geology needs a time scale a brief history of geology relative

Geology Needs a Time ScaleA Brief History of GeologyRelative Dating—Key PrinciplesCorrelation of Rock LayersFossils: Evidence of Past LifeDating with RadioactivityThe Geologic Time ScaleDifficulties in Dating the Geologic

Time Scale

• Box 10.1 Understanding Earth: Deciphering the Past by Understanding the Present

• Box 10.2 People and the Environment: Radon—A Threat to Human Health

• Box 10.3 Understanding Earth: Using Tree Rings to Date and Study the Recent Past

The strata exposed in Arizona’s Grand Canyon contain clues to hundreds of millions of years of Earth history. (Photo© by Carr Clifton)

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UNIT 4: DECIPHERINGEARTH’S HISTORY

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286 Unit Four Deciphering Earth’s History

In the eighteenth century, James Hutton recog-nized the immensity of Earth history and the im-portance of time as a component in all geologicalprocesses. In the nineteenth century, others ef-fectively demonstrated that Earth had experi-

enced many episodes of mountain building and erosion,which must have required great spans of geologic time.Although these pioneering scientists understood thatEarth was very old, they had no way of knowing its trueage. Was it tens of millions, hundreds of millions, oreven billions of years old? Rather, a geologic time scalewas developed that showed the sequence of eventsbased on relative dating principles. What are these prin-ciples? What part do fossils play? With the discoveryof radioactivity and the development of radiometricdating techniques, geologists now can assign fairly ac-curate dates to many of the events in Earth history.What is radioactivity? Why is it a good “clock” for dat-ing the geologic past? In this chapter we shall answerthese questions.

Geology Needs a Time ScaleIn 1869 John Wesley Powell, who was later to head theU.S. Geological Survey, led a pioneering expeditiondown the Colorado River and through the GrandCanyon (Figure 10.1). Writing about the rock layers thatwere exposed by the downcutting of the river, Powellsaid that “the canyons of this region would be a Book of

Revelations in the rock-leaved Bible of geology.” Hewas undoubtedly impressed with the millions of yearsof Earth history exposed along the walls of the GrandCanyon (see chapter-opening photo).

Powell realized that the evidence for an ancientEarth is concealed in its rocks. Like the pages in a longand complicated history book, rocks record the geo-logical events and changing life forms of the past. Thebook, however, is not complete. Many pages, especial-ly in the early chapters, are missing. Others are tattered,torn, or smudged. Yet enough of the book remains toallow much of the story to be deciphered.

Interpreting Earth history is a prime goal of the sci-ence of geology. Like a modern-day sleuth, the geologistmust interpret clues found preserved in the rocks. Bystudying rocks, especially sedimentary rocks, and thefeatures they contain, geologists can unravel the com-plexities of the past.

Geological events by themselves, however, have lit-tle meaning until they are put into a time perspective.Studying history, whether it be the Civil War or the Ageof Dinosaurs, requires a calendar. Among geology’smajor contributions to human knowledge is the geolog-ic time scale and the discovery that Earth history is ex-ceedingly long.

The geologists who developed the geologic timescale revolutionized the way people think about timeand how they perceive our planet. They learned thatEarth is much older than anyone had previously imag-ined and that its surface and interior have been changed

B.A.

Figure 10.1 A. Start of the expedition from Green River station. A drawing from Powell’s 1875 book. B. Major John Wesley Powell,pioneering geologist and the second director of the U.S. Geological Survey. (Courtesy of the U.S. Geological Survey, Denver)

Chapter 10 Geologic Time 287

over and over again by the same geological processesthat operate today.

A Brief History of GeologyIn the mid-1600s, James Ussher, Anglican Archbishopof Armagh, Primate of all Ireland, published a work thathad immediate and profound influence on people’s viewof Earth’s age. A respected scholar of the Bible, Ussherconstructed a chronology of human and Earth historyin which he determined that Earth was only a few thou-sand years old, having been created in 4004 B.C. Ussher’streatise earned widespread acceptance among Europe’sscientific and religious leaders, and his chronology wassoon printed in the margins of the Bible itself.

During the seventeenth and eighteenth centuriesthe doctrine of catastrophism strongly influenced peo-ple’s thinking about Earth. Briefly stated, catastrophistsbelieved that Earth’s landscapes had been developedprimarily by great catastrophes. Features such as moun-tains and canyons, which today we know take great pe-riods of time to form, were explained as having beenproduced by sudden and often worldwide disasterstriggered by unknowable causes that no longer oper-ate. This philosophy was an attempt to fit the rate of

Earth processes to the prevailing ideas on the age ofEarth.

