Chapter 8-1 PDF

UNIT IV DECI P H ERI N G E A RT H’S HIS TO RY FOCUS ON CONCEPTS Each statement represents the primary learning objective for the corresponding major heading 8 within the chapter. After you complete the chapter, you sho...

UNIT IV DECI P H ERI N G E A RT H’S HIS TO RY FOCUS ON CONCEPTS Each statement represents the primary learning objective for the corresponding major heading 8 within the chapter. After you complete the chapter, you should be able to: 8.1 Explain the principle of uniformitarianism and discuss how it differs from catastrophism. 8.2 Distinguish between numerical and relative dating and apply relative dating principles to determine a time sequence of geologic events. 8.3 Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various fossil types. 8.4 Explain how rocks of similar age that are in different places can be matched up. 8.5 Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates. 8.6 Distinguish among the four basic time units that make up the geologic time scale and explain why the time scale is considered to be a dynamic tool. 8.7 Explain how reliable numerical dates are determined for layers of sedimentary rock. This hiker is on the Kaibab Trail in Arizona’s Grand Canyon ­National Park. Millions of years of Earth history are exposed in the canyon’s rock walls. (Photo by Michael Collier) 248 M08_LUTG4814_8E_SE_C08.indd 248 23/01/16 2:30 pm  Geologic Time 249 M08_LUTG4814_8E_SE_C08.indd 249 23/01/16 2:30 pm  I     n the eighteenth century, James Hutton recognized the immensity of Earth history and the   importance of time as a component in all geologic processes. In the nineteenth century, others effectively demonstrated that Earth had experienced many episodes of mountain building and erosion, which must have required great spans of geologic time. Although these pioneering scientists understood that Earth was very old, they had no way of knowing its true age. Was it tens of millions, hundreds of millions, or even billions of years old? Because they didn’t know exactly how old Earth was, they developed a geologic time scale that showed the sequence of events based on relative dating principles. What were these principles? What part do fossils play? With the discovery of radioactivity and the development of radiometric dating techniques, geologists can now assign quite accurate dates to many of the events in Earth history. What is radioactivity? Why is it a good “clock” for dating the geologic past? This chapter will answer these questions. 8.1 A Brief History of Geology E xplain the principle of uniformitarianism and discuss how it differs from catastrophism.  This chapter begins with two images from the Grand to form, were explained as having been produced by sud- Canyon. The chapter-opening photo shows a hiker along den and often worldwide disasters of unknowable causes the Kaibab Trail, deep in the canyon. By contrast, the that no longer operate. This philosophy was an attempt hiker in Figure 8.1 is perched on the rim of the canyon. to fit the rate of Earth’s processes to the prevailing ideas The layers of sedimentary rock separating the two hikers about Earth’s age. represent millions of years of Earth history. Geologists have developed the knowledge and skills needed to deci- pher the clues contained in these strata that tell the long The Birth of Modern Geology and complex story of this region. Modern geology began in the late 1700s, when James The nature of our Earth—its materials and processes— Hutton, a Scottish physician and gentleman farmer, has been a focus of study for centuries. However, the late published his Theory of the Earth. In this work, Hut- 1700s is generally regarded as the beginning of modern ton put forth a fundamental principle that is a pillar of geology. It was during this time that James Hutton pub- geology today: uniformitarianism. It simply states that lished his important work Theory of the Earth. Prior to the physical, chemical, and biological laws that oper- that time, a great many explanations about Earth history ate today have also operated in the geologic past. This relied on supernatural events. means that the forces and processes that we observe presently shaping our planet have been at work for a very long time. Thus, to understand ancient rocks, we must Catastrophism first understand present-day processes and their results. In the mid-1600s, James Ussher, Anglican Archbishop of This idea is commonly expressed by saying “The present Armagh, Primate of All Ireland, published a work that is the key to the past.” had immediate and profound influence. A respected Prior to Hutton’s Theory of the Earth, no one had ef- scholar of the Bible, Ussher constructed a chronology of fectively demonstrated that geologic processes occur over human and Earth history in which he determined that extremely long periods of time. However, Hutton persua- Earth was only a few thousand years old, having been sively argued that processes that appear to be weak and created in 4004 b.c. Ussher’s treatise earned widespread slow acting can, over long spans of time, produce effects acceptance among Europe’s scientific and religious lead- that are just as great as those resulting from sudden cata- ers, and his chronology was soon printed in the margins strophic events. Unlike his predecessors, Hutton cited of the Bible itself. verifiable