how are canals the paleozoic era connected?paleontologists, scientists who study fossils, can learn...

358

How AreCanals &

the Paleozoic EraConnected?

How AreCanals &

the Paleozoic EraConnected?

Before the invention of the locomotive, canals, such as the one at upper right, were animportant means of transportation. In the 1790s, an engineer traveled around England

to study new canals. The engineer noticed something odd: All across the country, certaintypes of rocks seemed to lie in predictable layers, or strata. And the same strata always hadthe same kinds of fossils in them. Since each layer of sedimentary rock typically forms ontop of the previous one, scientists realized that the strata recorded the history of life onEarth. By the mid-1800s, the known rock strata had been organized into a system that wenow know as the geologic time scale. In this system, Earth’s history is divided into unitscalled eras, which in turn are divided into periods. Many of the rock layers in the GrandCanyon (background) date from the Paleozoic, or “ancient life,” Era.

Coco McCoy from Rainbow/PictureQuest

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Visit to find project ideas and resources.Projects include:• History Discover some of Earth’s inhabitants of different time periods using

the fossil record. Create a drawing of a scene in Earth’s history.• Technology Choose an extinct animal to investigate. How has technology

allowed paleontologists to learn about how it lived?• Model As a group, design a wall mural or diorama depicting the layers of the

geologic time scale, or a particular scene of interest from an era.Use online resources to form your own opinion concerning platetectonics. Investigate the Fossils of Antarctica and what theycould tell us about its ancient climate and location.

earth.msscience.com/unit_project

(background)Coco McCoy from Rainbow/PictureQuest, (t)Mary Evans Picture Library

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360

sections

1 Fossils

2 Relative Ages of RocksLab Relative Ages

3 Absolute Ages of RocksLab Trace Fossils

Virtual Lab How can fossiland rock data determine whenan organism lived?

Reading the PastThe pages of Earth’s history, much like thepages of human history, can be read if you lookin the right place. Unlike the pages of a book,the pages of Earth’s past are written in stone. Inthis chapter you will learn how to read thepages of Earth’s history to understand what theplanet was like in the distant past.

List three fossils that you would expectto find a million years from now in the place you live today.Science Journal

Clues to Earth’s Past

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361

Age of Rocks Make the follow-ing Foldable to help you under-stand how scientists determinethe age of a rock.

Fold a sheet of paper in halflengthwise.

Fold paper down 2.5 cm from thetop. (Hint: Fromthe tip of yourindex finger to yourmiddle knuckle isabout 2.5 cm.)

Open and drawlines along the2.5-cm fold.Label as shown.

Summarize in a Table As you read the chapter,in the left column, list four different ways inwhich one could determine the age of a rock. Inthe right column, note whether each methodgives an absolute or a relative age.

Determining

Age

Absoluteor Relative

STEP 3

STEP 2

STEP 1

1. Fill a small jar (about 500 mL) one-thirdfull of plaster of paris. Add water until thejar is half full.

2. Drop in a few small shells.

3. Cover the jar and shake it to model a swift,muddy stream.

4. Now model the stream flowing into a lakeby uncovering the jar and pouring the con-tents into a paper or plastic bowl. Let themixture sit for an hour.

5. Crack open the hardened plaster to locatethe model fossils.

6. Think Critically Remove the shells fromthe plaster and study the impressions theymade. In your Science Journal, list what theimpressions would tell you if found in a rock.

Clues to Life’s PastFossil formation begins when dead plants oranimals are buried in sediment. In time, ifconditions are right, the sediment hardensinto sedimentary rock. Parts of the organismare preserved along with the impressions ofparts that don’t survive. Any evidence ofonce-living things contained in the rockrecord is a fossil.

Start-Up Activities

Preview this chapter’s contentand activities at earth.msscience.com

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362

Traces of the Distant PastA giant crocodile lurks in the shallow water of a river. A herd

of Triceratops emerges from the edge of the forest and cautiouslymoves toward the river. The dinosaurs are thirsty, but dangerwaits for them in the water. A large bull Triceratops moves intothe river. The others follow.

Does this scene sound familiar to you? It’s likely that you’veread about dinosaurs and other past inhabitants of Earth. Buthow do you know that they really existed or what they were like?What evidence do humans have of past life on Earth? Theanswer is fossils. Paleontologists, scientists who study fossils, canlearn about extinct animals from their fossil remains, as shownin Figure 1.

■ List the conditions necessary forfossils to form.

■ Describe several processes of fossil formation.

■ Explain how fossil correlation isused to determine rock ages.

■ Determine how fossils can beused to explain changes inEarth’s surface, life forms, andenvironments.

Fossils help scientists find oil andother sources of energy necessaryfor society.

Review Vocabularypaleontologist: a scientist whostudies fossils

New Vocabulary

• fossil • mold

• permineralized • castremains • index fossil

• carbon film

Fossils

Figure 1 Scientists canlearn how dinosaurs lookedand moved using fossilremains. A skeleton canthen be reassembled anddisplayed in a museum.

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Formation of FossilsFossils are the remains, imprints,

or traces of prehistoric organisms.Fossils have helped scientists deter-mine approximately when life firstappeared, when plants and animalsfirst lived on land, and when organismsbecame extinct. Fossils are evidence ofnot only when and where organismsonce lived, but also how they lived.

For the most part, the remains ofdead plants and animals disappearquickly. Scavengers eat and scatter theremains of dead organisms. Fungi and bacteria invade, causingthe remains to rot and disappear. If you’ve ever left a banana onthe counter too long, you’ve seen this process begin. In time,compounds within the banana cause it to break down chemi-cally and soften. Microorganisms, such as bacteria, cause it todecay. What keeps some plants and animals from disappearingbefore they become fossils? Which organisms are more likely tobecome fossils?

Conditions Needed for Fossil Formation Whether ornot a dead organism becomes a fossil depends upon how well itis protected from scavengers and agents of physical destruction,such as waves and currents. One way a dead organism can beprotected is for sediment to bury the body quickly. If a fish diesand sinks to the bottom of a lake, sediment carried into the lakeby a stream can cover the fish rapidly. As a result, no waves orscavengers can get to it and tear it apart. The body parts thenmight be fossilized and included in a sedimentary rock likeshale. However, quick burial alone isn’t always enough to makea fossil.

Organisms have a better chance of becoming fossils if theyhave hard parts such as bones, shells, or teeth. One reason is thatscavengers are less likely to eat these hard parts. Hard parts alsodecay more slowly than soft parts do. Most fossils are the hardparts of organisms, such as the fossil teeth in Figure 2.

Types of PreservationPerhaps you’ve seen skeletal remains of Tyrannosaurus rex

towering above you in a museum. You also have some idea ofwhat this dinosaur looked like because you’ve seen illustrations.Artists who draw Tyrannosaurus rex and other dinosaurs basetheir illustrations on fossil bones. What preserves fossil bones?

