Objectives

1. The Geologic Time Scale Objectives • Describe the geologic time scale. • Distinguish among the following geologic time scale divisions: eon, era, period, and epoch. Vocabulary • geologic time scale • eon • era • period • epoch

2. The Geologic Time Scale The Geologic Time Scale • By studying the characteristics of rocks and the fossils within them, geologists can interpret the environments in which the rocks were deposited, reconstruct Earth’s history, and possibly predict events or conditions in the future.

3. The Geologic Time Scale The Rock Record • Geologists have divided the history of Earth into time units based upon the fossils contained within the rocks. The geologic time scale is a record of Earth’s history from its origin 4.6 billion years ago to the present. • This scale allows the correlation of geologic events, environmental changes, and the development of life-forms that are preserved in the rock record.

4. The Geologic Time Scale The Rock Record

5. The Geologic Time Scale Geologic Time • The oldest division of time is at the bottom of the geologic time scale. • The time scale is divided into units called eons, eras, periods, and epochs. • An eon,measured in billions of years, is the longest time unit of the geologic time scale. • An era, defined by the differences in life-forms found in rock and measured in hundreds of millions to billions of years, is the second-longest span of time on the geologic time scale.

6. The Geologic Time Scale Geologic Time • Precambrian Time, which makes up approximately 90 percent of geologic time, is divided into the Archean and Proterozoic Eons. • The end of the Proterozoic, the more recent of the two, is marked by the first appearance of organisms with hard parts.

7. The Geologic Time Scale Geologic Time Plants and Animals Evolve • During the Paleozoic Era, the oceans became full of a wide diversity of plants and animals; land plants appeared and were followed by land animals. • The Mesozoic Era is known for the emergence and evolution of dinosaurs, reef-building corals, large predatory reptiles, and flowering plants and trees. • During the Cenozoic Era, mammals increased both in number and diversity, human ancestors developed, and grasses and flowering plants expanded on land.

8. The Geologic Time Scale Geologic Time Periods of Geologic Time • Periods, usually measured in terms of tens of millions of years to hundreds of millions of years, are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited. • The Cenozoic is divided into three periods: the Paleogene, Neogene, and Quaternary. • The boundaries between the periods of the Cenozoic are not marked by extinction events.

9. The Geologic Time Scale Geologic Time Epochs of Geologic Time • Epochs are smaller divisions of geologic time and are usually measured in millions of years to tens of millions of years. • The Cenozoic Periods have been further divided into epochs including the Paleocene and the Oligocene. • Different groups of organisms have been used to distinguish the various epochs. • Regardless of how a geologic period was defined, each unit contains specific characteristics that set it apart from the rest of geologic history.

10. The Geologic Time Scale Section Assessment 1. Match the following terms with their definitions. ___ eon ___ period ___ era ___ epoch A. time periods defined by the life forms that were present; usually measured in terms of tens of millions to hundreds of millions of years B.smaller divisions of time; usually measured in millions to tens of millions of years C.the longest period of time; measured in billions of years D. second longest period of time; measured in hundreds of millions to billions of years

11. The Geologic Time Scale Section Assessment 2. How does the geologic time scale correspond with the rock record?

12. The Geologic Time Scale Section Assessment 3. Identify whether the following statements are true or false. ______ Precambrian Time represents 90 percent of geologic time. ______ Human ancestors developed during the Paleozoic Era. ______ The boundaries between the periods of the Cenozoic are marked by mass extinctions. ______ Organisms in Precambrian Time had soft bodies with no shells or skeletons.

13. End of Section 1

14. Relative-Age Dating of Rocks Objectives • Apply the principles for determining relative age to interpret rock sequences. • Describe an unconformity and how it is formed within the rock record. Vocabulary • uniformitarianism • original horizontality • superposition • cross-cutting relationships • unconformity • correlation

15. Relative-Age Dating of Rocks Relative-Age Dating of Rocks The principle of uniformitarianism states that the forces that continually change the surface features of Earth today have been occurring since Earth formed. • Only the rate, intensity, and scale with which the forces occur have changed. • The resulting sediments and rocks all record an environment and fossils within the rocks preserve evidence of the life-forms that lived during the time of deposition.

