1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Rock Record Chapter 8 Table of Contents Section 1 Determining Relative Age Section 2 Determining Absolute Age Section 3 The Fossil Record

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Objectives State the principle of uniformitarianism. Explain how the law of superposition can be used to determine the relative age of rocks. Compare three types of unconformities. Apply the law of crosscutting relationships to determine the relative age of rocks.

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Uniformitarianism uniformitarianism a principle that geologic processes that occurred in the past can be explained by current geologic processes. One of the basic foundations of geology.(5)(6) Geologists estimate that Earth is about 4.6 billion years old, an idea that was first proposed by James Hutton in the 18th century after observing geologic changes taking place on his farm. These forces that shaped his land took place very slowly. (1)(2)(3)(10) Hutton theorized that the same forces that change Earth’s surface now, such as volcanism and erosion, are the same forces that were at work in the past.(4)

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Uniformitarianism, continued Earth’s Age Hutton’s ideas raised serious questions about Earth’s age, because up until that time, most scientists thought Earth was only 6,000 years old, and that all Earth’s geologic features had formed at the same time.(8)(9) Hutton’s ideas about uniformitarianism encouraged other scientists to learn more about Earth’s history. They pointed out that although the processes of the past and present are the same, the rates at which the processes work may vary over time. (7) (11)

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Relative Age, continued relative age the age of an object in relation to the ages of other objects One way to learn about Earth’s past is to determine the order in which rock layers and other rock structures formed. (12) Layers of rock, called strata, show the sequence of events that took place in the past. (13)

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Relative Age, continued Scientists can determine the order in which rock layers formed by using a few basic principles. Once they know the order, a relative age can be determined for each layer.(14) Relative age indicated that one layer is older or younger than another layer but does not indicate the rock’s age in years.(15)

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Law of Superposition, continued law of superposition the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed. Scientists commonly study the layers in sedimentary rocks to determine the relative age of rocks.(16) Sedimentary rocks form when new sediments are deposited on top of old layers of sediment.(17) As the sediments accumulate, they harden into layers called beds. The boundary between two beds is called a bedding plane.(18)(19)

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Law of Superposition, continued Scientists use a basic principle called the law of superposition to determine the relative age of a layer of sedimentary rock.(20) The law of superposition states that an undeformed rock layer is older than the layers above it and younger than the layers below it.

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Law of Superposition, continued The diagram below illustrates the law of Superposition. Section 1 Determining Relative Age

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Principle of Original Horizontality Scientist know that sedimentary rock generally forms in horizontal layers. (21) The principle of original horizontality states that sedimentary rocks left undisturbed will remain in horizontal layers. So, scientists can assume that sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.(22)(23)(24)

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Principle of Original Horizontality, continued Graded Bedding In some cases, tectonic forces push older layers on top of younger ones or overturn a group of rock layers. So, scientists must look for clues to the original position of the layers.(25)(26) One clue is the size of particles in the layers. In some environments, the largest particles of sediment are deposited in the bottom layers.

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Principle of Original Horizontality, continued Graded Bedding The arrangement of layers in which coarse and heavy particles are in the bottom layers is called graded bedding.(27) If larger particles are located in the top layers, the layers may have been overturned by tectonic forces.(28)

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Principle of Original Horizontality, continued Cross-Beds Another clue is the shape of the bedding planes. When sand is deposited, sandy sediment forms curved beds at an angle to the bedding plane. (29 These beds are called cross-beds. Scientists can study the shape of the cross-beds to determine the original position of the layers.(29)(31) Cross-beds appear curved at the bottom and cut off at the top because the tops of the sandy layers commonly erode before new layers are deposited. (30)

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Principle of Original Horizontality, continued Ripple Marks Ripple marks are small waves that form on the surface of sand because of the action of water or wind. When the sand becomes sandstone, the ripple marks may be preserved.(32) In undisturbed sedimentary rock layers, the crests of the ripple marks point upward. Scientists study the orientation of the ripple marks to determine the original position of the layers and use the law of superposition.(33)(34)

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Unconformities, continued unconformity a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time(36)(41) Movements of Earth’s crust can lift up rock layers that were buried and expose them to erosion, which is a natural force that can cause breaks in the geologic record.(35)(44) Then, during deposition, in which there is an accumulation of sediments, new rock layers form in place of the eroded layers.(42) The missing layers create a break in the geologic record, called an unconformity. Acoording to the law of superposition, all rocks beneath an unconformity are older than the rocks above it. (37)

