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geologic time v olcanic eruptions glaciations tectonic collisions and mountain building have all occurred many times in various places on earth what information do we have today that tells us of these events the rocks themselves it has been said that a picture tells a thousand words the photograph at right is worth many more than that recorded in the rock layers is evidence of geologic events that have helped to shape earth s history along with these dramatic events countless species of plants and animals have appeared and disappeared fossils of these organisms are evidence of these occurrences by studying the characteristics of layered rocks and the changes in life through time geologists have been able to unravel earth s history unit contents 21 fossils and the rock record 22 the precambrian earth 23 the paleozoic era 24 the mesozoic and cenozoic eras go to the national geographic expedition on page 892 to learn more about topics that are connected to this unit 550

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capital reef national park utah 551

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21 what you ll learn · how geologists divide earth s long history · how certain geologic principles can be used to interpret age relations in layered rocks · how different techniques to determine the ages of rocks are used · what fossils are how they form and how they are used to interpret earth s history why it s important fossils and rocks contain a record of earth s history and can be used to make predictions about earth s future some fossils can help identify potential sites of energy resources fossils and the rock record to find out more about fossils and the rock record visit the glencoe science web site at science.glencoe.com 552

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discovery lab have you ever found shells at a beach along a river or by a pond if so did you wonder where they came from or what type of animal might have lived in them the shape size and composition of shells provide clues about the environment in which individual animals once lived in this activity you will make inferences about shells that you examine 1 obtain a mixture of sand and microfossils from your teacher 2 place the mixture on a petri dish or a small shallow tray 3 use tweezers or a small dry paintbrush to separate the fossils from the sandy sediment fossil hunt activity 4 categorize the fossils by shape size and composition caution always wear safety goggles and an apron in the lab wash your hands when lab is completed observe in your science journal explain how fossils can help determine the age of sediment or a rock does categorizing the fossils provide any further clues about the environment in which the fossiliferous sediment formed explain 21.1 objectives the geologic time scale a hike down the kaibab trail in the grand canyon reveals the multicolored layers of rock that make up the canyon walls these layers or strata are made of different types of sedimentary rock some of the rock layers have fossils in them at the bottom of the grand canyon is the colorado river which has been cutting downward through the rocks of the canyon for millions of years also at the bottom are rocks that date back 400 million years or more these rocks record the many advances and retreats of oceans and the development of plants and animals by studying the characteristics of rocks and the fossils within them geologists can interpret the environments the rocks were deposited in reconstruct earth s history and possibly predict events or conditions in the future · describe the geologic time scale · distinguish among the following geologic time scale divisions eon era period and epoch vocabulary geologic time scale eon era period epoch the rock record to help in the analysis of earth s rocks geologists have divided the history of earth into time units based upon the fossils contained 21.1 the geologic time scale 553

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phanerozoic eon cenozoic era homo sapiens evolves most recent ice ages occur grand canyon forms quaternary period 1.6 m.y.b.p recent holocene epoch 0.01 m.y.b.p pleistocene epoch 1.6 m.y.b.p neogene period 23 m.y.b.p pliocene epoch 5 m.y.b.p miocene epoch 23 m.y.b.p paleogene period 66 m.y.b.p oligocene epoch 35 m.y.b.p eocene epoch 56 m.y.b.p paleocene epoch 66 m.y.b.p mammals are abundant angiosperms are dominant alps and the himalayas begin to rise dinosaurs become extinct dinosaurs are dominant first birds appear mountain building continues in western north america many marine invertebrates become extinct building of appalachians ends glaciers retreat amphibians are dominant glacial advances occur corals and other invertebrates are dominant warm shallow seas cover much of north america trilobites brachiopods other marine invertebrates are abundant thick sediments deposited in inland seas mesozoic era cretaceous period 146 m.y.b.p jurassic period 208 m.y.b.p triassic period 245 m.y.b.p angiosperms appear rocky mountains begin to form first mammals and cycads appear atlantic ocean begins to form pangaea breaks up reptiles evolve coal swamps form shallow seas begin to withdraw fish are dominant first amphibians appalachians continue to form in north america and europe first land plants form first insects first fish appear appalachians begin to form ediacara organisms develop bacteria-like organisms form several episodes of mountain building occur within the rocks these time units are part of the geologic time scale a record of earth s history from its origin 4.6 billion years ago to the present since the naming of the first geologic time period the jurassic in 1797 development of the time scale has continued to the present the names of the periods do not change but the years marking the beginning and end of each unit of time are continually being refined the geologic time scale is shown in figure 21-1 this scale enables scientists from around the world to correlate the geologic events environmental changes and the development of life-forms that are preserved in the rock record geologic time the oldest division of time is at the bottom of the geologic time scale moving upward on the scale each division is younger just as the rock layers in the rock record grow generally younger as you move upward the time scale is divided into units called eons eras periods and epochs an eon is the longest time unit and is measured in billions of years the archean the proterozoic and the phanerozoic are eons an era is the next-longest span of time it is measured in hundreds of millions to billions of years eras are defined by the differences in life-forms found in rocks the names of eras are based on the relative ages of these lifeforms for example in greek paleo means old meso means middle and ceno means recent zoic means of life in greek and thus mesozoic means middle life precambrian time which makes up approximately 90 percent of geologic time is divided into the archean and proterozoic eons the proterozoic is the more recent of the two and the end of it is marked by the first appearance of organisms with hard parts all life-forms up until then had soft bodies and no shells or skeletons some of paleozoic era permian period 290 m.y.b.p pennsylvanian period 323 m.y.b.p mississippian period 362 m.y.b.p devonian period 408 m.y.b.p silurian period 439 m.y.b.p ordovician period 510 m.y.b.p cambrian period 540 m.y.b.p precambrian time proterozoic eon 2500 m.y.b.p millions of years before present archean eon 4600 m.y.b.p figure 21-1 earth s long history is divided into specific units of time in the geologic time scale 554 chapter 21 fossils and the rock record