observations to support his ideas. During the 1600s and 1700s, the doctrine of For example, when he argued that mountains are ­catastrophism strongly influenced people’s thinking sculpted and ultimately destroyed by weathering and the about Earth. Briefly stated, catastrophists believed that erosional work of running water and that their wastes are Earth’s varied landscapes had been fashioned primarily carried to the oceans by processes that can be observed, by great catastrophes. Features such as mountains and Hutton said, “We have a chain of facts which clearly canyons, which today we know take great periods of time demonstrates that the materials of the wasted mountains 250 M08_LUTG4814_8E_SE_C08.indd 250 23/01/16 2:30 pm  8.2 Creating a Time Scale—Relative Dating Principles 251 Geology Today Did You Know? The basic tenets of uniformitarianism are just as viable Early attempts at de- today as in Hutton’s day. We realize more strongly than termining Earth’s age ever before that the present gives us insight into the past proved to be unreliable. and that the physical, chemical, and biological laws that One method reasoned govern geologic processes remain unchanging through that if the rate at which time. However, we also understand that the doctrine sediment accumulates should not be taken too literally. To say that geologic pro- could be determined, as cesses in the past were the same as those occurring today well as the total thick- is not to suggest that they always had the same relative ness of sedimentary rock importance or that they operated at precisely the same that had been deposited rate. Moreover, some important geologic processes are during Earth history, an not currently observable, but evidence that they occur is estimate of Earth’s age well established. For example, we know that Earth has could be made. All that experienced impacts from large meteorites even though was necessary was to we have no human witnesses. Such events altered Earth’s divide the rate of sedi- crust, modified its climate, and strongly influenced life ment accumulation into on the planet. the total thickness of The acceptance of the concept of uniformitarianism, sedimentary rock. This however, meant the acceptance of a very long history for method was riddled with Earth. Although Earth’s processes vary in their inten- difficulties. Can you think sity, they still take a long time to create or destroy major of some? landscape features. For example, geologists have estab- lished that mountains once existed in portions of pres- ent-day Minnesota, Wisconsin, Michigan, and ­Manitoba. Today, the region consists of low hills and plains. Ero- sion (processes that wear away land) gradually destroyed those peaks. The rock record contains evidence that shows Earth has experienced many cycles of mountain building and erosion. Concerning the ever-changing nature of Earth through great expanses of geologic time, Hutton made a statement that was to become his most famous. In concluding his classic 1788 paper published Figure 8.1 Contemplating geologic time This hiker is rest- in the Transactions of the Royal Society of Edinburgh, ing atop the Kaibab Formation, the uppermost layer in the he stated, “The results, therefore, of our present enquiry Grand Canyon. Beneath him are thousands of meters of sed- is, that we find no vestige of a beginning—no prospect imentary strata that go back more than 540 million years. of an end.” These strata rest atop even older sedimentary, metamorphic, It is important to remember that although many and igneous rocks, some as old as 2 billion years. Although features of our physical landscape may seem to be un- the canyon’s rock record has numerous interruptions, the rocks beneath the hiker contain clues to great spans of changing over the decades we observe them, they are Earth history. (Photo by Michael Collier) nevertheless changing—but on time scales of hundreds, thousands, or even many millions of years. have traveled through the rivers”; and, further, “There is not one step in all this progress that is not to be actually perceived.” He went on to summarize these thoughts by 8.1 CONCEPT CHECKS asking a question and immediately providing the answer: 1. Contrast catastrophism and uniformitarianism. “What more can we require? Nothing but time.” 2. How did each philosophy view the age of Earth? 8.2 Creating a Time Scale—Relative Dating Principles Distinguish between numerical and relative dating and apply relative dating principles to determine a time sequence of geologic events. Like the pages in a long and complicated history book, pages, especially in the early chapters, are missing. Others rocks record the geologic events and changing life-forms are tattered, torn, or smudged. Yet enough of the book re- of the past. The book, however, is not complete. Many mains to allow much of the story to be deciphered. M08_LUTG4814_8E_SE_C08.indd 251 23/01/16 2:30 pm  252 Chapter 8 Geologic Time Interpreting Earth history is a prime goal of the sci- the development of radiometric dating, geologists had no ence of geology. Like a modern-day sleuth, a geologist reliable method of numerical dating and had to rely solely must interpret the clues found preserved in the rocks. By on relative dating. studying rocks and the features they contain, geologists can unravel the complexities of the