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Geology Needs a Time Scale • Box 10.1 Understanding Earth: A Brief History of Geology Deciphering the Past by Relative Dating—Key Principles Understanding the Present Correlation of Rock Layers • Box 10.2 People and the Environment: Fossils: Evidence of Past Life Radon—A Threat to Human Dating with Radioactivity Health The Geologic Time Scale • Box 10.3 Understanding Earth: Using Difficulties in Dating the Geologic Tree Rings to Date and Study Time Scale the Recent Past The strata exposed in Arizona’s Grand Canyon contain clues to hundreds of millions of years of Earth history. (Photo © by Carr Clifton) EARTH’S HISTORY EARTH’S UNIT 4: DECIPHERING CHAPTER10 Geologic Time 285 286 Unit Four Deciphering Earth’s History n the eighteenth century, James Hutton recog- Revelations in the rock-leaved Bible of geology.” He nized the immensity of Earth history and the im- was undoubtedly impressed with the millions of years portance of time as a component in all geological of Earth history exposed along the walls of the Grand Iprocesses. In the nineteenth century, others ef- Canyon (see chapter-opening photo). fectively demonstrated that Earth had experi- Powell realized that the evidence for an ancient enced many episodes of mountain building and erosion, Earth is concealed in its rocks. Like the pages in a long which must have required great spans of geologic time. and complicated history book, rocks record the geo- Although these pioneering scientists understood that logical events and changing life forms of the past. The Earth was very old, they had no way of knowing its true book, however, is not complete. Many pages, especial- age. Was it tens of millions, hundreds of millions, or ly in the early chapters, are missing. Others are tattered, even billions of years old? Rather, a geologic time scale torn, or smudged. Yet enough of the book remains to was developed that showed the sequence of events allow much of the story to be deciphered. based on relative dating principles. What are these prin- Interpreting Earth history is a prime goal of the sci- ciples? What part do fossils play? With the discovery ence of geology. Like a modern-day sleuth, the geologist of radioactivity and the development of radiometric must interpret clues found preserved in the rocks. By dating techniques, geologists now can assign fairly ac- studying rocks, especially sedimentary rocks, and the curate dates to many of the events in Earth history. features they contain, geologists can unravel the com- What is radioactivity? Why is it a good “clock” for dat- plexities of the past. ing the geologic past? In this chapter we shall answer Geological events by themselves, however, have lit- these questions. tle meaning until they are put into a time perspective. Studying history, whether it be the Civil War or the Age of Dinosaurs, requires a calendar. Among geology’s Geology Needs a Time Scale major contributions to human knowledge is the geolog- ic time scale and the discovery that Earth history is ex- In 1869 John Wesley Powell, who was later to head the ceedingly long. U.S. Geological Survey, led a pioneering expedition The geologists who developed the geologic time down the Colorado River and through the Grand scale revolutionized the way people think about time Canyon (Figure 10.1). Writing about the rock layers that and how they perceive our planet. They learned that were exposed by the downcutting of the river, Powell Earth is much older than anyone had previously imag- said that “the canyons of this region would be a Book of ined and that its surface and interior have been changed A. B. Figure 10.1 A. Start of the expedition from Green River station. A drawing from Powell’s 1875 book. B. Major John Wesley Powell, pioneering geologist and the second director of the U.S. Geological Survey. (Courtesy of the U.S. Geological Survey, Denver) Chapter 10 Geologic Time 287 over and over again by the same geological processes Earth processes to the prevailing ideas on the age of that operate today. Earth. Birth of Modern Geology A Brief History of Geology Modern geology began in the late 1700s when James In the mid-1600s, James Ussher, Anglican Archbishop Hutton, a Scottish physician and gentleman farmer, of Armagh, Primate of all Ireland, published a work that published his Theory of the Earth. In this work, Hutton had immediate and profound influence on people’s view put forth a fundamental principle that is a pillar of ge- of Earth’s age. A respected scholar of the Bible, Ussher ology today: uniformitarianism. It simply states that constructed a chronology of human and Earth history the physical, chemical, and biological laws that operate today in which he determined that Earth was only a few thou- have also operated in the geologic past. This means that the sand years old, having been created in 4004 