Introduction

Learning objectives

After carefully reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Apply basic geological principles to the determination of the relative ages of rocks
  • Explain the difference between relative and absolute age-dating techniques
  • Summarize the history of the geological time scale and the relationships between eons, eras, periods, and epochs
  • Understand the importance and significance of unconformities
  • Estimate the age of a rock based on the fossils that it contains
  • Describe some applications and limitations of isotopic techniques for geological dating
  • Use isotopic data to estimate the age of a rock
  • Describe the techniques for dating geological materials using tree rings and magnetic data
  • Explain why an understanding of geological time is critical to both geologists and the public in general

Time is the dimension that sets geology apart from most other sciences. Geological time is vast, and Earth has changed enough over that time that some of the rock types that formed in the past could not form today. Furthermore, as we’ve discussed, even though most geological processes are very, very slow, the vast amount of time that has passed has allowed for the formation of extraordinary geological features, as shown in Figure 8.1.

Figure 8.1 Arizona’s Grand Canyon is an icon for geological time; 1,450 million years are represented by this photo. The light-colored layered rocks at the top formed at around 250 Ma, and the dark ones at the bottom (within the steep canyon) at around 1,700 Ma. [SE]

Figure 8.1 Arizona’s Grand Canyon is an icon for geological time; 1,450 million years are represented by this photo. The light-colored layered rocks at the top formed at around 250 Ma, and the dark ones at the bottom (within the steep canyon) at around 1,700 Ma. [SE]

We have numerous ways of measuring geological time. We can tell the relative ages of rocks (for example, whether one rock is older than another) based on their spatial relationships; we can use fossils to date sedimentary rocks because we have a detailed record of the evolution of life on Earth; and we can use a range of isotopic techniques to determine the actual ages (in millions of years) of igneous and metamorphic rocks.

But just because we can measure geological time doesn’t mean that we understand it. One of the biggest hurdles faced by geology students, and geologists as well, in understanding geology, is to really come to grips with the slow rates at which geological processes happen and the vast amount of time involved.