Time and Geology PDF

C H A P T E R 8 Time and Geology Kaibab Limestone...

C H A P T E R 8 Time and Geology Kaibab Limestone Coconino Sandstone Supai Formation Redwall Limestone Bright Angel Shale ne Sandsto Tapeats Vishnu Schist Gran d Ca nyon Serie s Grand Canyon, Arizona. Horizontal Paleozoic beds (top of photo) overlie tilted Precambrian beds (Grand Canyon Series) and older, Precambrian metamorphic rock (Vishnu Schist). Photo © Craig Aurness/Corbis The Key to the Past Numerical Age Relative Time Isotopic Dating Principles Used to Determine Relative Age Uses of Isotopic Dating Unconformities Combining Relative and Numerical Ages Correlation Age of the Earth The Standard Geologic Time Scale Comprehending Geologic Time Summary 178  Time and Geology 179 LEARNING OBJECTIVES Differentiate between relative age and numerical age. Describe radioactive decay and how radiogenic isotopes can be Know the relative dating principles and use them to determine a used to determine numerical age. sequence of geologic events. Know the age of the Earth and the major subdivisions of Discuss how fossils are used to determine relative age and how geologic time. paleontology contributed to the development of the geologic time scale. T he immensity of geologic time is hard for humans to per- what has now been determined scientifically. In the Christendom ceive. It is unusual for someone to live a hundred years, of the seventeenth and eighteenth centuries, formation of all rocks but a person would have to live 10,000 times that long and other geologic events were placed into a biblical chronology. to observe a geologic process that takes a million years. In this This required that features we observe in rocks and landscapes chapter, we try to help you develop a sense of the vast amounts were created supernaturally and catastrophically. The sedimen- of time over which geologic processes have been at work. tary rocks with marine fossils (clams, fish, etc.) that we find in Geologists working in the field or with maps or illustrations mountains thousands of meters above sea level were believed to in a laboratory are concerned with relative time—unraveling have been deposited by a worldwide flood (Noah’s flood) that the sequence in which geologic events occurred. For instance, inundated all of Earth, including its highest mountains, in a mat- a geologist looking at the photo of Arizona’s Grand Canyon on ter of days. Because no known physical laws could account for the facing page can determine that the tilted sedimentary rocks such events, they were attributed to divine intervention. In the are older than the horizontal sedimentary rocks and that the eighteenth century, however, James Hutton, a Scotsman often lower layers of the horizontal sedimentary rocks are older than regarded as the father of geology, realized that geologic features the layers above them. But this tells us nothing about how long could be explained through present-day processes. He recognized ago any of the rocks formed. To determine how many years ago that our mountains are not permanent but have been carved into rocks formed, we need the specialized techniques of radioac- their present shapes and will be worn down by the slow agents of tive isotope dating. Through isotopic dating, we have been able erosion now working on them. He realized that the great thick- to determine that the rocks in the lowermost part of the Grand nesses of sedimentary rock we find on the continents are products Canyon are well over a billion years old. of sediment removed from land and deposited as mud and sand This chapter explains how to apply several basic principles in seas. The time required for these processes to take place had to to decipher a sequence of events responsible for geologic be incredibly long. Hutton broke from conventional thinking that features. These principles can be applied to many aspects of Earth is no more than a few thousand years old when he wrote in geology—as, for example, in understanding geologic structures 1788, “We find no sign of a beginning—no prospect for an end.” (chapter 15). Understanding the complex history of mountain Hutton’s writings were not widely read, but his concept of geo- belts (chapter 20) also requires knowing the techniques for logical processes requiring vast amounts of time was popularized determining relative ages of rocks. in the 1800s when Charles Lyell published his landmark book, Determining age relationships between geographically Principles of Geology, which refined and expanded upon many widely separated rock units is necessary for understanding the of Hutton’s ideas. Charles Darwin was among those influenced geologic history of a region, a continent, or the whole Earth. by Lyell’s writing. His evolutionary theory involving survival of Substantiation of the plate tectonics theory depends on intercon- the fittest, published in the mid-1800s, required the great amount tinental correlation of rock units and geologic events, piecing of time that the works of Hutton and Lyell proposed. together evidence that the continents were once one great body. The concept that geologic processes operating at pres- Widespread use of fossils led to the development of the ent are the same processes that operated in the past eventually standard geologic time scale. Originally based on relative age became known as the principle of uniformitarianism. The relationships, the subdivisions of the standard geologic time principle is stated more succinctly as “The present is the key scale have now been assigned numerical ages in thousands, to the past.” The term uniformitarianism is a bit unfortunate, millions, and billions of years through isotopic dating. Think because it suggests that changes take place at a uniform rate. It of the geologic time scale as a sort of calendar to which events does not allow for the fact that sudden, violent events, such as a and rock units can be referred. major, short-lived volcanic eruption, can also influence Earth’s history. Many geologists prefer actualism in place of uniformi- tarianism. The term actualism comes closer to conveying the THE KEY TO THE PAST principle that the same processes and natural laws that oper- ated in the past are those we can actually observe or infer from Until the 1800s, people living in Western culture did not question observation as operating at present. It is based on the assump- the religious perception of Earth being only a few thousand years tion, central to the sciences, that physical laws are independent old. On the other hand, Chinese and Hindu cultures believed the of time and location. Under present usage, uniformitarianism age of Earth was vast beyond comprehension—more in line with has the same meaning as actualism for most geologists. 