past. Relative Dates When we place rocks in their proper Geologic events by themselves, however, have little sequence of formation—which formed first, second, meaning until they are put into a time perspective. third, and so on—we are establishing relative dates. Studying history, whether it is the Civil War or the age Such dates do not indicate how long ago something of dinosaurs, requires a calendar. Among geology’s major took place, only that it followed one event and preceded contributions to human knowledge are the geologic time another. The relative dating techniques that were de- scale and the discovery that Earth history is exceedingly veloped are valuable and still widely used. Numerical long. dating methods did not replace these techniques; rather, they supplemented them. To establish a relative time scale, a few basic principles or rules had to be discov- Numerical and Relative Dates ered and applied. Although they may seem obvious to The geologists who developed the geologic time scale us today, they were major breakthroughs in thinking at revolutionized the way people think about time and per- the time, and their discovery was an important scientific ceive our planet. They learned that Earth is much older achievement. than anyone had previously imagined and that its surface and interior have been changed over and over again by the same geologic processes that operate today. Principle of Superposition Nicolas Steno, a Danish anatomist, geologist, and priest Numerical Dates During the late 1800s and early (1638–1686), is credited with being the first to recog- 1900s, attempts were made to determine Earth’s age. nize a sequence of historical events in an outcrop of Although some of the methods appeared promising at sedimentary rock layers. Working in the mountains of the time, none of these early efforts proved to be reli- western Italy, Steno applied a very simple rule that has able. What these scientists were seeking was a numeri- come to be the most basic principle of relative dating— cal date. Such dates specify the actual number of years the principle of superposition. The principle simply that have passed since an event occurred. Today, our states that in an undeformed sequence of sedimentary understanding of radiometric dating techniques allows rocks, each bed is older than the one above and younger us to accurately determine numerical dates for rocks that than the one below. Although it may seem obvious represent important events in Earth’s distant past. We that a rock layer could not be deposited with nothing will study these techniques later in this chapter. Prior to beneath it for support, it was not until 1669 that Steno clearly stated this Figure 8.2 Superposition principle. Applying the principle of This rule also ap- superposition to these plies to other surface- ­layers in the upper portion deposited materials, of the Grand Canyon, the such as lava flows and Supai Group is oldest and the Kaibab Limestone is beds of ash from volca- youngest. nic eruptions. Applying superposition to the beds exposed in the upper portion of the Grand Canyon, we can easily place the layers in their proper order. Among those that are pictured in Figure 8.2, the sedimentary rocks in the Supai Group are the oldest, followed in order by the ­Hermit Shale, Coconino Sandstone, Toroweap Formation, and Kaibab Dennis Tasa Limestone. M08_LUTG4814_8E_SE_C08.indd 252 23/01/16 2:31 pm  8.2 Creating a Time Scale—Relative Dating Principles 253 Principle of Original Figure 8.3 Original hori- zontality Most layers of Horizontality ­sediment are deposited in a nearly horizontal Steno is also credited with ­position. When we see recognizing the importance strata that are folded or of another basic principle, tilted, we can assume that called the principle of they were moved into that original horizontality. position by crustal distur- It simply states that layers bances after their deposi- of sediment are generally tion. (Photo by Marco Simoni/ deposited in a horizontal Robert Harding World Imagery) position. Thus, if we ob- serve rock layers that are flat, we know that they have not been disturbed and still have their original horizontality. The layers in the Grand Canyon shown in Figures 8.1 and 8.2 illus- trate this. But if layers are folded or inclined at a steep angle, they must have been moved into that position by Layer ends by thinning at Figure 8.4 Lateral crustal disturbances some- Layer ends by grading margin of sedimentary basin ­continuity Sediments are time after their deposition into a different kind of sediment deposited over a large area (Figure 8.3). in a continuous sheet. Sedimentary strata extend Principle of Lateral continuously in all direc- tions until they thin out at Continuity the edge of a depositional basin or grade into a dif- The principle of lateral ferent type of sediment. continuity refers to the fact that sedimentary beds origi- Lateral continuity allows us nate as continuous layers that to infer that the layers were originally continuous extend in all directions until across the canyon they eventually grade into a different type of sediment or until they thin out at the edge of the basin of deposi- tion (Figure 8.4). For example, when we look at the two walls of a canyon and see strata that are identical or similar, we can assume that those SmartFigure 8.5 