Birth of Modern GeologyModern geology began in the late 1700s when JamesHutton, a Scottish physician and gentleman farmer,published his Theory of the Earth. In this work, Huttonput forth a fundamental principle that is a pillar of ge-ology today: uniformitarianism. It simply states thatthe physical, chemical, and biological laws that operate todayhave also operated in the geologic past. This means that theforces and processes that we observe presently shapingour planet have been at work for a very long time. Thus,to understand ancient rocks, we must first understandpresent-day processes and their results. This idea iscommonly expressed by saying “the present is the keyto the past” (see Box 10.1).

Prior to Hutton’s Theory of the Earth, no one had ef-fectively demonstrated that geological processes occurover extremely long periods of time. However, Huttonpersuasively argued that weak, slow-acting processescould, over long spans of time, produce effects just asgreat as those resulting from sudden catastrophicevents. Unlike his predecessors, Hutton carefully citedverifiable observations to support his ideas.

UNDERSTANDING EARTH:Deciphering the Past by Understanding the Present

BO

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Louis Agassiz, aSwiss scientist

born in 1807, was instrumental in for-mulating modern ideas about the IceAge (Figure 10.A). The development ofthis knowledge provides an excellentexample of the application of the prin-ciple of uniformitarianism.

In 1821 Agassiz heard another sci-entist present a paper in which he indi-cated that glacial features occurred inplaces that were a significant distancefrom existing glaciers in the Alps. This,of course, implied that the glaciers hadonce occupied areas considerably be-yond their present limits. Agassiz wasskeptical about this hypothesis and setout to invalidate it. Ironically, his field-work in the Alps convinced him of themerits of his colleague’s hypothesis.Agassiz found the same unique de-

posits and features that can be seenforming in association with active glac-iers in places far beyond the limits ofthe ice. Subsequent work led Agassiz tohypothesize that a great ice age had oc-curred in response to a period of world-wide climate change and had affectedlarge parts of the globe. Agassiz’s ideaseventually developed into our present-day glacial theory.

The proof of the glacial theory pro-posed by Agassiz and others constitutesa classic example of applying the prin-ciple of uniformitarianism. Realizingthat certain landforms and other fea-tures are produced by no other knownprocess but glacial activity, they wereable to reconstruct the extent of nowvanished ice sheets. Clearly, under-standing the present was the key to de-ciphering the past.

Figure 10.A Louis Agassiz (1807–1873)played a major role in the development ofglacial theory. (Courtesy of HarvardUniversity Archives)


For example, when he argued that mountains aresculpted and ultimately destroyed by weathering andthe work of running water, and that their wastes arecarried to the oceans by processes that can be observed,Hutton said, “We have a chain of facts which clearlydemonstrates that the materials of the wasted moun-tains have traveled through the rivers”; and further,“There is not one step in all this progress that is not tobe actually perceived.” He then went on to summarizethis thought by asking a question and immediately pro-viding the answer: “What more can we require? Noth-ing but time.”

Geology TodayToday the basic tenets of uniformitarianism are just as vi-able as in Hutton’s day. Indeed, we realize more strong-ly than ever that the present gives us insight into thepast and that the physical, chemical, and biological lawsthat govern geological processes remain unchangingthrough time. However, we also understand that thedoctrine should not be taken too literally. To say that ge-ological processes in the past were the same as those oc-curring today is not to suggest that they always had thesame relative importance or that they operated at pre-cisely the same rate. Moreover, some important geolog-ic processes are not currently observable, but evidencethat they occur is well established. For example, weknow that Earth has experienced impacts from large me-teorites even though we have no human witnesses. Suchevents altered Earth’s crust, modified its climate, andstrongly influenced life on the planet.

The acceptance of uniformitarianism meant the ac-ceptance of a very long history for Earth. AlthoughEarth’s processes vary in intensity, they still take a verylong time to create or destroy major landscape features.

For example, geologists have established thatmountains once existed in portions of present-day Min-nesota, Wisconsin, and Michigan. Today the regionconsists of low hills and plains. Erosion gradually de-stroyed these peaks. Estimates indicate that the NorthAmerican continent is being lowered at a rate of about3 centimeters per 1000 years. At this rate, it would take100 million years for water, wind, and ice to lowermountains that were 300 meters (10,000 feet) high.