16. Relative-Age Dating of Rocks Principles for Determining Relative Age • The concept of relative-age dating places the ages of rocks and the events that formed them in order, but without exact dates. • This is done by comparing one event or rock layer to another.

17. Relative-Age Dating of Rocks Principles for Determining Relative Age Geologic Principles • The principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers. • The principle of superposition states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath.

18. Relative-Age Dating of Rocks Principles for Determining Relative Age Geologic Principles • The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across.

19. Relative-Age Dating of Rocks Principles for Determining Relative Age Inclusions • Relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it. • These particles, called inclusions, indicate that the rocks in the lower layer are older than those on top.

20. Relative-Age Dating of Rocks Other Means of Determining Relative Age • The fact that Earth is constantly changing makes it difficult to find an undisturbed sequence of rock layers. An unconformity is a gap in the rock record usually caused by an erosional surface becoming buried by the deposition of younger rocks.

21. Relative-Age Dating of Rocks Other Means of Determining Relative Age • The unconformity is called a disconformity when horizontal sedimentary rocks overlie horizontal sedimentary rocks.

22. Relative-Age Dating of Rocks Other Means of Determining Relative Age • A different type of unconformity exists when sedimentary rocks overlie nonsedimentary rocks. • The contact point between the nonsedimentary and sedimentary rock is called a nonconformity.

23. Relative-Age Dating of Rocks Other Means of Determining Relative Age • An angular unconformity is created when horizontal sedimentary rocks are uplifted and tilted, are exposed to erosional processes, and deposition resumes.

24. Relative-Age Dating of Rocks Other Means of Determining Relative Age Correlation of Rock Strata • Correlation is the matching of outcrops of one geographic region to another. • Geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers. • Correlation allows geologists to accurately locate that same rock layer in another location.

25. Relative-Age Dating of Rocks Section Assessment 1. Match the following terms with their definitions. ___ original horizontality ___ superposition ___ unconformity ___ correlation A. principle which states that sedimentary rocks are deposited in horizontal layers B.a gap in the rock record C.principle which states that oldest rocks are at the bottom and that each successive layer is younger D. matching of outcrops from one geographic region to another

26. Relative-Age Dating of Rocks Section Assessment 2. What is the principle of uniformitarianism?

27. Relative-Age Dating of Rocks Section Assessment 3. Identify whether the following statements are true or false. ______ Relative-age dating allows geologists to determine the age of rock formations. ______ A limestone layer is older than a shale layer that is above it. ______ Rock layers are often found undisturbed if you dig deep enough. ______ The grains in a rock layer can be from a younger layer of rock.

28. End of Section 2

29. Absolute-Age Dating of Rocks Objectives • Explain the several different methods used by scientists to determine absolute age. • Describe how objects are dated by the use of certain radioactive elements. • Explain how annual tree rings and glacial varves are used to date geologic events. Vocabulary • radioactive decay • radiometric dating • half-life • dendrochronology • varve • key bed

30. Absolute-Age Dating of Rocks Absolute-Age Dating of Rocks • Absolute-age dating enables scientists to determine the actual age of a rock, fossil, or other object using the decay rate of radioactive isotopes. • Radioactive substances emit nuclear particles at a constant rate regardless of any physical or environmental changes. • The original radioactive element is referred to as the “parent,” and the new element is referred to as the “daughter.” • As the numbers of protons and neutrons change with each nuclear emission, the element is converted to a different element.

31. Absolute-Age Dating of Rocks Absolute-Age Dating of Rocks Radioactive decay is the emission of radioactive particles and the resulting change into other elements over time.

32. Absolute-Age Dating of Rocks Use of Radioactive Isotopes In a process called radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil to determine its absolute age. • Because it often takes a long time for the entire amount of an isotope to decay, geologists use the half-life of an isotope.

33. Absolute-Age Dating of Rocks Use of Radioactive Isotopes Half-life is the length of time it takes for one-half of the original amount of an isotope to decay.

34. Absolute-Age Dating of Rocks Use of Radioactive Isotopes Carbon-14 • Carbon-14 (C-14) is a radioactive isotope that is commonly used to determine the absolute age of an object, especially one that is of organic origin. • C-14 is accurate for dating objects up to 75 000 years old. • For the dating of a particularly old rock sample, a radioactive isotope with a longer half-life must be used.