Cross- strata were continuous before the canyon was carved. cutting fault The rocks are older than the fault that ­A lthough rock outcrops may be separated by a consider- displaced them. (Morley able distance, the principle of lateral continuity tells us Read/Alamy) (https://goo.gl/ that they once formed a continuous layer. This principle BiFVHa) allows geologists to relate rocks in isolated outcrops to one another. Combining the principles of lateral continu- Condor ity and superposition lets us extend relative age relation- Video ships over broad areas. This process, called correlation, is Fault examined later in the chapter. Principle of Cross-Cutting Relationships Figure 8.5 shows a mass of rock that is offset by a fault, a fracture in rock along which displacement occurs. It is clear that the rocks must be older than the fault that M08_LUTG4814_8E_SE_C08.indd 253 23/01/16 2:31 pm  254 Chapter 8 Geologic Time Figure 8.6 Cross-cutting is logical and straightforward. dikes An igneous intrusion Dikes The rock mass adjacent to is younger than the rocks the one containing the inclu- that are intruded. (Photo by sions must have been there Jonathan. S Kt) first in order to provide the rock fragments. Therefore, the rock mass that contains the inclusions is the younger of the two. For example, when magma intrudes into surrounding rock, blocks of the surrounding rock may become dislodged and incor- porated into the magma. If these pieces do not melt, they remain as inclusions, known as xenoliths. In another example, when sediment is deposited atop a weathered mass of bedrock, pieces broke them. The principle of cross-cutting relation- of the weathered rock become incorporated into the ships states that geologic features that cut across rocks younger sedimentary layer (Figure 8.7). must form after the rocks they cut through. Igneous intrusions provide another example. The dikes shown in Figure 8.6 are tabular masses of igneous rock that cut Unconformities through the surrounding rocks. The magmatic heat from When we observe layers of rock that have been de- igneous intrusions often creates a narrow “baked” zone posited essentially without interruption, we call them of contact metamorphism on the adjacent rock, also indi- conformable. Particular sites exhibit conformable beds cating that the intrusion occurred after the surrounding representing certain spans of geologic time. However, rocks were in place. no place on Earth has a complete set of conformable strata. Throughout Earth history, the deposition of sedi- Principle of Inclusions ment has been interrupted over and over again. All such Sometimes inclusions can aid in the relative dating pro- breaks in the rock record are termed unconformities. An cess. Inclusions are fragments of one rock unit that have unconformity represents a long period during which been enclosed within another. The principle of inclusions deposition ceased, erosion removed previously formed rocks, and then deposition resumed. In each case, uplift and erosion are followed by subsidence and renewed SmartFigure 8.7 Inclusions These inclusions of igneous rock contained in sedimentation. Unconformities are important features The rock containing inclu- the adjacent sedimentary layer indicate that the because they represent significant geologic events in sions is younger than the sediments were deposited atop the weathered Earth history. Moreover, their recognition helps us iden- inclusions. (https://goo.gl/ igneous mass and thus are younger. tify what intervals of time are not represented by strata Okfrm6) and thus are missing from the geologic record. Tutorial There are three basic types of unconformities. Sedimentary Angular Unconformity Perhaps the most easily rec- layers ognized unconformity is an angular unconformity. It consists of tilted or folded sedimentary rocks that are overlain by younger, more flat-lying strata. An angular Igneous intrusion unconformity indicates that during the pause in deposi- tion, a period of deformation (folding or tilting) and ero- sion occurred (Figure 8.8). When James Hutton studied an angular unconfor- Xenoliths are inclusions in an igneous intrusion mity in Scotland more than 225 years ago, it was clear to that form when pieces of surrounding rock are him that it represented a major episode of geologic activ- incorporated into magma. ity (Figure 8.9). He and his colleagues also appreciated the M08_LUTG4814_8E_SE_C08.indd 254 23/01/16 2:31 pm  8.2 Creating a Time Scale—Relative Dating Principles 255 SmartFigure 8.8 Formation of an angular unconformity An Tutorial Deposition angular unconformity repre- 5 sents an extended period dur- 4 3 ing which deformation and ero- 2 sion occurred. (https://goo.gl/arrwhC) 1 Figure 8.9 Siccar Point, 5 T 4 Above the unconformity Scotland James Hutton 3 2 I lie gently dipping beds studied this famous Uplift of reddish sandstone 1 unconformity in the late M and conglomerate 1700s. (Photo by Marli Miller) Erosion E 5 Angular 4 unconformity 3 2 1 Angular unconformity Rock (#6) hammer Deposition 9 8 7 5 4 3 Below the unconformity 2 lie nearly vertical beds of 1 sandstone and shale immense time span implied by such relationships. When a companion later wrote of their visit to this site, he stated that “the mind seemed to grow giddy by looking so far into the abyss of time.” Disconformity A disconformity is a