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1. Section 1 Geologic History Section 2Looking at Fossils Section 3Time Marches On The Fossil Record Preview Concept Mapping

2. “The Present is the Key to the Past.” This phrase was the cornerstone of the uniformitarianist theory developed by geologist James Hutton in the late 1700s. Write a few sentences in yourscience journal about how studying the present could reveal the story of Earth’s history. Use sketches to illustrate processes that occurred millions of years ago that you can still see today. Section 1 Geologic History Bellringer

3. Compareuniformitarianism with catastrophism. Describehow the science of geology has changed over the past 200 years. Contrastrelative dating with absolute dating. Section 1 Geologic History Objectives

4. In Theory of the Earth (1788), James Hutton introduced the idea of uniformitarianism. Uniformitarianism assumes that geologic processes that are shaping the Earth today have been at work throughout Earth’s history. Section 1 Geologic History The Principle of Uniformitarianism, continued

5. Section 1 Geologic History

6. Uniformitarianism Versus Catastrophism During Hutton’s time, most scientists supported catastrophism, the principle that all geologic change occurs suddenly. Supporters of catastrophism thought that Earth’s mountains, canyons, and seas formed during rare, sudden events called catastrophes. Section 1 Geologic History The Principle of Uniformitarianism, continued

7. Most people also believed that Earth was only a few thousand years old. Hutton’s work suggested a very different reality. According to his theories, Earth had to be much older, because gradual geologic processes would take much longer than a few thousand years. Section 1 Geologic History The Principle of Uniformitarianism, continued

8. A Victory for Uniformitarianism Catastrophism remained the guiding principle of geology in the early 19th century. But uniformitarianism became geology’s guiding principle after Charles Lyell reintroduced the concept in his Principles of Geology (1830-1833). Section 1 Geologic History The Principle of Uniformitarianism, continued

9. Using Hutton’s notes and evidence of his own, Lyell successfully challenged the principle of catastrophism. He saw no reason to doubt that major geologic change happened at the same rate in the past as it happens in the present—gradually. Section 1 Geologic History The Principle of Uniformitarianism, continued

10. During the late 20th century, scientists such as Stephen J. Gould challenged the principle of uniformitarianism. They believed that catastrophes sometimes play an important role in shaping Earth’s history. Neither theory completely accounts for all geologic change. Section 1 Geologic History Modern Geology—A Happy Medium

11. Most geologic change is gradual and uniform. But catastrophes that cause geologic change have occurred during Earth’s long history. Asteroid and comet strikes to Earth, for example, have caused rapid change. Section 1 Geologic History Modern Geology—A Happy Medium, continued

12. Some scientists think an asteroid strike 65 million years ago caused the dinosaurs to become extinct. Section 1 Geologic History Modern Geology—A Happy Medium, continued

13. Scientists can use two methods to determine the age of objects in sedimentary rocks. One of those methods is known as relative dating. Relative dating examines a fossil’s position within rock layers to estimate its age. Section 1 Geologic History Relative Dating

14. The bottom layers of rock are usually the oldest, and the top layers are usually the youngest. Scientists can use the order of these rock layers to determine the relative age of objects within the layers. For example, fossils in the bottom layers are usually older than fossils in the top layers. Section 1 Geologic History Relative Dating, continued

15. The Geologic Column To make relative dating easier, geologists combine data from all of the known rock sequences around the world. From this information, geologists create the geologic column—an ideal sequence of rock layers that contains all of the known fossils and rock formations on Earth. These layers are arranged from oldest to youngest. Section 1 Geologic History Relative Dating, continued

16. Section 1 Geologic History

17. Scientists can use absolute dating to more precisely determine the age of a fossil or rock. In absolute dating, scientists examine atoms to measure the age of fossils or rocks in years. Atoms are the particles that make up all matter. Section 1 Geologic History Absolute Dating

18. Some atoms are unstable, and will decay over time. When an atom decays, it becomes a different and more stable kind of atom. Each kind of unstable atom decays at its own rate. Section 1 Geologic History Absolute Dating, continued

