particularly useful when drilling deep wells. Due to their small size the entire fossil can be collected and studied in the drill cuttings Section 24.1
16 Relative Geologic Time Relative geologic time is determined by placing the sequence of rocks and geologic events into sequential order without knowing their actual dates. Several common sense principles are used to help determine the relative ages of the rocks and relative sequence of geologic events: The principle of superposition The principle of cross-cutting relationships Section 24.2
17 Principle of Superposition Principle of superposition in a sequence of undisturbed sedimentary rocks, lavas, or ash the oldest layer is on the bottom with each ascending layer progressively younger In other words, the bottom layer was deposited first and is therefore the oldest layer; the top layer was deposited last and is therefore the youngest layer. If the layers have been disturbed (faulted or folded) this must be taken into account Section 24.2
18 Principle of Cross-Cutting Relationships Principle of cross-cutting relationships an igneous rock is younger than the rock layers that it has intruded This principle also applies to faults and folds, where the fault or fold is younger than any rocks that are affected Section 24.2
19 Principle of Cross-Cutting Relationships The igneous dike is younger than the layers that it cuts across and the fault is younger than the dike, since it cuts across the dike Section 24.2
20 Unconformities Unconformities represent gaps or breaks in the geologic rock record. Nowhere on Earth is the rock record for all geologic time complete. In any given area, there are missing layers due to nondeposition or due to erosion of some of the layers. The amount of missing geologic time represented by an unconformity is usually very difficult to determine Section 24.2
21 Applying Principles of Relative Dating An Example Using the principles of relative dating, analyze the figure below and put the rocks marked 1 through 5 in order from youngest to oldest. Note that rock layer 5 is above rock layer 4, rock layer 4 is above rock layer 3, and rock layer 3 is above rock 1. Also notice that rock 2 cuts across rock Section 24.2
22 Applying Principles of Relative Dating An Example (cont.) The principle of superposition indicates that rock layer 5 is the youngest, followed by rock layers 4, then 3, then 1. The principle of crosscutting relationships indicate that rock 2 is younger than rock 1. Therefore the correct order from oldest to youngest is 1, 2, 3, 4, Section 24.2
23 Finding an Unconformity Confidence Exercise Using the figure below, find the unconformity and determine when (relatively) it must have been formed. Note that the top of rock 2 is even with rock 1. This indicates that both rocks 1 & 2 underwent erosion together. Rock layer 3 was deposited after this episode of erosion Section 24.2
24 Finding an Unconformity Confidence Exercise (cont.) The time of erosion occurred after rocks 1 and 2 were present but before rock layer 3 was deposited. Thus the unconformity was formed before rock layer 3 and after rocks 1 and Section 24.2
25 Correlation After geologists determine the relative ages of the rocks at several separate localities they attempt to match the layers by age. Correlation is the process of determining age equivalence between different localities. For example, if the rock at location A is known and the rock in location B is correlated to A, then the age of the rock at B is the same as the age of the rock at A Section 24.2
26 Index Fossils Index fossils fossils that are wide-spread in distribution, easily identified, and limited to a particular time segment of the Earth s history These fossils can be of major assistance during the process of correlation. Once a particular index fossil has been thoroughly established, geologists immediately know the age of any rocks containing this index fossil anywhere in the world Section 24.2
27 Using the Process of Correlation An Example Four fossils, labeled A through D, are shown in the figure below, along with their time ranges. a) Which fossil would be the most useful as an index fossil? b) If a rock layer from a certain locality contains both fossils C and D, what is the age of the rock? Section 24.2
