336 Chapter 12

12.1 Discovering Earth’s History

Reading StrategyIdentifying Main Ideas Copy and expandthe table below. As you read, fill in the firstcolumn with a main idea and add details thatsupport it in the second column.

Key ConceptsHow do rocks allowgeologists to interpretEarth’s history?

How does uniformitarianismhelp explain Earth’sfeatures?

How do geologists userelative dating in theirwork?

What are the key principlesof relative dating?

What do unconformitiesrepresent?

Vocabulary◆ uniformitarianism◆ relative dating◆ law of

superposition◆ principle of original

horizontality◆ principle of

cross-cuttingrelationships

◆ unconformity◆ correlation

In the 18th and 19th centuries, scientists recognized that Earth hada very long history and that Earth’s physical features must have takena long time to form. But they had no way of knowing Earth’s true age.A geologic time scale was developed that showed the sequence, ororder, of events based on several principles of relative dating. What arethese principles? What part do fossils play? In this chapter you willlearn the answers to these questions.

Rocks Record Earth HistoryIn 1869, Major John Wesley Powell, shown in Figure 1A, led an expedi-tion down the Colorado River and through the Grand Canyon, shown inFigure 1B. Powell realized that the evidence for an ancient Earth was con-cealed in its rocks. Powell was impressed with the record of Earth’s historycontained in the rocks exposed along the walls of the Grand Canyon.

Rocks record geological events and changing life forms of thepast. Erosion has removed a lot of Earth’s rock record but enough ofit remains to allow much of the story to be studied and interpreted.

Geological events by themselves, however, have little meaning untilthey are put into a time perspective. The geologic time scale revolu-tionized the way people think about time and how they perceive ourplanet. We have learned that Earth is much older than anyonehad previously imagined and that its surface and interior have beenchanged by the same geological processes that continue today.

Figure 1 Exploring the GrandCanyon A John Wesley Powell,pioneering geologist and thesecond director of the U.S.Geological Survey. B Start of the expedition from Green River station.

A

B

Main Idea Details

1.

2.

3. f. ?e. ?

d. ?c. ?

b. ?a. ?

336 Chapter 12

FOCUS

Section Objectives12.1 Explain how rocks allow

geologists to interpret Earth’shistory.

12.2 Recognize how uniformitari-anism helps explain Earth’sfeatures.

12.3 List the key principles ofrelative dating and describehow geologists use relativedating in their work.

12.4 Describe the importance ofunconformities in unravelingEarth’s history.

Build VocabularyWord Forms Point out that the worduniform is closely tied to this section’svocabulary term uniformitarianism. Askstudents to state the meaning of uniformin their own words. (Sample answer:always the same) Tell them that themeaning of uniform can help them toremember the meaning of uniformitar-ianism, which refers to processes thathave operated in a similar mannerthroughout Earth’s history.

Reading StrategySample answers: a. rock record; b. cluesto geological events and changing lifeforms; c. uniformitarianism; d. today’sprocesses mimic those of the past; e.relative dating; f. the relative order ofevents

INSTRUCT

Rocks RecordEarth HistoryBuild Science SkillsUsing Analogies Have students createanalogies that describe the followingkey concept: “Rocks record geologicalevents and changing life forms of thepast.” A sample analogy might be thatrocks are like a long and complicatedwell-worn novel. Even though some ofthe pages might be missing or torn,enough of the book remains for thestory to be understood.Verbal

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2

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Reading Focus

1

Section 12.1

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A Brief History of GeologyThe primary goal of geologists is to interpret Earth’s history. By study-ing rocks, especially sedimentary rocks, geologists can begin tounderstand and explain the past.

In the mid-1600s, Archbishop James Ussher constructed a chronol-ogy or time line of both human and Earth history in which hedetermined that Earth was more than five thousand years old. Hebelieved Earth had been created in 4004 B.C. Ussher published hischronology, and his book earned widespread acceptance amongEurope’s scientific and religious leaders.

In the late 1700s, James Hutton, a Scottish physician and gentlemanfarmer, published his Theory of the Earth. In this work, Hutton put forththe fundamental principle of uniformitarianism, which simply statesthat the physical, chemical, and biological laws that operate today havealso operated in the geologic past. Uniformitarianism means thatthe forces and processes that we observe today have been at work for avery long time. To understand the geologic past, we must first under-stand present-day processes and their results.

