The Age of Earth's Rocks

LONumber=6E1_0410; CorrectionKey=NL-A DO NOT EDIT--Changes mustbemadethrough "Fileinfo" 96 The Age of Earth’s Rocks Unit 2EarthThrough Time determine theagesofEarth’s rocks andfossils. you willbeabletoexplain howscientists By theendofthislesson . . contain cluesaboutthehistoryofarea. The colorfulrock layersthatmake upthesehillsinOregon LESSON 1 LESSON

© Houghton Mifflin Harcourt • Image Credits: (r) ©Barrett & MacKay Photo/All Canada Photos/Getty Images nearby sea. many riversthatflowedintoa lush vegetation,aswell consisted ofswampsand small mammals.Thearea turtles, crocodiles, and dinosaurs livedalongside About 76millionyearsago, 1. explain howwe knowwhentheseancient animalslived. EVIDENCE NOTEBOOK shown abovewere determined? past environments, plants, andanimals.Howdoyouthinktheagesofanimals The rocks andfossils foundtodayinDinosaurProvincial Park tellusaboutthepark's How dowe ancient know animals lived? when these History of Dinosaur Provincial of ParkHistory CAN YOU EXPLAIN IT? IT? YOUCAN EXPLAIN As youexplore thislesson, gatherevidencetohelp as thisplesiosaur. and large marinereptiles such creatures suchasammonites area. Inthesealivedshelled the searose andcovered the Around 75millionyearsago, download additionallabresources. the Hands-OnLabforthislesson andto Go onlinetoviewthedigitalversionof sedimentary layers. lived here are foundwithinthe plants andanimalsthatonce park. Fossils oftheancient are exposedthroughout the Today, sedimentaryrock layers Lesson 1TheAge ofEarth’s Rocks 97 LONumber=6E1_0410; CorrectionKey=NL-A DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 98 2. Discuss rocks thatcanforminbodiesofwaterasmaterialssettle inlayers andharden overtime. forms whenlavacoolsaboveEarth’s surface.Limestoneandsiltstoneare sedimentary that formswhenmagmacoolsbeneathEarth’s surface.Basaltisanigneousrock that Different rocks formindifferent environments. For example,granite isanigneousrock erosion, deposition,melting,andburialtake placeoverthousandstomillionsofyears. you seetodaywere notalwaysthere. Rocksformand changeasprocesses suchas humans, andevenbefore manyoftheplantsoranimalsthatyouseelivedonEarth. range from hundreds tobillionsofyearsold!Manytherocks youseeexistedbefore Why wouldanyonewanttostudyrocks? Rockstellusalotaboutthepast.Earth’s rocks Unit 2EarthThrough Time their ages.Whichrock layer formedfirst? Earth isever-changing. Althoughrocks seemlike theyare permanent,therocks Describing Formation the of Sedimentary With aclassmate, lookattherock layersinthepicture. Noteanypatterns in Rocks and Fossils EXPLORATION 1 seashore. rocks formedinanarea near a in theserocks suggest thatthe ago. Fossils andpatterns shown formed about250millionyears The lowerbrown-red layer once existed. where tropical lakes andstreams scientists cantellitformed studying thisrock layer, 200 millionyearsago.By This rock layerformedabout today’s Sahara Desert. with hugesanddunes,similarto formed inadesertenvironment Patterns intherock showthatit about 145millionyearsago. This rock layerbeganforming