28 Using the Process of Correlation An Example (solution) a) Fossil A would be the best index fossil due to the narrow range of time that it existed. b) A rock that contains both fossils C and D must be Silurian in age. Only during the Silurian did both organisms live Section 24.2
29 Using the Process of Correlation Confidence Exercise a) Primitia would be the least use as an index fossil. It lived in a wide range of time. b) The presence of Phacops indicates that a rock is either Silurian or Devonian in age. c) Phacops is a trilobite Section 24.2
30 Relative Geologic Time Scale As geologists all over the world worked to correlate rocks over large areas, the relative ages of most rocks on the Earths surface have been determined. Using fossils and principles of relative dating techniques geologists have established a relative time scale for the Earth s geologic history Section 24.2
31 Relative Geologic Time Scale Eon the largest unit of geologic time The Phanerozoic Eon is the one we live in. The time before the Phanerozoic Eon is known as Precambrian time Section 24.2
32 Relative Geologic Time Scale Eons are divided into eras. There are three eras contained within the Phanerozoic Eon: Paleozoic Era the oldest and age of ancient life Mesozoic Era the age of reptiles Cenozoic Era the youngest and age of mammals Section 24.2
33 Relative Geologic Time Scale Each era, in turn, is divided into several smaller units of time called periods. The Paleozoic is split into seven periods. The Mesozoic is split into three periods. The Cenozoic is split into two periods Section 24.2
34 Measuring Absolute Geologic Time The development of the relative geologic time scale was a significant achievement and provided geologists with an excellent tool for the interpretation of rocks. In addition to knowing the order of geologic events, geologist also wanted to know how long ago these events occurred. Geologists wanted a tool that would enable them to know absolute ages Section 24.3
35 Measuring Absolute Geologic Time The need to determine absolute time became more crucial as James Hutton s concept of uniformitarianism and Darwin s theory of organic evolution became widely accepted. Both geologists and biologists concluded that these processes were very slow and indicated that the age of the Earth was much older than previously thought Section 24.3
36 Radiometric Dating Radiometric dating the determination of age by the measurement of the rate of decay of radionuclides in the rocks Recall that an atomic nuclei is said to be radioactive when it will naturally decay. The product of decay is generally called the daughter nuclei or daughter product. Daughter products may themselves be stable or radioactive (unstable.) Section 24.3
37 Radiometric Dating Half-life Half-life the length of time taken for half of the radionuclide in a sample to decay This rate of decay has been found to always be constant. Unaffected by temperature, pressure, and chemical environment The older the rock the less parent and the more daughter product is present. Different radioactive parents may have drastically different half-lives Section 24.3
38 Half-Life and Radiometric Dating As the parent nuclide decays the proportion of the parent decreases and the proportion of the daughter increases Section 24.3
39 Rock Clocks Condition #1 Radioactive decay can serve as a clock for dating rocks, if the following conditions are met Over the lifetime of the rock, no daughter or parent has been added or subtracted. This condition requires that there has been no contamination of the rock. If either parent or daughter nuclides are added or subtracted by metamorphism or fluid movement, the date obtained is not valid Section 24.3
40 Rock Clocks Condition #2 The age of the rock is reasonably close to the halflife of the parent radionuclide. If too many half-lives transpire it may become impossible to measure the amount of the remaining parent nuclide. If only a small portion of one half-life transpires then it may be impossible to measure the amount daughter product present. In either case, a valid date cannot be obtained Section 24.3
41 Rock Clocks - Condition #3 No daughter product was present when the rock initially formed. If daughter product was present when the rock formed, later analysis of the rock will result in an inaccurate parent to daughter ratio. Sometimes it may be possible to determine the amount of daughter nuclide initially present. In order to use radiometric dating techniques at all, the rocks must actually contain the appropriate radionuclides Section 24.3
