Working through this chapter of the study guide will enable you to:

1. Understand the importance of fossils in the study of geologic time.
2. Explain the concept of relative geologic time and tell the principles upon which it is based.
3. Define absolute geologic time and show how radiometric dating has become the fundamental process used to establish such a time scale.
4. Learn how the age of the Earth can be determined.
5. Integrate relative and absolute geologic time to form the master clock of Earth's evolutionary history, the geologic time scale.

Discussion

How can we understand the events of the past if we do not have a reliable time scale upon which to base our discussion? It is indeed not easy. This chapter allows us to learn about the development of just such a scale, beginning with the relative time scale first used in geology, detailing the development of an absolute time scale using radiometric dating methods, and ending with the interweaving of the two into one comprehensive evolutionary master clock of Earth's history called the geologic time scale. This is an important aspect of our study of physical science, because it gives us an overview of what has happened to our world from the time of its initial formation right up to the present day.


Section  24.1Fossils

Paleontology, the study of fossils, is a key component in the determination of the history and age of Earth. Fossil records of ancient animals and plants can be preserved in several forms including actual original remains, molds and casts of previous remains, remains that have been replaced by minerals such as silica, calcite, and pyrite, and trace fossils that show tracks, burrows, etc., belonging to ancient creatures. Fossil records have been found that date back over 3 billion years, which is nearly 3/4 as old as Earth itself is believed to be.


Section  24.2Relative Geologic Time

Relative geologic time is an attempt to arrange the sequence of geologic events in a given locality into a simple chronological order. This can be accomplished in stratified deposits by the use of two general tools: the principle of superposition and the principle of cross-cutting relationships. The principle of superposition is based on the simple observation that younger layers of sediments are always deposited on top of older layers. Thus the top layers in any stratified deposit must be the youngest. The principle of cross-cutting relationships requires that a rock formation or fault must be younger than any other rock structure or fault line through which it cuts.

Unfortunately, deposits of sedimentary rock do not always remain horizontal and perfectly layered. Over the years these deposits can become broken, warped, and folded by other geologic processes into new configurations, making the relative age of the various layers hard to untangle. Igneous rock can also work its way into cracks and faults in the original layered structure, which further complicates the dating process. In situations like this, however, the principle of cross-cutting relationships comes into play, helping us sort out the actual age of the various layers by adding radiometric dating to the available tools that can be brought to bear on this analysis. For various reasons covered in more detail in Section 24.5, sedimentary rock usually cannot be accurately dated using radiometric dating methods, but the age of the intrusive igneous rock can. This brings the principle of cross-cutting relationships into the picture again and adds the ability to work in absolute time units, such as years, to the dating process.

Once local geologic history has been established for many different locations on Earth's surface, the goal is to form a complete geologic history of the entire planet using a combination of all these local data. This process is called correlation. Although much progress has been made in recent years, the development of one geologic history for the entire Earth is far from complete. There are some very good correlations for large geologic areas and even of entire continents, but much work remains to be done.

Of the many ways that rock samples can be correlated over great distances, the most successful has been the use of fossils, which can be either of plant or animal origin. They are most useful when the particular organism used in the correlation study has evolved in a regular and identifiable way over the years in response to shifts in environmental conditions. Such fossils are very important to the accurate correlations of widely separated rock samples, and so these fossils have been given the special designation of index fossils.

Another important tool used in establishing relative geologic time involves unconformities, which are localized breaks or unrecorded intervals in the geologic record. Unconformities represent periods of time when the depositing of sediment may have stopped, erosion may have removed some of the previous rock layers, and, finally, depositing of sediment began again. The importance of such unrecorded intervals lies in the fact that they invariably mark some noteworthy geologic event in the local history of crustal activity. Unconformities can be very valuable in the correlation of geologic dating over wide geographic areas.

Geologic time is divided into units, similar to the way that our normal time scale is divided into days, hours, minutes, and seconds. Geologic time units are, however, much longer. The longest geologic unit of time is the eon, which is then subdivided into eras, periods, and epochs. Figure 24.9 and the chapter's Spotlight feature show the basic structure of geologic time using these units.


Section  24.3Radiometric Dating

Relative geologic time is useful, but it is also important to know the actual age of a given geologic formation or event in years. Early attempts at absolute dating involved dividing the amount of geologic work that had been done, such as erosion, by the rate at which that work is known to occur today. These methods were not particularly successful and often led to very misleading estimates. Radiometric decay has proved to be the best available solution to the absolute geologic time problem. It was discovered that the ratio of the amount of radioactive nuclei left, compared with the amount of daughter nuclei produced by the original decay, could be used together with the well-known decay rate for various radionuclides to determine the age of geologic specimens. This decay rate is referred to as the half-life. Although some of these decay processes lead to long chains of radioactive products, careful analysis can sort out these complications, and radiometric dating has become the primary tool used by geologists to find absolute dates for events and specimen formation.

Several radionuclides are used in this process, some of the most important of which are uranium-238, thorium-232, potassium-40, rubidium-87, and carbon-14. Details on the use and reliability of radiometric dating using these radionuclides are presented in the textbook. Note that each radionuclide has its own niche in the overall radiometric dating process, because each has a unique half-life and other characteristics that make it more useful in some geologic dating situations than in others.


Section  24.4The Age of Earth

Many attempts have been made to estimate the age of Earth. A particularly poor result was obtained when Lord Kelvin compared the estimated heat content of molten Earth with the heat loss rate for Earth's surface. This determination was off by several billion years, not because of careless work or poor data collection, but because the internal radioactive decay processes that continuously add heat to Earth's interior were unknown and not taken into account. Today, scientists believe that the age of Earth is about 4.56 billion years. This age is supported by several forms of evidence including the radiometric dating of Earth's rocks, the age of meteorites, and the age of rock samples brought back from the Moon. It is believed that in its earliest state, Earth had a molten surface that took several hundred million years to cool. It is hard to find samples of rock that old because weathering and subduction have probably destroyed most of these earliest rock formations. Nevertheless, we can consider ourselves fortunate to have found collaborating evidence of this ancient formation of Earth's crust in rock samples as much as 4 billion years old and fossils over 3 billion years old.


Section  24.5The Geologic Time Scale

The combination of relative and absolute geologic time gives us a comprehensive chronology that shows the important events in Earth's history and gives us a good indication of when they occurred. Such an integrated combination is called the geologic time scale. This scale is summarized in the chapter's Spotlight feature. Fine tuning of the geologic time scale is still being done today, but it has already proved to be a surprisingly accurate tool for geologists to work with, considering the great age of our planet and the number of variables that are involved in the study of its history.

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