© Houghton Mifflin Harcourt • Image Credits: (t) ©Emmanuel LATTES/Alamy; (c) ©Peter Essick/Aurora/Getty Images; (b) ©Corbin17/Alamy is atthetop. oldest layerisatthebottom andtheyoungestlayer sedimentary rocks stayinhorizontal layers.The one byone.IfundisturbedEarthprocesses, harden, actingasagluetoformrock. layers. Thesubstancescomeoutofthesolutionand dissolved substancesseepsthrough thesediment under theweightofupperlayers.Water containing and becomescompressed aslowerlayersare buried and lake bottoms. Thesedimentsettles inlayers moves sedimenttolow-lyingareas suchasvalleys organisms. limestone can be made of the remains of microscopic from theremains ofplantsandanimals.For example, called sediment.Somesedimentaryrocks are made Sedimentary rocks are madeoftinyrock pieces Formation Rock Sedimentary fossils aswell. preserved inrock. Ice,tar, andtree sapcancontain such asfootprintsorburrows. Notallfossils are Fossils canalsoshowtraces ofananimal’s activity, by minerals thatform arock versionofthepart. Bone, shell,orwood,forexample,canbereplaced organism butare nottheorganism’s actualparts. sedimentary rock. Somefossils looklike partsofan remains ofanorganism thatlivedlongago. rock, whichcontainsafossil. Fossils are thetraces or preserved. Thesedimenthardens andbecomesa by sediment,evidenceoftheorganism canbe is eatenbyanimals.However, ifitisquicklyburied When aplantoranimaldies,itoften decomposesor Rock Sedimentary in Fossils As sedimentarylayersform,theystackup Sedimentary rock canformwhenerosion Fossils are mostcommonlypreserved in solution tothisproblem. Canyouthinkofanyhelpfultoolsortechnology? where youcanseetherock layers.Theyare allbelowtheground. Propose a clues aboutthepast.There isoneproblem: There are noexposedcliffs orareas EngineerIt 3. You wanttoexplore rock layersinaregion tofindfossils and and oceans.Thesepiecescanformrocks suchascoquina. Over time,shellsandsedimentpileupatthebottom oflakes cemented intorocks like thislimestone. shelled animalsthatlivedinthepastmaybecompressed and After thousandstomillionsofyears,sedimentandpartsfrom solids. Thiscanformsedimentaryrock layerssuchasgypsum. water evaporates, dissolved substancesinthe waterbecome Over verylongperiods,bodiesofwatercandryup.Asthe Lesson 1TheAge ofEarth’s Rocks 99 LONumber=6E1_0410; CorrectionKey=NL-B DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 100 A. 5. fern lookssimilartofernsfoundintropical areas today. Fossils provide evidenceaboutpastlifeandenvironments. For example,thisfossilized Analyze Fossils to Describe Earth’s Past 4. the ocean. the inacorallives reef in starfish inthisphoto years. The 400 million foron Earth over starfish lived have Different of types Unit 2EarthThrough Time name oftheenvironment inwhichitlikely formed. Look ateachimage.Usethewords from theword banktolabeleachfossil withthe for thesecondandthird photosbelowtoexplainhowthelivingstarfishbecameafossil. Under certainconditions,deadorganisms canbepreserved asfossils. Write captions B. C. grassland • a lake • a • a tropical forest WORD BANK

© Houghton Mifflin Harcourt • Image Credits: (tl) ©Vilainecrevette/Shutterstock; (tc) ©Danita Delimont/Gallo Images/Getty Images; (tr) ©Roberto Nistri/Alamy; (bl) ©CM Dixon/The Print Collector/Getty Images; (bc) ©Snowshill/Shutterstock; (br) ©Kevin Schafer/Alamy LONumber=6E1_0410; CorrectionKey=NL-B DO NOT EDIT--Changes mustbemadethrough “Fileinfo”

© Houghton Mifflin Harcourt • Image Credits: ©Lori Andrews/Moment/Getty Images 6. Discuss You candescriberelative agewithoutknowingactualage. older thanthebabyandyoungeradult.Thisisadescriptionofyourrelative age. how oldsomethingelseis.For example,thinkofababyandanadultyouknow. You are determining therelative agesofrocks. Relativeageishowoldsomethingcompared to But howcanrocks andfossils telluswheneventslike thesehappened?Onewayisby rocks eventellusabouteventssuchasmeteoriteimpactsandvolcaniceruptions. Rocks andfossils tellusalotaboutEarth’s pastenvironments andorganisms. Some Age Relative the oldestandwhichisyoungest? Determining the Relative Ages of Rocks Think abouttherelative ages ofthesepancakes. Canyouexplainwhich is EXPLORATION 2 Lesson 1TheAge ofEarth’s Rocks while stackingthemonaplate. He cooked thepancakes oneatatime Taral madepancakes forhisfriends. 101 DO NOT EDIT--Changes must be made through “File info” LONumber=6E1_0410; CorrectionKey=NL-B

