Introduction

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 13.7 billion years ago. In this chapter we discuss rather precise techniques that enable us to determine the age of the Earth and other solid matter in the solar system with even higher accuracy and perhaps more confidence: some 4.5682 billion years ago, the planet we live on began to take shape in the form of tiny solids condensed from a hot, gaseous disk.

Overview of age dating

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 order in which a series of objects is found – and then assuming that the series represents a temporal ordering. In sediments on Earth, older layers of soil, sand, and rock are by definition those which are deposited first, hence they lie at the bottom of a sequence of layers progressing upward from oldest to youngest. If there is no disturbance, one can reasonably assume that the layers have been preserved in the order in which they were deposited. 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 and hence their inversion by some geological event. We discuss relative geologic dating in Chapter 8.