1. How a Fossil Forms - Fossils
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.

2. Changes Over Time - Fossils
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.

3. Changes Over Time - Fossils
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.

4. Using Prior Knowledge - Fossils
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.

5. The Position of Rock Layers
- 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.

6. Determining Relative Age
- 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.

7. Determining Relative Age
- 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.

8. Using Fossils to Date Rocks
- 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.

9. Asking Questions - The Relative Age of Rocks
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.
Question
Answer
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.

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

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

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

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

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

15. Determining Absolute Ages
- 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.

16. Identifying Main Ideas
- 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 using potassium-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.

17. 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.

18. Sequencing - The Geologic Time Scale
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

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

20. Life Develops - Early Earth
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.

21. Comparing and Contrasting
- 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
Feature
Early Earth
Later Precambrian Earth
Atmosphere
Hydrogen and helium
Carbon dioxide, nitrogen, and water vapor
Oceans
Earth’s surface is too hot. All water evaporates into water vapor.
Earth cools, water vapor condenses, and rain falls. Rain forms oceans.
Continents
Less dense rock at surface forms continents.
Old continents break apart, and new continents form as a result of continental drift.

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

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

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

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

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

27. - Eras of Earth’s History
Geologic History

28. Previewing Visuals - Eras of Earth’s History
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.

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