But even this time span is relatively short on thetime scale of Earth history, for the rock record containsevidence that shows Earth has experienced many cy-cles of mountain building and erosion. Concerning theever changing nature of Earth through great expansesof geologic time, Hutton made a statement that was tobecome his most famous. In concluding his classic 1788paper published in the Transactions of the Royal Society ofEdinburgh, he stated, “The results, therefore, of our pres-ent enquiry is, that we find no vestige of a beginning—no prospect of an end.”

It is important to remember that although manyfeatures of our physical landscape may seem to be un-changing over our lifetimes, they are nevertheless

changing, but on time scales of hundreds, thousands,or even many millions of years.

Relative Dating—Key Principles

During the late 1800s and early 1900s, various attemptswere made to determine the age of Earth. Althoughsome of the methods appeared promising at the time,none proved reliable. What these scientists were seek-ing was a numerical date. Such dates specify the actu-al number of years that have passed since an eventoccurred—for example, the extinction of the dinosaursabout 65 million years ago. Today our understanding ofradioactivity allows us to accurately determine numer-ical dates for rocks that represent important events inEarth’s distant past. We will study radioactivity later inthis chapter. Prior to the discovery of radioactivity, ge-ologists had no accurate and dependable method of nu-merical dating and had to rely solely on relative dating.

Relative dating means placing rocks in their prop-er sequence of formation—which ones formed first, sec-ond, third, and so on. Relative dating cannot tell us howlong ago something took place, only that it followed oneevent and preceded another. The relative dating tech-niques that were developed are valuable and still wide-ly used. Numerical dating methods did not replace thesetechniques; they simply supplemented them. To estab-lish a relative time scale, a few basic principles or ruleshad to be discovered and applied. Although they mayseem obvious to us today, they were major break-throughs in thinking at the time, and their discovery andacceptance was an important scientific achievement.

Geologic TimeRelative DatingEE

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?S T U D E N T S S O M E T I M E S A S K . . .You mentioned early attempts at

determining Earth’s age that proved unreliable. How did

nineteenth-century scientists go about making such calculations?

One method that was attempted several times involved therate at which sediment is deposited. Some reasoned that ifthey could determine the rate that sediment accumulates andcould further ascertain the total thickness of sedimentary rockthat had been deposited during Earth history, they could es-timate the length of geologic time. All that was necessary wasto divide the rate of sediment accumulation into the totalthickness of sedimentary rock.

Estimates of Earth’s age varied each time this method wasattempted. The age of Earth as calculated by this methodranged from 3 million to 1.5 billion years! Obviously thismethod was riddled with difficulties. Can you suggest whatsome might have been?


Law of SuperpositionNicolaus Steno, a Danish anatomist, geologist, andpriest (1636–1686), is credited with being the first to rec-ognize a sequence of historical events in an outcrop ofsedimentary rock layers. Working in the mountains ofwestern Italy, Steno applied a very simple rule that hascome to be the most basic principle of relative dating—the law of superposition. The law simply states that inan undeformed sequence of sedimentary rocks, eachbed is older than the one above it and younger than theone below. Although it may seem obvious that a rocklayer could not be deposited unless it had somethingolder beneath it for support, it was not until 1669 thatSteno clearly stated the principle.

This rule also applies to other surface-deposited ma-terials, such as lava flows and beds of ash from volcaniceruptions. Applying the law of superposition to the bedsexposed in the upper portion of the Grand Canyon (Fig-ure 10.2), you can easily place the layers in their properorder. Among those that are shown, the sedimentaryrocks in the Supai Group must be the oldest, followed inorder by the Hermit Shale, Coconino Sandstone,Toroweap Formation, and Kaibab Limestone.

Principle of Original HorizontalitySteno is also credited with recognizing the importanceof another basic principle, called the principle of orig-inal horizontality. Simply stated, it means that layers ofsediment are generally deposited in a horizontal posi-tion. Thus, if we observe rock layers that are flat, itmeans they have not been disturbed and thus still havetheir original horizontality. The layers in the GrandCanyon illustrate this in the chapter-opening photo and

in Figure 10.2. But if they are folded or inclined at asteep angle, they must have been moved into that po-sition by crustal disturbances sometime after their de-position (Figure 10.3).