Figure 2 These fossil sharkteeth are hard parts. Soft parts ofanimals do not become fossilizedas easily.

SECTION 1 Fossils 363

Predicting FossilPreservationProcedure1. Take a brief walk outside

and observe your neighborhood.

2. Look around and noticewhat kinds of plants andanimals live nearby.

Analysis1. Predict what remains from

your time might be preserved far into thefuture.

2. Explain what conditionswould need to exist for these remains to be fossilized.

Jeffrey Rotman/CORBIS

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Mineral Replacement Most hard parts of organisms suchas bones, teeth, and shells have tiny spaces within them. In life,these spaces can be filled with cells, blood vessels, nerves, or air.When the organism dies and the soft materials inside the hardparts decay, the tiny spaces become empty. If the hard part isburied, groundwater can seep in and deposit minerals in thespaces. Permineralized remains are fossils in which the spacesinside are filled with minerals from groundwater. In perminer-alized remains, some original material from the fossil organism’sbody might be preserved—encased within the minerals fromgroundwater. It is from these original materials that DNA, thechemical that contains an organism’s genetic code, can some-times be recovered.

Sometimes minerals replace the hard parts of fossil organ-isms. For example, a solution of water and dissolved silica (thecompound SiO2) might flow into and through the shell of adead organism. If the water dissolves the shell and leaves silica inits place, the original shell is replaced.

Often people learn about past forms of life from bones,wood, and other remains that became permineralized orreplaced with minerals from groundwater, as shown in Figure 3,but many other types of fossils can be found.

Carbon Films The tissues of organisms aremade of compounds that contain carbon.Sometimes fossils contain only carbon. Fossilsusually form when sediments bury a deadorganism. As sediment piles up, the organism’sremains are subjected to pressure and heat.These conditions force gases and liquids fromthe body. A thin film of carbon residue is left,forming a silhouette of the original organismcalled a carbon film. Figure 4 shows the car-bonized remains of graptolites, which weresmall marine animals. Graptolites have beenfound in rocks as old as 500 million years.

Figure 3 Opal and various miner-als have replaced original materialsand filled the hollow spaces in thispermineralized dinosaur bone.Explain why this fossil retained theshape of the original bone.

Figure 4 Graptolites lived hun-dreds of millions of years ago anddrifted on currents in the oceans.These organisms often are pre-served as carbon films.

364 CHAPTER 13 Clues to Earth’s Past(t)Dr. John A. Long, (b)A.J. Copley/Visuals Unlimited

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Coal In swampy regions, large volumes of plant matter accu-mulate. Over millions of years, these deposits become completelycarbonized, forming coal. Coal is an important fuel source, butsince the structure of the original plant is usually lost, it cannotreveal as much about the past as other kinds of fossils.

In what sort of environment does coal form?

Molds and Casts In nature, impressions form whenseashells or other hard parts of organisms fall into a soft sedi-ment such as mud. The object and sediment are then buried bymore sediment. Compaction, together with cementation,which is the deposition of minerals from water into the porespaces between sediment particles, turns the sediment intorock. Other open pores in the rock then let water and air reachthe shell or hard part. The hard part might decay or dissolve,leaving behind a cavity in the rock called a mold. Later, min-eral-rich water or other sediment might enter the cavity, formnew rock, and produce a copy or cast of the original object, asshown in Figure 5.

Figure 5 A castresembling the originalorganism forms whena mold fills with sedi-ment or minerals fromgroundwater.

The fossil begins to dissolve as water moves throughspaces in the rock layers.

The fossil has been dissolvedaway. The harder rock oncesurrounding it forms a mold.

Sediment washes into themold and is deposited, ormineral crystals form.

A cast results.

Coal Mining Many of thefirst coal mines in theUnited States were locatedin eastern states likePennsylvania and WestVirginia. In your ScienceJournal, discuss how theenvironments of the pastrelate to people’s livestoday.

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366 CHAPTER 13 Clues to Earth’s Past

Original Remains Sometimes conditionsallow original soft parts of organisms to be pre-served for thousands or millions of years. Forexample, insects can be trapped in amber, ahardened form of sticky tree resin. The ambersurrounds and protects the original material ofthe insect’s exoskeleton from destruction, asshown in Figure 6. Some organisms, such as themammoth, have been found preserved in frozenground in Siberia. Original remains also havebeen found in natural tar deposits, such as the La Brea tar pits in California.

Trace Fossils Do you have a handprint in plaster that youmade when you were in kindergarten? If so, it’s a record that tellssomething about you. From it, others can guess your size andmaybe your weight at that age. Animals walking on Earth longago left similar tracks, such as those in Figure 7. Trace fossils arefossilized tracks and other evidence of the activity of organisms.In some cases, tracks can tell you more about how an organismlived than any other type of fossil. For example, from a set oftracks at Davenport Ranch, Texas, you might be able to learnsomething about the social life of sauropods, which were large,plant-eating dinosaurs. The largest tracks of the herd are on theouter edges and the smallest are on the inside. These tracks ledsome scientists to hypothesize that adult sauropods surroundedtheir young as they traveled—perhaps to protect them frompredators. A nearby set of tracks might mean that another typeof dinosaur, an allosaur, was stalking the herd.

Figure 6 The original soft partsof this mosquito have been pre-served in amber for millions ofyears.

Figure 7 Tracks made in softmud, and now preserved in solidrock, can provide informationabout animal size, speed, andbehavior.

The dinosaur track below is from the Glen Rose Formation in north-central Texas.

The tracks to the rightare located on a Navajoreservation in Arizona.

(t)PhotoTake, NYC/PictureQuest, (b)Louis Psihoyos/Matrix

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Trails and Burrows Other trace fossils include trails andburrows made by worms and other animals. These, too, tellsomething about how these animals lived. For example, byexamining fossil burrows you can sometimes tell how firm thesediment the animals lived in was. As you can see, fossils can tella great deal about the organisms that have inhabited Earth.

How are trace fossils different from fossils thatare the remains of an organism’s body?

Index FossilsOne thing you can learn by studying fossils is that species of

organisms have changed over time. Some species of organismsinhabited Earth for long periods of time without changing.Other species changed a lot in comparatively short amounts oftime. It is these organisms that scientists use as index fossils.

Index fossils are the remains of species that existed on Earthfor relatively short periods of time, were abundant, and werewidespread geographically. Because the organisms that becameindex fossils lived only during specific intervals of geologic time,geologists can estimate the ages of rock layers based on the par-ticular index fossils they contain. However, not all rocks containindex fossils. Another way to approximate the age of a rock layeris to compare the spans of time, or ranges, over which more thanone fossil appears. The estimated age is the time interval wherefossil ranges overlap, as shown in Figure 8.

