<H1>Geography 111 - Geologic Time</H1>

Geologic time - chapter 9

We have already discussed geologic time at a very simple descriptive level and at a philosophical level: we know the age of the earth in billions of years, and we know that up until the 16th century Western civilization generally accepted the biblical account of an earth that was only a few thousand years old. James Hutton was the first modern scientist to look into the abyss of geologic time and recognize it for what it was: a virtually infinite span, at least compared with the length of a human lifetime. Yet even with this recognition he had no way of actually measuring that span, and it was not until the twentieth century that the tools became available that would make this possible.

How, then, were geologists of the past two centuries able to construct the elaborate table of eras, periods, and epochs that we know as the geological time scale? Before absolute dating was possible, several important principles were devised that allowed geologists to determine the relative ages of different rock layers. These principles of relative age dating, together with other rules for correlation of rocks of similar ages over a broad area, made it possible to construct a cataloguing system that could be applied over large areas of the globe. Long before geologists and paleontologists knew the beginning and ending dates of the Cambrian or Ordovician periods, they were able to determine whether a particular sedimentary rock was deposited in the Cambrian, the Ordovician, the Silurian or the Devonian. When the discovery of the phenomenon of radioactive decay made it possible to develop methods for radiometric dating of rocks, it suddenly became possible to start attaching absolute dates to all of the time periods that were already included in the time scale. In this chapter we focus first on the basic principles and tools of stratigraphy (the study of layered sequences of sedimentary rocks) and relative age dating; then on the geologic time scale; and then on the use of radiometric dating techniques together with other tools like the sequence of reversals of the earth's magnetic field.

Relative age dating often relies on a small number of basic principles, some of which have been around since Nicolaus Steno formulated them in Italy in the late 1600's. These include the principle of original horizontality; the law of superposition; the principle of faunal succession (this last one is discussed back on page 6 in the text); and the principle of cross-cutting relationships. Be prepared to explain these principles and be prepared to use them in deciphering the age sequence in a block diagram such as fig. 9.4, 9.8 or 9.19. The drills and questions in the CD-ROM are very useful in working out how to do this.
What is an unconformity and why is it significant? How are the major varieties of unconformities (angular unconformity, nonconformity, and disconformity) distinguished from each other?
What is an outcrop, what is a geologic formation, and what do geologists mean by the term "correlation"? How is it possible to correlate rocks of a particular age with rocks of a similar age in another area? What role do fossils play in correlation? What are index fossils?
What are the basic processes involved in radioactive decay, and what are parent and daughter isotopes, and what is meant by "half-life"? Be prepared to calculate the age of a rock sample based on the ratio of parent to daughter product; this will be reviewed in class. The CD-ROM also does a good job of explaining this topic.
What are the advantages and drawbacks of carbon-14 dating as opposed to other prominent radioactive isotopes like rubidium/strontium 87, potassium/argon 40, and uranium 238/lead 206?
How can radiometric dating be used, together with the principles of relative age dating, to establish a time sequence for a particular set of rocks? Toward the end of the chapter you will also find a discussion of how igneous intrusions and volcanic ash layers can be used to estimate the ages of sedimentary rocks that otherwise may not be datable by radiometric techniques.
Make sure that you are familiar with the major subdivisions of the geologic time scale, including the age and significance of the Precambrian/Cambrian boundary (marking the beginning of the Phanerozoic eon); the Paleozoic, Mesozoic, and Cenozoic eras; the age and significance of the Permian/Triassic boundary and the Cretaceous/Tertiary boundary; the subdivision of the Cenozoic into Tertiary and Quaternary periods; and the further subdivision of the Quaternary into the Pleistocene and the Holocene epochs. The importance of major extinctions or major transitions in the history of life is expressed in these major divisions or boundaries in geologic time.