Hands-On Lab Model Rock Layers to Determine Relative Age

How can building a model help you determine the order in which rocks MATERIALS form? A physical model can help you see how a sequence of rocks can • items such as beads or shells form over time, one by one. It can also help you make observations to represent fossils about the rocks’ relative ages. • modeling clay (at least five In the real world, rock sequences can span large areas. Often, different colors) rocks are not visible because they are beneath Earth’s surface, but • plastic knife sometimes they are exposed along cliffs or where a hill was cut through • tray or container to hold for road construction. the model

Procedure STEP 1 Discuss Gather your materials. Discuss with a group or partner how you can use your materials to make a model of four sedimentary rock layers.

STEP 2 Choose four rocks from the list. Note the environment in which each type of rock formed. This is also known as a rock’s depositional environment. • Sandstone with fossils formed in a sandy ocean bottom (yellow clay with fossil materials) • Shale formed in a deep, muddy lake (brown clay) • Siltstone formed in a river floodplain (red clay) • Sandstone formed in a sandy desert (yellow clay) • Coal formed where a tropical swamp once existed (black clay) • Limestone formed in a shallow sea (white clay) STEP 3 Build your model. Starting at the bottom of the table, complete Rows 1–4 with the information listed above about each of your rock layers.

Order of Rock type Depositional Material used Events or event environment 5. Fifth

4. Fourth

3. Third © Houghton Mifflin Harcourt

2. Second

1. First

102 Unit 2 Earth Through Time DO NOT EDIT--Changes must be made through “File info” LONumber=6E1_0410; CorrectionKey=NL-B

STEP 4 Choose one of these events to model in volcanic eruption your set of rock layers. Be sure to add the new layer of igneous rock event to the table in the top row. Use the plastic knife or another color of clay to igneous intrusion represent the chosen event. • Igneous intrusion: Magma rose and intruded through some of the sedimentary rock layers. The magma cooled into igneous rock.

• Lava flow: Magma intruded through magma chamber all rock layers making its way to the surface (where it is then called lava). This diagram shows both an igneous intrusion below Earth’s The lava flowed over the top of the surface and a lava flow on Earth’s surface. existing rock layers and cooled into a new layer of igneous rock.

• Fault: Tectonic plate movements can fault cause rock to break and move along planes called faults. Rock layers can shift up, down, sideways, or at an angle along faults. Analysis STEP 5 Language SmArts Use your table and knowledge of relative age to write a short, informative paragraph that explains how your model represents Forces deep within Earth can form a fault environmental changes in an area over time. where rocks shift up or down.

STEP 6 Draw Exchange models with another group. Analyze the other group’s model to determine the order in which each rock formed. Make a sketch of the model and label it to show the relative ages of the © Houghton Mifflin Harcourt rocks and the event. When you are done, exchange information with the other group to check if you correctly sequenced the layers and features.