Figure 8 The fossils in a sequence of sedimentary rock can be used toestimate the ages of each layer. The chart shows when each organisminhabited Earth. Explain why it is possible to say that the middle layer of rock wasdeposited between 440 million and 410 million years ago.

Fossil Range Chart

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Fossils and Ancient EnvironmentsScientists can use fossils to determine what the environment

of an area was like long ago. Using fossils, you might be able tofind out whether an area was land or whether it was covered byan ocean at a particular time. If the region was covered by ocean,it might even be possible to learn the depth of the water. Whatclues about the depth of water do you think fossils could provide?

Fossils also are used to determine the past climate of aregion. For example, rocks in parts of the eastern United Statescontain fossils of tropical plants. The environment of this partof the United States today isn’t tropical. However, because of thefossils, scientists know that it was tropical when these plantswere living. Figure 9 shows that North America was located nearthe equator when these fossils formed.

Shallowseas

Shallowseas

Shallowseas

Coal swamps

Exposedland

Exposedland

Exposedland

AFRICA

Deepocean

Deepocean

Deepocean

Equator

Figure 9 The equator passedthrough North America 310 millionyears ago. At this time, warm,shallow seas and coal swamps cov-ered much of the continent, andferns like the Neuropteris, below,were common.

Ancient EcologyEcology is the study ofhow organisms interactwith each other and withtheir environment. Somepaleontologists study the ecology of ancientorganisms. Discuss thekinds of information youcould use to determinehow ancient organismsinteracted with theirenvironment.

David M. Dennis

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Self Check1. Describe the typical conditions necessary for fossil

formation.

2. Explain how a fossil mold is different from a fossil cast.

3. Discuss how the characteristics of an index fossil areuseful to geologists.

4. Describe how carbon films form.

5. Think Critically What can you say about the ages oftwo widely separated layers of rock that contain thesame type of fossil?

6. Communicate what you learn about fossils. Visit amuseum that has fossils on display. Make an illustra-tion of each fossil in your Science Journal. Write a briefdescription, noting key facts about each fossil and howeach fossil might have formed.

7. Compare and contrast original remains with otherkinds of fossils. What kinds of information would only be available from original remains? Are there any limitations to the use of original remains?

Shallow Seas How would you explain the presence of fos-silized crinoids—animals that lived in shallow seas—in rocksfound in what is today a desert? Figure 10 shows a fossil crinoidand a living crinoid. When the fossil crinoids were alive, a shal-low sea covered much of western and central North America.The crinoid hard parts were included in rocks that formed fromthe sediments at the bottom of this sea. Fossils provide informa-tion about past life on Earth and also about the history of therock layers that contain them. Fossils can provide informationabout the ages of rocks and the climate and type of environmentthat existed when the rocks formed.

Figure 10 The crinoid on theleft lived in warm, shallow seasthat once covered part of NorthAmerica. Crinoids like the one onthe right typically live in warm,shallow waters in the PacificOcean.

SummaryFormation of Fossils

• Fossils are the remains, imprints, or traces ofpast organisms.

• Fossilization is most likely if the organism hadhard parts and was buried quickly.

Fossil Preservation

• Permineralized remains have open spacesfilled with minerals from groundwater.

• Thin carbon films remain in the shapes ofdead organisms.

• Hard parts dissolve to leave molds.

• Trace fossils are evidence of past activity.

Index Fossils

• Index fossils are from species that were abun-dant briefly, but over wide areas.

• Scientists can estimate the ages of rocks con-taining index fossils.

Fossils and Ancient Environments

• Fossils tell us about the environment in whichthe organisms lived.

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Relative Ages of Rocks

Figure 11 The pile of magazines illustratesthe principle of superposition. According tothis principle, the oldest rock layer (or maga-zine) is on the bottom.

SuperpositionImagine that you are walking to your favorite store and you

happen to notice an interesting car go by. You’re not sure whatkind it is, but you remember that you read an article about it.You decide to look it up. At home you have a stack of magazinesfrom the past year, as seen in Figure 11.

You know that the article you’re thinking of came out in theJanuary edition, so it must be near the bottom of the pile. Asyou dig downward, you find magazines from March, thenFebruary. January must be next. How did you know that theJanuary issue of the magazine would be on the bottom? To findthe older edition under newer ones, you applied the principleof superposition.

Oldest Rocks on the Bottom According to the principleof superposition, in undisturbed layers of rock, the oldest rocksare on the bottom and the rocks become progressively youngertoward the top. Why is this the case?

■ Describe methods used to assignrelative ages to rock layers.

■ Interpret gaps in the rock record.■ Give an example of how rock

layers can be correlated withother rock layers.

Being able to determine the age ofrock layers is important in trying tounderstand a history of Earth.

Review Vocabularysedimentary rock: rock formedwhen sediments are cementedand compacted or when mineralsare precipitated from solution

New Vocabulary

• principle of superposition

• relative age

• unconformity

370Aaron Haupt

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SECTION 2 Relative Ages of Rocks 371

Rock Layers Sediment accumulates in horizontal beds,forming layers of sedimentary rock. The first layer to form is onthe bottom. The next layer forms on top of the previous one.Because of this, the oldest rocks are at the bottom. However,forces generated by mountain formation sometimes can turnlayers over. When layers have been turned upside down, it’s nec-essary to use other clues in the rock layers to determine theiroriginal positions and relative ages.

Relative AgesNow you want to look for another magazine. You’re not sure

how old it is, but you know it arrived after the January issue. Youcan find it in the stack by using the principle of relative age.

The relative age of something is its age in comparison to theages of other things. Geologists determine the relative ages ofrocks and other structures by examining their places in asequence. For example, if layers of sedimentary rock are offset bya fault, which is a break in Earth’s surface, you know that the lay-ers had to be there before a fault could cut through them. Therelative age of the rocks is older than the relative age of the fault.Relative age determination doesn’t tell you anything about theage of rock layers in actual years. You don’t know if a layer is 100million or 10,000 years old. You only know that it’s younger thanthe layers below it and older than the fault cutting through it.

Other Clues Help Determination of relative age is easy if therocks haven’t been faulted or turned upside down. For example,look at Figure 12. Which layer is the oldest? In cases where rocklayers have been disturbed you might have to look for fossils andother clues to date the rocks. If you find a fossil in the top layerthat’s older than a fossil in a lower layer, you can hypothesizethat layers have been turned upside down by folding duringmountain building.

Figure 12 In a stack of undis-turbed sedimentary rocks, the old-est rocks are at the bottom. Thisstack of rocks can be folded byforces within Earth.Explain how you can tell if an olderrock is above a younger one.

Topic: Relative DatingVisit for Weblinks to information about relativedating of rocks and othermaterials.