42 Condition #3 Sometimes a Problem In the case of uranium-lead dating, there are many different isotopes of lead. For example, we know that primordial lead consists of 1.4% lead-204, 24.1% lead-206, 22.1% lead-207, and 52.4% lead-208. We also know that lead-204 is never created from radioactive decay. Therefore if any lead-204 is present we know that the other three lead isotopes are also present and we know their ratios Section 24.3
43 Primordial and Radiogenic Lead Since lead-204 is present, we know how much of the other isotopes are primordial and how much are radiogenic Section 24.3
44 Major Radionuclides Used for Radiometric Dating Note that since the half-lives vary the range of ages also varies. Not all rocks can be radiometrically dated, only those with the appropriate mineral present Section 24.3
45 Major Radionuclides Used for Radiometric Dating Note that since the half-lives vary the range of ages also varies. Not all rocks can be radiometrically dated, only those with the appropriate mineral present. Section 24.3
46 Potassium-Argon Dating Potassium (K) is one of the most abundant elements in minerals of the Earth s crust. A very small percentage (0.012%) is the radioactive isotope, potassium-40. Potassium-40 has a half-life of 1.25 billion years and decays to Argon-40. K-Ar dating can be used in a variety of minerals including orthoclase, muscovite, biotite, hornblende, and others Section 24.3
47 Potassium-Argon Dating Limitations Recall that Ar is an inert gas. When K-40 decays to Ar-40 the minerals are susceptible to Ar-40 leakage, especially if the mineral has been heated. If some of Ar-40 (the daughter product) leaks out, the resulting date will not be valid. K-Ar dating may reveal the last time the rock was heated and not the time of original crystalliztion of the mineral Section 24.3
48 Rubidium-Strontium Dating Rubidium-87 is a common constituent of many crustal minerals. Rubidium-87 has a half-life of 49 billion years and decays to Strontium-87. A significant portion of Sr-87 is primordial and therefore corrections are necessary. Rb-87 is found in many of the same minerals as K-40. Therefore Rb-Sr dating is commonly used as a check against K-Ar age determinations Section 24.3
49 Using Radiometric Dating An Example The ratio of U-235 to its daughter, Pb-207, is 1 to 3 in a certain rock. That is, only 25% of the original U-235 remains. (The half-life of U-235 is 704 x 10 6 years.) How old is the rock? To decay from 100% to 25% takes 2 half-lives. 100% 50% 25% (2) x (704 x 10 6 years) = 1408 x 10 6 years or 1.41 billion years = age of the rock Section 24.3
50 Using Radiometric Dating Confidence Exercise The ratio of U-235 to its daughter, Pb-207, is 1 to 7 in a certain rock. That is, only 12.5% of the original U- 235 remains. (The half-life of U-235 is 704 x 10 6 years.) How old is the rock? To decay from 100% to 12.5% takes 3 half-lives. 100% 50% 25% 12.5% (3) x (704 x 10 6 years) = 2112 x 10 6 years or 2.1 billion years = age of the rock Section 24.3
51 Carbon Dating Developed in 1950 by American, Willard Libby Carbon-14 ( 14 C) dating is the only radiometric dating technique that can be used to date once-living organisms. 14 C is a radionuclide with a half-life of 5730 years. The age of an ancient organic remain is measured by comparing the amount of 14 C in the ancient sample compared to the amount of 14 C in modern organic matter Section 24.3
52 Carbon Dating 14 C is a natural product formed in the atmosphere. About one in a trillion C atoms in plants is 14 C. 14 C is incorporated into all living organisms. Living matter has an activity of about 15.3 counts/minute/gram C. At death the 14 C present begins to decay Section 24.3
53 Carbon Dating Modern Methods In the newest carbon dating techniques, the amounts of both 14 C and 12 C are measured. The ratio of these two isotopes in the ancient sample is compared to the ratio in living matter. Using this method only very small samples are needed and specimens as old as 75,000 years can be accurately dated. Beyond 75,000 years, the amount of 14 C still not decaying is too small to measure Section 24.3
54 Carbon Dating - Limitations Carbon dating techniques assumes that the amount of 14 C in the atmosphere (and therefore in living organisms) has been constant for the past 75,000 years. We now know that the amount of 14 C in the atmosphere has varied by (+) or (-) 5%. These variations in 14 C levels have been due to changes in solar activity and changes in the Earth s magnetic field Section 24.3