Lesson 1 The Age of Earth’s Rocks 103 LONumber=6E1_0410; CorrectionKey=NL-B DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 104 discovered. provide isknownasthefossilrecord. Thefossil record grows asfossils are continually on Earthhaschangedovertime.AllofEarth’s knownfossils andtheinformationthey 75 millionyearsold. rocks inwhichtheyare found,andanyotherfossils inthoserocks, are alsolikely about Provincial Park. Theyare marine indexfossils thatare about75millionyearsold.The of rocks andotherfossils. For example,specificammonitefossils are foundinDinosaur for about1millionyearsorless. Theageofanindexfossil canhelpestablishtheages fossil istheremains ofanorganism thatwascommonandwidespread, butonlyexisted In mostcases,fossils are thesameageasrock inwhichtheyare found.Anindex Fossils scientists topiecetogethersomeofEarth’s pastenvironments andevents. is allofEarth’s knownrocks andtheinformationtheycontain.Therock record allows That is,thefeature isyoungerthantherocks itcutsacross. pancakes. Thesameistrueforafaultoranigneousintrusionthatcutsacross rock layers. Thecuthappenedafter thepancakes were made,sothecutisyoungerthan half? of rocks, fossils, andfeatures suchasfaults.Whatifyoucutthestackofpancakes in than somethingelse.Geologistsuserelative datingtodeterminetherelative ages Relative datingisanymethodofdeterminingwhethersomethingolderoryounger That is,therock layershavenotbeengreatly deformedbygeologicprocesses. bottom andtheyoungestisattop.Thistrueaslongrock layersare undisturbed. It istheyoungest.Thislike astackofsedimentaryrock layers;theoldestlayerisat placed atthebottom ofthestack.Thelastpancake madeendedupontopofthestack. Think aboutthestackofpancakes. Theoldestpancake istheonethatwasmadefirstand Dating Relative Unit 2EarthThrough Time Organizing allofEarth’s knownfossils from oldesttoyoungestshowshowlife Scientists findtherelative agesofrocks tocompiletherock record. Therock record When youdeterminedtherelative agesofthepancakes, youusedrelative dating. 8. EVIDENCE NOTEBOOK park’s ancientanimals. Dinosaur Provincial Park canprovide informationabouttheagesof Explain howundisturbedsedimentary rock layersandindex fossils in igneous intrusion 7. through. the sedimentaryrocks itcuts is of igneousrock. Theintrusion and formedadiagonalband sedimentary rocks, cooled, Magma intrudedintothese older /younger than

9. This type of trilobite is an index fossil that lived about 440 million years ago. Fossils of the trilobite trilobite and the brittle star were found in the same rock layer. What can you infer about these two organisms? A. They likely lived at the same time. B. They are likely closely related. C. They likely lived in different habitats.

D. They are likely younger than the rock layer. brittle star brittle stars

Unconformities Some rock layers are missing, forming gaps in the rock record. Such a gap is called an unconformity. These gaps can occur when rock layers are eroded or when sediment is not deposited for a period of time. In this way, rock layers are like pages in a book of Explore Earth’s history—only some pages were torn out or never written in the first place! ONLINE!

sedimentary layers new sedimentary layers ocean floor unconformity

eroded rock

Over millions of years, sediment settles Sea level drops and exposes the New layers form over the old set on the sea floor, forming layers. sediment. Some layers are eroded. of layers—an unconformity now exists.

Determine Relative Age The positions of rock layers, fossils, faults, and igneous intrusion intrusions can be used to determine their relative ages. Scientists use relative dating to piece together Earth’s history. Look at the rock layers and features in the diagram. fault

10. Number the diagram to show the relative ages of the rocks and features. Use the number 1 for the first (oldest) rock layer or feature. Use the number 7 for the most © Houghton Mifflin Harcourt • Image Credits: (t) ©Chris Hellier/Corbis © Houghton Mifflin Harcourt • Image Credits: recent (youngest) rock layer or feature. 2 Sandstone Limestone Igneous intrusion Shale with Limestone Andesite fossils with fossils