Activity Imagine yourself at anarchaeological dig. You havefound a rare artifact and want toknow its age. Make a list of cluesyou might look for to provide arelative date and explain howeach would allow you to approxi-mate the artifact’s age.

earth.msscience.com

Limestone

Coal

Sandstone

Undisturbed Layers

Limestone

Coal

Sandstone

Folded Layers

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UnconformitiesA sequence of rock is a record of past events.

But most rock sequences are incomplete—layersare missing. These gaps in rock sequences arecalled unconformities (un kun FOR muh teez).Unconformities develop when agents of erosionsuch as running water or glaciers remove rocklayers by washing or scraping them away.

How do unconformities form?

Angular Unconformities Horizontal layersof sedimentary rock often are tilted anduplifted. Erosion and weathering then weardown these tilted rock layers. Eventually,younger sediment layers are deposited horizon-tally on top of the tilted and eroded layers.Geologists call such an unconformity an angularunconformity. Figure 13 shows how angularunconformities develop.

Disconformity Suppose you’re looking at astack of sedimentary rock layers. They lookcomplete, but layers are missing. If you lookclosely, you might find an old surface of erosion.This records a time when the rocks wereexposed and eroded. Later, younger rocksformed above the erosion surface when deposi-tion of sediment began again. Even though allthe layers are parallel, the rock record still has agap. This type of unconformity is called a dis-conformity. A disconformity also forms when aperiod of time passes without any new deposi-tion occurring to form new layers of rock.

Nonconformity Another type of unconfor-mity, called a nonconformity, occurs whenmetamorphic or igneous rocks are uplifted anderoded. Sedimentary rocks are then depositedon top of this erosion surface. The surfacebetween the two rock types is a nonconformity.Sometimes rock fragments from below areincorporated into sediments deposited abovethe nonconformity. All types of unconformitiesare shown in Figure 14.

Figure 13 An angular unconformity resultswhen horizontal layers cover tilted, eroded layers.

Angular unconformity

Sedimentary rocks are deposited originally ashorizontal layers.

The horizontal rock layers are tilted as forceswithin Earth deform them.

The tilted layers erode.

An angular unconformity results when new layersform on the tilted layers as deposition resumes.

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NGS TITLE

Figure 14

VISUALIZING UNCONFORMITIES


An unconformity is a gap in the rockrecord caused by erosion or a pausein deposition. There are three major

kinds of unconformities—nonconformity,angular unconformity, and disconformity.

Disconformity

In a nonconformity, horizontal layers of sedimen-tary rock overlie older igneous or metamorphic rocks.A nonconformity in Big Bend National Park, Texas, isshown above.

An angular unconformity develops when newhorizontal layers of sedimentary rock form on topof older sedimentary rock layers that have beenfolded by compression. An example of an angularunconformity at Siccar Point in southeastern Scot-land is shown above.

A disconformity develops whenhorizontal rock layers are exposedand eroded, and new horizontal layers of rock are deposited on theeroded surface. The disconformityshown below is in the Grand Canyon.

▼

▼▼

Nonconformity

Angular unconformity

(l)IPR/12-18 T. Bain, British Geological Survey/NERC. All rights reserved., (r)Tom Bean/CORBIS, (bkgd.)Lyle Rosbotham

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Matching Up Rock LayersSuppose you’re studying a layer of sandstone in Bryce

Canyon in Utah. Later, when you visit Canyonlands NationalPark, Utah, you notice that a layer of sandstone there looks justlike the sandstone in Bryce Canyon, 250 km away. Above thesandstone in the Canyonlands is a layer of limestone and thenanother sandstone layer. You return to Bryce Canyon and findthe same sequence—sandstone, limestone, and sandstone. Whatdo you infer? It’s likely that you’re looking at the same layers ofrocks in two different locations. Figure 15 shows that these rocksare parts of huge deposits that covered this whole area of thewestern United States. Geologists often can match up, or corre-late, layers of rocks over great distances.

Evidence Used for Correlation It’s not always easy to saythat a rock layer exposed in one area is the same as a rock layerexposed in another area. Sometimes it’s possible to walk alongthe layer for kilometers and prove that it’s continuous. In othercases, such as at the Canyonlands area and Bryce Canyon as seenin Figure 16, the rock layers are exposed only where rivers havecut through overlying layers of rock and sediment. How can youshow that the limestone sandwiched between the two layers ofsandstone in Canyonlands is likely the same limestone as atBryce Canyon? One way is to use fossil evidence. If the sametypes of fossils were found in the limestone layer in both places,it’s a good indication that the limestone at each location is thesame age, and, therefore, one continuous deposit.

How do fossils help show that rocks at differentlocations belong to the same rock layer?

Figure 15 These rock layers,exposed at Hopi Point in GrandCanyon National Park, Arizona, canbe correlated, or matched up, withrocks from across large areas of thewestern United States.

Topic: Correlating withIndex FossilsVisit for Weblinks to information about usingindex fossils to match up layers ofrock.

Activity Make a chart thatshows the rock layers of both theGrand Canyon and Capitol ReefNational Park in Utah. For eachlayer that appears in both parks,list an index fossil you could find tocorrelate the layers.

earth.msscience.com

UTAH

ARIZONA

Bryce CanyonNational Park

Canyonlands National Park

Grand CanyonNational Park

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Self Check1. Discuss how to find the oldest paper in a stack of papers.

2. Explain the concept of relative age.

3. Illustrate a disconformity.

4. Describe one way to correlate similar rock layers.

5. Think Critically Explain the relationship between theconcept of relative age and the principle of superposition.

SummarySuperposition

• Superposition states that in undisturbed rock,the oldest layers are on the bottom.

Relative Ages

• Rock layers can be ranked by relative age.

Unconformities

• Angular unconformities are new layersdeposited over tilted and eroded rock layers.

• Disconformities are gaps in the rock record.

• Nonconformities divide uplifted igneous ormetamorphic rock from new sedimentary rock.

Matching Up Rock Layers

• Rocks from different areas may be correlatedif they are part of the same layer.

6. Interpret data to determine the oldest rock bed. A sandstone contains a 400-million-year-old fossil. Ashale has fossils that are over 500 million years old. Alimestone, below the sandstone, contains fossilsbetween 400 million and 500 million years old. Which rock bed is oldest? Explain.

Can layers of rock be correlated in other ways? Sometimesdetermining relative ages isn’t enough and other dating meth-ods must be used. In Section 3, you’ll see how the numerical agesof rocks can be determined and how geologists have used thisinformation to estimate the age of Earth.

Figure 16 Geologists havenamed the many rock layers, orformations, in Canyonlands and inBryce Canyon, Utah. They also havecorrelated some formationsbetween the two canyons.List the labeled layers present atboth canyons.