gap in the rock record that represents a period during which erosion rather than deposition occurred. Imagine that a series Disconformity Figure 8.10 Disconformity Gap in the rock record represents a The layers on both sides of of sedimentary layers are deposited in a shallow marine period of nondeposition and erosion this gap in the rock record setting. Following this period of deposition, sea level are essentially parallel. falls or the land rises, exposing some of the sedimentary layers. During this span, when the sedimentary beds are above sea level, no new sediment accumulates, and some of the existing layers are eroded away. Later sea level Younger, rises or the land subsides, submerging the landscape. horizontal Now the surface is again below sea level, and a new sedimentary rocks series of sedimentary beds is deposited. The boundary separating the two sets of beds is a disconformity— a span for which there is no rock record (Figure 8.10). ­Because the layers above and below a disconformity Older, horizontal are parallel, these features are sometimes difficult to sedimentary identify unless you notice evidence of erosion such as a rocks buried stream channel. M08_LUTG4814_8E_SE_C08.indd 255 23/01/16 2:31 pm  256 Chapter 8 Geologic Time Figure 8.11 Nonconformity Nonconformity disconformities imply crustal movements, so too do Younger sedimentary rocks Period of uplift and erosion that nonconformities. Intrusive igneous masses and meta- rest atop older metamor- exposed the deep rocks at the surface morphic rocks originate far below the surface. Thus, for phic or igneous rocks. a nonconformity to develop, there must be a period of uplift and erosion of overlying rocks. Once the igneous or metamorphic rocks are exposed at the surface, they are subjected to weathering and erosion prior to subsid- Younger, sedimentary ence and a period of sedimentation. layers deposited atop erosion surface Unconformities in the Grand Canyon The rocks ex- posed in the Grand Canyon of the Colorado River repre- Older, igneous and/or sent a tremendous span of geologic history. It is a wonder- metamorphic rocks ful place to take a trip through time. The canyon’s colorful that formed deep strata record a long history of sedimentation in a variety within the crust of environments—advancing seas, rivers and deltas, tidal flats, and sand dunes. But the record is not continuous. Nonconformity The third basic type of unconformity is Unconformities represent vast amounts of time that have a nonconformity, in which younger sedimentary strata not been recorded in the canyon’s layers. Figure 8.12 is a overlie older metamorphic or intrusive igneous rocks geologic cross section of the Grand C­ anyon. All three (Figure 8.11). Just as angular unconformities and some types of unconformities can be seen in the canyon walls. Figure 8.12 Cross section of the Grand Canyon All Disconformity three types of unconformi- ties are present. (Center Kaibab Plateau Kaibab photo by Marli Miller; other photos Limestone by E. J. Tarbuck) Toroweap Coconino Angular unconformity Sandstone Hermit Shale Supai Disconformity Group Redwall Nonconformity Disconformity Limestone Muav Limestone Tonto Bright Angular unconformity Group Angel Shale Tapeats Nonconformity Sandstone Inner gorge Non Unkar con Group form ity Colorado Zoroaster Granite River Vishnu Schist M08_LUTG4814_8E_SE_C08.indd 256 23/01/16 2:31 pm  8.2 Creating a Time Scale—Relative Dating Principles 257 Angular SmartFigure 8.13 Applying unconformity principles of relative dating (https://goo.gl/w4HtAw) Tutorial K J I H G Working out the F E geologic history of a D Uplift hypothetical region Sill Angular C B unconformity Interpretation: Dike A Ocean K I HJ 6. Finally, a period of uplift and erosion. The G Ocean irregular surface and stream valley indicate that E another gap in the rock record is being created E F C by erosion. D Subsidence B Sill C A B Dike A 1. Beneath the Rock 5. Next, beds, G, H, ocean, beds A, B, eroded I, J, and K were C, and E were away deposited in that deposited in that E order atop the order (law of F D Sill erosion surface superposition). C E to produce an D B Sill C angular A B Dike unconformity. Uplift E D A Sill 2. Uplift and intrusion of a sill C 4. Layers A through F were (layer D). We know that sill D B F tilted and exposed layers is younger than beds C and E A were eroded. Dike because of the inclusions in the sill of fragments from 3. Next is the intrusion of dike F. beds C and E. Because the dike cuts through layers A through E, it must be younger (principle of cross-cutting relationships). Applying Relative Dating Principles 8.2 CONCEPT CHECKS By applying the principles of relative dating to the hypo- 1. Distinguish between numerical dates and relative thetical geologic cross section shown in Figure 8.13, the dates. rocks and the events in Earth history they represent can 2. Sketch and label five simple diagrams that be placed into their proper sequence. The statements in illustrate each of the following: superposition, the figure summarize the logic used to interpret the cross original horizontality, lateral continuity, cross- section. In this example, we establish a relative time scale cutting relationships, and inclusions. for the rocks and events in the area of the cross section. 