Lesson 1 The Age of Earth’s Rocks 105 LONumber=6E1_0410; CorrectionKey=NL-A DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 106 11. Discuss Howdoyouthinkscientistsfigured out found inthesesamerock layers mustalsobe76millionyearsold. conclude thatsomeoftherock layersintheparkformed76millionyearsago.Fossils rock inDinosaurProvincial Park. The absoluteageoftheashallowsscientiststo understanding ofEarth’s history. For example,weathered volcanicashisfound within as longsomeoftheunstablematerialexists. 1.3 billion yearsforhalfoftheremaining potassium tobreak down.Thispattern repeats 1.3 billion yearsforhalfofthepotassium tobreak downintoargon. Ittakes another at aspecificrate. For example,unstable potassium breaks downintoargon. Ittakes which iswhentherock formed. ratio isusedtocalculatehowlongagotheunstableparticlesbeganbreaking down, relative amountsofunstableparticlesandthemore stableparticlestheybecome.This particles thatbegintobreak downassoonthecrystalforms. Scientistsmeasure the when moltenrock coolsand formsnewcrystals.Thecrystalscontainunstable Absolute datingistypicallyusedtofindtheagesofigneousrocks. Igneousrocks form Absolute datingisanymethodofmeasuringtheactualagesomethinginyears. In the1950s,technologymadeitpossible tofindtheabsoluteagesofsomerocks. Relative and Dating Dating Absolute Write an 12. age istheactualofsomething. expressed inunitsoftime.Inotherwords, absolute Absolute ageidentifieshowoldanobjectis,as object isolderoryoungerthanotherobjects. Relative ageisdescribedintermsofwhetheran Unit 2EarthThrough Time the absoluteageofthiszircon crystal? Write an Absolute datingandrelative datingare usedtogethertoprovide amore complete Different typesofunstableparticlesexistinrocks, andeachtype breaks down R R A nexttostatementsthatdescribeabsoluteage. nexttothosethatdescriberelative age. That istheoldestbicycleI’veeverseen. I am14yearsold. I amyoungerthanmycousin. Using Absolute and Relative Age EXPLORATION 3 holding itisSimonWilde,whodiscovered itin1984. rock inAustralia. It’s about4.2billionyearsold.Theperson This tinyblackspeckisacrystalthatwasfoundinsandstone This isthelastbookinseries. This coinisthenewestinmycollection. My catlivedtobe15yearsold.

Do the Math Determine Absolute Age

The first step in finding a rock’s absolute age is to measure the amounts of unstable particles and the stable particles they form. A = n × h Next, the rate at which the unstable particles break down must be A = Age of rock found. This rate of change is called half-life. Half-life is the amount of n number of half-lives passed time needed for half the amount of unstable particles to change into = h half-life of unstable particle more stable particles. = The rock below formed when molten rock cooled. It contains unstable uranium particles that break down into more stable lead particles. It takes 704 million years for half the rock’s uranium to change to lead. That is, the half-life is 704 million years. At any point in time, the amounts of uranium and lead can be measured to find the rock’s age.

The Breakdown of Uranium to Lead over Three Half-Lives

Uranium

Uranium Lead

IgneousLead rock forms This 704 million years 1.4 billion years later 2.1 billion years later igneous rock just formed. later One half-life Another half-life has The pattern continues. It contains unstable has passed. Half of the passed, so half of the For every half-life, half of uranium particles. At this unstable uranium has remaining uranium has the remaining uranium time, none of the uranium changed into a more stable changed into lead. The changes to lead. has changed into lead. form of lead. The rock is rock is now 1,408 million 704 million years old. years old.

13. Fill in the table as you explore the half-life diagram.

Half-Lives Unstable Particles Stable Form of Time Passed Passed in the Rock the Particle (Millions of Years) 0 16 0 0

2 4 12 1,408

3 14

Lesson 1 The Age of Earth’s Rocks 107 LONumber=6E1_0410; CorrectionKey=NL-A DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 108 Recallthezircon crystalfoundinthesandstoneAustralia. Thezircon formedlong 15. these otherbodies,Earthisabout4.6billionyearsold. same ageasEarth.Absolutedatingofmeteoritesandmoonrocks suggeststhat,like formed atthesametimeasotherbodiesinoursolarsystem,meteoritesshouldbe to Earth’s surface.Theabsoluteagesofmeteoritescanbedetermined.BecauseEarth itself—with oneexception:meteorites. under youngerrocks longago.Therefore, mostrocks onEarthare youngerthanEarth This isbecausethefirstrocks thatformedonEarthhadbeeneroded, melted,orburied Absolute datingcanbeusedtofindtheageofEarth,butnotbyusingrocks from Earth. The Absolute Age of Earth