180 CHAPTER 8 We now realize that geology involves time periods much pattern; however, a geologist has learned to approach seem- greater than a few thousand years. But how long? For instance, ingly formidable problems by breaking them down to a num- were rocks near the bottom of the Grand Canyon (chapter open- ber of simple problems. (In fact, a geologic education trains ing photo) formed closer to 10,000 or 100,000 or 1,000,000 or students in a broad spectrum of problem-solving techniques, 1,000,000,000 years ago? What geologists needed was some useful for a wide variety of applications and career opportuni- “clock” that began running when rocks formed. Such a “clock” ties.) As an example, the geology of the Grand Canyon, shown was found shortly after radioactivity was discovered. Dating based in the chapter opening photo, can be analyzed in four parts: on radioactivity (discussed later in this chapter) allows us to deter- (1) horizontal layers of rock; (2) inclined layers; (3) rock under- mine a rock’s numerical age (also known as absolute age)—age lying the inclined layers (plutonic and metamorphic rock); and given in years or some other unit of time. Geologists working in the (4) the canyon itself, carved into these rocks. field or in a laboratory with maps, cross sections, and photographs After you have studied the following section, return to the are more often concerned with relative time, the sequence in photo of the Grand Canyon and see if you can determine the which events took place, rather than the number of years involved. sequence of geologic events that took place. These statements show the difference between numerical age and relative time: “The American Revolutionary War took place after the Principles Used to Determine signing of the Magna Carta but before World War II.” This Relative Age statement gives the time of an event (the Revolutionary War) relative to other events. Most of the individual parts of the larger problem are solved by But in terms of numerical age, we could say: “The Revolu- applying several simple principles while studying the exposed tionary War took place about two and a half centuries ago.” Note rock. In this way, the sequence of events or the relative time that a numerical age does not have to be an exact age, merely age involved can be determined. Contacts are particularly useful given in units of time. Because most geologic problems are con- for deciphering the geologic history of an area. (Contacts, as cerned with the sequence of events, we discuss relative age first. described in previous chapters, are the surfaces separating two different rock types or rocks of different ages.) To explain various principles, we will use a fictitious place that bears some resem- RELATIVE TIME blance to the Grand Canyon. We will call this place, represented by the block diagram of figure 8.1, Minor Canyon. The formation The geology of an area may seem, at first glance, to be hope- names are also fictitious. (Formations, as described in chapter lessly complex. A nongeologist might think it impossible to 6, are bodies of rock of considerable thickness with recogniz- decipher the sequence of events that created such a geologic able characteristics that make each distinguishable from adjacent Bed that tapers tio n L im e s to ne Ju n c e r G u lc h Leet m S k in n F Lee v il le F m t Ju H a m li n nct nd Fm Fm ion kla r C it y Bir Fm Fo s te Fm n to n nd Fm L a rs o B ir k la e Dik i te G ra n rg F m u Ta r b L u Contact tg ra Contact metamorphosed d metamorphosed zone F zone m FIGURE 8.1 Block diagram representing the Minor Canyon area. (Tilted layers that are exposed in the canyon and are younger than the Leet Junction Formation are not named because they are not discussed or part of the figures that follow.) A key to the symbols representing rock types can be found on the page facing the inside back cover of this book.  Time and Geology 181 rock units. They are named after local geographic features, such be there before the next layer can be deposited on top of it. as towns or landmarks. Grand Canyon’s formation names are The principle of superposition also applies to layers formed by shown on the chapter’s opening photo.) The symbols in this dia- multiple lava flows, where one lava flow is superposed on a gram represent different rock types. For example, the dashes in previously solidified flow. the Foster City Fm represent shale. A key to these symbols is Applying the principle of superposition, we can deter- found on the page facing the inside front cover of this book. Note mine that the Skinner Gulch Limestone is the youngest layer the contacts between the tilted formations, the horizontal forma- of sedimentary rock in the Minor Canyon area. The Hamlin- tions, the granite, and the dike. What sequence of events might ville Formation is the next oldest formation, and the Larsonton be responsible for the geology of Minor Canyon? (You might Formation is the oldest of the still-horizontal sedimentary rock briefly study the block diagram and see how much of the geo- units. Similarly, we assume that the inclined layers were origi- logic history of the area you can decipher before reading further.) nally horizontal (by the