55 Carbon Dating - Limitations These slight variations in 14 C abundance have been corrected by careful analyses of California s 5,000 year-old bristlecone pines. An extremely accurate calibration curve has been developed for 14 C dates back to about 5000 B.C. Carbon dating is widely used in archaeology, and has been used to date bones and other organic remains, charcoal from fires, beams in pyramids, the Dead Sea Scrolls, and the Shroud of Turin Section 24.3
56 Lord Kelvin & the Age of the Earth In the middle to late 1800 s Lord Kelvin was the most distinguished physicist in the world. He attempted to determine the Earth s age by using the rate of heat loss from its interior. As a basic assumption, Lord Kelvin considered that the entire Earth began in a molten state and slowly became solid as it lost this residual heat from the still hot interior. He was trying to determine the elapsed time from the molten beginning to the present day Section 24.4
57 Lord Kelvin & the Age of the Earth Lord Kelvin s calculations and measurements led him to conclude that the Earth was between 20 and 40 million years old. Many of the geologists and biologists of the time thought that the slow pace of geologic and organic evolution indicated a much older age. Due to his considerable prestige and actual data, Lord Kelvin s conclusion was difficult to argue against Section 24.4
58 Radioactivity & the Age of the Earth Radioactivity was discovered in 1896 and it soon became apparent that all the heat in the Earth s interior was not residual. This discovery invalidated Lord Kelvin s basic assumption and it became evident that his estimate of the Earth s age was in error. We now know that most of the Earth s interior heat is due to radioactive decay. With the addition of radioactive-generated heat, there is much more heat to account for and much more time needed Section 24.4
59 Age of the Earth After many years of study, geologists are now confident that the Earth s age is 4.56 billion years. (4560 million years) There are three main lines of evidence that support this age for the Earth: The age of Earth rocks The age of meteorites The age of the Moon rocks Section 24.4
60 Age of Earth Rocks With the advent of radiometric dating techniques, it is possible to put absolute dates on many igneous and metamorphic rocks, To date, the oldest rocks yet analyzed are zircon crystals from northwest Australia. Dated at approximately billion years Other exceedingly old rocks on Earth include 4.0- billion-year-old rocks in Canada, 3.8-billion-year-old granites in Greenland, and 3.4-billion-year-old granites in South Africa Section 24.4
61 Age of Meteorites Meteorites from our solar system are thought to have formed at the same time as Earth. These meteorites have been reliably dated at 4.56 billion years using both U-Pb and Rb-Sr methods Section 24.4
62 Age of Moon Rocks A number of Moon rocks have been meticulously analyzed. Rocks from the lunar highlands are the oldest, yielding an age of 4.55 billion years Section 24.4
63 Age of the Earth Several different lines of evidence strongly suggest that the planets, the moons, and the asteroids of our solar system were all formed approximately 4.56 billion years ago. It is doubtful that rocks on Earth will ever be found that are fully 4.56 billion years old. In its early history the Earth s surface was likely molten for several hundred million years. Plate tectonics, weathering, metamorphism, and other processes have destroyed many ancient rocks Section 24.4
64 Geologists and Scripture Some people argue for a very young Earth. In the range of 5,000 to 10,000 years, according to their scriptural interpretation Geologists are consistently drawn into the young Earth versus old Earth debate. Ultimately, it really doesn t matter to most geologists how old the Earth is. They simply want to reliably know how old it is. At the present time the physical and biological evidence on Earth points overwhelmingly to a long Earth (4.56 billion years) perspective Section 24.4
65 Geologic Time Scale The modern geologic time scale has been constructed using both relative geologic time and absolute geologic time. Most of the accepted dates are estimated values and are subject to minor changes as new data is acquired Section 24.5