19. The time it takes for half of the unstable atoms in a sample to decay is known as the half-life of that atom. Scientists can examine a sample of rock or fossil, and look at the ratio of unstable to stable atoms. Since they know the half-life, they can determine the approximate age of the sample. Section 1 Geologic History Absolute Dating, continued

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21. Uranium-238 has a half-life of 4.5 billion years. Scientists can use uranium-238 to date rocks or fossils that are millions of years old. Carbon-14 has a half-life of only 5,780 years. Scientists use carbon-14 to date fossils and other objects that are less than 50,000 years old, such as human artifacts. Section 1 Geologic History Absolute Dating, continued

22. Section 1 Geologic History Radiometric Dating Click below to watch the Visual Concept. Visual Concept

23. Paleontology is the science involved with the study of past life. Scientists who study past life are called paleontologists. Paleontologists collect data by studying fossils, the remains of organisms preserved by geological processes. Section 1 Geologic History Paleontology—The Study of Past Life

24. Vertebrate and invertebrate paleontologists study the remains of animals. Paleobotanists study fossils of plants. Other paleontologists reconstruct past ecosystems, study the traces that animals left behind, and piece together the conditions under which fossils formed. Section 1 Geologic History Paleontology—The Study of Past Life, continued

25. Describe the fossil record of your own life that might be found 65 million years from now. What items, or artifacts, might be likely to survive? What kinds of things would decay and disappear? Do you think your fossil record would produce an accurate picture of your life? What might be missing? Write your description in yourscience journal. Later, you will share your description with the class. Section 2 Looking at Fossils Bellringer

26. Describefive ways in which different types of fossils form. Listthree types of fossils that are not part of organisms. Explainhow fossils can be used to determine the history of changes in environments and organisms. Explainhow index fossils can be used to date rock layers. Section 2 Looking at Fossils Objectives

27. The remains or physical evidence of an organism preserved by geologic processes is called a fossil. Fossils in rocks can form when an organism dies and is quickly covered by sediment. When the sediment becomes rock, hard parts of the organism are preserved. Section 2 Looking at Fossils Fossilized Organisms

28. If an insect is caught in sticky tree sap, the sap covers its entire body and hardens quickly. Fossils in amber are entire organisms preserved inside hardened tree sap, called amber. Some of the best insect fossils, as well as frogs and lizards, have been found in amber. Section 2 Looking at Fossils Fossilized Organisms, continued

29. Organisms can also be preserved by petrifaction. Petrifaction is a process in which minerals replace the organism’s tissues. Permineralization and replacement are forms of petrifaction. Section 2 Looking at Fossils Fossilized Organisms, continued

30. In the process of permineralization, pore space in an organism’s hard tissue (like bone or wood) is filled up with mineral. In the process of replacement, minerals completely replace the tissues of the organism. Some samples of petrified wood are composed completely of minerals. Section 2 Looking at Fossils Fossilized Organisms, continued

31. In some places, asphalt wells up and forms thick, sticky pools at Earth’s surface. These asphalt pools can trap and preserve many organisms. The La Brea asphalt deposits in Los Angeles, California have preserved organisms for at least 38,000 years. Section 2 Looking at Fossils Fossilized Organisms, continued

32. Frozen Fossils In 1999, scientists removed a 20,000-year-old woolly mammoth that was frozen in the Siberian tundra. These mammoths became extinct about 10,000 years ago. Because cold temperatures slow down decay, the mammoth was almost perfectly preserved. Section 2 Looking at Fossils Fossilized Organisms, continued

33. Trace fossils are any naturally preserved evidence of animal activity. Tracks are an example of a trace fossil. They form when animal footprints fill with sediment. Tracks can reveal size and speed of an animal, and whether it traveled in groups. Section 2 Looking at Fossils Other Types of Fossils

34. Burrows are another trace fossil. Burrows are shelters made by animals that bury themselves in sediment, such as clams. Another type of trace fossil is coprolite, or preserved animal dung. Section 2 Looking at Fossils Other Types of Fossils, continued