first principle). By mentally restoring Our interpretations are based mainly on layered rock (sedi- them to their horizontal position (or “untilting” them), we can mentary or volcanic). The subdiscipline of geology that uses see that the youngest formation of the sequence is the Leet interrelationships between layered rock (mostly) or sediment to Junction Formation and that the Tarburg, Birkland, and Lutgrad interpret the history of an area or region is known as stratigraphy Formations are progressively older. (from the Latin word stratum, meaning a thing spread out, or a cover). Four of stratigraphy’s principles are used to determine Lateral Continuity the geologic history of a locality or a region. These are the prin- The principle of lateral continuity states that an original sedi- ciples of (1) original horizontality, (2) superposition, (3) lateral mentary layer extends laterally until it tapers or thins at its continuity, and (4) cross-cutting relationships. These principles edges. This is what we expect at the edges of a depositional will be used in interpreting figure 8.1. environment, or where one type of sediment interfingers later- ally with another type of sediment as environments change. In Original Horizontality figure 8.1, the bottom bed of the Hamlinville Formation (repre- The principle of original horizontality (as described in chapter sented by red dots), tapers as we would expect from this princi- 6) states that beds of sediment deposited in water formed as hor- ple. We are not seeing any other layers taper, either because we izontal or nearly horizontal layers. (The sedimentary rocks in are not seeing their full extent within the diagram or because figure 8.1 were originally deposited in a marine environment.) they have been truncated (cut off abruptly) due to later events. Note in figure 8.1 that the Larsonton Formation and overly- ing rock units (Foster City Formation, Hamlinville Formation, and Cross-Cutting Relationships Skinner Gulch Limestone) are horizontal. Evidently, their origi- The fourth principle can be applied to determine the remain- nal horizontal attitude has not changed since they were deposited. ing age relationships at Minor Canyon. The principle of cross- However, the Lutgrad, Birkland, Tarburg, and Leet Junction For- cutting relationships states that a disrupted pattern is older mations must have been tilted after they were deposited as hori- than the cause of disruption. A layer cake (the pattern) has to zontal layers. By applying the principle of original horizontality, be baked (established) before it can be sliced (the disruption). we have determined that a geologic event—tilting of bedrock— To apply this principle, look for disruptions in patterns of occurred after the Leet Junction, Tarburg, Birkland, and Lutgrad rock. Note that the valley in figure 8.1 is carved into the hori- Formations were deposited on a sea floor. We can also see that the zontal rocks as well as into the underlying tilted rocks. The sed- tilting event did not affect the Larsonton and overlying formations. imentary beds on either side of the valley appear to have been (A reasonable conclusion is that tilting was accompanied by uplift sliced off, or truncated, by the valley. (The principle of lateral and erosion, all before renewed deposition of younger sediment.) continuity tells us that sedimentary beds normally become thin- ner toward the edges rather than stop abruptly.) So the event Superposition that caused the valley must have come after the sedimentation The principle of superposition states that within a sequence of responsible for deposition of the Skinner Gulch Limestone and undisturbed sedimentary or volcanic rocks, the oldest layer is at the underlying formations. That is, the valley is younger than these bottom and layers are progressively younger upward in the stack. layers. We can apply the principle of cross-cutting relationships Obviously, if sedimentary rock is formed by sediment set- to contacts elsewhere in figure 8.1, with the results shown in tling onto the sea floor, then the first (or bottom) layer must table 8.1. TABLE 8.1 Relative Ages of Features in Figure 8.1 Determinable by Cross-Cutting Relationships Feature Is Younger Than But Older Than Valley (canyon) Skinner Gulch Limestone Foster City Formation Dike Hamlinville Formation Dike Larsonton Formation Foster City Formation Larsonton Formation Leet Junction Formation and granite Dike Granite Tarburg Formation Larsonton Formation 182 CHAPTER 8 Water o n Fm Juncti Leet r g Fm Tarbu Sea floor FIGURE 8.2 m nd F Birkla The area during deposition of the initial sedimentary layer of the Lutgrad Formation. m rad F Lutg We can now describe the geological history of the Minor Canyon area represented in figure 8.1 on the basis of what we have learned through applying the principles. Figures 8.2 FIGURE 8.3 through 8.11 show how the area changed over time, progress- The area after deposition of the four formations shown but before intrusion of the ing from oldest to youngest events. granite. By superposition, we know that the Lutgrad Formation, the lowermost rock unit in the tilted sequence, must be the oldest of the sedimentary rocks as well as the oldest rock unit in the diagram. From the principle of original horizontality, we infer that these layers must have been tilted after they formed. Figure 8.2 shows initial sedimentation of the Lutgrad Formation taking place. If the entire depositional basin were shown, the layer would be tapered at its edges, according to the o n Fm principle of lateral continuity. Juncti Leet Superposition indicates that the Birkland Formation was deposited on top of the Lutgrad Formation. Deposition of the Tarburg and Leet Junction Formations followed in turn (figure 8.3). The truncation of bedding in the Lutgrad, Birkland, and r g Fm Tarbu Tarburg Formations by the granite tells us that the granite intruded sometime after the Tarburg Formation was formed (this is an intrusive contact). Although