The immensity of geologic time is hard for humans to perceive. It is unusual for someone to live a hundred years, but a person would have to live 10,000 times that long to observe a geologic process that takes a million years. In this chapter, we try to help you develop a sense of the vast amounts of time over which geologic processes have been at work.
Geologists working in the field or with maps or illustrations in a laboratory are concerned with relative time - unraveling the sequence in which geologic events occurred. For instance, a geologist looking at a photo of the Grand Canyon can determine that the tilted sedimentary rocks at the bottom of the canyon are older than the horizontal sedimentary rocks above them, and that the lower layers of the horizontal sedimentary rocks are older than those above them. But this tells us nothing about how long ago any of the rocks formed. To determine how many years ago rocks formed, we need the specialized techniques of radioactive isotope dating. Through isotopic dating we have been able to determine that the rocks in the lowermost part of the Grand Canyon are well over a billion years old.
This chapter explains how to apply several basic principles to decipher a sequence of events responsible for geologic features. These principles can be applied to many aspects of geology - as, for example, in understanding geologic structures (chapter 6). Understanding the complex history of mountain belts (chapter 5) also requires knowing the techniques for determining relative ages of rocks.
Determining age relationships between geographically widely separated rock units is necessary for understanding the geologic history of a region, a continent, or the whole Earth. Substantiation of the plate tectonics theory depends on intercontinental correlation of rock units and geologic events, piecing together evidence that the continents were once one great body.
Widespread use of fossils led to the development of the standard geologic time scale. Originally based on relative age relationships, the subdivisions of the standard geologic time scale have now been assigned numerical ages in thousands, millions, and billions of years through isotopic dating. Think of the geologic time scale as a sort of calendar to which events and rock units can be referred. Its major subdivisions are referred to elsewhere in this book.