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these resembled organisms that exist today such as sponges snails and worms while others cannot be accurately assigned to any known animal or plant group plants and animals evolve during the paleozoic era the oceans became full of a wide diversity of plants and animals trilobites dominated the oceans in the cambrian period land plants appeared and were followed by land animals and swamps provided the plant material that became the coal deposits of the pennsylvanian the end of the paleozoic era is marked by the largest extinction event in earth s history as many as 90 percent of all marine invertebrate species became extinct the era following the paleozoic era the mesozoic era is known for the emergence of dinosaurs but other important developments occurred then as well reef-building corals and large predatory reptiles developed in the oceans amphibians left the water for the land dinosaur populations began a slow decline in numbers throughout the cretaceous period as mammals evolved and grew in number flowering plants and trees evolved during the cretaceous the end of the mesozoic is also marked by a large extinction event in addition to the remaining dinosaurs many other groups of organisms became extinct mammals increased both in number and diversity in the cenozoic human ancestors developed at this time grasses and flowering plants expanded on land while ocean life remained relatively unchanged throughout this era periods of geologic time periods are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited periods are usually measured in terms of tens of millions of years to hundreds of millions of years some were named for the geographic region in which the rocks of that age were first observed studied and described for example the mississippian period was named for the distinctive limestone bluffs along the mississippi river as shown in figure 21-2 the jurassic period was named for the rocks that were described in the jura mountains in europe to learn more about dinosaurs and their evolution go to the national geographic expedition on page 892 to find out more about the geologic time scale visit the glencoe science web site at science.glencoe.com figure 21-2 these mississippianaged limestone bluffs border the mississippi river in iowa 21.1 the geologic time scale 555

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historically the cenozoic era was divided into two periods the tertiary and the quaternary currently however the cenozoic is divided into three periods the paleogene neogene and quaternary in contrast to the boundaries between the paleozoic and the mesozoic eras the boundaries between the periods of the cenozoic are not marked by extinction events epochs of geologic time epochs are even smaller divisions of geologic time and are usually measured in millions of years to tens of millions of years the fossil record of the cenozoic era is relatively complete because there has been less time for weathering and erosion to remove evidence of this part of earth s history thus rocks and fossils from this era are easily accessed and studied accordingly the cenozoic periods have been further divided into epochs such as the paleocene and the oligocene different groups of organisms have been used to distinguish the various epochs for example marine fossils were used to mark the oligocene epoch and terrestrial plant fossils such as those shown in figure 21-3 were used to mark the eocene epoch regardless of how a geologic period was defined each unit contains specific characteristics that set it apart from the rest of geologic history in the design your own geolab at the end of this chapter you will find out what makes each time unit unique figure 21-3 this fossil sycamore leaf is preserved in the eocene-aged green river formation in wyoming 1 what is the basis for the development of the geologic time scale 2 what does the geologic time scale indicate about the change in life-forms over time 3 what do the names of the three eras of the phanerozoic mean 4 what major change occurred in life-forms at the end of the proterozoic 5 how were the geologic time periods named on what basis are they defined 6 thinking critically explain why the use of living faunas is acceptable for defining the periods and epochs of the cenozoic era skill review 7 graphing make a bar graph that shows the relative percentages of time that each period of the geologic time scale spans for more help refer to the skill handbook 556 chapter 21 fossils and the rock record