figure 8.4 shows that the granite was emplaced before tilting of the layered rock, we cannot determine from looking at figure 8.1 whether the granite intruded the sedimentary rocks before or after tilting. We can, m nd F however, determine through cross-cutting relationships that Birkla tilting and intrusion of the granite occurred before deposition of the Larsonton Formation. Figure 8.5 shows the rocks in the area rad F m have been tilted and erosion has taken place. Sometime later, Lutg sedimentation was renewed, and the lowermost layer of the Contact metamorphosed zone Larsonton Formation was deposited on the erosion surface, as shown in figure 8.6. Contacts representing buried erosion sur- FIGURE 8.4 faces such as these are called unconformities and are discussed The area before layers were tilted and after intrusion of granite, if the intrusion took in more detail in the Unconformities section of this chapter. place before tilting.  Time and Geology 183 Rock removed Larsonton Water Sea floor by erosion sediment Previous land surface Erosion surface Granite Granite FIGURE 8.6 FIGURE 8.5 The area at the time the Larsonton Formation was being deposited. The area before deposition of the Larsonton Formation. Dashed lines show rock probably lost through erosion. Rock later removed Rock later removed Larsonton Fm by erosion by erosion Larsonton Fm ? Dike FIGURE 8.7 FIGURE 8.8 Area before intrusion of dike. Thickness of layers above the Larsonton Formation Dike intruded into the Larsonton Formation and preexisting, overlying layers of is indeterminate. indeterminate thickness. After the Larsonton Formation was deposited, an unknown some rocks that are no longer present, as shown in figure 8.8. additional thickness of sedimentary layers was deposited, as Figure 8.9 shows the area after the erosion that truncated the shown in figure 8.7. This can be determined through applica- dike took place. tion of cross-cutting relationships. The dike is truncated by the Once again, sedimentation took place as the lowermost Foster City Formation; therefore, it must have extended into layer of the Foster City Formation blanketed the erosion surface 184 CHAPTER 8 Dike exposed Sediment for Sea floor Water on surface Foster City Fm Larsonton Fm Dike FIGURE 8.9 The area after rock overlying the Larsonton Formation, along with part of the dike, was removed by erosion. Dike FIGURE 8.10 Sediment being deposited that will become part of the Foster City Formation. (figure 8.10). Sedimentation continued until the uppermost layer (top of the Skinner Gulch Limestone) was deposited. At some later time, the area was raised above sea level, and the stream began to carve the canyon (figure 8.11). Because can be observed between the Leet Junction Formation and the valley sides truncated the youngest layers of rock, we can the granite, we cannot say whether the granite is younger or determine from figure 8.1 that the last event was the carving of older than the Leet Junction Formation. Nor, as mentioned the valley. earlier, can we determine whether the granite formed before, Note that there are limits on how precisely we can during, or after the tilting of the lower sequence of sedimen- determine the relative age of the granite body. It definitely tary rocks. intruded before the Larsonton Formation was deposited and Now, if you take another look at the chapter opening after the Tarburg Formation was deposited. As no contacts photo of the Grand Canyon (and figure 8.16), you should be Stream Skinner Gulch Limestone Hamlinville Fm m ity F er C Fo s t n Fm onto Lars Tarburg Fm FIGURE 8.11 The same area after all of the rocks had formed and then had risen above sea level. The stream is beginning to form the Dike valley visible in figure 8.1.  Time and Geology 185 able to determine the sequence of events. The sequence (going overlying the granite has granite pebbles in it. Therefore, the from older to younger) is as follows: Regional metamorphism granite is older than the horizontal sedimentary rock. took place resulting in the Vishnu Schist of the lower part of the Grand Canyon (you cannot tell these are schists from the photograph). Erosion followed and leveled the land sur- Unconformities face. Sedimentation followed, resulting in the Grand Canyon In this and earlier chapters, we noted the importance of con- Series rocks. These sedimentary layers were subsequently tacts for deciphering the geologic history of an area. In chap- tilted (they were also faulted, although this is not evident in ters 3 and 6 we described intrusive contacts and sedimentary the photograph). Once again, erosion took place. The lower- contacts. Faults (described in chapter 15) are a third type of most of the presently horizontal layers of sedimentary rock contact. The final important type of contact is an unconformity. was deposited (the Tapeats Sandstone followed by the Bright Each type of contact has a very different implication about Angel Shale). Subsequently, each of the layers progressively what took place in the geologic past. higher up the sequence formed. Finally, the stream (the Col- An unconformity is a surface (or contact) that represents orado River) eroded its way through the rock, carving the a gap in the geologic record, with the rock unit immediately Grand Canyon. above the contact being considerably younger than the rock beneath. Most unconformities are buried erosion surfaces. Other Time Relationships Unconformities are classified into three types—disconformities, Other characteristics of geology can be applied to help deter- angular unconformities, and nonconformities—with each type mine relative ages (figure 8.12). The tilted layers in figure 8.12 having important implications for the geologic history of the immediately adjacent to the granite body have been contact area in which it occurs. metamorphosed (think “seared” or “baked”). This indicates that the Tarburg Formation and older formations shown in Disconformities figure 8.1 had to be there before