Principle of Cross-CuttingRelationshipsWhen a fault cuts through other rocks, or when magmaintrudes and crystallizes, we can assume that the faultor intrusion is younger than the rocks affected. Forexample, in Figure 10.4, the faults and dikes clearlymust have occurred after the sedimentary layers weredeposited.

This is the principle of cross-cutting relationships.By applying the cross-cutting principle, you can see thatfault A occurred after the sandstone layer was deposit-ed, because it “broke” the layer. However, fault A oc-curred before the conglomerate was laid down, becausethat layer is unbroken.

We can also state that dike B and its associated sillare older than dike A, because dike A cuts the sill. In thesame manner, we know that the batholith was emplacedafter movement occurred along fault B, but before dikeB was formed. This is true because the batholith cutsacross fault B, and dike B cuts across the batholith.

InclusionsSometimes inclusions can aid the relative datingprocess. Inclusions are pieces of one rock unit that arecontained within another. The basic principle is logicaland straightforward. The rock mass adjacent to the onecontaining the inclusions must have been there first inorder to provide the rock fragments. Therefore, the rock

B.

Kaibab Limestone

Toroweap Formation

Youn

ger

Coconino Sandstone

Hermit Shale

Supai Group

A.

Figure 10.2 Applying the law of superposition to these layers exposed in the upper portion of the Grand Canyon, the SupaiGroup is oldest and the Kaibab Limestone is youngest. (Photo by E. J. Tarbuck)

Figure 10.3 Most layers of sedimentare deposited in a nearly horizontalposition. Thus, when we see rock layersthat are folded or tilted, we canassume that they must have beenmoved into that position by crustaldisturbances after their deposition.These folded layers are exposed in theNamib Desert (southwestern Africa).(Photo by Michael Fogden/DRK Photo)

mass containing inclusions is the younger of the two.Figure 10.5 provides an example. Here the inclusionsof intrusive igneous rock in the adjacent sedimentarylayer indicate that the sedimentary layer was depositedon top of a weathered igneous mass rather than beingintruded from below by magma that later crystallized.

UnconformitiesWhen we observe layers of rock that have been deposit-ed essentially without interruption, we call them con-formable. Particular sites exhibit conformable bedsrepresenting certain spans of geologic time. However, noplace on Earth has a complete set of conformable strata.

Throughout Earth history, the deposition of sedi-ment has been interrupted again and again. All suchbreaks in the rock record are termed unconformities. An

unconformity represents a long period during whichdeposition ceased, erosion removed previously formedrocks, and then deposition resumed. In each case upliftand erosion are followed by subsidence and renewedsedimentation. Unconformities are important featuresbecause they represent significant geologic events inEarth history. Moreover, their recognition helps us iden-tify what intervals of time are not represented by strataand thus are missing from the geologic record.

The rocks exposed in the Grand Canyon of the Col-orado River represent a tremendous span of geologic his-tory. It is a wonderful place to take a trip through time.The canyon’s colorful strata record a long history of sed-imentation in a variety of environments—advancingseas, rivers and deltas, tidal flats, and sand dunes. Butthe record is not continuous. Unconformities represent

Conglomerate

Shale

Sandstone

Sill

Batholith

Fault A

Dike A Fault B

Dike B

Figure 10.4 Cross-cuttingrelationships are an important principleused in relative dating. An intrusiverock body is younger than the rocks itintrudes. A fault is younger than therock layers it cuts.

290


A. Intrusive igneous rock

B. Exposure and weathering of intrusive igneous rock

C. Deposition of sedimentary layers

Sedimentarylayers

Inclusions ofigneous rock

Intrusive igneousrock

Nonconformity

Figure 10.5 These three diagrams illustrate one way thatinclusions can form, and a type of unconformity termed anonconformity. In the third diagram we know the igneous rockmust be older because pieces of it are included in the overlyingsedimentary bed. When older intrusive igneous rocks are overlainby younger sedimentary layers, a noncomformity is said to exist.The photo shows an inclusion of dark igneous rock in a lighter-colored and younger host rock. (Photo by Tom Bean)

vast amounts of time that have not been recorded in thecanyon’s layers. Figure 10.6 is a geologic cross section ofthe Grand Canyon. Refer to it as you read about the threebasic types of unconformities: angular unconformities,disconformities, and nonconformities.

Angular Unconformity. Perhaps the most easilyrecognized unconformity is an angular unconfor-mity. It consists of tilted or folded sedimentary rocksthat are overlain by younger, more flat-lying strata.