Today, scientists understand that these same processes may notalways have had the same relative importance or operated at preciselythe same rate. Moreover, some important geologic processes are notcurrently observable, but evidence that they occur is well established.For example, we know that Earth has been hit by large meteorites eventhough we have no human witnesses. Such events altered Earth’s crust,modified its climate, and strongly influenced life on the planet.

The acceptance of uniformitarianism meant the acceptance of avery long history for Earth. It is important to remember that althoughmany features of our physical landscape may seem to be unchangingover our lifetimes, they are still changing, but on time scales of hun-dreds, thousands, or even millions of years.

Relative Dating—Key PrinciplesDuring the late 1800s and early 1900s, several attempts were made todetermine the age of Earth. To establish a relative time scale, a few basicprinciples or rules had to be discovered and applied. These principleswere major breakthroughs in thinking at the time, and their discoveryand acceptance was an important scientific achievement.

Relative dating means identifying which rock units formed first,second, third, and so on. Relative dating tells us the sequence inwhich events occurred, not how long ago they occurred.

How do the laws that govern geologicalprocesses change through time?

Geologic Time 337

A Brief Historyof GeologyUse CommunityResourcesArrange for students to visit a localmuseum or historical society to viewartifacts that relate to important eventsin your community’s geologic history.Kinesthetic

Relative Dating—Key Principles

As students read about the law ofsuperposition, make sure they clearlydistinguish between theories and laws.Some students mistakenly think thatwhen theories gain enough supportingevidence, they automatically becomelaws. In reality, a theory never becomesa law. A scientific law simply describesthe behavior of an event or process innature. A theory attempts to explain thisbehavior.Verbal

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Geologic Time 337

Customize for English Language Learners

Use a cloze strategy to extract information fromthe text about the key principles of relativetime. For example, after reading pp. 336–337,have ELL students fill in the blanks in thefollowing paragraph: Rocks recordevents and changing of the past.

Uniformitarianism means that theand processes that operate havebeen at work for a very long time.(geological, life forms, forces, today) Use studentanswers to pinpoint misunderstandings andclarify key concepts.

Answer to . . .

The same laws thatoperated in the past

still operate today.

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338 Chapter 12

Law of Superposition Nicolaus Steno, a Danish anatomist,geologist, and priest (1636–1686), is credited with describing a set ofgeologic observations that are the basis of relative dating. The firstobservation is the law of superposition. The law of superposi-tion states that in an undeformed sequence of sedimentary rocks,each bed is older than the one above it and younger than the onebelow it. Although it may seem obvious that a rock layer could not bedeposited unless it had something older beneath it for support, it wasnot until 1669 that Steno stated the principle. This rule also applies toother surface-deposited materials, such as lava flows and beds of ashfrom volcanic eruptions. Applying the law of superposition to the bedsexposed in the upper portion of the Grand Canyon, shown in Figure2, you can easily place the layers in their proper order.

Principle of Original HorizontalityAnother of Steno’s observations is called theprinciple of original horizontality. Theprinciple of original horizontality means thatlayers of sediment are generally deposited ina horizontal position. If you see rock layersthat are flat, it means they haven’t been dis-turbed and they still are in their originalhorizontal position. The layers in the GrandCanyon shown on pages 334–335 and inFigure 2 clearly demonstrate this. However, therock layers shown in Figure 3 have been tiltedand bent. This tilting means they must havebeen moved into this position sometime aftertheir deposition.

To what rock type can the law of superposition and the principle of original horizontality be best applied?

Figure 3 Disturbed Rock LayersRock layers that are folded ortilted must have been moved into that position by crustaldisturbances after theirdeposition. These folded layersare exposed in the Namib Desert(southwestern Africa).

Figure 2 Orderingthe Grand Canyon’sHistory The law ofsuperposition can be applied to thelayers exposed in the Grand Canyon. InterpretingIllustrations Whichlayer is the oldest?youngest?

Kaibab Limestone

Toroweap Formation

Youn

ger

Coconino Sandstone

Hermit Shale

Supai GroupA B

338 Chapter 12

Use VisualsFigure 2 Have students state the lawof superposition in their own words.(Unless layers of sedimentary rock aredisturbed, the oldest rocks will be on thebottom. The rocks get younger frombottom to top.) Then have studentssequence the layers of rock from oldestto youngest. (Supai Group, Hermit Shale,Coconino Sandstone, ToroweapFormation, Kaibab Limestone)Visual

Build Reading LiteracyRefer to the Build Reading Literacystrategy for Chapter 1, which providesthe guidelines for an anticipation guide.