intrusion of the hot, granite In a disconformity, the contact representing missing rock strata magma. The base of the Larsonton Formation in contact with separates beds that are parallel to one another. Probably what the granite would not be contact metamorphosed because it was has happened is that older rocks were eroded away parallel to deposited after the granite had cooled (and had been exposed the horizontal bedding plane; renewed deposition later buried by erosion). the erosion surface (figure 8.13). The principle of inclusion states that fragments included Because it often appears to be just another sedimentary in a host rock are older than the host rock. In figure 8.12, the contact (or bedding plane) in a sequence of sedimentary rock, granite contains inclusions of the tilted sedimentary rock. a disconformity is the hardest type of unconformity to detect in Therefore, the granite is younger than the tilted rock. The rock the field. Rarely, a telltale weathered zone is preserved immedi- ately below a disconformity. Usually, the disconformity can be detected only by studying fossils from the beds in a sequence Pebbles of granite Earth’s surface Sequence of sedimentary Sequence shows a break rock with complete record in the record as indicated of deposition by correlatable fossils Disconformity Inclusion in granite Granite (xenolith) Dashed lines indicate correlation of rock units between the Contact metamorphosed zone two areas FIGURE 8.12 FIGURE 8.13 Age relationships indicated by contact metamorphism, inclusions (xenoliths) in Schematic representation of a disconformity. The disconformity is in the block on granite, and pebbles of granite. the right. 186 CHAPTER 8 of sedimentary rocks. If certain fossil beds are absent, indicat- youngest: (1) deposition and lithification of sedimentary rock ing that a portion of geologic time is missing from the sedimen- (or solidification of successive lava flows if the rock is volca- tary record, it can be inferred that a disconformity is present nic); (2) uplift accompanied by folding or tilting of the layers; in the sequence. Although it is most likely that some rock lay- (3) erosion; and (4) renewed deposition (usually preceded by ers are missing because erosion followed deposition, in some subsidence) on top of the erosion surface (figure 8.14). Fig- instances neither erosion nor deposition took place for a sig- ures 8.1 and 8.12 also show angular unconformities but with nificant amount of geologic time. simple tilting rather than folding of the older beds. Angular Unconformities Nonconformities An angular unconformity is a contact in which younger strata A nonconformity is a contact in which an erosion surface on overlie an erosion surface on tilted or folded layered rock. plutonic or metamorphic rock has been covered by younger It implies the following sequence of events, from oldest to sedimentary or volcanic rock (figure 8.15). A nonconformity generally indicates deep or long-continued erosion before sub- sequent burial, because metamorphic or plutonic rocks form at Sea level considerable depths in Earth’s crust. The geologic history implied by a nonconformity, shown in figure 8.15, is (1) crystallization of igneous or metamorphic rock at depth; (2) erosion of at least several kilometers of overlying rock (the great amount of erosion further implies considerable uplift of this portion of Earth’s crust); and (3) deposition of new A Sedimentation sediment, which eventually becomes sedimentary rock, on the B Folding Erosion surface C Erosion Angular conformity Younger horizontal beds Sea level E New layers Rock debris eroded from of sediment above covers red beds Tilted older Angular red beds unconformity D Renewed deposition of sediment Geologist’s View FIGURE 8.14 A particular sequence of events (A–D) producing an angular unconformity. Marine deposited sediments are uplifted and folded (probably during plate-tectonic convergence). Erosion removes the upper layers. The area drops below sea level (or sea level rises) and renewed sedimentation takes place. (An angular unconformity can also involve terrestrial sedimentation.) (E) is an angular unconformity at Cody, Wyoming. Photo by C. C. Plummer  Time and Geology 187 Sea level Pluton Metamorphosed A Sedimentation B Deep burial rock C During mountain-building Erosion Part eroded away episode: Intense deformation, surface intrusion of a pluton, and metamorphism of lower rocks D Uplift Plutonic rock accompanied by erosion Paleozoic sedimentary rock Erosion surface Nonconformity E Continued erosion Sea level Precambrian metamorphic rock Nonconformity F Renewed G deposition FIGURE 8.15 (A–F) Sequence of events implied by a nonconformity underlain by metamorphic and plutonic rock. (G) A nonconformity in Grand Canyon, Arizona. Paleozoic sedimentary rocks overlie vertically foliated Precambrian metamorphic rocks. Photo by C. C. Plummer ancient erosion surface. Figures 8.1 and 8.12 also show noncon- a region, a continent, and even between continents. Various formities; however, these represent erosion to a relatively shallow methods of correlation are described along with examples of depth as the rocks intruded by the pluton have not been region- how the principles we described earlier in this chapter are used ally metamorphosed, as was the case for those in figure 8.15. to determine whether rocks in one area are older or younger than rocks in another area. Correlation Physical Continuity In geology, correlation usually means determining time Finding physical continuity—that is, being able to trace phys- equivalency of rock units. Rock units may be correlated within ically the course of a rock unit—is one way to correlate rocks 188 CHAPTER 8 Coconino Sandstone Navajo Sandstone Navajo Sandstone 0 50 100 Km ZION AREA Bright Coconino Vishnu Angel GRAND Sandstone Schist Shale CANYON FIGURE 8.16 Schematic cross section through part of the Colorado Plateau showing the relationship of the Coconino Sandstone, the white cliff-forming