TABLE OF CONTENTS The concept of geologic time is central to an understanding of all geologic phenomena and features. Many nongeologists find it difficult to grasp the idea that the earth is a dynamic environment that changes over long time intervals, because daily, and even lifetime, changes on and beneath the earth's surface are imperceptibly small. However, there is a large body of evidence that geologic processes do operate over vast periods of time to continually shape and reshape the earth. This chapter presents an overview of some fundamentals of geologic time, so that the reader can better comprehend topics in later chapters concerning the origins of sedimentary environments and their deposits (including reservoirs) and postdepositional processes that modify these deposits. Also, the broad field of biostratigraphy is touched upon in this chapter, because it is so indelibly linked with geologic time.
Table 4.1 lists some of the key events in the origin of the earth and its inhabiting organisms, including humans. The earth is thought to have formed about 4.5 billion years ago, although this date is subject to change as new evidence is uncovered. The oldest age-dated rocks (again, subject to change as older rocks are discovered and verified) are thought to have formed on the order of 3.8 billion years ago, suggesting a long time interval between the earth's formation in a molten state and its cooling to form solid rock. The atmosphere formed a few hundreds of thousands of years afterward.