Date deposited (millions of years ago)

CanyonlandsNational Park

Bryce CanyonNational Park

Wasatch Fm

Kaiparowits Fm

Straight Cliffs Ss

Dakota SsWinsor FmEntrada Ss

Navajo SsCarmel Fm

Older rocksnot exposed

Morrison Fm

Entrada Ss

Navajo Ss

Wingate SsChinle FmMoenkopi Fm

Cutler Gp

Rico GpHermosa Gp

2– 65

65–136

136–190

190–225

225–280

280 –320

Canyonlands National ParkCanyonlands National Park Bryce Canyon National ParkBryce Canyon National Park

earth.msscience.com/self_check_quiz(l)Michael T. Sedam/CORBIS, (r)Pat O'Hara/CORBIS

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Which of your two friends is older? To answerthis question, you’d need to know their relativeages. You wouldn’t need to know the exact ageof either of your friends—just who was bornfirst. The same is sometimes true for rock layers.

Real-World QuestionCan you determine the relative ages of rock layers?

Goals■ Interpret illustrations of rock layers and

other geological structures and determinethe relative order of events.

Materialspaper pencil

Procedure1. Analyze Figures A and B.

2. Make a sketch of Figure A. On it, identifythe relative age of each rock layer, igneousintrusion, fault, and unconformity. Forexample, the shale layer is the oldest, somark it with a 1. Mark the next-oldest fea-ture with a 2, and so on.

3. Repeat step 2 for Figure B.

Conclude and ApplyFigure A

1. Identify the type of unconformity shown. Is it possible that there were originally morelayers of rock than are shown?

2. Describe how the rocks above the faultmoved in relation to rocks below the fault.

3. Hypothesize how the hill on the left side ofthe figure formed.

Figure B

4. Is it possible to conclude if the igneousintrusion on the left is older or younger thanthe unconformity nearest the surface?

5. Describe the relative ages of the twoigneous intrusions. How did you know?

6. Hypothesize which two layers of rockmight have been much thicker in the past.

Compare your results with other students’results. For more help, refer to the ScienceSkill Handbook.


Granite

A

B

Sandstone

Limestone

Shale

Relative Ages

512-S2-MSS05ges 8/20/04 12:39 PM Page 376

Absolute Ages As you sort through your stack of magazines looking for that

article about the car you saw, you decide that you need to restackthem into a neat pile. By now, they’re in a jumble and no longerin order of their relative age, as shown in Figure 17. How canyou stack them so the oldest are on the bottom and the newestare on top? Fortunately, magazine dates are printed on the cover.Thus, stacking magazines in order is a simple process.Unfortunately, rocks don’t have their ages stamped on them. Ordo they? Absolute age is the age, in years, of a rock or otherobject. Geologists determine absolute ages by using propertiesof the atoms that make up materials.

Radioactive DecayAtoms consist of a dense central regioncalled the nucleus, which is surrounded by a

cloud of negatively charged particles called electrons. Thenucleus is made up of protons, which have a positive charge, andneutrons, which have no electric charge. The number of protonsdetermines the identity of the element, and the number of neu-trons determines the form of the element, or isotope. For exam-ple, every atom with a single proton is a hydrogen atom.Hydrogen atoms can have no neutrons, a single neutron, or twoneutrons. This means that there are three isotopes of hydrogen.

What particles makeup an atom’s nucleus?

Some isotopes are unstable andbreak down into other isotopes and particles. Sometimes a lot of energy isgiven off during this process. Theprocess of breaking down is calledradioactive decay. In the case ofhydrogen, atoms with one proton andtwo neutrons are unstable and tend tobreak down. Many other elementshave stable and unstable isotopes.

Absolute Ages of Rocks

■ Identify how absolute age dif-fers from relative age.

■ Describe how the half-lives ofisotopes are used to determine a rock’s age.

Events in Earth’s history can be bet-ter understood if their absolute agesare known.

Review Vocabularyisotopes: atoms of the same ele-ment that have different numbersof neutrons

New Vocabulary

• absolute age

• radioactive decay

• half-life

• radiometric dating

• uniformitarianism

Figure 17 The magazines thathave been shuffled through nolonger illustrate the principle ofsuperposition.

SECTION 3 Absolute Ages of Rocks 377Aaron Haupt

512-S3-MSS05ges 8/20/04 12:40 PM Page 377

Modeling Carbon-14DatingProcedure 1. Count out 80 red jelly

beans.2. Remove half the red jelly

beans and replace themwith green jelly beans.

3. Continue replacing half thered jelly beans with greenjelly beans until only 5 redjelly beans remain. Countthe number of times youreplace half the red jellybeans.

Analysis1. How did this activity model

the decay of carbon-14atoms?

2. How many half lives of carbon-14 did you modelduring this activity?

3. If the atoms in a boneexperienced the samenumber of half lives asyour jelly beans, how oldwould the bone be?


Alpha and Beta Decay In some isotopes, a neutron breaksdown into a proton and an electron. This type of radioactive decayis called beta decay because the electron leaves the atom as a betaparticle. The nucleus loses a neutron but gains a proton. When thenumber of protons in an atom is changed, a new element forms.Other isotopes give off two protons and two neutrons in the formof an alpha particle. Alpha and beta decay are shown in Figure 18.

Half-Life In radioactive decay reactions, the parent isotopeundergoes radioactive decay. The daughter product is produced byradioactive decay. Each radioactive parent isotope decays to itsdaughter product at a certain rate. Based on this decay rate, it takesa certain period of time for one half of the parent isotope to decayto its daughter product. The half-life of an isotope is the time ittakes for half of the atoms in the isotope to decay. For example, thehalf-life of carbon-14 is 5,730 years. So it will take 5,730 years forhalf of the carbon-14 atoms in an object to change into nitrogen-14 atoms. You might guess that in another 5,730 years, all of theremaining carbon-14 atoms will decay to nitrogen-14. However,this is not the case. Only half of the atoms of carbon-14 remainingafter the first 5,730 years will decay during the second 5,730 years.So, after two half-lives, one fourth of the original carbon-14 atomsstill remain. Half of them will decay during another 5,730 years.After three half-lives, one eighth of the original carbon-14 atomsstill remain. After many half-lives, such a small amount of the par-ent isotope remains that it might not be measurable.

Figure 18 In beta decay, aneutron changes into a protonby giving off an electron. Thiselectron has a lot of energy andis called a beta particle.

Unstable parent isotope

Proton

Beta decay Daughter product

Beta particle(electron)

Neutron

Unstable parent isotope

Proton

Neutron

Alpha decay Daughter product

Alpha particle

In the process of alpha decay, an unsta-ble parent isotope nucleus gives off analpha particle and changes into a newdaughter product. Alpha particles containtwo neutrons and two protons.