16. A fieldgeologistmodeledarock sequencetohelpdetermine theirages. Scientists usebothrelative andabsolutedatingtofindtheagesofrocks andfossils. Together Dating Relative Absolute Use and Unit 2EarthThrough Time compared totheagesofrocks around it. sandstone. Completeeachstatementtomake ittrue. before thesandstone.Itwaspartofanigneousrock before itbecamepartofthe calculate theactualageofzircon mineral. statement(s) thatare true: ages oftheigneousrocks given? Checkthe What canyouconcludebasedontheabsolute Meteorites are small,rocky bodiesthathavetraveled through spaceandfallen 14. EVIDENCE NOTEBOOK The rocks shifted alongthefaultless The sandstonemustbeless than The limestoneisbetween200and The shalewithfossils is 175 million than 175millionyearsago. 200 millionyearsold. 175 millionyearsold. years old. in DinosaurProvincial Park lived?Explain. How canabsolutedatingbeusedtodescribewhenthefossil organisms datingwasusedtodeterminetheageofsandstonerock fossils Shale with Sandstone with fossils Limestone Limestone 200 millionyears Andesite intrusion Igneous datingwasusedto 175 millionyears

TAKE IT FURTHER

Continue Your Exploration

Name: Date:

Check out the path below or go online to choose one of the other paths shown.

Exploring the • Hands-on Labs Go online to Ashfall Fossil choose one • Exploring Local Geology of these other Beds • Propose Your Own Path paths.

Although most fossils are preserved by sedimentary rock, some are preserved by igneous rock. Look at these fossils of ancient animals from the Ashfall Fossil Beds in Nebraska. The animals were killed by hot volcanic ash that covered the area during an eruption. As the ash settled into thick layers, it cooled and hardened into rock. Volcanic ash can fall from the sky or flow downhill during an eruption. Ash flows can burn everything in their path, but sometimes they will preserve animals such as these.

These rocks contain fossils of rhinos and horses that died in a volcanic ash flow.

young adult male rhino “Tusker” rhino calf

large three-toed horse “Cormo” rhino calves (possibly twins) “T.L.” and “R.G.C.” © Houghton Mifflin Harcourt • Image Credits: ©Jim West/Alamy ©Jim © Houghton Mifflin Harcourt • Image Credits:

adult female rhino “Sandy” with baby “Justin”

Lesson 1 The Age of Earth’s Rocks 109 DO NOT EDIT--Changes must be made through “File info” LONumber=6E1_0410; CorrectionKey=NL-B

TAKE IT FURTHER

Continue Your Exploration

The following field notes were recorded in the area:

Date: October 9 Location: Ashfall Fossil Beds State Historical Park, Nebraska Observations and Notes: • The fossils in the photo were uncovered in the ash layer. • Other fossils exist above and below the ash layer as shown in the table. • The ash layer is as thick as three meters in some places. • Absolute dating shows that the ash layer formed 12 million years ago.

Rocks and Fossils from the Ashfall Fossil Beds State Historical Park Rock Layers Fossils Found zebras, lemmings, giant camels, muskrats, loose sand and gravel giant beavers sandstone layer barrel-bodied rhinos, giant land tortoises, ash layer camels, rodents, horses sandstone layer alligators, fish, hornless rhinos, giant sandy and silty sedimentary rock layers salamanders Source: The University of Nebraska State Museum and Nebraska Game and Parks Commission, “Geologic Setting of Ashfall Fossil Beds and Vicinity,” 2015

1. The ash layer is igneous rock. Absolute dating shows the ash layer is 12 million years old. What can you infer about the animals found in the ash layer?