An angular unconformity indicates that during thepause in deposition, a period of deformation (foldingor tilting) and erosion occurred (Figure 10.7).

When James Hutton studied an angular unconfor-mity in Scotland more than 200 years ago, it was clearto him that it represented a major episode of geologic ac-tivity (Figure 10.7E). He also appreciated the immensetime span implied by such relationships. When a com-panion later wrote of their visit to the site, he stated that“the mind seemed to grow giddy by looking so far intothe abyss of time.”

Disconformity. When contrasted with angular un-conformities, disconformities are more common, butusually far less conspicuous because the strata on eitherside are essentially parallel. For example, look at thedisconformities in the cross section of the Grand Can-yon in Figure 10.6. Many disconformities are difficult toidentify because the rocks above and below are similarand there is little evidence of erosion. Such a breakoften resembles an ordinary bedding plane. Other dis-conformities are easier to identify because the ancienterosion surface is cut deeply into the older rocks below.

Nonconformity. The third basic type of unconfor-mity is a nonconformity. Here the break separatesolder metamorphic or intrusive igneous rocks fromyounger sedimentary strata (Figures 10.5 and 10.6).Just as angular unconformities and disconformitiesimply crustal movements, so too do nonconformities.Intrusive igneous masses and metamorphic rocks ori-ginate far below the surface. Thus, for a nonconfor-mity to develop, there must be a period of uplift andthe erosion of overlying rocks. Once exposed at thesurface, the igneous or metamorphic rocks are sub-jected to weathering and erosion prior to subsidenceand the renewal of sedimentation.

Using Relative Dating PrinciplesIf you apply the principles of relative dating to the hy-pothetical geologic cross section in Figure 10.8 (p. 294),you can place in proper sequence the rocks and theevents they represent. The statements within the figuresummarize the logic used to interpret the cross section.

In this example, we establish a relative time scalefor the rocks and events in the area of the cross section.Remember that this method gives us no indication asto how many years of Earth history are represented, forwe have no numerical dates. Nor do we know how thisarea compares to any other.

Correlation of Rock LayersTo develop a geologic time scale that is applicable tothe entire Earth, rocks of similar age in different regionsmust be matched up. Such a task is referred to as cor-relation.


KaibabFormation

ToroweapFormation

CoconinoSandstone

Hermit Shale

Supai Group

Redwall Limestone

Muav Limestone

Bright Angel ShaleTontoGroup

Tapeats Sandstone

Permian

Pennsylvanian

Mississippian

Devonian

Cambrian

Precambrian

Disconformity

Disconformity

Angular unconformity

Nonconformity

Vishnu Schist

UnkarGroup

Zoroaster Granite

InnerGorge

ColoradoRiver

Kaibab Plateau

Figure 10.6 This cross section through the Grand Canyon illustrates the three basic types of unconformities. An angularunconformity can be seen between the tilted Precambrian Unkar Group and the Cambrian Tapeats Sandstone. Twodisconformities are marked, above and below the Redwall Limestone. A nonconformity occurs between the igneous andmetamorphic rocks of the Inner Gorge and the sedimentary strata of the Unkar Group.

Within a limited area, correlating the rocks of onelocality with those of another may be done simply bywalking along the outcropping edges. However, thismight not be possible when the rocks are mostly con-cealed by soil and vegetation. Correlation over shortdistances is often achieved by noting the position of adistinctive rock layer in a sequence of strata. Or, a layermay be identified in another location if it is composedof very distinctive or uncommon minerals.

By correlating the rocks from one place to another,a more comprehensive view of the geologic history of aregion is possible. Figure 10.9, for example, shows thecorrelation of strata at three sites on the Colorado Plateauin southern Utah and northern Arizona. No single lo-cale exhibits the entire sequence, but correlation revealsa more complete picture of the sedimentary rock record.

Many geologic studies involve relatively small areas.Such studies are important in their own right, but theirfull value is realized only when the rocks are correlatedwith those of other regions. Although the methods justdescribed are sufficient to trace a rock formation over rel-

atively short distances, they are not adequate for match-ing rocks that are separated by great distances. When cor-relation between widely separated areas or betweencontinents is the objective, geologists must rely on fossils.