B.C.Ussher’s forces and processes that we observe presently shaping treatise earned widespread acceptance among Europe’s our planet have been at work for a very long time. Thus, scientific and religious leaders, and his chronology was to understand ancient rocks, we must first understand soon printed in the margins of the Bible itself. present-day processes and their results. This idea is During the seventeenth and eighteenth centuries commonly expressed by saying “the present is the key the doctrine of catastrophism strongly influenced peo- to the past” (see Box 10.1). ple’s thinking about Earth. Briefly stated, catastrophists Prior to Hutton’s Theory of the Earth, no one had ef- believed that Earth’s landscapes had been developed fectively demonstrated that geological processes occur primarily by great catastrophes. Features such as moun- over extremely long periods of time. However, Hutton tains and canyons, which today we know take great pe- persuasively argued that weak, slow-acting processes riods of time to form, were explained as having been could, over long spans of time, produce effects just as produced by sudden and often worldwide disasters great as those resulting from sudden catastrophic triggered by unknowable causes that no longer oper- events. Unlike his predecessors, Hutton carefully cited ate. This philosophy was an attempt to fit the rate of verifiable observations to support his ideas. UNDERSTANDING EARTH: Deciphering the Past by Understanding the Present BOX 10.1 ouis Agassiz, a posits and features that can be seen LSwiss scientist forming in association with active glac- born in 1807, was instrumental in for- iers in places far beyond the limits of mulating modern ideas about the Ice the ice. Subsequent work led Agassiz to Age (Figure 10.A). The development of hypothesize that a great ice age had oc- this knowledge provides an excellent curred in response to a period of world- example of the application of the prin- wide climate change and had affected ciple of uniformitarianism. large parts of the globe. Agassiz’s ideas In 1821 Agassiz heard another sci- eventually developed into our present- entist present a paper in which he indi- day glacial theory. cated that glacial features occurred in The proof of the glacial theory pro- places that were a significant distance posed by Agassiz and others constitutes from existing glaciers in the Alps. This, a classic example of applying the prin- of course, implied that the glaciers had ciple of uniformitarianism. Realizing once occupied areas considerably be- that certain landforms and other fea- yond their present limits. Agassiz was tures are produced by no other known skeptical about this hypothesis and set process but glacial activity, they were out to invalidate it. Ironically, his field- able to reconstruct the extent of now work in the Alps convinced him of the vanished ice sheets. Clearly, under- merits of his colleague’s hypothesis. standing the present was the key to de- Agassiz found the same unique de- ciphering the past. Figure 10.A Louis Agassiz (1807–1873) played a major role in the development of glacial theory. (Courtesy of Harvard University Archives) 288 Unit Four Deciphering Earth’s History For example, when he argued that mountains are changing, but on time scales of hundreds, thousands, sculpted and ultimately destroyed by weathering and or even many millions of years. the work of running water, and that their wastes are carried to the oceans by processes that can be observed, Relative Dating—Key Principles Hutton said, “We have a chain of facts which clearly demonstrates that the materials of the wasted moun- Geologic Time E A tains have traveled through the rivers”; and further, E Relative Dating R C T N “There is not one step in all this progress that is not to H S C I E be actually perceived.” He then went on to summarize During the late 1800s and early 1900s, various attempts this thought by asking a question and immediately pro- were made to determine the age of Earth. Although viding the answer: “What more can we require? Noth- some of the methods appeared promising at the time, ing but time.” none proved reliable. What these scientists were seek- Geology Today ing was a numerical date. Such dates specify the actu- al number of years that have passed since an event Today the basic tenets of uniformitarianism are just as vi- occurred—for example, the extinction of the dinosaurs able as in Hutton’s day. Indeed, we realize more strong- about 65 million years ago. Today our understanding of ly than ever that the present gives us insight into the radioactivity allows us to accurately determine numer- past and that the physical, chemical, and biological laws ical dates for rocks that represent important events in that govern geological processes remain unchanging Earth’s distant past.

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