Anticipation Guide Before studentsread about the principle of cross-cuttingrelationships and unconformities, havethem review the law of superposition.Point out that this law only pertains toan undisturbed sequence of sedimentaryrocks. Ask: What might happen if therocks were disturbed? (Sample answer:The youngest rocks might not necessarilybe on top.) Have students read the texton pp. 339–342 to see if theirpredictions were correct.Verbal, Logical

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Section 12.1 (continued)

The work of Swiss scientist Louis Agassizprovides an excellent example of the applicationof the principle of uniformitarianism. In 1821,Agassiz heard another scientist present a paperstating that glacial features occurred in placesthat were far from existing glaciers in the Alps.The hypothesis implied that these glaciershad once been much larger in size. Agassizdoubted the hypothesis and set out to proveit wrong. Ironically, he was the one who was

wrong. In the Alps, Agassiz found the sameunique deposits and features that could beseen forming with active glaciers far beyondthe limits of the ice in the Alps. Subsequentwork led Agassiz to hypothesize that a greatice age had occurred in response to a periodof worldwide climate change. Agassiz’s ideaseventually developed into our present-dayglacial theory.

Facts and Figures

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Geologic Time 339

Principle of Cross-Cutting RelationshipsThe principle of cross-cutting relationships is Steno’sthird observation. The principle of cross-cuttingrelationships states that when a fault cuts through, orwhen magma intrudes other rocks and crystallizes, wecan assume that the fault or intrusion is younger thanthe rocks affected. For example, in Figure 4 you can seethat fault A occurred after the sandstone layer wasdeposited because it “broke” the layer. However, fault Aoccurred before the conglomerate was laid down,because that layer is unbroken. Because they cutthrough the layers of sedimentary rock, the faults anddikes clearly must have occurred after the sedimentarylayers were deposited.

Inclusions Sometimes inclusions can help the relative datingprocess. Inclusions are pieces of one rock unit that are containedwithin another. The rock unit next to the one containing the inclu-sions must have been there first in order to provide the rock fragments.Therefore, the rock unit containing inclusions is the younger of thetwo. Figure 5 provides an example. The photograph in Figure 5C showsinclusions of igneous rock within a layer of sedimentary rock. Howdid they get there? The inclusions indicate that the sedimentary layerwas deposited on top of the weathered igneous mass. The sedimen-tary layer must be younger than the igneous rock because thesedimentary layer contains pieces of the igneous rock. We know thelayer was not intruded upon by magma from below that later crystal-lized because the sedimentary rock is still horizontal.

ConglomerateShale

Sandstone

Sill

Batholith

Fault A

Dike A Fault B

Dike B

Formation of Inclusions

Figure 5 A A mass of igneous rock formed from magma that intruded an olderrock body. B The older rock erodes and exposes the igneous rock to weathering.C Sedimentary rock layers form on top of the weathered igneous rock.

Figure 4 Applying Steno’sPrinciples Cross-cuttingrelationships are an importantprinciple used in relative dating.An intrusive rock body is youngerthan the rocks it intrudes. A faultis younger than the rock layers it cuts. Interpreting Diagrams What is the age relationship betweenthe batholith, dike B, dike A, and the sill?

Intrusive igneous rock Exposure and weathering ofintrusive igneous rock

Deposition ofsedimentarylayers

Sedimentarylayers Inclusions of

igneous rock

Intrusive igneousrock Nonconformity

A B C

Cross-CuttingRelationshipsPurpose Students will observe howcross-cutting relationships can be usedin relative dating.

Materials wooden stick, piece of foam

Procedure To represent a dike, insert awooden stick into a corner of the foamfrom underneath. Then break off thatcorner and realign the pieces so that thefoam and stick are close to their originalpositions. Ask students to explain whateach element represents, and therelative ages of those elements.

Expected Outcomes Students willrecognize that the foam represents theoldest layer of rock. The wooden stickrepresents a younger intrusion. Thebroken foam represents a fault; it isthe youngest element.Visual

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Geologic Time 339

Answer to . . .

Figure 2 The oldest layer is theSupai Group. The youngest layer isthe Kaibab Limestone.

Figure 4 The batholith is oldest.The sill and dike B are the same unit.Dike A is the youngest.

undisturbed sedimentaryrocks

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340 Chapter 12

Unconformities Casual observation of layers of rock may looklike they represent a complete geologic history of an area. However, noplace on Earth is geologically complete. Throughout Earth’s history, thedeposition of sediment has been interrupted again and again. All suchbreaks in the rock record are termed unconformities. An uncon-formity represents a long period during which deposition stopped,erosion removed previously formed rocks, and then depositionresumed. In each case uplift and erosion are followed by subsidenceand renewed sedimentation, as shown in Figure 6. Unconformities areimportant features because they represent significant geologic eventsin Earth history. Moreover, their recognition helps us identify whatintervals of time are not represented in the rock record.