Fossils: Evidence of Past LifeFossils, the remains or traces of prehistoric life, are im-portant inclusions in sediment and sedimentary rocks.They are important tools for interpreting the geologicpast. Knowing the nature of the life forms that existedat a particular time helps researchers understand pastenvironmental conditions. Further, fossils are impor-tant time indicators and play a key role in correlatingrocks of similar ages that are from different places.

Types of FossilsFossils are of many types. The remains of relatively re-cent organisms may not have been altered at all. Suchobjects as teeth, bones, and shells are common exam-


E.

Sea level

1234

A. Deposition

5

1234

B. Folding and uplifting

5

1234

C. Erosion

5

1234

D. Subsidence and renewed deposition

7

6 (Angularunconformity)

Sea level89

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Figure 10.7 Formation of an angular unconformity. An angularunconformity represents an extended period during whichdeformation and erosion occurred. Part E shows an angularunconformity at Siccar Point, Scotland, that was first described byJames Hutton more than 200 years ago. (Photo by A. P.Trujillo/APT Photos)

ples. Far less common are entire animals, flesh included,that have been preserved because of rather unusual cir-cumstances. Remains of prehistoric elephants calledmammoths that were frozen in the Arctic tundra ofSiberia and Alaska are examples, as are the mummifiedremains of sloths preserved in a dry cave in Nevada.

Given enough time, the remains of an organism arelikely to be modified. Often fossils become petrified (lit-erally, “turned into stone”), meaning that the small in-ternal cavities and pores of the original structure are filledwith precipitated mineral matter (Figure 10.10A, p. 296).In other instances replacement may occur. Here the cellwalls and other solid material are removed and replacedwith mineral matter. Sometimes the microscopic detailsof the replaced structure are faithfully retained.

Molds and casts constitute another common class offossils. When a shell or other structure is buried in sed-iment and then dissolved by underground water, a moldis created. The mold faithfully reflects only the shapeand surface marking of the organism; it does not revealany information concerning its internal structure. Ifthese hollow spaces are subsequently filled with min-eral matter, casts are created (Figure 10.10B).

A type of fossilization called carbonization is partic-ularly effective in preserving leaves and delicate ani-mal forms. It occurs when fine sediment encases theremains of an organism. As time passes, pressuresqueezes out the liquid and gaseous components andleaves behind a thin residue of carbon (Figure 10.10C).Black shales deposited as organic-rich mud in oxygen-poor environments often contain abundant carbonizedremains. If the film of carbon is lost from a fossil pre-served in fine-grained sediment, a replica of the sur-face, called an impression, may still show considerabledetail (Figure 10.10D).

Delicate organisms, such as insects, are difficult topreserve, and consequently they are relatively rare inthe fossil record. Not only must they be protected fromdecay but they must not be subjected to any pressurethat would crush them. One way in which some insectshave been preserved is in amber, the hardened resin ofancient trees. The fly in Figure 10.10E was preservedafter being trapped in a drop of sticky resin. Resin sealedoff the insect from the atmosphere and protected the re-mains from damage by water and air. As the resin hard-ened, a protective pressure-resistant case was formed.

In addition to the fossils already mentioned, thereare numerous other types, many of them only tracesof prehistoric life. Examples of such indirect evidenceinclude:

1. Tracks—animal footprints made in soft sedimentthat was later lithified (Figure 10.10F).

2. Burrows—tubes in sediment, wood, or rock madeby an animal. These holes may later become filledwith mineral matter and preserved. Some of the old-est-known fossils are believed to be worm burrows.


KJI HG

FED

C

B

A

Sill

F

BA

E

DC

E

C

B

A

F

Ocean

OceanE

DC

B

A

Angularunconformity

Rockeroded away

Sill

Sill

Following the intrusion of sill D,the intrusion of dike F occurred.

Because the dike cuts throughbeds A through E, it must be

younger than all of them (principleof cross-cutting relationships).

Bed D is a sill (a concordantigneous intrusion). Applyingthe principle of cross-cuttingrelationships, sill D must beyounger than the rocks thatwere intruded. Further evidencethat sill D is younger than bedsC and E are the inclusions (arrows)in the sill of fragments from thesebeds. If this igneous masscontains pieces of adjacentstrata, then the adjacent stratamust have been there first.

Applying the law of superposition,beds A, B, C, and E weredeposited in that order.

1.

2.

3.

4.

5.

6.

Next, the rocks were tilted and eroded.The tilting happened first becausethe upturned ends of the strata havebeen eroded. The tilting and erosion,followed by further deposition,produced an angular unconformity.