35. Absolute-Age Dating of Rocks Other Ways to Determine Age • Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also be used to help geologists determine the age of an object or event.

36. Absolute-Age Dating of Rocks Other Ways to Determine Age Tree Rings • The age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree. • The widths of tree rings are directly related to the climatic conditions during growth periods. • Dendrochronology is the science of comparing annual growth rings in trees to date events and changes in past environments.

37. Absolute-Age Dating of Rocks Other Ways to Determine Age Seasonal Climatic Changes • About 11 000 years ago, continental glaciers covered the northern part of the United States. • Varves are bands of alternating light- and dark-colored sediments of sand, clay, and silt found in lakes that resulted from summer and winter runoff from glaciers. • Varves from different lakes can be compared to determine the ages of glacial lake sediments from about 15 000 to 12 000 years ago.

38. Absolute-Age Dating of Rocks Other Ways to Determine Age Distinctive Sediment Layers • When a widespread sediment layer is formed by an instantaneous or short-lived event, geologists may be able to determine the time of the event through radiometric dating. • A key bed is a layer which has been dated and acts as a time marker, which can be used to correlate rock layers across large areas.

39. Absolute-Age Dating of Rocks Section Assessment 1. Match the following terms with their definitions. ___ half-life ___ dendrochronology ___ varve ___ key bed A. alternating light- and dark-colored sedimentary deposits in glacial lakes B.the time period until the ratio of parent-to-daughter atoms is equal C.a widespread layer that has been accurately dated D. the science of comparing annual growth rings in trees to date events and environmental changes

40. Absolute-Age Dating of Rocks Section Assessment 2. How old is an object of organic origin if it has 25 percent of carbon-14 remaining? Why?

41. Absolute-Age Dating of Rocks Section Assessment 3. Why would rubidium-87 with a half-life of 48.6 billion years probably not be useful in dating an object that is 100 000 years old?

42. End of Section 3

43. Remains of Organisms in the Rock Record Objectives • Define fossil. • Explainseveralmethodsbywhichfossilscanbepreserved. • Describe the characteristics of an index fossil. • Discuss how fossils can be used to interpret Earth’s past physical and environmental history. Vocabulary • fossil • evolution • original preservation • altered hard part • permineralization • index fossil • mold • cast

44. Remains of Organisms in the Rock Record Remains of Organisms in the Rock Record Fossils are the evidence or remains of once-living plants or animals. • The fossil record provides evidence of evolution. • Evolution is an adaptive change in the DNA of populations as a result of mutation and/or environmental change. • Fossils preserved in the rock record also provide information about past environmental conditions and can be used to correlate rock layers from one area to another.

45. Remains of Organisms in the Rock Record Types of Fossils Fossils with original preservation are the soft and hard parts of plant and animal remains that have not undergone any kind of change since the organisms’ deaths. • Such fossils are uncommon because their preservation requires extraordinary circumstances such as freezing, drying out, or oxygen-free environments.

46. Remains of Organisms in the Rock Record Types of Fossils Altered Hard Parts • Altered hard parts are fossils in which all the organic material has been removed and the hard parts of a plant or animal have been changed either by mineral replacement or by recrystallization. • Permineralization is the process by which pore spaces in a fossil are filled in with mineral substances. • During the process of recrystallization, the exterior of the shell or a bone remains the same, but the shell microstructures are destroyed.

47. Remains of Organisms in the Rock Record Types of Fossils Altered Hard Parts

48. Remains of Organisms in the Rock Record Types of Fossils Index Fossils • Index fossils are remains of plants or animals that can be used by geologists to correlate rock layers over large geographic areas or to date a particular rock layer. • An index fossil is easily recognized, abundant, and widely distributed geographically and must also have lived during a short period of time.

49. Remains of Organisms in the Rock Record Types of Fossils Molds and Casts • A mold is formed when the original shell parts of an organism within a sedimentary rock are weathered and eroded. • A hollowed-out impression, or mold, of the shells is left in their place. • A cast of an organism is created if the cavity later becomes filled with minerals or sediment.

50. Remains of Organisms in the Rock Record Types of Fossils Molds and Casts