A geologic cross section of the Grand Canyon is shown in Figure 7. It shows the three basic types of unconformities: angularunconformities, disconformities, and nonconformities. Perhaps themost easily recognized unconformity is an angular unconformity. Itappears as tilted or folded sedimentary rocks that are overlain byyounger, more flat-lying strata. An angular unconformity indi-cates that during the pause in deposition, a period of deformation(folding or tilting) and erosion occurred.

Two sedimentary rock layers that are separated by an erosional sur-face are called a disconformity. Disconformities are more commonthan angular unconformities, but they are more difficult to recognize.The third basic type of unconformity is a nonconformity.Nonconformities mean the erosional surface separates older meta-morphic or intrusive igneous rocks from younger sedimentary rocks.

What are the three basic types of unconformities?

KaibabFormationToroweapFormation

CoconinoSandstone

HermitShale

Supai Group

Redwall Limestone

MuavLimestone

Bright AngelShale

TontoGroup

TapeatsSandstone

Permian

Pennsylvanian

Mississippian

Devonian

Cambrian

Precambrian

Disconformity

Disconformity

Angularunconformity

NonconformityVishnu Schist

UnkarGroup

Zoroaster Granite

InnerGorge

ColoradoRiver

Kaibab Plateau

Figure 7 A Record of Uplift,Erosion, and Deposition Thiscross section through the GrandCanyon illustrates the three basictypes of unconformities.

Sea level

1234

Deposition

5

1234

Folding and uplifting

5

1234

Erosion

5

1234

Subsidence and renewed deposition

7

6 (Angularunconformity)

Sea level89

5

A

B

C

D

Figure 6 Formation of an Angular Conformity An angular unconformityrepresents an extended periodduring which deformation anderosion occurred.

340 Chapter 12

Build Reading LiteracyRefer to p. 530D in Chapter 19, whichprovides the guidelines for makinginferences.

Make Inferences Have students drawon their prior knowledge to infer whysedimentary rocks in particular helpscientists learn about Earth’s past.Ask: How would heat, pressure, andmelting affect fossils? (They woulddestroy fossils.) What type of rock likelycontains most fossils? Explain youranswer. (Sedimentary rock likely containsmost fossils because it has not beenaffected by melting, heat, or pressure.)Verbal, Logical

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Section 12.1 (continued)

Although Earth processes vary in intensity,they still take a very long time to create ordestroy major landscape features. Forexample, geologists have established thatmountains once existed in portions of present-day Minnesota, Wisconsin, and Michigan.

Today the region consists of low hills andplains. Erosion gradually wore down thesepeaks. Scientists estimate that the NorthAmerican continent is eroding at a rate ofabout 3 cm every 1000 years.

Facts and Figures

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Geologic Time 341

Grand Canyon National Park Zion National Park Bryce Canyon National Park

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Devonian

Pennsylvanian

Mississippian

Cambrian

PrecambrianColoradoRiver

Moenkopi Fm

Kaibab Ls

Coconino SsHermit Shale

Supai Fm

Redwall Ls

Temple Butte Ls

Bright Angel Shale

Tapeats Ss

Vishnu Schist

Carmel Fm

Navajo Ss

Kayenta Fm

Wingate Ss

Chinle Fm

Moenkopi Fm

Kaibab Ls

Older rocks not exposed

Wasatch Fm

Kaiparowits Fm

Wahweap Ss

Straight Cliffs Ss

Tropic Shale

Dakota Ss

Winsor Fm

Curtis Fm

Entrada Ss

Carmel Fm

Navajo Ss

Older rocks not exposed

Muav Fm

Toroweap Fm

NEVADA

UTAH

ARIZONA

Grand CanyonNational

Park

ZionNational

Park

BryceCanyonNational

Park

NEVADA

UTAH

ARIZONA

Grand CanyonNational

Park

ZionNational

Park

BryceCanyonNational

Park

Figure 8 Correlation of strata at three locations onthe Colorado Plateau reveals a more complete viewof the extent of sedimentary rocks in the region.