unit in the left photo, in Grand Canyon to the Navajo Sandstone, white unit in the right photo, at Zion National Park. Photos by C. C. Plummer between two different places. The prominent white layer of Cross-bedding indicates that both were once a series of sand cliff-forming rock in figure 8.16 is the Coconino Sandstone, dunes. It is tempting to correlate them and conclude that both exposed along the upper part of the Grand Canyon. It can be formed at the same time. But if you were to drive or walk from seen all the way across the photograph. You can physically fol- the rim of the Grand Canyon (where the Coconino Sandstone low this unit for several tens of kilometers, thus verifying that, is below you), you would get to Zion by ascending a series of wherever it is exposed in the Grand Canyon, it is the same rock layers of sedimentary rock stacked on one another. In other unit. The Grand Canyon is an ideal location for correlating rock words, you would be getting into progressively younger rock, units by physical continuity. However, it is not possible to fol- as shown diagrammatically in figure 8.16. In short, you have low this rock unit from the Grand Canyon into another region shown through superposition that the sandstone in Zion (called because it is not continuously exposed. We usually must use the Navajo Sandstone) is younger than the Coconino Sandstone. other methods to correlate rock units between regions. Correlation by similarity of rock types is more reliable if a very unusual sequence of rocks is involved. If you find in one area Similarity of Rock Types a layer of green shale on top of a red sandstone that, in turn, over- Under some circumstances, correlation between two regions lies basalt of a former lava flow and then find the same sequence can be made by assuming that similar rock types in two regions in another area, you probably would be correct in concluding that formed at the same time. This method must be used with the two sequences formed at essentially the same time. extreme caution, especially if the rocks being correlated are When the hypothesis of continental drift was first proposed common ones. (see chapters 1 and 19), important evidence was provided by To show why correlation by similarity of rock type does not correlating a sequence of rocks (figure 8.17) consisting of gla- always work, we can try to correlate the white, cliff-forming cially deposited sedimentary rock (tillites, described in chapter Coconino Sandstone in the Grand Canyon with a rock unit 12 on glaciation), overlain by continental sandstones, shales, of similar appearance in Zion National Park about 100 kilo- and coal beds. These strata are in turn overlain by basalt flows. meters away (figure 8.16). Both units are white sandstone. The sequence is found in parts of South America, Australia,  Time and Geology 189 Basalt flows layers anywhere in the world can be assigned to their correct (early Mesozoic) place in geologic history by identifying the fossils they contain. Ideally, a geologist hopes to find an index fossil, a fos- sil from a very short-lived, geographically widespread species Continental known to exist during a specific period of geologic time. A single sandstones, Glossopteris shales, and fossils index fossil allows the geologist to correlate the rock in which it coal beds is found with all other rock layers that contain that fossil. Many fossils are of little use in time determination because Tillites the species thrived during too large a portion of geologic time. (late Paleozoic) Sharks, for instance, have been in the oceans for a long time, so FIGURE 8.17 discovering an ordinary shark’s tooth in a rock is not very help- Rock sequences similar to this are found in India, Africa, South America, Australia, ful in determining the rock’s relative age. and Antarctica. The rocks in each of these localities contain the fossil plant A single fossil that is not an index fossil is not very useful Glossopteris. for determining the age of the rock it is in. However, finding several species of fossils (a fossil assemblage) in a layer of rock Africa, Antarctica, and India. It is very unlikely that an identi- is generally more useful for dating rocks than a single fossil is, cal sequence of rocks could have formed on each of the con- because the sediment must have been deposited at a time when tinents if they were widely separated, as they are at present. all the species represented existed. Figure 8.18 depicts five spe- Therefore, the continents on which the sequence is found are cies of fossils, each of which existed over a long time span. likely to have been part of a single supercontinent on which Where various combinations of these fossils are found in three the rocks were deposited. Fossils found in these rocks further rocks, the time of formation of each rock can be assigned to a strengthened the correlation. narrow span of time. In some regions, a key bed, a very distinctive layer, can be Some fossils are restricted in geographic occurrence, rep- used to correlate rocks over great distances. An example is a resenting organisms adapted to special environments. But many layer of volcanic ash produced from a very large eruption and former organisms apparently lived over most of the Earth, and distributed over a significant portion of a continent. fossil assemblages from these may be used for worldwide cor- relation. Fossils in the lowermost horizontal layers of the Grand Correlation by Fossils Canyon are comparable to ones collected in Wales, Great Britain, Fossils are common in sedimentary rock, and their presence is and many other places in the world (the trilobites in figure 8.19 important for correlation. Plants and animals that lived at the are an example). We can, therefore, correlate these rock units and time the rock formed were buried by sediment, and their fossil say they formed during the same general span of geologic time. remains are preserved in sedimentary rock. Most of the fos- sil