66 Geologic Time Scale Time is given in millions of years before present, along with major geologic and biologic events Section 24.5
67 Construction of the Geologic Time Scale Sedimentary rocks have been used primarily to establish the relative time scale. Sedimentary rocks are rarely suitable for radiometric dating since they are composed of erosional debris. Igneous rocks have been used primarily to attain radiometric absolute dates. In some cases metamorphic rocks have been used to attain radiometric absolute dates for deformational (marking intense heating and recrystallization) events Section 24.5
68 Dating Sedimentary Rocks In many cases the only way to attain an absolute date on sedimentary rocks is to relate the sedimentary rocks to igneous rocks. This process is called bracketing. The absolute dates that are obtainable from several igneous rocks serve to bracket the minimum and maximum age of the sedimentary layers of interest Section 24.5
69 Using Igneous Rocks to Date Sedimentary Rocks - An Example In the figure below, the two igneous dikes have been dated: X = 400 My and Y = 350 My. What can be said about the age of the Devonian stratum labeled B? Igneous dike X intruded Silurian strata. Part of dike X and the Silurian strata were eroded. Strata B was deposited later, therefore it is younger than 400 My Section 24.5
70 Using Igneous Rocks to Date Sedimentary Rocks - An Example (cont.) Strata B must be older than the age of dike Y. (350 My) The Devonian strata B is between My. We have therefore bracketed the age of the Devonian stratum Section 24.5
71 Using Igneous Rocks to Date Sedimentary Rocks What can be said about the absolute age of the sedimentary rock layer A from the Mississippian period? Part of dike Y was eroded before layer A was deposited. Thus, rock layer A was deposited after Y. (350 My) From the Geologic Time Scale we know that the Mississippian Period extended from My. Layer A is younger than 350 and older than 320 My Section 24.5
72 Highlights of Geologic Time The beginning of the Archeon eon, at 4000 m.y.a., marks the date of the oldest Earth rocks. The Protoerozoic eon began 2500 m.y.a. and coincides with the formation of the North American continental core. Phanerozoic eon began at about 545 m.y.a Section 24.5
73 Proterozoic Supercontinent Rhodinia Rhodinia formed in the late Proterozoic. It broke apart during the early Paleozoic Era Section 24.5
74 Highlights of Geologic Time The Phanerozoic eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. Hard-shelled marine invertebrates first became abundant at the start of the Paleozoic. Life began to proliferate during the Cambrian Period and continued throughout the Paleozoic Section 24.5
75 Highlights of Geologic Time The Paleozoic came to an abrupt and possibly catastrophic end 245 m.y.a. The end of the Paleozoic is sometimes called the Great Dying. 90% of the ocean species and 70% of the land species became extinct. Perhaps a huge asteroid hit the Earth Section 24.5
76 Highlights of Geologic Time During the Mesozoic era the global climates were mild. Corals grew in what is now Europe. Dinosaurs were common in the western U.S. and Canada, as well as many other areas. The Mesozoic came to a catastrophic end at about 65 m.y.a Section 24.5
77 Highlights of Geologic Time About 70% of the world s plant and animal species, including the dinosaurs became extinct. Evidence strongly indicates that an asteroid struck the Earth on the NW side of the Yucatan peninsula of Mexico Section 24.5
78 Highlights of Geologic Time The Cenozoic era began at 65 m.y.a. and is commonly known as the age of mammals. Our present period is called the Quaternary. It began about 2 m.y.a. with the first appearance of the genus Homo. The Cenozoic is subdivided into epochs. The last two are of special interest Section 24.5
79 Highlights of Geologic Time The Pleistocene epoch is also known as the ice age and is marked by significant worldwide glaciation. Our present epoch, the Holocene began about 10,000 years ago when the glaciers retreated from Europe and North America Section 24.5
80 Geologic Time in Perspective A Timeline Each kilometer represents about 1 million years. Note how long geologic time is compared to what we call recorded history Section 24.5
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