512-S3-MSS05ges 8/20/04 12:40 PM Page 378

Radiometric AgesDecay of radioactive isotopes is

like a clock keeping track of timethat has passed since rocks haveformed. As time passes, the amountof parent isotope in a rock decreasesas the amount of daughter productincreases, as in Figure 19. By mea-suring the ratio of parent isotope todaughter product in a mineral andby knowing the half-life of the par-ent, in many cases you can calculatethe absolute age of a rock. Thisprocess is called radiometric dating.

A scientist must decide which parent isotope to use whenmeasuring the age of a rock. If the object to be dated seems old,then the geologist will use an isotope with a long half-life. Thehalf-life for the decay of potassium-40 to argon-40 is 1.25 bil-lion years. As a result, this isotope can be used to date rocks thatare many millions of years old. To avoid error, conditions mustbe met for the ratios to give a correct indication of age. Forexample, the rock being studied must still retain all of theargon-40 that was produced by the decay of potassium-40.Also, it cannot contain any contamination of daughter productfrom other sources. Potassium-argon dating is good for rockscontaining potassium, but what about other things?

Radiocarbon Dating Carbon-14 is useful for dating bones,wood, and charcoal up to 75,000 years old. Living things take incarbon from the environment to build their bodies. Most of thatcarbon is carbon-12, but some is carbon-14, and the ratio ofthese two isotopes in the environment is always the same. Afterthe organism dies, the carbon-14 slowly decays. By determiningthe amounts of the isotopes in a sample, scientists can evaluatehow much the isotope ratio in the sample differsfrom that in the environment. For example, dur-ing much of human history, people built camp-fires. The wood from these fires often is preservedas charcoal. Scientists can determine the amountof carbon-14 remaining in a sample of charcoalby measuring the amount of radiation emitted by the carbon-14 isotope in labs like the one in Figure 20. Once they know the amount ofcarbon-14 in a charcoal sample, scientists can deter-mine the age of the wood used to make the fire.

100%

50%

50% 75% 87.5% 93.75%

25%

12.5% 6.25%

Parentmaterial

1 half-life

2 half-lives

3 half-lives

4 half-lives

12

14

18 16

1

12

34

78 16

15

Figure 19 During each half-life,one half of the parent materialdecays to the daughter product. Explain how one uses both parent and daughter material to estimate age.

Figure 20 Radiometric ages aredetermined in labs like this one.

379

Jam

es K

ing-

Hol

mes

/Sci

ence

Pho

to L

ibra

ry/P

hoto

Res

earc

hers

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Age Determinations Aside from carbon-14 dating, rocksthat can be radiometrically dated are mostly igneous andmetamorphic rocks. Most sedimentary rocks cannot be datedby this method. This is because many sedimentary rocks aremade up of particles eroded from older rocks. Dating thesepieces only gives the age of the preexisting rock from which itcame.

The Oldest Known Rocks Radiometric dating has beenused to date the oldest rocks on Earth. These rocks are about3.96 billion years old. By determining the age of meteorites,and using other evidence, scientists have estimated the age ofEarth to be about 4.5 billion years. Earth rocks greater than3.96 billion years old probably were eroded or changed by heatand pressure.

Why can’t most sedimentary rocks be datedradiometrically?

When did the Iceman die?

Carbon-14 dating has been used to date charcoal, wood, bones,

mummies from Egypt and Peru, theDead Sea Scrolls, and the ItalianIceman. The Iceman was found in 1991 in the Italian Alps, near theAustrian border. Based on carbon-14analysis, scientists determined that the Iceman is 5,300 years old.Determine approximately in what year the Iceman died.

Identifying the ProblemThe half-life chart shows the decay of carbon-14 over time. Half-life is the time it

takes for half of a sample to decay. Fill in the years passed when only 3.125 percent ofcarbon-14 remain. Is there a point at which no carbon-14 would be present? Explain.

Solving the Problem1. Estimate, using the data table, how much carbon-14 still was present in the Iceman’s

body that allowed scientists to determine his age.2. If you had an artifact that originally contained 10.0 g of carbon-14, how many

grams would remain after 17,190 years?

Reconstruction of Iceman

Topic: Isotopes in Ice CoresVisit for Weblinks to information about ice coresand how isotopes in ice are used tolearn about Earth’s past.

Activity Prepare a report thatshows how isotopes in ice corescan tell us about past Earth envi-ronments. Include how these find-ings can help us understandtoday’s climate.

earth.msscience.com

Half-Life of Carbon-14

Percent Years Carbon-14 Passed

100 0

50 5,730

25 11,460

12.5 17,190

6.25 22,920

3.125

Kenneth Garrett

512-S3-MSS05ges 8/20/04 12:40 PM Page 380


SECTION 3 Absolute Ages of Rocks 381

Self Check1. Evaluate the age of rocks. You find three undisturbed

rock layers. The middle layer is 120 million years old.What can you say about the ages of the layers aboveand below it?

2. Determine the age of a fossil if it had only one eighthof its original carbon-14 content remaining.

3. Explain the concept of uniformitarianism.

4. Describe how radioactive isotopes decay.

5. Think Critically Why can’t scientists use carbon-14 todetermine the age of an igneous rock?

SummaryAbsolute Ages

• The absolute age is the actual age of an object.

Radioactive Decay

• Some isotopes are unstable and decay intoother isotopes and particles.

• Decay is measured in half-lives, the time ittakes for half of a given isotope to decay.

Radiometric Ages

• By measuring the ratio of parent isotope todaughter product, one can determine theabsolute age of a rock.

• Living organisms less than 75,000 years oldcan be dated using carbon-14.

Uniformitarianism

• Processes observable today are the same asthe processes that took place in the past.

6. Make and use a table that shows the amount of parent material of a radioactive element that is leftafter four half-lives if the original parent material had a mass of 100 g.

UniformitarianismCan you imagine trying to determine the age of

Earth without some of the information you knowtoday? Before the discovery of radiometric dating,many people estimated that Earth is only a fewthousand years old. But in the 1700s, Scottish scientist James Hutton estimated that Earth is much older. He used the principle ofuniformitarianism. This principle states thatEarth processes occurring today are similar to thosethat occurred in the past. Hutton’s principle is oftenparaphrased as “the present is the key to the past.”

Hutton observed that the processes that changed the land-scape around him were slow, and he inferred that they were justas slow throughout Earth’s history. Hutton hypothesized that ittook much longer than a few thousand years to form the layersof rock around him and to erode mountains that once stoodkilometers high. Figure 21 shows Hutton’s native Scotland, aregion shaped by millions of years of geologic processes.

Today, scientists recognize that Earth has been shaped by twotypes of change: slow, everyday processes that take place overmillions of years, and violent, unusual events such as the colli-sion of a comet or asteroid about 65 million years ago thatmight have caused the extinction of the dinosaurs.