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Unconformities, continued There are three types of unconformities.(38) A nonconformity is an unconformity in which stratified rock rests upon unstratified rock. Forms when unstatified igneous or metamorphic rock is uplifted, erodes, and then sediments are deposited on the eroded surface.(39)(40)(45) An angular unconformity is the boundary between a set of tilted layers and a set of horizontal layers.(39)(43) A disconformity is the boundary between horizontal layers of old rock and younger, overlying layers that are deposited on an eroded surface.(39)(46)

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Types of Unconformities, continued The diagram below illustrates the three types of unconformities. Section 1 Determining Relative Age

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Determining Relative Age Chapter 8 Unconformities, continued Crosscutting Relationships law of crosscutting relationships the principle that a fault or body of rock is younger than any other body of rock that it cuts through.(51) A fault is a break or crack in Earth’s crust along which rocks shift their position. An intrusion is a mass of igneous rock that forms when magma is injected into rock.(48)(49) In these cases, scientists use the law of crosscutting relationships, or the fact that a fault or an intrusion is always younger than the layers it cuts through, to determine the order of the layers.(50)(52) When rock layers have been disturbed by faults or intrusions the layers on either side of the boundary are nearly horizontal, and the rock layers may look as though they were deposited continuously. But a large time gap exists where the upper and lower layers meet.(47)

19. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Crosscutting Relationships, continued The law of crosscutting relationships can be used to determine the relative ages of rock layers. Section 1 Determining Relative Age

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Objectives Summarize the limitations of using the rates of erosion and deposition to determine the absolute age of rock formations. Describe the formation of varves. Explain how the process of radioactive decay can be used to determine the absolute age of rocks.

21. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Absolute Dating Methods absolute age the numeric age of an object or event, often stated in years before the present, as established by an absolute-dating process, such as radiometric dating(3) Scientists use a variety of ways to determine absolute age, or the numeric age, of a rock formation.(2) Relative age only indicates that one rock formation is younger or older than another rock formation. (1)

22. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Absolute Dating Methods, continued Rates of Erosion One way scientists use to estimate absolute age is to study rates of erosion. The age of a stream can be measured using rates of erosion by measuring the rate at which the stream erodes its bed. (6) Studying the rates of erosion is practical only for geologic features that formed within the past 10,000 to 20,000 years. Niagara Falls is a good example for using erosion rates. (7)(8) For older surface features, the method is less dependable because rates of erosion can vary over millions of years. An example is the Grand Canyon. (9)

23. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Absolute Dating Methods, continued Rates of Deposition Scientists can also date rocks by measuring the chemical composition of certain materials in rock.(5) Scientists can also estimate absolute age by calculating the rate of sediment deposition. By using data collected over a long period of time, geologists can estimate the average rates of deposition for common sedimentary rocks.(4)(10) In general, the rate of deposition for limestone, shale, or sandstone is about 30 cm of sedimentary rock over a period of 1,000 years. (11) This method is not always accurate because not all sediment is deposited at an average; therefore it provides only an estimate of absolute age. Ex: Floods deposit many centimeters of sediment in a single day or rates of deposit change over time.(12)

24. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Absolute Dating Methods, continued Varve Count varve a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age. Some sedimentary deposits show two definite annual layers, called varves. They consist of light-colored band of coarse particles and a dark-colored band of finer particles. (14)(16) The varves can be counted much like tree rings due to formations of distinct annual layers to determine the age of the sedimentary deposit. By counting varves in a deposit, scientists can estimate age of deposit(13) (17) They often form in glacial lakes. A rush of melted water during summer carries large amounts of sediment into lake, where coarse particles quickly settle to bottom. When winter comes, ice forms and fine particles settle in thin layer on top of coarse particles.(15)

25. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating radiometric dating a method of determining the absolutes age of an object by comparing the relative percentages of a radioactive (parent) isotope(original radioactive isotope) and a stable (daughter) isotope.(25)(26) Rocks generally contain small amounts of radioactive material that can act as natural clocks.(18) Atoms of the same element that have different numbers of neutrons are called isotopes.(19) Radioactive isotopes can be used to determine age.

26. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Radioactive isotopes have nuclei that emit particles and large amounts of energy at a constant rate regardless of surrounding conditions.(20)(21) Scientists use the natural breakdown of isotopes to accurately measure the absolute age of rock, which is called radiometric dating.(23) To do this, scientists measure the concentration of the parent isotope or original isotope, and of the newly formed daughter isotopes. Then, using the known decay rate, they can accurately determine the absolute age of the rock.(22)

27. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Radiometric Dating Section 2 Determining Absolute Age

28. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Half-Life When atoms emit particles and energy, the atoms change into different isotope of the same element or into an isotope of a different element.(24) half-life the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope(different isotope of the same element or into an isotope of a different element in which a radioactive atom has changed as it emits particles and energy.(27)(31) Scientists have determined that the time required for half of any amount of a particular radioactive isotope to decay is always the same or and can be determined for any isotope. Nothing changes it! (29)(30)

29. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Half-Life Scientist use the known decay rate of atoms and by comparing the amounts of parent and daughter isotopes in a rock sample, scientists can determine the age of the sample.(28)(34) The greater the percentage of daughter isotopes present in the sample, the older the rock is.(35) A parent or daughter isotope can be gained or lost through leaking or contamination.(36) The amount of time passed since a rock has formed determines which radioactive element will give a more accurate measurement of a rock’s age. (37)

30. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Radioactive Decay and Half-Life, continued Section 2 Determining Absolute Age

31. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Radioactive Decay and Half-Life, continued If you began with a 10 gram sample of a parent isotope, how much of that isotope would be left after one half-life? 10g x ½ = 5g (32) How much of an original isotope remains at the end of a second half-life? 1 half-life = ½ 2 half lives = ½ x ½ = ¼ (33)

32. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Half-Life Section 2 Determining Absolute Age

33. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Radioactive Isotopes Uranium-238, or 238 U, is an isotope of uranium that has an extremely long half-life of 4.5 billion years, and is most useful for dating geologic samples that are more than 10 million years old due to it’s long half-life.(38)(39) Potassium-40, or 40 K, has a half-life of 1.25 billion years; is found in mica, clay, and feldspar; and is used to date rock that are between 50,000 and 4.6 billion years old. (40)(41)(42)

34. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Radioactive Isotopes Rubidium-87 has a half-life of about 49 billion years; it is commonly found in minerals that contain 40 K, and is used to verify the age of rocks previously dated by using 40 K.(43)

35. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Carbon Dating Younger rock layers may be dated indirectly by dating organic material found within the rock. Organic remains, such as wood, bones, and shells that are less than 70,000 years old can be determined by using a method known as carbon-14 dating, or radiocarbon dating.(44) Carbon dioxide in the atmosphere contains nonradioactive carbon-12 and no radioactive isotope carbon-14.(46) Carbon-14 combines with oxygen to form radioactive carbon dioxide.(45)

36. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Determining Absolute Age Chapter 8 Radiometric Dating, continued Carbon Dating All living organisms have both the carbon-12 and carbon-14 isotope. To find the age of a sample of organic material, scientists first compare the ratio of 14 C to 12 C and then the second step is to compare this with the ratio of 14 C to 12 C known to exist in a living organism.(48)(49) Once a plant or animal dies, the ratio begins to change, and scientist can determine the age from the difference between the ratios of 14 C to 12 C in the dead organism, which can no longer absorb carbon-12 or carbon- 14.(47)(51) Caron-14 has a half-life of about 5,730 years.(50) Carbon-14 in the tissues of dead plants and animals decreases steadily as the radioactive carbon-14 decays into nonradioactive nitrogen-14.(52)

37. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Objectives Describe four ways in which entire organisms can be preserved as fossils. List five examples of fossilized traces of organisms. Describe how index fossils can be used to determine the age of rocks.

38. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Interpreting the Fossil Record fossils the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock, but rarely found in igneous or metamorphic rock(3)(5)(6) paleontology the scientific study of fossils(4) Fossils are an important source of information for finding the relative and absolute ages of rocks.(1) Fossils also provide clues to past geologic events, climates, and the evolution of living things over time.(2)

39. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Interpreting the Fossil Record, continued Almost all fossils are discovered in sedimentary rock. This is due to the fact that the sediments that cover fossils slow or stop the process of decay and protect the bodies of dead organisms from damage.(7) Fossils are rarely found in igneous or metamorphic rocks because the intense heat, pressure, and chemical reactions that occur during the formation of igneous and metamorphic rocks destroy all organic structures.(8) The fossil record provides information about the geologic history of Earth. (9)

40. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Interpreting the Fossil Record, continued A way that scientists can tell if an area of land was once covered by an ocean is when fossils of marine plants and animals are discovered in areas far from any ocean. (11) Fossils reveal ways that organisms have changed throughout the geologic past.(10) Scientists can use this information to learn about how environmental changes have affected living organisms. (12)

41. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Only dead organisms that are buried quickly or protected from decay can become fossils. (14) Typically, dead plants and animals are eaten or decomposed by bacteria.(13) Generally only the hard parts of organisms, such as wood, bones, shells, and teeth, become fossils. (15) In rare cases, an entire organism may be preserved. In some types of fossils, only a replica of the original organism remains. Others merely provide evidence that life once existed.

42. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Mummification Mummified remains are often found in very dry places, because most bacteria which cause decay cannot survive in these places.(16) Some ancient civilizations mummified their dead by carefully extracting the body’s internal organs and then wrapping the body in carefully prepared strips of cloth.(17)

43. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Amber Hardened tree sap is called amber. Insects become trapped in the sticky sap and are preserved when the sap hardens.(18) In many cases, delicate features such as legs and antennae have been preserved. In rare cases, DNA has been recovered from amber.(19)

44. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Tar Seeps When thick petroleum oozes to Earth’s surface, the petroleum forms a tar seep.(20) Tar seeps are commonly covered by water. Animals that come to drink the water can become trapped in the sticky tar. (21) The remains of the trapped animals are covered by the tar and preserved.

45. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Freezing The low temperatures of frozen soil and ice can protect and preserve organisms. Because most bacteria cannot survive freezing temperatures, organisms that are buried in frozen soil or ice do not decay.

46. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Fossilization Petrification Mineral solutions such as groundwater replace the original organic materials that were covered by layers of sediment with new materials. Some common petrifying minerals are silica, calcite, and pyrite.(22) The substitution of minerals for organic material other results in the formation of a nearly perfect mineral replica of the original organism.

47. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Types of Fossils trace fossil a fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment(28) In some cases, no part of the original organism survives in fossil form. But the fossilized evidence of past animal movement can still provide information about prehistoric life. They have been found for reptiles, amphibians, birds, and mammals.(31) A trace fossils in an important clue to the animal’s appearance and activities.(29) An example is a bird’s footprint. (30)

48. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Types of Fossils Imprints Carbonized imprints of leaves, stems, flowers, and fish made in soft mud or clay have been found preserved in sedimentary rock. When original organic material partially decays, it leaves behind a carbon-rich film. An imprint displays the surface features of the organism.(23)

49. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Types of Fossils Molds and Casts Shells often leave empty cavities called molds within hardened sediment. When a shell is buried, its remains eventually decay and leave an empty space. When sand or mud fills a mold and hardens, a natural cast forms. (24) A cast is a replica of the original organism.(25)

50. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Types of Fossils Coprolites Fossilized dung or waste materials from ancient animals are called coprolites. They can be cut into thin sections and observed through a microscope. The materials identified in these sections reveal the feeding habits of ancient animals, such as dinosaurs.(26)

51. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Types of Fossils Gastroliths Some dinosaurs had stones in their digestive systems to help grind their food. In many cases, these stones, which are called gastroliths, survives as fossils. Gastroliths can often be recognized by their smooth, polished surfaces and by their close proximity to dinosaurs remains. (27)

52. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Index Fossils Index fossils Index fossil a fossil that is used to establish the age of rock layers because it is distinct, abundant, and widespread and existed for only a short span of geologic time. (32)(38) Paleontologists can use index fossils to determine the relative ages of the rock layers in which the fossils are located.(37)

53. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Index Fossils Index fossils To be an index fossil, a fossil must be present in rocks scattered over a large region, and it must have features that clearly distinguish it from other fossils. (33)(34) In addition, organisms from which the fossil formed must have lived during a short span of geologic time, and the fossil must occur in fairly large numbers within the rock layers.(35)(36)

54. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 The Fossil Record Chapter 8 Index Fossils and Absolute Age Scientists can use index fossils to estimate absolute ages of specific rock layers. Because organisms that formed index fossils lived during short spans of geologic time, the rock layer in which an index fossil was discovered can be dated accurately. Scientists can also use index fossils to date rock layers in separate area. An example is ammonite fossils are found in rock layers 180 to 206 million years. (39) Index fossils are used to help locate rock layers that are likely to contain oil and natural gas deposits. (41)

55. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Index Fossils and Absolute Age, continued Scientists use index fossils to determine the absolute age of rock layers in different parts of the world because index fossils are known to come from a particular period of geologic time, if an index fossil is found in rock layers anywhere in the world, scientists know that the rock layers in which it was found formed during the same time period. (40)

56. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 8 Index Fossils Section 3 The Fossil Record

57. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Law of Superposition Chapter 8

58. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Types of Unconformities Chapter 8

59. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Crosscutting Relationships Chapter 8

60. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Radioactive Decay and Half-Life Chapter 8

61. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Geologic Map of Bedrock in Ohio Chapter 8