Geologic Time opens with a brief history of geology that spans from James Ussher (mid-1600s) to James Hutton (late-1700s) and Sir Charles Lyell (mid-1800s). The chapter continues with a discussion of the fundamental principles of relative dating, including the law of superposition, principle of original horizontality, principle of cross-cutting relationships, and the uses of inclusions and unconformities. How rock units in different localities can be correlated is also investigated. The types of fossils and their significance to understanding geologic time precedes a discussion of the conditions favoring preservation. Also examined is the use of fossils in correlating and dating rock units. Following an explanation of radioactivity, the fundamentals and importance of radiometric dating are presented. The chapter concludes with an examination of the geologic time scale.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•Describe the doctrine of uniformitarianism.
•Explain the difference between absolute and relative dating.
•List the laws and principles used in relative dating.
•Discuss unconformities.
•Explain correlation of rock layers.
•Describe fossils, fossilization, and the uses of fossils.
•Explain radioactivity and radiometric dating.
•Describe the geologic time scale.
Chapter Summary
•The doctrine of uniformitarianism, one of the fundamental principles of modern geology put forth by James Hutton in his published work Theory of the Earth in the late 1700s, states that the physical, chemical, and biological laws that operate today have also operated in the geologic past. The idea is often summarized as “the present is the key to the past." Hutton argued that processes that appear to be slow-acting could, over long spans of time, produce effects that were just as great as those resulting from sudden catastrophic events. Catastrophism, on the other hand, states that Earth’s landscapes have been developed primarily by great catastrophes. Sir Charles Lyell (mid-1800s) is given the most credit for advancing the basic principles of modern geology with the publication of the eleven editions of his great work, Principles of Geology.
•The two types of dates used by geologists to interpret Earth history are 1) relative dates, which put events in their proper sequence of formation, and 2) absolute dates, which pinpoint the time in years when an event took place.
•Relative dates can be established using the law of superposition, principle of original horizontality, principle of cross-cutting relationships, inclusions, and unconformities.
•Correlation, the matching up of two or more geologic phenomena in different areas, is used to develop a geologic time scale that applies to the whole Earth.
•Fossils are the remains or traces of prehistoric life. Some fossils form by petrification, replacement, or carbonization. Others are impressions, casts, or have been preserved in amber. Fossils can also be indirect evidence of prehistoric life such as tracks or burrows. The special conditions that favor preservation are rapid burial and the possession of hard parts such as shells, bones, or teeth.
•Fossils are used to correlate sedimentary rocks that are from different regions by using the rocks’ distinctive fossil content and applying the principle of fossil succession. The principle of fossil succession, which is based on the work of William Smith in the late 1700s, states that fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. The use of index fossils, those that are widespread geographically and are limited to a short span of geologic time, provides an important method for matching rocks of the same age.
•Each atom has a nucleus containing protons (positively charged particles) and neutrons (neutral particles). Orbiting the nucleus are negatively charged electrons. The atomic number of an atom is the number of protons in the nucleus. The mass number is the number of protons plus the number of neutrons in an atom's nucleus. Isotopes are variants of the same atom, but with a different number of neutrons, and hence a different mass number.
•Radioactivity is the spontaneous breaking apart (decay) of certain unstable atomic nuclei. Three common forms of radioactive decay are 1) emission of alpha particles from the nucleus, 2) emission of a beta particle (or electron) from the nucleus, and 3) capture of an electron by the nucleus.
•An unstable radioactive isotope, called the parent, will decay and form daughter products. The length of time for one-half of the nuclei of a radioactive isotope to decay is called the half-life of the isotope. If the half-life of the isotope is known, and the parent/daughter ratio can be measured, the age of a sample can be calculated. Carbon-14, the radioactive isotope of carbon that is absorbed by living matter, is used to date very recent events. Although the procedure is quite complex, radiometric dating has vindicated the ideas of Hutton, Darwin, and others who have inferred that geologic time must be immense.
•The geologic time scale divides Earth's history into units of varying magnitude. It is commonly presented in chart form, with the oldest time and event at the bottom and the youngest at the top. The principal subdivisions of the geologic time scale, called eons, include the Hadean, Archean, Proterozoic (together, these three eons are commonly referred to as the Precambrian), and, beginning about 570 million years ago, the Phanerozoic. The Phanerozoic (meaning “visible life") eon is divided into the following eras: Paleozoic (“ancient life”), Mesozoic (“middle life"), and Cenozoic (“recent life").
•The primary problem in assigning absolute dates to units of time is that not all rocks can be dated radiometrically. A sedimentary rock may contain particles of many ages that have been weathered from different rocks that formed at various times. One way geologists assign absolute dates to sedimentary rocks is to relate them to datable igneous masses, such as volcanic ash beds.