Figure 21 The rugged high-lands of Scotland were shaped byerosion and uplift.

earth.msscience.com/self_check_quizWildCountry/CORBIS

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Model and InventModel and Invent

Real-World QuestionTrace fossils can tell you a lot about the activitiesof organisms that left them. They can tell you how an organism fed or what kind of home it had.How can you model trace fossils that can provide information about the behavior of organisms? What materials can youuse to model trace fossils? What types of behavior could you showwith your trace fossil model?

Make a Model1. Decide how you are going to make your model. What materials

will you need?

2. Decide what types of activities you will demonstrate with yourmodel. Were the organisms feeding? Resting? Traveling? Werethey predators? Prey? How will your model indicate the activitiesyou chose?

3. What is the setting of your model? Are you modeling the organ-ism’s home? Feeding areas? Is your model on land or water? Howcan the setting affect the way you build your model?

4. Will you only show trace fossils from a single species or multiplespecies? If you include more than one species, how will you pro-

vide evidence of any interactionbetween the species?

Check the Model Plans1. Compare your plans with those

of others in your class. Didother groups mention detailsthat you had forgotten to thinkabout? Are there any changesyou would like to make to yourplan before you continue?

2. Make sure your teacherapproves your plan before youcontinue.

Goals■ Construct a model of

trace fossils.■ Describe the informa-

tion that you can learnfrom looking at yourmodel.

Possible Materialsconstruction paperwireplastic (a fairly rigid type)scissorsplaster of paristoothpickssturdy cardboardclaypipe cleanersglue

Safety Precautions


Trace Fxssils

(t)A.J. Copley/Visuals Unlimited, (b)Lawson Wood/CORBIS

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Ask other students in your class or anotherclass to look at your model and describewhat information they can learn from thetrace fossils. Did their interpretations agreewith what you intended to show?

Test Your Model1. Following your plan, construct your model of trace fossils.

2. Have you included evidence of all the behaviors you intended to model?

Analyze Your Data1. Evaluate Now that your model is complete, do you think that it adequately

shows the behaviors you planned to demonstrate? Is there anything that youthink you might want to do differently if you were going to make the modelagain?

2. Describe how using different kinds of materials might have affected yourmodel. Can you think of other materials that would have allowed you to showmore detail than you did?

Conclude and Apply1. Compare and contrast your model of trace fossils with

trace fossils left by real organisms. Is one more easilyinterpreted than the other? Explain.

2. List behaviors that might not leave any trace fossils.Explain.

LAB 383Matt Meadows

512-S3-MSS05ges 8/20/04 12:40 PM Page 383

O n a December day in 1938, just before Christmas, Marjorie Courtenay-Latimer went to say hello

to her friends on board a fishing boat thathad just returned to port in South Africa.Courtenay-Latimer, who worked at amuseum, often went aboard her friends’ shipto check out the catch. On this visit, shereceived a surprise Christmas present—anodd-looking fish. As soon as the woman spot-ted its strange blue fins among the piles ofsharks and rays, she knew it was special.

Courtenay-Latimer took the fish back toher museum to study it. “It was the mostbeautiful fish I had ever seen, five feet long,and a pale mauve blue with iridescent silvermarkings,” she later wrote. Courtenay-Latimer sketched it and sent the drawing to a friend of hers, J. L. B. Smith.

Smith was a chemistry teacher who waspassionate about fish. After a time, he real-ized it was a coelacanth (SEE luh kanth).Fish experts knew that coelacanths had firstappeared on Earth 400 million years ago. But the experts thought the fish were extinct. People had found fossils of coelacanths, but

no one had seen one alive. It was assumedthat the last coelacanth species had died out65 million years ago. They were wrong. Theship’s crew had caught one by accident.

Smith figured there might be more livingcoelacanths. So he decided to offer a rewardfor anyone who could find a living specimen.After 14 years of silence, a report came inthat a coelacanth had been caught off theeast coast of Africa.

Today, scientists know that there are at leastseveral hundred coelacanths living in the IndianOcean, just east of central Africa. Many of thesefish live near the Comoros Islands. The coela-canths live in underwater caves during the daybut move out at night to feed. The rare fish arenow a protected species. With any luck, theywill survive for another hundred million years.

SOMETIMES GREAT

DISCOVERIES HAPPEN BY ACCIDENT!

Write a short essay describing the discovery of thecoelacanths and describe the reaction of scientists tothis discovery. For more information, visit

earth.msscience.com/oops

Seconddorsal fin

Anal fin

Pelvic fin

Camouflagemarks

First dorsal fin

Pectoralfin

Some scientists call the coelacanth“Old Four Legs.” It got its nickname

because the fish has paired finsthat look something like legs.

The World's

StoryA catch-of-the-day

set science onits ears

Oldest Fish

Jacques Bredy

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Fossils

1. Fossils are more likely to form if hard partsof the dead organisms are buried quickly.

2. Some fossils form when original materialsthat made up the organisms are replaced withminerals. Other fossils form when remainsare subjected to heat and pressure, leavingonly a carbon film behind. Some fossils arethe tracks or traces left by ancient organisms.

Relative Ages of Rocks

1. The principle of superposition states that,in undisturbed layers, older rocks lie under-neath younger rocks.

2. Unconformities, or gaps in the rock record, are due to erosion or periods of time during which no depositionoccurred.

3. Rock layers can be correlated using rocktypes and fossils.

Absolute Ages of Rocks

1. Absolute dating provides an age in years for the rocks.

2. The half-life of a radioactive isotope is the time it takes for half of the atoms of the isotope to decay into another isotope.

CHAPTER STUDY GUIDE 385

Hollow spaces

filled withminerals

types of remains types of remains

Original parts Molds and casts

Carbon films Trails

Trace fossils

Burrows

Copy and complete the following concept map on fossils.

Fossils

kinds of evidence kinds of evidence

preserved by preserved by preserved by preserved by

remains oforganisms

evidence of activitiesof organisms

Body fossils

Frozen in ice or

trapped inamber

earth.msscience.com/interactive_tutor(tl)François Gohier/Photo Researchers, (b)Mark E. Gibson/DRK Photo, (tr)Sinclair Stammers/Photo Researchers

512-CR-MSS05ges 8/20/04 12:37 PM Page 385

http://earth.msscience.com/interactive_tutor

Write an original sentence using the vocabulary word to which each phrase refers.

1. thin film of carbon preserved as a fossil

2. older rocks lie under younger rocks

3. processes occur today as they did in the past

4. gap in the rock record

5. time needed for half the atoms to decay

6. fossil organism that lived for a short time

7. gives the age of rocks in years

8. minerals fill spaces inside fossil

9. a copy of a fossil produced by filling amold with sediment or crystals

Choose the word or phrase that best answers the

question.