species found in rock layers are now extinct—99.9% of all species that ever lived are extinct. (The concept of species for Time intervals fossils is similar to that in biology.) over which species existed First area In a thick sequence of sedimentary rock layers, the fossils nearer the bottom (that is, in the older rock) are more unlike Second area Younger today’s plants and animals than are those near the top. As early 3 Z as the end of the eighteenth century, naturalists realized that the fossil remains of creatures of a series of “former worlds” were preserved in Earth’s sedimentary rock layers. In the early nineteenth century, a self-educated English surveyor named TIME William Smith realized that different sedimentary layers are 2 Y Z characterized by distinctive fossil species and that fossil spe- cies succeed one another through the layers in a predictable 1 X X order. Smith’s discovery of this principle of faunal succession Older allowed rock layers in different places to be correlated based on their fossils. We now understand that faunal succession works Disconformity because there is an evolutionary history to life on Earth. Species evolve, exist for a time, and go extinct. Because the same spe- FIGURE 8.18 cies never evolves twice (extinction is forever), any period of The use of fossil assemblages for determining relative ages. time in Earth history can be identified by the species that lived Rock X contains. Therefore, it must have formed during time interval 1. at that time. Paleontologists, specialists in the study of fos- Rock Y contains. Therefore, it must have formed during time interval 2. sils, have patiently and meticulously over the years identified Rock Z contains. Therefore, it must have formed during time interval 3. many thousands of species of fossils and determined the time In the second area, fossils of time interval 2 are missing. Therefore, the surface sequence in which they existed. Therefore, sedimentary rock between X and Z is a disconformity. 190 CHAPTER 8 TABLE 8.2 Geologic Time Scale Eon Era Period Epoch Quaternary Holocene (Recent) Pleistocene Neogene** Pliocene Cenozoic Miocene Oligocene Paleogene** Eocene Paleocene Cretaceous Phanerozoic Mesozoic Jurassic Triassic Permian Pennsylvanian FIGURE 8.19 Elrathia kingii trilobites from the Middle Cambrian Wheeler Formation of Utah. The Paleozoic Mississippian } Carboniferous* Devonian larger one is 10 mm in diameter. Photo by Robert R. Gaines Silurian Ordovician Cambrian The Standard Geologic Time Scale Precambrian Time (consists of the Proterozoic, Archean, Geologists can use fossils in rock to refer the age of the rock and Hadean Eons) to the standard geologic time scale, a worldwide relative time *Outside of North America, Carboniferous Period is used rather than scale. Based on fossil assemblages, the geologic time scale subdi- Pennsylvanian and Mississippian. vides geologic time. On the basis of fossils found, a geologist can **In 2003, the International Commission on Stratigraphy recommended say, for instance, that the rocks of the lower portion of horizontal dropping Tertiary and Quaternary as periods and replacing them with layers in the Grand Canyon formed during the Cambrian Period. Paleogene and Neogene. This proposal caused some controversy, and in This implicitly correlates these rocks with certain rocks in Wales 2009 the commission decided to replace the Tertiary with the Paleogene and Neogene while retaining the Quaternary as a separate era. Currently, (in fact, the period takes its name from Cambria, the Latin name the Geological Society of America annually updates the geologic time for Wales) and elsewhere in the world where similar fossils occur. scale and posts it on www.geosociety.org/science/timescale/ The geologic time scale, shown in a somewhat abbrevi- ated form in table 8.2, has had tremendous significance as a unifying concept in the physical and biological sciences. The working out of the evolutionary chronology by successive gen- erations of geologists and other scientists has been a remark- It is noteworthy that the fossil record indicates mass able human achievement. The geologic time scale consists of extinctions, in which a large number of species became extinct, four eons. The Phanerozoic Eon (meaning “visible life”) is occurred a number of times in the geologic past. The two great- divided into three eras, which are divided into periods, which est mass extinctions define the boundaries between the three are, in turn, subdivided into epochs. (Remember that this is a eras (see boxes 8.1 and 8.2). relative time scale. We will add dates later in this chapter after Fossils have been used to determine ages of the horizontal discussing isotopic dating.) rocks in the Grand Canyon. All are Paleozoic. The lowermost Precambrian denotes the vast amount of time that preceded horizontal formations (chapter opening photo) are Cambrian, the Paleozoic Era (which begins with the Cambrian Period). The above which are Devonian, Mississippian, Pennsylvanian, and Paleozoic Era (meaning “old life”) began with the appearance Permian rock units. By referring to the geologic time scale of complex life (trilobites, for example), as indicated by fossils. (table 8.2), we can see that Ordovician and Silurian rocks Rocks older than Paleozoic contain few fossils. This is because are not represented. Thus, an unconformity (a disconformity) creatures with shells or other hard parts, which are easily preserved is present within the horizontally layered rocks of the Grand as fossils, did not evolve until the beginning of the Paleozoic. Canyon. The Mesozoic Era (meaning “middle life”) followed the Paleozoic. On land, dinosaurs became the dominant animals of the Mesozoic. We live in the Holocene (or Recent) Epoch of NUMERICAL AGE the Quaternary Period of the Cenozoic Era (meaning “new life”). The Quaternary also includes the most recent ice ages, Counting annual growth rings in a tree trunk will tell you which were part of the