Unveiling Earth's History Through Fossils and Rocks

1. Table of Contents • Fossils • The Relative Age of Rocks • Radioactive Dating • The Geologic Time Scale • Early Earth • Eras of Earth’s History

2. - Fossils How a Fossil Forms • Most fossils form when living things die and are buried by sediment. The sediment slowly hardens into rock and preserves the shapes of the organisms.

3. - Fossils Changes Over Time • The fossils record provides evidence about the history of life and past environments on Earth. The fossil record also shows that different groups of organisms have changed over time. Fossils of many different kinds of organisms were formed in this ancient lakeshore environment that existed in Wyoming about 50 million years ago.

4. - Fossils Changes Over Time • From fossils, scientists have reconstructed the paleomastodon. This animal had a short trunk and short tusks on both the upper and lower jaws. The paleomastodon is an ancestor of the modern elephant.

5. - Fossils Using Prior Knowledge • Before you read, look at the section headings and visuals to see what this section is about. Then write what you know about fossils in a graphic organizer like the one below. As you read, write what you learn. What You Know Fossils come from ancient organisms. Fossils are found in hardened rock. Fossils show us how some present-day organisms looked different in the past. What You Learned Molds and casts are types of fossils. Organisms are also preserved in amber, tar, and ice. Fossils tell us about past climates, changes in Earth’s surface, and how organisms have changed over time.

6. - Fossils Links on Fossils • Click the SciLinks button for links on fossils.

7. End of Section:Fossils

8. - The Relative Age of Rocks The Position of Rock Layers • According to the law of superposition, in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

9. - The Relative Age of Rocks Determining Relative Age • To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record.

10. - The Relative Age of Rocks Determining Relative Age • An unconformity occurs where erosion wears away layers of sedimentary rock. Other rock layers then form on top.

11. - The Relative Age of Rocks Using Fossils to Date Rocks • Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

12. - The Relative Age of Rocks Index Fossil Activity • Click the Active Art button to open a browser window and access Active Art about index fossils.

13. Question Answer - The Relative Age of Rocks Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. What does the position of rock layer reveal? The oldest layers—and the oldest fossils—are at the bottom. How do geologists determine the relative age of a rock? They examine the position of rock layer, extrusions and intrusions of igneous rock, faults, and gaps in the geologic record. How are fossils used to date rocks? The age of an index fossil tells the age of the rock layer in which it occurs.

14. - The Relative Age of Rocks Rock Layers • Click the Video button to watch a movie about rock layers.

15. - The Relative Age of Rocks Index Fossils • Click the Video button to watch a movie about index fossils.

16. End of Section:The Relative Age of Rocks

17. - Radioactive Dating Radioactive Decay • During radioactive decay, the atoms of one element break down to form atoms of another element.

18. - Radioactive Dating Radioactive Decay • The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay.

19. - Radioactive Dating Determining Absolute Ages • Geologists use radioactive dating to determine the absolute ages of rocks.

20. What percentage of a radioactive element will be left after three half-lives? First multiply 1/2 three times to determine what fraction of the element will remain. You can convert this fraction to a percentage by setting up a proportion: To find the value of d, begin by cross-multiplying, as for any proportion: 1 X 100 = 8 X d d = d = 12.5% - Radioactive Dating Percentages

21. Practice Problem What percent of a radioactive element will remain after five half-lives? 3.125% - Radioactive Dating Percentages

22. - Radioactive Dating Determining Absolute Ages • The age of a sedimentary rock layer can be determined relative to the absolute age of an igneous intrusion or extrusion near the sedimentary rock.

23. - Radioactive Dating Identifying Main Ideas • As you read the section “Determining Absolute Ages,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea. Main Idea Using radioactive dating, scientists can determine… Detail Detail Detail the absolute ages of the most ancient rocks usingpotassium-40. the absolute ages of fossils up to about 50,000 years ago using carbon-14. the ages of sedimentary rocks by dating the igneous intrusions and extrusions near the sedimentary rock.

24. - Radioactive Dating More on Radioactive Dating • Click the PHSchool.com button for an activity about radioactive dating.

25. End of Section:Radioactive Dating

26. - The Geologic Time Scale The Geologic Time Scale • Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

27. - The Geologic Time Scale Sequencing • As you read, make a flowchart like the one below that shows the eras and periods of geologic time. Write the name of each era and period in the flowchart in the order in which it occurs. Geologic Time Scale Precambrian Time Paleozoic Era: Permian Paleozoic Era: Cambrian Period Mesozoic Era: Triassic Paleozoic Era: Ordovician Period Mesozoic Era: Jurassic Paleozoic Era: Silurian Mesozoic Era: Cretaceous Paleozoic Era: Devonian Cenozoic Era: Tertiary Paleozoic Era: Carboniferous Cenozoic Era: Quaternary

28. - The Geologic Time Scale More on the Geologic Time Scale • Click the PHSchool.com button for an activity about the geologic time scale.

29. End of Section:The Geologic Time Scale

30. - Early Earth Earth’s Surface Forms • During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form.

31. - Early Earth Life Develops • Scientists have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present-day bacteria.

32. - Early Earth Comparing and Contrasting • As you read, compare and contrast the different types of mass movement by completing a table like the one below. Precambrian Earth Later Precambrian Earth Feature Early Earth Atmosphere Hydrogen and helium Carbon dioxide, nitrogen, and water vapor Oceans Earth cools, water vapor condenses, and rain falls. Rain forms oceans. Earth’s surface is too hot. All water evaporates into water vapor. Continents Old continents break apart, and new continents form as a result of continental drift. Less dense rock at surface forms continents.

33. - Early Earth Links on Precambrian Earth • Click the SciLinks button for links on Precambrian Earth.

34. End of Section:Early Earth

35. - Eras of Earth’s History Mass Extinctions • The graph shows how the number of families of animals in Earth’s oceans has changed.

36. The x-axis shows time in millions of years before the present; the y-axis shows the number of families of ocean animals. Reading Graphs: What variable is shown on the x-axis of the graph? On the y-axis? - Eras of Earth’s History Mass Extinctions

37. Slightly more than 50 million years ago Interpreting Data: How long ago did the most recent mass extinction occur? - Eras of Earth’s History Mass Extinctions

38. The one that occurred about 230 million years ago Interpreting Data: Which mass extinction produced the greatest drop in the number of families of ocean animals? - Eras of Earth’s History Mass Extinctions

39. The number of families of ocean animals immediately dropped but then increased. Relating Cause and Effect: In general, how did the number of families change between mass extinctions? - Eras of Earth’s History Mass Extinctions

40. - Eras of Earth’s History Geologic History

41. - Eras of Earth’s History Continental Drift Activity • Click the Active Art button to open a browser window and access Active Art about continental drift.

42. - Eras of Earth’s History Previewing Visuals • Before you read, preview Figure 22. Then write three questions you have about Earth’s history in a graphic organizer like the one below. As you read, answer your questions. Earth’s History Q. What geologic events happened during Precambrian Time? A. Earth, the oceans, and the first sedimentary rocks formed. Q. When did the dinosaurs appear on Earth? A. About 225 million years ago Q. What caused the mass extinction at the end of the Cretaceous Period? A. An object from space struck Earth and blocked the sunlight.

43. End of Section:Eras of Earth’s History

44. Graphic Organizer Fossils include Rock fossils Preserved fossils include include Trace fossils Carbon films Petrified fossils Molds and casts Ice Amber Tar

45. End of Section:Graphic Organizer