Introduction

Prior to the invention of radioisotopic techniques for dating rock samples, geologists determined relative ages for rocks using simple principles of how rocks and their fragments are deposited, and using remains or records of extinct life to correlate samples from different locations. When combined later with the dating of rocks by radioisotopic techniques, a detailed history of Earth could be developed. We work with this history repeatedly throughout the rest of the book. This chapter serves as an introduction to the techniques used to assemble such a record.

Catastrophism versus uniformitarianism

When we look at Earth's landforms, we are viewing a snapshot, a moment in a vast span of time during which mountains rise and fall, seas expand over land areas and contract again, and continents shift their positions and grow slowly from new rock added by volcanoes. These processes all require vast amounts of time for their completion, but most do not proceed in a smooth, gradual manner. Instead, geologic processes are a combination of gradual effects and sudden catastrophes. The earthquakes that shake California represent sudden failures of rock after the build up of stresses over time as one portion of California slowly glides past the other, as we discuss in Chapter 9.

The realization that Earth changes in this hybrid fashion was long in coming. Much of the history of the development of geology was a battle between those who argued in favor of uniformitarianism, and hence gradual change over enormous spans of time, and those who claimed that Earth was young and shaped by catastrophic processes.