3. What is the significance of an unconformity? Remember, we do not know how many years of Earth 4. Distinguish among angular unconformity, history are represented, nor do we know how this area disconformity, and nonconformity. compares to any other. M08_LUTG4814_8E_SE_C08.indd 257 23/01/16 2:31 pm  258 Chapter 8 Geologic Time 8.3 Fossils: Evidence of Past Life Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various fossil types. Fossils, the remains or traces of prehistoric life, are im- have been preserved because of rather unusual circum- portant inclusions in sediment and sedimentary rocks. stances. Remains of prehistoric elephants called mam- They are basic and important tools for interpreting the moths that were frozen in the Arctic tundra of Siberia geologic past. The scientific study of fossils is called and Alaska are examples, as are the mummified remains ­paleontology. It is an interdisciplinary science that of sloths preserved in a dry cave in Nevada. blends geology and biology in an attempt to understand all aspects of the succession of life over the vast expanse Permineralization When mineral-rich groundwater per- of geologic time. Knowing the nature of the life-forms meates porous tissue such as bone or wood, minerals pre- that existed at a particular time helps researchers under- cipitate out of solution and fill pores and empty spaces in stand past environmental conditions. Further, fossils are a process called permineralization. The formation of pet- important time indicators and play a key role in correlat- rified wood involves permineralization with silica, often ing rocks of similar ages that are from different places. from a volcanic source such as a surrounding layer of vol- canic ash. The wood is gradually transformed into chert, sometimes with colorful bands from impurities such as Types of Fossils iron or carbon (Figure 8.14A). The word petrified literally Fossils are of many types. The remains of relatively re- means “turned into stone.” Sometimes the microscopic cent organisms may not have been altered at all. Such details of the petrified structure are faithfully retained. objects as teeth, bones, and shells are common examples. Far less common are entire animals, flesh included, that Molds and Casts Another common class of fossils is molds and casts. When a shell or another structure is Figure 8.14 Types of fossils buried in sediment and then dis- (Photo A by Bernhard Edmaier/ solved by underground water, a Science Source; photo B by E. J. mold is created. The mold faith- Tarbuck; photo C by Florissant fully reflects only the shape Fossil Beds National Monument; and surface marking of the or- photo D by E. J. Tarbuck; photo E by Colin Keates/Dorling Kindersley ganism; it does not reveal any Ltd; photo F by E. J. Tarbuck) information concerning its in- ternal structure. If these hol- low spaces are subsequently B. A trilobite preserved as a filled with mineral matter, casts mold and cast are created (Figure 8.14B). A. Petrified wood preserved by permineralization Carbonization and Impres- Did You Know? sions A type of fossilization People frequently con- called carbonization is par- fuse paleontology and ticularly effective in preserving archaeology. Paleontolo- leaves and delicate animal forms. gists study fossils and It occurs when fine sediment are concerned with all encases the remains of an or- life-forms in the geo- D. Fishes preserved as ganism. As time passes, pres- C. A fossil bee detailed impressions logic past. By contrast, preserved as a sure squeezes out the liquid archaeologists focus on thin carbon film and gaseous components and the material remains of leaves behind a thin residue of carbon past human life. These (Figure 8.14C). Black shale deposited as remains include both the organic-rich mud in oxygen-poor en- objects used by people vironments often contains abundant long ago, called artifacts, carbonized remains. If the film of and the buildings and carbon is lost from a fossil preserved other structures associ- in fine-grained sediment, a replica of ated with where people the surface, called an impression, lived, called sites. F. A coprolite (fossil dung)– may still show considerable detail E. Spider preserved in amber an example of a trace fossil (Figure 8.14D). M08_LUTG4814_8E_SE_C08.indd 258 23/01/16 2:31 pm  8.4 Correlation of Rock Layers 259 Amber Delicate organisms, such as insects, are difficult special conditions favor preservation: rapid burial and the to preserve, and consequently they are relatively rare in possession of hard parts. Did You Know? the fossil record. However, amber—the hardened resin of When an organism perishes, its soft parts are usually Even when organisms die ancient trees—can preserve them in exquisite three-di- quickly eaten by scavengers or decomposed by bacteria. and their tissues decay, mensional detail. The spider in Figure 8.14E was preserved Occasionally, however, the remains are buried by sedi- some of their organic after being trapped in a drop of sticky resin. Resin sealed ment. When this occurs, the remains are protected from compounds may survive off the spider from the atmosphere and protected the the environment, where destructive processes operate. in sediments. Some of remains from damage by water and air. As the resin hard- Rapid burial, therefore, is an important condition favor- these compounds resist ened, a protective pressure-resistant