Use VisualsFigure 8 To fully understand thisdiagram, which includes geologicsubdivisions, students will need toexamine a copy of the geologic timescale. Either obtain a copy or referstudents to Figure 17 on p. 353. Ask:Which site has the oldest exposedrocks? (Grand Canyon National Park)Visual

Build Science SkillsMaking Judgments Build on priorknowledge to ensure that studentsunderstand the concept of relativedating. Ask: If you were told thata person has an older sister anda younger brother, what can youdetermine about their ages? (You candetermine the relative ages of the threesiblings. You would need additionalinformation, however, to determinethe actual ages.)Logical

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Geologic Time 341

Answer to . . .

angular unconformity,disconformity, and

nonconformity

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342 Chapter 12

Section 12.1 Assessment

Reviewing Concepts1. What information do rocks provide to

geologists?

2. What does uniformitarianism tell us aboutprocesses at work on Earth’s surface today?

3. How can relative dating be used ingeology?

4. List and briefly describe Steno’s principles.

5. What is an unconformity?

6. What is the name of the process in which rocklayers in different regions are matched?

Critical Thinking7. Applying Concepts How did the

acceptance of uniformitarianism change theway scientists viewed Earth?

8. Comparing and Contrasting How doesArchbishop Ussher’s work compare to that ofJames Hutton? What did each do to back uptheir ideas?

9. Summarizing What features would you lookfor to correlate rocks from one area toanother?

Correlation of Rock LayersTo develop a geologic time scale that can be applied to the entire Earth,rocks of similar age in different regions must be matched up. This taskis called correlation.

Within a small area, you can correlate the rocks of one locality withthose of another by simply walking along the outcropping edges.However, this might not be possible when the rocks are covered by soiland vegetation. You can correct this problem by noting the position ofa distinctive rock layer in a sequence of strata. Or, you might be ableto identify a rock layer in another location if it is composed of verydistinctive or uncommon minerals.

By correlating the rocks from one place to another, it is possibleto create a more complete view of the geologic history of a region.Figure 8 on page 341, for example, shows the correlation of strata atthree sites on the Colorado Plateau in southern Utah and northernArizona. No single location contains the entire sequence. But correla-tion reveals a more complete picture of the sedimentary rock record.

The methods just described are used to trace a rock formation overa relatively short distance. But they are not adequate for matchingrocks that are separated by great distances. The use of fossils comes into play when trying to correlate rocks separated by great distances.For: Links on relative dating

Visit: www.SciLinks.org

Web Code: cjn-4122

Descriptive Paragraph Imagine that youare hiking down into the Grand Canyon.Use some of Steno’s principles to write aparagraph describing what you see, howold it all is, and how it was deposited.

342 Chapter 12

Correlation ofRock LayersBuild Science SkillsApplying Concepts Review Steno’sprinciples. Then ask: Why must therock layers be undisturbed in order toapply them? (Sample answer: Tectonismcan cause entire sequences of rock to beoverturned, in which case the oldest layerswould be on the top.)Logical

ASSESSEvaluateUnderstandingHave students use clay to model thethree basic types of unconformities.

ReteachHave students summarize how our viewsof Earth’s age have changed over time.(People once believed that Earth was onlya few thousand years old. We now knowthat Earth is 4.6 billion years old.)

A sample paragraph might use the lawof superposition to describe the layersfrom bottom to top; the principle oforiginal horizontality to describe therock layers on either side of the canyonand how they match up; and theprinciple of cross-cutting relationshipsto explain the relative ages of the dikestoward the bottom of the canyon.

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Section 12.1 (continued)

original horizontality states that sedimentaryrocks are generally deposited horizontally. Theprinciple of cross-cutting relationships statesthat features such as faults and intrusions areyounger than the features they cut across.5. a break in the rock record6. correlation7. They learned that Earth was very old, thatEarth’s landscape is always changing, andthat the processes they observed had alsobeen at work in the past.

Section 12.1 Assessment

1. They provide information about geologicalevents and changing life forms of the past.2. The processes also operated in thegeologic past.3. It helps to identify the order of events,such as which rock layer was deposited first.4. The law of superposition states that in asequence of undisturbed rock layers, the old-est layer is on the bottom; the upper layersare progressively younger. The principle of

Download a worksheet on relativedating for students to complete,and find additional teacher supportfrom NSTA SciLinks.

8. Ussher made a timeline of human historyand Earth history. He did not consider Earth’sphysical features. Hutton’s ideas came fromdirect observations of Earth and Earthprocesses.9. same rock type, distinctive layers,uncommon minerals, specific fossils

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