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21.2 relative-age dating of rocks objectives as late as the turn of the nineteenth century the majority of the world believed that earth was only about 6000 years old this age had been determined by archbishop james ussher of ireland who used a chronology of human and earth history to calculate earth s age as early as 1770 james hutton a scottish physician and geologist had begun to observe and to attempt to explain earth s landscapes hutton s observations in great britain helped him to develop the principle of uniformitarianism which attempts to explain the forces that continually change the surface features of earth such processes include mountain building erosion earthquakes and sea-level changes the principle of uniformitarianism states that the processes occurring today have been occurring since earth formed only the rate intensity and scale with which they occur have changed for example if you stand on the shore of an ocean watching the waves come in you are observing a process that has not changed since the oceans were formed the waves crashing on a cambrian shore a jurassic shore and a modern shore all share the same process the resulting sediments and rocks all record a beach environment where the sediments become finer with distance from shore and the fossils within the rocks preserve evidence of the lifeforms that lived during the time of deposition · apply the principles for determining relative age to interpret rock sequences · describe an unconformity and how it is formed within the rock record vocabulary uniformitarianism original horizontality superposition cross-cutting relationships unconformity correlation principles for determining relative age the concept of relative-age dating places the ages of rocks and the events that formed them in order but without exact dates this is done by comparing one event or rock layer to another figure 21-4 the colorado river in grand canyon national park has cut through rock layers that span the triassic through the precambrian 557

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how is relative age determined demonstrate how the principles of superposition original horizontality and crosscutting relationships are used to determine the relative ages of rock layers procedure 1 draw a diagram of an outcrop with four horizontal layers label the layers 1 through 4 2 draw a vertical intrusion from layer 1 to layer 3 3 label the bottom-left corner of the diagram x and the top-right corner y 4 cut the paper diagonally from x to y move the left-hand piece 1.5 cm along the cut analyze and conclude 1 how can you determine the relative ages of the strata in your diagram 2 how does the principle of cross-cutting relationships explain the age of the vertical intrusion 3 what does line xy represent is line xy older or younger than the vertical intrusion and surrounding strata explain geologic principles many different horizontal or nearly horizontal layers of rocks make up the walls of the grand canyon shown in figure 21-4 most of the rocks are sedimentary and were originally deposited millions of years ago by water or wind the principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers while we may not know the actual ages of the rock layers we can assume that the oldest rocks are at the bottom and that each successive layer going toward the top is younger thus we can infer that the moenkopi formation which rims the top of the grand canyon is much younger than the vishnu group found at the bottom of the gorge as shown in figure 21-5 this is an application of the principle of superposition which states that in an undisturbed rock sequence the oldest rocks are at the bottom and each successive layer is younger than the layer beneath triassic moenkopi formation kaibab limestone permian coconino sandstone hermit shale pennsylvanian redwall limestone mississippian muav limestone cambrian bright angel shale tapeats sandstone figure 21-5 using the principle of superposition geologists have determined the relative ages of these rock layers zoroaster granite precambrian vishnu group 558 chapter 21 fossils and the rock record

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rocks exposed in the deepest part of the grand canyon are some of the oldest in north america these are mostly igneous and metamorphic rocks within the vishnu group at the bottom of the grand canyon sequence are dikes of granite the principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across therefore the granite is younger than the schist because the granite cuts across the schist in earthquake-prone areas such as california and in ancient mountainous regions such as the adirondacks of new york there are many faults as you learned in chapter 20 a fault is a fracture in earth along which movement takes place a fault is younger than the strata and surrounding geologic features because it cuts across them you used geologic principles to determine relative ages of rocks in the minilab on the previous page inclusions relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it the bottom layer was eroded and the loose material on the surface became incorporated in the newly deposited top layer these particles called inclusions indicate that the rocks in the lower layer are older than those on top as you learned in chapter 6 once a rock has been eroded the resulting sediment may be transported and redeposited and recemented many kilometers away in this case although this newly formed rock may be jurassic in age the grains that make up the rock may be cambrian in age another example of the use of eroded sediments to determine the relative ages of rocks is a cooled lava flow that has bedrock particles trapped within it an inclusion layer that is formed during a lava flow is illustrated in figure 21-6 other means of determining relative age the fact that earth is constantly changing as a result of processes such as weathering erosion earthquakes and volcanism makes it difficult to find an undisturbed sequence of rock layers for example if rocks that record a volcanic eruption or the last occurrence of a particular fossil are eroded away then the record of that particular a b figure 21-6 this pahoehoe lava flow in hawaii most likely contains pieces or inclusions of the aa lava flow beneath it a when lava or sediments are deposited on top of an eroded surface that contains loose fragments the fragments become incorporated as inclusions in the top layer b 21.2 relative-age dating of rocks 559

