Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgments
- PART ONE THE ASTRONOMICAL PLANET: EARTH'S PLACE IN THE COSMOS
- PART TWO THE MEASURABLE PLANET: TOOLS TO DISCERN THE HISTORY OF EARTH AND THE PLANETS
- 5 Determination of Cosmic and Terrestrial Ages
- 6 Other Uses of Isotopes for Earth History
- 7 Relative Age Dating of Cosmic and Terrestrial Events: The Cratering Record
- 8 Relative Age Dating of Terrestrial Events: Geologic Layering and Geologic Time
- 9 Plate Tectonics: An Introduction to the Process
- PART THREE THE HISTORICAL PLANET: EARTH AND SOLAR SYSTEM THROUGH TIME
- PART FOUR THE ONCE AND FUTURE PLANET
- Index
- Plate section
5 - Determination of Cosmic and Terrestrial Ages
from PART TWO - THE MEASURABLE PLANET: TOOLS TO DISCERN THE HISTORY OF EARTH AND THE PLANETS
- Frontmatter
- Contents
- Preface
- Acknowledgments
- PART ONE THE ASTRONOMICAL PLANET: EARTH'S PLACE IN THE COSMOS
- PART TWO THE MEASURABLE PLANET: TOOLS TO DISCERN THE HISTORY OF EARTH AND THE PLANETS
- 5 Determination of Cosmic and Terrestrial Ages
- 6 Other Uses of Isotopes for Earth History
- 7 Relative Age Dating of Cosmic and Terrestrial Events: The Cratering Record
- 8 Relative Age Dating of Terrestrial Events: Geologic Layering and Geologic Time
- 9 Plate Tectonics: An Introduction to the Process
- PART THREE THE HISTORICAL PLANET: EARTH AND SOLAR SYSTEM THROUGH TIME
- PART FOUR THE ONCE AND FUTURE PLANET
- Index
- Plate section
Summary
OVERVIEW OF AGE DATING
To understand the history of Earth in the cosmos, we must be able to establish ages of physical evidence and timescales over which processes have occurred. The task is daunting because of the enormous spans of time over which the physical universe and Earth have existed, and several different approaches must be used. In chapter 2, we discussed observations leading to the conclusion that the universe is in an overall state of expansion, which began some 10 billion to 20 billion years ago. In this chapter we discuss rather precise techniques that enable us to determine the age of Earth and other solid matter in the solar system very confidently: Some 4.56 billion years ago, the planet we live on began to take shape.
It is useful to distinguish between two kinds of chronologies that are constructed in regard to Earth's history, because the techniques and uncertainties are quite different. A relative chronology is derived by observing the relative position in which the remains of an event lie. In sediments on Earth, older layers of soil, sand, and rock are deposited first, and then overlain by subsequent layers. Geologic processes might turn a whole stack of layers upside down, but fossils present in the layers, which can be compared to those in other layers worldwide, enable us to determine the age progression of the layers. We discuss relative geologic dating in chapter 8.
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- EarthEvolution of a Habitable World, pp. 47 - 53Publisher: Cambridge University PressPrint publication year: 1998