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.