case was formed. ing preservation. alteration and can be In addition, animals and plants have a much better analyzed to determine Trace Fossils In addition to the fossils already men- chance of being preserved as part of the fossil record if the kinds of organisms tioned, there are numerous other types, many of them they have hard parts. Although traces and imprints of they are derived from. only traces of prehistoric life. Examples of such indirect soft-bodied animals such as jellyfish, worms, and insects These are called chemi- evidence include: exist, they are not common. Flesh usually decays so rap- cal fossils. idly that preservation is exceedingly unlikely. Hard parts Tracks—animal footprints made in soft sediment such as shells, bones, and teeth predominate in the re- that later turned into sedimentary rock. cord of life in the past. Burrows—tubes in sediment, wood, or rock made by Because preservation is contingent on special condi- an animal. These holes may later become filled with tions, the record of life in the geologic past is biased. The mineral matter and preserved. fossil record of those organisms with hard parts that lived Coprolites—fossil dung and stomach contents that in areas of sedimentation is quite abundant. However, we can provide useful information pertaining to the size get only an occasional glimpse of the vast array of other and food habits of organisms (Figure 8.14F). life-forms that did not meet the special conditions favor- Gastroliths—highly polished stomach stones that ing preservation. some organisms use in the grinding of food. 8.3 CONCEPT CHECKS Conditions Favoring Preservation 1. Describe several ways that an animal or a plant Only a tiny fraction of the organisms that lived during can be preserved as a fossil. the geologic past have been preserved as fossils. The 2. List three examples of trace fossils. remains of an animal or a plant are normally destroyed. 3. What conditions favor the preservation of an Under what circumstances are they preserved? Two organism as a fossil? 8.4 Correlation of Rock Layers Explain how rocks of similar age that are in different places can be matched up. To develop a geologic time scale that is applicable to strata. Or a layer may be identified in another location if the entire Earth, rocks of similar age in different re- it is composed of distinctive or uncommon minerals. gions must be matched up. Such a task is referred to Many geologic studies involve relatively small areas. as c­ orrelation. Correlating the rocks from one place Although they are important in their own right, their to another makes possible a more comprehensive view full value is realized only when they are correlated with Did You Know? The word fossil comes of the geologic history of a region. Figure 8.15, for ex- other regions. Although the methods just described are from the Latin fossilium, ample, shows the correlation of strata at three sites on sufficient to trace a rock formation over relatively short which means “dug the C ­ olorado Plateau in southern Utah and northern distances, they are not adequate for matching up rocks up from the ground.” ­Arizona. No single locale exhibits the entire sequence, that are separated by great distances. When correlation As originally used by but correlation reveals a more complete picture of the between widely separated areas or between continents is medieval writers, a fossil sedimentary rock record. the objective, geologists must rely on fossils. was any stone, ore, or gem that came from an Correlation Within Limited Areas Fossils and Correlation underground source. In fact, many early books Within a limited area, correlating rocks of one locality The existence of fossils had been known for centuries, but on mineralogy are called with those of another may be done simply by walking it was not until the late 1700s and early 1800s that their books of fossils. The along the outcropping edges. However, this may not be significance as geologic tools was made evident. During current meaning of fossil possible when the rocks are mostly concealed by soil this period, an English engineer and canal builder, Wil- came about during the and vegetation. Correlation over short distances is often liam Smith, discovered that each rock formation in the 1700s. achieved by noting the position of a bed in a sequence of canals he worked on contained fossils unlike those in the M08_LUTG4814_8E_SE_C08.indd 259 23/01/16 2:31 pm  260 Chapter 8 Geologic Time Grand Canyon National Park Zion National Park Bryce Canyon National Park Bryce Canyon Paleogene Wasatch Fm National Park Kaiparowits Fm Correlation of strata at three sites on the Wahweap Ss Colorado Plateau builds a more complete Straight Cliffs Ss Cretaceous picture of the sedimentary record. Tropic Shale Dakota Ss Winsor Fm Zion National Park Curtis Fm Entrada Ss Jurassic Carmel Fm Carmel Fm Navajo Ss Navajo Ss Kayenta Fm Older rocks not exposed Wingate Ss Grand Canyon National Park Triassic Chinle Fm Moenkopi Fm Moenkopi Fm Kaibab Ls Kaibab Ls Permian Toroweap Fm Coconino Ss Older rocks not exposed Hermit Shale Supai Fm Pennsylvanian Bryce Redwall Ls Zion Canyon Mississippian National National Temple Butte Ls Park Park Devonian Muav Fm UTAH Bright Angel Shale ARIZONA Cambrian Tapeats Ss NEVADA Grand Canyon National Colorado Park River Vishnu Schist Precambrian Figure 