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event has been lost further changes may occur if the area is covered by a river during a flood or by the sea additionally an erosional surface might become buried by the deposition of younger rocks this buried erosional surface results in a gap in the rock record and is called an unconformity when horizontal sedimentary rocks overlie horizontal sedimentary rocks the unconformity is called a disconformity a different type of unconformity exists when sedimentary rocks overlie nonsedimentary rocks such as granite or marble such an unconformity suggests a possible uplifting of the marble or granite and exposure at the surface by weathering and erosion the contact point between the nonsedimentary and sedimentary rock is called a nonconformity the formation of unconformities are illustrated in figure 21-7 on the following page when horizontal sedimentary rocks are uplifted and tilted they are exposed to the processes of weathering and erosion when deposition resumes horizontal layers of sedimentary rocks are laid down on top of the erosional surface the layers beneath the eroded surface of the folded layers remain intact but they are at an angle to the eroded surface this type of unconformity is called an angular unconformity you will use several geologic principles to interpret the geologic history of an area in the problem-solving lab on the this page interpreting diagrams interpret the relative ages of rock layers use the diagram to answer the following questions n analysis 1 which is the oldest rock unit in the outcrop 2 an unconformity exists between which two layers of rock explain 3 what happened to the rock that came in contact with the molten material of the intruded dike 4 explain why the rock layers and features on the west side of the outcrop do not match the rock layers and features on the east side thinking critically 5 which is the younger feature in the outcrop the dike or the folded strata explain 6 list the order of geologic events represented by this outcrop which geologic principles did you use 560 chapter 21 fossils and the rock record

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b c sedimentary rock erosional surface erosional surface a igneous rock correlation of rock strata the permian kaibab formation rims the top of the grand canyon but is also found about 300 km away at the bottom of a 200-m gorge in capitol reef national park in utah how do geologists match rock layers such as these which are far apart from each other one method is by correlation correlation is the matching of outcrops of one geographic region to another geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers this information can be used to help in the exploration for oil or valuable minerals for example if a sandstone layer in one area contains oil it is possible that the same layer in a different area also contains oil correlation allows geologists to accurately locate that same sandstone layer in another location figure 21-7 this disconformity on san salvador island bahamas was formed by a soil that developed on top of a fossilized coral reef a a disconformity forms when a sedimentary rock layer is deposited on top of an eroded sedimentary rock layer b a nonconformity forms when a sedimentary rock layer is deposited on top of an eroded metamorphic or igneous rock layer c 1 how would a geologist use the principle of superposition to determine the relative ages of the rocks in the grand canyon 2 what is an unconformity 3 explain how inclusions at the base of a lava flow can help determine the relative age of the layers 4 a fault or a dike cuts across a sequence of rocks what does this suggest about the relative ages of the rocks 5 thinking critically explain how the principle of uniformitarianism is used to interpret earth s past skill review 6 interpreting data discuss how a sequence of strata can be correlated from one side of a canyon to another for more help refer to the skill handbook 21.2 relative-age dating of rocks 561