Pleistocene Epoch. how old a tree is. Similarly, layers of sediment deposited  Time and Geology 191 E A R T H S Y S T E M S 8.1 Highlights of the Evolution of Life through Time T he following is a very condensed preview of what you are likely to learn about if you take a historical geology course. The history of the biosphere is preserved in the fossil record. The Paleozoic ended with the greatest mass extinction ever to occur on Earth. Around 80% of marine species died out as the Permian Period ended. Through fossils, we can determine their place in the evolution of During the Mesozoic, new creatures evolved to occupy eco- plants and animals as well as get clues as to how extinct creatures logical domains left vacant by extinct creatures. Dinosaurs and lived. The oldest readily identifiable fossils found are prokaryotes— mammals evolved from the animal species that survived the great microscopic, single-celled organisms that lack a nucleus. These extinction. Dinosaurs became the dominant group of land animals. date back to around 3.5 billion years (b.y.) ago, so life on Earth is Birds likely evolved from dinosaurs in the Mesozoic. Large, now at least that old. It is likely that even more primitive organisms date extinct, marine reptiles lived in Mesozoic seas. Ichthyosaurs, for back further in time but are not preserved in the fossil record. Fos- example, were up to 20 meters long, had dolphinlike bodies, and sils of much more complex, single-celled organisms that contained were probably fast swimmers. Flying reptiles, pterosaurs, some of a nucleus (eukaryotes) are found in rocks as old as 1.4 b.y. These which had wingspans of almost 10 meters, soared through the air. are the earliest living creatures to have reproduced sexually. Colo- The Cretaceous Period (and Mesozoic Era) ended with the sec- nies of unicellular organisms likely evolved into multicellular organ- ond-largest mass extinction (around 75% of species were wiped out). isms. Multicellular algae fossils date back at least a billion years. The Cenozoic is often called the age of mammals. Mammals, Imprints of larger multicellular creatures appear in rocks of late which were small, insignificant creatures during the Mesozoic, Precambrian age, about 700 to 550 million years ago (m.y.). These evolved into the many groups of mammals (whales, bats, canines, resemble jellyfish and worms. cats, elephants, primates, and so forth) that occupy Earth at present. Abundant fossil evidence of life does not appear until the Many species of mammals evolved and became extinct throughout beginning of the Paleozoic era, 541 million years ago. Large num- the Cenozoic. Hominids (modern humans and our extinct ancestors) bers of fossils appeared early in the Cambrian Period. Trilobites have a fossil record dating back 6 m.y. and likely evolved from a now (see figure 8.19) evolved into many species and were particularly extinct ancestor common to hominids, chimpanzees, and other apes. abundant during the Cambrian. Trilobites were arthropods that We tend to think of mammals’ evolution as being the great suc- crawled on muddy sea floors and are the oldest fossils with eyes. cess story (because we are mammals); mammals, however, pale in They became less significant later in the Paleozoic, and finally, all comparison to insects. Insects have been around far longer than trilobites became extinct by the end of the Paleozoic. mammals and now account for an estimated 1 million species. The most primitive fishes, the first vertebrates, date back to late in the Cambrian. Fishes similar to presently living species Additional Resources (including sharks) flourished during the Devonian (named after University of California Museum of Paleontology Devonshire, England). The Devonian is often called the “age of Find pictures of the fossils named in this box. fishes.” Amphibians evolved from air-breathing fishes late in the www.ucmp.berkeley.edu/ Devonian. These were the first land vertebrates. However, inver- tebrate land animals date back to the latest Cambrian, and land The Paleontology Portal plants first appeared in the Ordovician. Reptiles and early ances- Another site to find out about fossils. You can search by type tors of mammals evolved from amphibians in Pennsylvanian time of creature, by time, or by location. or perhaps earlier. www.paleoportal.org/ annually in glacial lakes can be counted to determine how In 2008, a rock from Hudson Bay in northern Canada long those lakes existed (varves, as these deposits are called, (its location is indicated on the inside front cover) was dated are explained in chapter 12). But only within the few decades as being 4.28 billion years old. This rock is nearly 300 mil- following the discovery of radioactivity in 1896 have scien- lion years older than the previously dated oldest rock (age tists been able to determine numerical ages of rock units. We 4.03 billion years old). However, the age of 4.28 billion years have subsequently been able to assign numerical values to has been challenged, with some suggesting that an age of 3.8 the geologic time scale and determine how many years ago billion is more likely. In 2014, the oldest known mineral was the various eras, periods, and epochs began and ended: The dated at 4.4 billion years old, which is considerably older than Cenozoic Era began some 66 million years ago, the Meso- the oldest rock dated so far. The mineral, a zircon crystal from zoic Era started about 252 million years ago, and the Pre- Australia, was likely originally in a granite. Scientists who have cambrian ended (or the Paleozoic began) about 541 million studied this mineral think that its chemical makeup indicates years ago. The Precambrian includes most of geologic time, that the granite formed from a magma that had a component of because the age of Earth is commonly regarded as about 4.5 melted sedimentary rock. This would indicate that seas existed to 4.6 billion years. much earlier than geologists had previously thought possible.