8.15 Correlation Matching strata at three locations on the Colorado Plateau. (Photos by E. J. Tarbuck) M08_LUTG4814_8E_SE_C08.indd 260 23/01/16 2:31 pm  8.4 Correlation of Rock Layers 261 beds either above or below. Further, he noted that sedi- Figure 8.16 Index fossils Since mentary strata in widely separated areas could be identi- microfossils are often very abun- fied—and correlated—by their distinctive fossil content. dant, widespread, and quick to appear and become extinct, Principle of Fossil Succession Based on Smith’s clas- they constitute ideal index fos- sic observations and the findings of many geologists who sils. This scanning electron followed, one of the most important and basic principles micrograph shows marine in historical geology was formulated: Fossil organisms microfossils from the Miocene epoch (see the geologic time succeed one another in a definite and determinable scale in Figure 8.23). (Photo by order, and therefore any time period can be recognized Biophoto Associates/Science Source) by its fossil content. This has come to be known as the principle of fossil succession. In other words, when fossils are arranged according to their age, they do not sea. Also, by using what we know of living organisms, present a random or haphazard picture. To the contrary, we can conclude that fossil animals with thick shells, fossils document the evolution of life through time. capable of withstanding pounding and surging waves, For example, an Age of Trilobites is recognized quite inhabited shorelines. On the other hand, animals with early in the fossil record. Then, in succession, paleontolo- thin, delicate shells probably indicate deep, calm offshore gists recognize an Age of Fishes, an Age of Coal Swamps, waters. Hence, by looking closely at the types of fossils, an Age of Reptiles, and an Age of Mammals. These “ages” the approximate position of an ancient shoreline may be pertain to groups that were especially plentiful and char- identified. acteristic during particular time periods. Within each of Further, fossils can be used to indicate the former the “ages” are many subdivisions based, for example, on temperature of the water. Certain kinds of present-day certain species of trilobites and certain types of fish, rep- corals must live in warm and shallow tropical seas like tiles, and so on. This same succession of dominant organ- those around Florida and the Bahamas. When similar isms, never out of order, is found on every continent. types of coral are found in ancient limestones, they in- dicate the marine environment that must have existed Index Fossils and Fossil Assemblages When fossils when they were alive. These examples illustrate how fos- were found to be time indicators, they became the most sils can help unravel the complex story of Earth history. useful means of correlating rocks of similar age in differ- ent regions. Geologists pay particular attention to certain SmartFigure 8.17 Fossil fossils called index fossils (Figure 8.16). These fossils are Age ranges of some fossil groups assemblage Overlapping widespread geographically ranges of fossils help date rocks more exactly than Younger and are limited to a short using a single fossil. (https:// span of geologic time, so their goo.gl/dUqgP3) presence provides an im- Age Rock of r portant method of matching unit A ock unit A Tutorial rocks of the same age. Rock formations, however, do not always contain an index fos- sil. In such situations a group of fossils, called a fossil as- semblage, is used to establish TIME the age of the bed. F­ igure 8.17 ck illustrates how an assemblage of ro Age of fossils may be used to date B unit rocks more precisely than could be accomplished by the Rock unit B use of any one of the fossils. Older Environmental Indica- tors In addition to being important, and often essential, tools for correlation, fossils are important environmental indicators. Although much can be deduced about past 8.4 CONCEPT CHECKS environments by studying the nature and characteristics 1. What is the goal of correlation? of sedimentary rocks, a close examination of the fossils 2. State the principle of fossil succession in your present can usually provide a great deal more informa- own words. tion. For example, when the remains of certain clam 3. Contrast index fossil and fossil assemblage. shells are found in limestone, a geologist quite reasonably 4. In addition to being important time indicators, assumes that the region was once covered by a shallow how else are fossils useful to geologists? M08_LUTG4814_8E_SE_C08.indd 261 23/01/16 2:31 pm  262 Chapter 8 Geologic Time 8.5 Determining Numerical Dates with Radioactivity Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates. In addition to establishing relative dates by using the Practically all (99.9 percent) of an atom’s mass is principles described in the preceding sections, it is also found in the nucleus, indicating that electrons have prac- possible to obtain reliable numerical dates for events in tically no mass at all. By adding together the number of the geologic past. We know that Earth is about 4.6 bil- protons and neutrons in the nucleus, the mass number of lion years old and that dinosaurs became extinct about the atom is determined. The number of neutrons in the 66 million years ago. Dates that are expressed in millions nucleus can vary.