1. Layers of rock are deposited horizontally, like the layers of the Grand Canyon shown here.

2. Determining the Age of Rocks
How do geologists determine the relative and absolute ages of rock layers?
Fossils are the preserved remains or traces of once living things
Geologists use the law of superposition to determine the relative ages of sedimentary rocks from the sequence of rock layers and the fossils within each layer.
A rock’s absolute age is the time that has passed since the rock formed.

3. Determining the Age of Rocks
Geologists use radioactive dating to determine the absolute ages of rocks.

4. Determining the Age of Rocks
Geologists determine the relative age of a fossil as well as that of the rock containing it.
The relative age of a rock is its age compared to the ages of other rocks above or below it in a sequence of rock layers.

5. Determining the Age of Rocks
The walls of the Grand Canyon consist of many different layers of rock.
Kaibab-Toroweap limestone
Coconino sandstone
Younger
Hermit shale
Supai sandstone
Colorado River
Redwall limestone
Muav limestone
Bright Angel shale
Tapeats sandstone
Older
Vishnu schist

6. Determining the Age of Rocks
The Law of Superposition
Sedimentary rocks form as horizontal layers.
The law of superposition states that if rock layers are undisturbed, younger rocks lie above older rocks, and the oldest rocks are at the bottom.
Geologists have examined sedimentary rocks from locations around the world to develop a relative time scale for many rock layers.

7. Determining the Age of Rocks
Index Fossils and Relative Dating
Geologists can also determine the relative ages of sedimentary rocks by examining the fossils that are found in them.
Most types of organisms preserved as fossils are now extinct. An extinct type of organism is one that no longer exists.

8. Determining the Age of Rocks
Fossils of organisms that are easily identified, occurred over a large area, and lived during a well-defined period of time are called index fossils.
Geologists use index fossils to determine the relative ages of rocks.
If a rock contains examples of an index fossil, then the rock must have formed during the time that that organism lived.

9. Determining the Age of Rocks
Radioactive Dating
When a rock forms, it has a known ratio of radioactive and stable isotopes.
Because a radioisotope decays into a stable isotope at a steady rate as the rock ages, scientists can measure this ratio to find the rock’s absolute age.

10. A Brief History of Earth
What forms the basis for the geologic time scale?
The geologic time scale is based on the relative ages of rock layers and the use of radioactive dating to find the absolute ages of rocks.

11. A Brief History of Earth
What are the major divisions of Earth’s history?
The major divisions of Earth’s history are Precambrian time and the Paleozoic, Mesozoic, and Cenozoic Eras.

12. A Brief History of Earth
Geologists have used information about the relative and absolute ages of rocks to develop a timeline for the history of Earth.
Earth’s history is divided into several large units, called eras. Each era is one major stage in Earth’s history.

13. A Brief History of Earth
An era is divided into smaller units called periods. Eras and periods help scientists locate changes and events in Earth’s history.
At some boundaries between eras, many different organisms became extinct within a relatively short time–an event called a mass extinction.
Theories to explain mass extinctions include asteroid impacts, volcanic activity, disease, and climate change.

14. A Brief History of Earth
The geologic time scale shows the major intervals in Earth’s history.

15. A Brief History of Earth
Precambrian Time: 4.6 Billion – 542 Million
Years ago
The earliest portion of Earth’s history, known as Precambrian time, includes the formation of Earth and the early development of life.
By 4 billion years ago, the forces that cause plate movement were already at work.
Soon after, one-celled organisms appeared in the oceans.
Later in the Precambrian, simple soft-bodied animals developed.

16. A Brief History of Earth
Paleozoic Era: Million Years Ago
Early in the Cambrian period, a variety of animals developed in the oceans.
More than 450 million years ago, fishes evolved in the oceans.
Plants and animals, including early reptiles, began to live on land.
Dense forests of mosses and cone-bearing plants covered much of the land.

17. A Brief History of Earth
At times during the Paleozoic, parts of many continents were flooded by seas.
Thick layers of sediment deposited in these seas formed much of the sedimentary rock found on the continents today.
During the last period of the Paleozoic Era, the Permian period, the supercontinent Pangaea formed.

18. A Brief History of Earth
During the early Paleozoic Era, life began to evolve into many different forms.
This scene shows how life might have looked on the sea floor during the Cambrian Period.

19. A Brief History of Earth
Mesozoic Era: Million Years Ago
The Mesozoic Era was the time of the dinosaurs.
The first dinosaurs appeared about 225 million years ago.
About this same time, Pangaea began to break up.
The first mammals also appeared in the Mesozoic.

20. A Brief History of Earth
Another major development was the appearance of flowering plants.
All the dinosaurs and many other types of organisms were killed at the end of the Mesozoic Era in a mass extinction. The leading hypothesis for the cause of the mass extinction is the impact of one or more large asteroids.

21. A Brief History of Earth
This spiral diagram provides an overview of Earth’s history.

22. A Brief History of Earth
Cenozoic Era: 65 Million Years Ago to the
Present
Since the end of the Mesozoic Era, Earth has been in the Cenozoic Era.
During this time, Earth’s climate has generally become cooler and drier.
Several ice ages have come and gone in the last 5 million years.

23. A Brief History of Earth
Mammals became very diverse and widespread during the Cenozoic Era.
About 150,000 years ago, modern humans first appeared in Africa. Since then, humans have migrated to every continent.

24. A Brief History of Earth
During the Mesozoic Era, dinosaurs roamed Earth’s surface.
During the Cenozoic Era, the diversity and size of mammals increased greatly.

25. Assessment Questions
Which process do geologists use to determine the absolute age of rocks?
analyzing index fossils
determining superposition
relative dating
radioactive dating

26. Assessment Questions
Which process do geologists use to determine the absolute age of rocks?
analyzing index fossils
determining superposition
relative dating
radioactive dating ANS: D

27. During which era of Earth’s history did dinosaurs live?
Assessment Questions
During which era of Earth’s history did dinosaurs live?
Precambrian era
Paleozoic era
Mesozoic era
Cenozoic era

28. During which era of Earth’s history did dinosaurs live?
Assessment Questions
During which era of Earth’s history did dinosaurs live?
Precambrian era
Paleozoic era
Mesozoic era
Cenozoic era ANS: C