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21.3 objectives absolute-age dating of rocks as you have learned relative-age dating is a method of comparing past geologic events based on the observed order of strata in the rock record in contrast absolute-age dating enables scientists to determine the actual age of a rock fossil or other object scientists have devised a method for dating very old objects using the decay rate of radioactive isotopes these isotopes are found in igneous and metamorphic rocks some fossils and organic remains radioactive substances emit nuclear particles at a constant rate as the numbers of protons and neutrons change with each nuclear emission the element is converted to a different element the original radioactive element is referred to as the parent and the new element is referred to as the daughter for example a radioactive isotope of uranium u-238 will decay into an isotope of lead pb-206 over a specific span of time as illustrated in figure 21-8 the emission of radioactive particles and the resulting change into other elements over time is called radioactive decay once the emission of these atomic particles begins the rate remains constant regardless of environment pressure temperature or any other physical changes thus these atomic particles become accurate indicators of the absolute age of an object · explain the several different methods used by scientists to determine absolute age · describe how objects are dated by the use of certain radioactive elements · explain how annual tree rings and glacial varves are used to date geologic events vocabulary radioactive decay radiometric dating half-life dendrochronology varve key bed use of radioactive isotopes figure 21-8 the decay of u-238 to pb-206 follows a specific and never-changing path in a process called radiometric dating scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil this ratio is then used to determine the absolute age of the rock or fossil as the number of parent atoms decreases the number of daughter atoms increases by the same amount and indicates the increasing age of an object because it often takes a long time for the entire amount of an isotope to decay geologists use the length of time it takes for one-half of the original amount to decay this uranium 238 u-238 radioactive decay uranium 238 thorium 234 protactinium 234 uranium 234 thorium 230 bismuth 214 radium 226 radon 222 polonium 218 lead 214 polonium 210 polonium 214 lead 210 bismuth 210 lead 206 562 chapter 21 fossils and the rock record

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table 21-1 half-lives of selected radioactive isotopes radioactive isotope rubidium-87 thorium-232 potassium-40 uranium-238 uranium-235 carbon-14 approximate half-life 48.6 billion years 14.0 billion years 1.3 billion years 4.5 billion years 0.7 billion years 5730 years decay product strontium-87 lead-208 argon-40 lead-206 lead-207 nitrogen-14 period of time is called the half-life table 21-1 lists some common radioactive isotopes and their half-lives carbon-14 another radioactive isotope that is commonly used to determine the absolute age of an object is carbon-14 c-14 this isotope is especially useful for finding the age of materials that are of organic origin such as amber humanoid bones papyrus and charcoal fragments this is because all organic materials contain carbon c-14 decays into the stable nonradioactive element nitrogen-14 n-14 the half-life of c-14 is 5730 years as shown in table 21-2 when state-of-the-art technology is used c-14 dating is accurate for objects up to 75 000 years old if u-238 is used for an object that is only a few hundred thousand years old the ratio of parent to daughter atoms will be too large to be useful therefore a radioactive isotope with a shorter half-life than u238 such as u-235 which has a half-life of 700 000 000 years must be used conversely for the dating of a particularly old rock sample a radioactive isotope with a longer half-life must be used otherwise there may come a point when the ratio of parent-to-daughter atoms is too small to measure but the age of the rock has not yet been determined in essence the isotope used for dating depends on the estimated general age of the rock or object being dated using numbers a granite sample from canada was dated using uranium-235 which has a half-life of 700 000 000 years the rock was calculated to be 2.8 billion years old how many half-lives have elapsed since the rock formed table 21-2 radioactive decay of carbon-14 to nitrogen-14 percent parent element time 1 time 2 time 3 time 4 100 50 25 12.5 percent daughter element 0 50 75 87.5 elapsed years 0 5730 11 560 17 090 number of half-lives 0 1 2 3 21.3 absolute-age dating of rocks 563

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other ways to find out more about the oldest rocks found to date visit the glencoe science web site at science.glencoe.com to determine age determining the relative or absolute age of an object or event is not limited to the use of rocks or chemical elements naturally occurring materials such as trees lake-bottom sediment and volcanic ash can also be used to help geologists determine the age of an object or event such as a forest fire a drought a flood or a volcanic eruption tree rings with the use of a technique from the science of forestry the age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree during the spring months a tree experiences its greatest growth while in the winter its growth is less thus the widths of tree rings are directly related to the climatic conditions during growth periods a pair of spring and winter growth rings represents an annual tree ring dendrochronology is the science of comparing annual growth rings in trees to date events and changes in past environments for example in mesa verde national park in colorado the age of the wooden rafters used to build the pueblos of the anasazi have been determined with the use of dendrochronology figure 21-9 shows a pueblo from mesa verde national park the anasazi were a group of native americans that lived in the southwestern united states it has been calculated by other methods that the pueblos were built between a.d 1150 and a.d 1200 it also has been determined that pueblos in the southwestern united states were abandoned by the anasazi around a.d 1300 most likely because of a severe drought that lasted from a.d 1276 to a.d 1299 seasonal climatic changes about 11 000 years ago continental glaciers covered the northern part of the united states during the summer months the ice would partially melt large volumes of water containing fine glacial sediment were carried downstream and deposited in large lakes summer deposits are generally light-colored and relatively thick compared to the thinner organically enriched figure 21-9 the cliff palace in mesa verde national park was built by an ancient group of native americans known as the anasazi 564 chapter 21

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