10. What is any evidence of ancient life called?A) half-life C) unconformityB) fossil D) disconformity

11. Which of the following conditions makesfossil formation more likely?A) buried slowlyB) attacked by scavengersC) made of hard partsD) composed of soft parts

12. What are cavities left in rocks when a shellor bone dissolves called?A) casts C) original remainsB) molds D) carbon films

13. To say “the present is the key to the past”is a way to describe which of the followingprinciples?A) superposition C) radioactivityB) succession D) uniformitarianism

14. A fault can be useful in determining whichof the following for a group of rocks?A) absolute age C) radiometric ageB) index age D) relative age

15. Which of the following is an unconfor-mity between parallel rock layers?A) angular unconformityB) faultC) disconformityD) nonconformity

Use the illustration below to answer question 16.

16. Which of the following puts the layers inorder from oldest to youngest?A) 5-4-3-2-1 C) 2-3-4-5-1B) 1-2-3-4-5 D) 4-3-2-5-1

17. Which process forms new elements?A) superpositionB) uniformitarianismC) permineralizationD) radioactive decay

386 CHAPTER REVIEW

absolute age p. 377carbon film p. 364cast p. 365fossil p. 363half-life p. 378index fossil p. 367mold p. 365permineralized remains

p. 364

principle of superpositionp. 370

radioactive decay p. 377radiometric dating p. 379relative age p. 371unconformity p. 372uniformitarianism p. 381

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CHAPTER REVIEW 387

28. Calculate how many half-lives have passed in arock containing one-eighth the originalradioactive material and seven-eighths of thedaughter product.

Use the graphs below to answer question 29.

29. Interpret Data Which of the above curves bestillustrates radioactive decay?

% re

mai

nin

g

# of half-lives

% re

mai

nin

g

# of half-lives

% re

mai

nin

g

# of half-lives

% re

mai

nin

g

# of half-lives

25. Discuss uniformitarianism in the followingscenario. You find a shell on the beach,and a friend remembers seeing a similarfossil while hiking in the mountains. Whatdoes this suggest about the past environ-ment of the mountain?

26. Illustrate Create a model that allows you toexplain how to establish the relative agesof rock layers.

27. Use a Classification System Start your own fossil collection. Label each find as to type,approximate age, and the place where itwas found. Most state geological surveyscan provide you with reference materialson local fossils.

18. Explain why the fossil record of life on Earthis incomplete. Give some reasons why.

19. Infer Suppose a lava flow was foundbetween two sedimentary rock layers.How could you use the lava flow to learnabout the ages of the sedimentary rocklayers? (Hint: Most lava contains radioac-tive isotopes.)

20. Infer Suppose you’re correlating rock layers in the western United States. Youfind a layer of volcanic ash deposits. Howcan this layer help you in your correlationover a large area?

21. Recognize Cause and Effect Explain how some woolly mammoths could have beenpreserved intact in frozen ground. Whatconditions must have persisted since thedeaths of these animals?

22. Classify each of the following fossils in the correct category in the table below:dinosaur footprint, worm burrow,dinosaur skull, insect in amber, fossilwoodpecker hole, and fish tooth.

23. Compare and contrast the three different kindsof unconformities. Draw sketches of eachthat illustrate the features that identifythem.

24. Describe how relative and absolute ages dif-fer. How might both be used to establishages in a series of rock layers?

Types of Fossils

Trace Fossils Body Fossils

earth.msscience.com/chapter_review

Do not write in this book.

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Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

Use the photo below to answer question 1.

1. Which type of fossil preservation is shownabove?A. trace fossil C. carbon filmB. original D. permineralized

remains remains

2. Which principle states that the oldest rocklayer is found at the bottom in an undis-turbed stack of rock layers?A. half-life C. superpositionB. absolute dating D. uniformitarianism

3. Which type of scientist studies fossils?A. meteorologist C. astronomerB. chemist D. paleontologist

4. Which are the remains of species thatexisted on Earth for relatively short periodsof time, were abundant, and were wide-spread geographically?A. trace fossils C. carbon filmsB. index fossils D. body fossils

5. Which term means matching up rock layersin different places?A. superposition C. uniformitarianismB. correlation D. absolute dating

6. Which of the following is least likely to befound as a fossil?A. clam shell C. snail shellB. shark tooth D. jellyfish imprint

7. Which type of fossil preservation is a thincarbon silhouette of the original organism?A. cast C. moldB. carbon film D. permineralized

remains

8. Which isotope is useful for dating woodand charcoal that is less than about 75,000 years old?A. carbon-14 C. uranium-238B. potassium-40 D. argon-40

Use the diagram below to answer questions 9–11.

9. Which sequence of letters describes therock layers in the diagram from oldest toyoungest?A. D, Q, A, Z, L C. Z, L, A, D, QB. L, Z, A, Q, D D. Q, D, L, Z, A

10. What does the wavy line between layers Aand Q represent?A. a disconformity C. a nonconformityB. a fault D. an angular

unconformity

11. Which of the following correctly describesthe relative age of the fault?A. younger than A, but older than QB. younger than Z, but older than LC. younger than Q, but older than AD. younger than D, but older than Q

Fault

388 STANDARDIZED TEST PRACTICETom Bean/CORBIS

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STANDARDIZED TEST PRACTICE 389

Record your answers on the answer sheetprovided by your teacher or on a sheet of paper.

12. What is a fossil?

13. How is a fossil cast different from a fossilmold?

14. Describe the principle of uniformitarianism.

15. Explain how the original remains of aninsect can be preserved as a fossil inamber.

16. Why do scientists hypothesize that Earthis about 4.5 billion years old?

17. Describe the process of radioactive decay.Use the terms isotope, nucleus, and half-lifein your answer.

Use the table below to answer questions 18–20.

18. What value should replace the letter X inthe table above?

19. What value should replace the letter Y inthe table above?

20. Explain the relationship between thenumber of half-lives that have elapsedand the amount of parent isotoperemaining.

21. Compare and contrast the three types ofunconformities.

22. Why are index fossils useful for estimatingthe age of rock layers?

Record your answers on a sheet of paper.

23. Why are fossils important? What informa-tion do they provide?

24. List three different types of trace fossils.Explain how each type forms.

Examine the graph below and answer questions 25–27.

25. How does the potential for remains to bepreserved change as the rate of burial bysediment increases?

26. Why do you think this relationship exists?

27. What other factors affect the potential forthe remains of organisms to become fossils?

28. How could a fossil of an organism thatlived in ocean water millions of years agobe found in the middle of North America?

Number of Half-lives Parent Isotope Remaining (%)

1 100

2 X

3 25

4 12.5

5 Y

Check It Again Double check your answers before turning inthe test.

Relationship Between Sediment Burial Rate and Potential for Remains to Become Fossils

Low

High

Preservation potential

Sed

imen

t b

uri

al ra

te

Low High

earth.msscience.com/standardized_test

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how are canals the paleozoic era connected?paleontologists, scientists who study fossils, can learn...

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