Beds, G, H, I, J, and K weredeposited in that order, again

using the law of superposition.Although the lava flow (bed H)is not a sedimentary rock layer,it is a surface-deposited layer,

and thus superposition maybe applied.

Finally, the irregular surfaceindicates that another gapin the rock record is being

produced by erosion.

DC

BA

E

KJI HG

F

BA

E

D

C

Angularunconformity

Sill

Interpretation:

KJ

IH

G

F

B

A

E

D

C

Angularunconformity

Sill

Figure 10.8 Geologic cross section of a hypothetical region.

3. Coprolites—fossil dung and stomach contents thatcan provide useful information pertaining to foodhabits of organisms.

4. Gastroliths—highly polished stomach stones thatwere used in the grinding of food by some extinctreptiles.

Conditions Favoring PreservationOnly a tiny fraction of the organisms that have livedduring the geologic past have been preserved as fossils.Normally the remains of an animal or plant are de-stroyed. Under what circumstances are they preserved?Two special conditions appear to be necessary: rapidburial and the possession of hard parts.

When an organism perishes, its soft parts usuallyare quickly eaten by scavengers or decomposed by bac-teria. Occasionally, however, the remains are buried bysediment. When this occurs, the remains are protectedfrom the environment, where destructive processes op-erate. Rapid burial therefore is an important conditionfavoring preservation.

In addition, animals and plants have a much betterchance of being preserved as part of the fossil record ifthey have hard parts. Although traces and imprints ofsoft-bodied animals such as jellyfish, worms, and in-sects exist, they are not common. Flesh usually decaysso rapidly that preservation is exceedingly unlikely.Hard parts such as shells, bones, and teeth predomi-nate in the record of past life.


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

Figure 10.9 Correlation of strata at three locations on the Colorado Plateau reveals a more complete view of the extent ofsedimentary rocks in the region. (After U.S. Geological Survey)

Because preservation is contingent on special con-ditions, the record of life in the geologic past is biased.The fossil record of those organisms with hard partsthat lived in areas of sedimentation is quite abundant.However, we get only an occasional glimpse of the vastarray of other life forms that did not meet the specialconditions favoring preservation.

Fossils and CorrelationThe existence of fossils had been known for centuries,yet it was not until the late 1700s and early 1800s thattheir significance as geologic tools was made evident.

During this period an English engineer and canalbuilder, William Smith, discovered that each rock for-mation in the canals he worked on contained fossils un-like those in the beds either above or below. Further, henoted that sedimentary strata in widely separated areascould be identified and correlated by their distinctivefossil content.

Based on Smith’s classic observations and the find-ings of many geologists who followed, one of the mostimportant and basic principles in historical geology wasformulated: Fossil organisms succeed one another in a def-inite and determinable order, and therefore any time period


A

B

C D

E F

Figure 10.10 There are many types of fossilization. Six examples are shown here. A. Petrified wood in Petrified Forest National Park,Arizona. B. Natural casts of shelled invertebrates. C. A fossil bee preserved as a thin carbon film. D. Impressions are common fossils andoften show considerable detail. E. Insect in amber. F. Dinosaur footprint in fine-grained limestone near Tuba City, Arizona. (Photo A byDavid Muench; Photos B, D, and F by E. J. Tarbuck; Photo C courtesy of the National Park Service; Photo E by Breck P. Kent)

296


can be recognized by its fossil content. This has come to beknown as the principle of fossil succession. In otherwords, when fossils are arranged according to their ageby applying the law of superposition to the rocks inwhich they are found, they do not present a random orhaphazard picture. To the contrary, fossils showchanges that document the evolution of life throughtime.

For example, an Age of Trilobites is recognizedquite early in the fossil record. Then, in succession, pa-leontologists recognize an Age of Fishes, an Age of CoalSwamps, an Age of Reptiles, and an Age of Mammals.These “ages” pertain to groups that were especiallyplentiful and characteristic during particular time pe-riods. Within each of the ages, there are many subdivi-sions based, for example, on certain species of trilobitesand certain types of fish, reptiles, and so on. This samesuccession of dominant organisms, never out of order,is found on every continent.