35. Molds and casts are two more examples of fossils. A cavity in rock where a plant or animal was buried is called a mold. A cast is an object that is created when sediment fills a mold and becomes rock. Section 2 Looking at Fossils Other Types of Fossils, continued

36. The Information in the Fossil Record The fossil record gives only a rough sketch of the history of life on Earth. Most organisms never become fossils. Many fossils have yet to be discovered. Section 2 Looking at Fossils Using Fossils to Interpret the Past

37. Organisms with hard body parts have left more fossils than those with soft body parts. Organisms that lived in areas that favored fossilization have also left more fossils. Section 2 Looking at Fossils Using Fossils to Interpret the Past, continued

38. But fossils can show a history of environmental change. For example, the presence of marine fossils on mountaintops in Canada means that these mountains formed at the bottom of the ocean. Marine fossils can also help scientists reconstruct ancient coastlines and detect the presence of ancient seas. Section 2 Looking at Fossils Using Fossils to Interpret the Past, continued

39. Scientists can use fossils of plants and land animals to reconstruct past climates. By examining fossils, scientists can tell whether the climate of an area was cooler or wetter than that climate is now. Section 2 Looking at Fossils Using Fossils to Interpret the Past, continued

40. History of Changing Organisms Scientists study the relationships between fossils to interpret how life has changed over time. Since the fossil record is incomplete, paleontologists look for similarities between fossils over time to try to track change. Section 2 Looking at Fossils Using Fossils to Interpret the Past, continued

41. Scientists have found that particular types of fossils appear only in certain layers of rock. By dating rock layers above and below these fossils, scientists can determine the time span in which the organism lived. If the organism lived for a relatively short period of time, its fossils would show up in limited layers. Section 2 Looking at Fossils Using Fossils to Date Rocks

42. Index fossils are fossils of organisms that lived for a relatively short, well-defined geologic time span. To be index fossils, these fossils must be found worldwide. Section 2 Looking at Fossils Using Fossils to Date Rocks, continued

43. Ammonites of the genus Tropites are index fossils. Section 2 Looking at Fossils Using Fossils to Date Rocks, continued

44. These ammonites were marine mollusks similar to modern squids. Tropites lived between 230 million and 208 million years ago. Fossils of these ammonites are index fossils for that time period. Section 2 Looking at Fossils Using Fossils to Date Rocks, continued

45. Trilobites of the genus Phacops are also index fossils. Trilobites are extinct. Their closest living relative is the horseshoe crab. Section 2 Looking at Fossils Using Fossils to Date Rocks, continued

46. Phacops lived about 400 million years ago. When scientists find fossils of trilobites anywhere on Earth, they assume the rock layers are also approximately 400 million years old. Section 2 Looking at Fossils Using Fossils to Date Rocks, continued

47. Archaeologists and paleontologists believe that modern humans have lived on Earth for 150,000 to 200,000 years. If we imagine the history of the Earth to be the length of one calendar year, on which date do you think modern humans arrived? Record your answer in yourscience journal. Section 3 Time Marches On Bellringer

48. Explainhow geologic time is recorded in layers of sedimentary rock. Explainhow the geologic time scale illustrates the occurrence of processes on Earth. Explainhow the fossil record provides evidence of changes that have taken place in organisms over time. Section 3 Time Marches On Objectives

49. Earth is about 4.6 billion years old. Paleontologists find a record of Earth’s history in rock formations and fossils around the world. Dinosaur National Monument in Utah contains the remains of thousands of dinosaurs that inhabited the area about 150 million years ago. Section 3 Time Marches On Geologic Time

50. Although 150 million years seems like an incredibly long period, it is little more than 3% of the time our planet has existed. The Rock Record and Geologic Time One of the best places in North America to see Earth’s history recorded in rock layers is Grand Canyon National Park in northwestern Arizona. Section 3 Time Marches On Geologic Time, continued