2. Language SmArts Write a short informative report applying what you’ve learned to explain the history of the area. • Use the observations and notes to explain what happened in the area over time. • How old are the fossils in the ash layer? What was the area like before the ash flow that formed the fossil beds? • How can absolute and relative dating help you explain how the area changed? © Houghton Mifflin Harcourt

3. Collaborate Many articles about the Ashfall Fossil Beds are available in magazines and on the Internet. Find several articles. In a group discussion, cite specific evidence that could help you identify the article that provides the most accurate and thorough information. Discuss the evidence with the group.

110 Unit 2 Earth Through Time DO NOT EDIT--Changes must be made through “File info” LONumber=6E1_0410; CorrectionKey=NL-B

LESSON 1 SELF CHECK

Can You Explain It?

Name: Date:

How do we know when these ancient animals lived?

76 million years ago 75 million years ago Present day

EVIDENCE NOTEBOOK Refer to the notes in your Evidence Notebook to help you construct an explanation for when these ancient animals lived.

1. State your claim. Make sure your claim fully explains how the ages of the animals shown above were determined. ©Barrett & MacKay Photo/All Canada

2. Summarize the evidence you have gathered to support your claim and explain your reasoning. © Houghton Mifflin Harcourt • Image Credits: (r) © Houghton Mifflin Harcourt • Image Credits: Photos/Getty Images

Lesson 1 The Age of Earth’s Rocks 111 LONumber=6E1_0410; CorrectionKey=NL-B DO NOT EDIT--Changes mustbemadethrough “Fileinfo” 112 Circle thecorrect termtocompleteeach 6. Which ofthefollowingstatementsare true? 5. features toanswer Questions5and 6. Use thediagram ofundisturbedrock layers and 4. 3. Use the photo to answer 3and 4. Questions lesson. of your the following to check the understanding questions Answer Unit 2EarthThrough Time formed fossils inthelimestone. that lived The fossils foundintheshaleare from animals younger /older younger The igneousintrusionis statement. Thesandstoneis201million yearsold. E. D. Allfossils formedbetween 175and C. The faultshifted therocks more than B. A. Choose allthatapply. Whatyeardidthefaultform? E. D. Whatare therelative agesoftherocks? C. When didtheoldestrock layerform? B. Whichistheoldestrock layer? A. Choose allthatapply. answered from theinformationinphoto? Which ofthefollowingquestionscouldbe D. thewhiterock layernearthebottom C. thefaultrunningthrough thecenter B. thethickblackrock layeratthetop A. most recently? Which rock layerorfeature ofthecliff formed Thesandstoneisolderthan200million Whatistheabsoluteageofmostrecent layer? thegray rock layerattheverybottom years old. 200 millionyearsago. 175 millionyearsago. All fossils are over175millionyearsold. before /after thanthefault. theanimalsthat LESSON 1 SELF CHECK 1 LESSON Checkpoints 200 millionyears fossils Shale with Sandstone with fossils Limestone Limestone 175 millionyears Andesite intrusion Igneous

© Houghton Mifflin Harcourt • Image Credits: (t) ©John Shaw/Science Source; (c) ©Chris Hellier/Corbis; (b) ©Andy Manis/AP Images age ofEarth. ages ofrocks andfossils. Absolutedatingprovides evidencethathelpsusestimate the The combinationofabsoluteandrelative datingallowsscientiststodeterminethe in thoselayers. Geologists userelative dating tocompare theagesofdifferent rock layersandthefossils Earth’s history. Sedimentary rock layers—andthefossils inthoserock layers—helpustounderstand Complete this interactive guide study to review the lesson. Interactive Review Howcanscientistsfindtheabsoluteagesofigneous rocks? C. Astudent makes asandwichwithseveral layersofbread and B. A. how theexampleofsandwichrelates torelative dating. a faultcutthrough rock layersafter therock layersformed.Explain cheese. Thenthestudentcutssandwichandsaysitmodelshow Summarize howsedimentaryrock andfossils form. LESSON 1 SELF CHECK 1 LESSON Lesson 1TheAge ofEarth’s Rocks 113