Once fossils were recognized as time indicators,they became the most useful means of correlatingrocks of similar age in different regions. Geologistspay particular attention to certain fossils called indexfossils. These fossils are widespread geographicallyand are limited to a short span of geologic time, sotheir presence provides an important method of

matching rocks of the same age. Rock formations,however, do not always contain a specific index fossil.In such situations, groups of fossils are used to estab-lish the age of the bed. Figure 10.11 illustrates how anassemblage of fossils can be used to date rocks moreprecisely than could be accomplished by the use ofonly one of the fossils.

In addition to being important and often essentialtools for correlation, fossils are important environmen-tal indicators. Although much can be deduced aboutpast environments by studying the nature and charac-teristics of sedimentary rocks, a close examination ofany fossils present can usually provide a great dealmore information.

For example, when the remains of certain clamshells are found in limestone, the geologist can assumethat the region was once covered by a shallow sea, be-cause that is where clams live today. Also, by usingwhat we know of living organisms, we can concludethat fossil animals with thick shells capable of with-standing pounding and surging waves must have in-habited shorelines. Conversely, animals with thin,delicate shells probably indicate deep, calm offshorewaters. Hence, by looking closely at the types of fossils,the approximate position of an ancient shoreline maybe identified.

Age of rock unit A

Age of rock unit B

Rock unit A

Rock unit B

TIM

EO

lder

Youn

ger

Age ranges of some fossil groups

Figure 10.11 Overlapping ranges of fossils help date rocks more exactly than using a single fossil.


Further, fossils can indicate the former temperatureof the water. Certain present-day corals require warmand shallow tropical seas like those around Florida andthe Bahamas. When similar corals are found in ancientlimestones, they indicate that a Florida-like marine en-vironment must have existed when the corals werealive. These examples illustrate how fossils can help un-ravel the complex story of Earth history.

Dating with Radioactivity

In addition to establishing relative dates by using theprinciples described in the preceding sections, it isalso possible to obtain reliable numerical dates forevents in the geologic past. For example, we knowthat Earth is about 4.5 billion years old and that the di-nosaurs became extinct about 65 million years ago.Dates that are expressed in millions and billions ofyears truly stretch our imagination because our per-sonal calendars involve time measured in hours,

Geologic TimeRadiometric DatingEE

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weeks, and years. Nevertheless, the vast expanse ofgeologic time is a reality, and it is radiometric datingthat allows us to measure it. In this section you willlearn about radioactivity and its application in radio-metric dating.

Reviewing Basic Atomic StructureRecall from Chapter 1 that each atom has a nucleus con-taining protons and neutrons and that the nucleus is or-bited by electrons. Electrons have a negative electricalcharge, and protons have a positive charge. A neutron isactually a proton and an electron combined, so it hasno charge (it is neutral).

The atomic number (each element’s identifying num-ber) is the number of protons in the nucleus. Every el-ement has a different number of protons and thus adifferent atomic number ( , ,

, , etc.). Atoms of the sameelement always have the same number of protons, sothe atomic number stays constant.

Practically all of an atom’s mass (99.9%) is in thenucleus, indicating that electrons have virtually no massat all. So, by adding the protons and neutrons in anatom’s nucleus, we derive the atom’s mass number. The

uranium = 92oxygen = 8carbon = 6hydrogen = 1

A. Alpha Emission

B. Beta Emission

C. Electron Capture

Neutron

Proton

Neutron

ProtonNeutron

Proton

Unstable parentnucleus

Daughternucleus

Alphaparticle

emission

Atomicnumber:2 fewer

Atomic mass:4 fewer

Daughternucleus-

Atomicnumber:1 more

Atomic mass:no change

Beta(electron)emission

Electron Atomicnumber:1 fewer

Atomic mass:no change

Daughternucleus-

Daughternucleus-


Daughternucleus

(–)

+–+

+

+

+–

+–

(–)


Daughternucleus

Figure 10.12 Common types ofradioactive decay. Notice that in eachcase the number of protons (atomicnumber) in the nucleus changes, thusproducing a different element.


number of neutrons can vary, and these variants, or iso-topes, have different mass numbers.

To summarize with an example, uranium’s nucle-us always has 92 protons, so its atomic number alwaysis 92. But its neutron population varies, so uraniumhas three isotopes: uranium-234

, uranium-235, and uranium-238. Allthree isotopes are mixed in nature. They look the sameand behave the same in chemical reactions.

RadioactivityThe forces that bind protons and neutrons together inthe nucleus usually are strong. However, in some iso-topes, the nuclei are unstable because the forces bindingprotons and neutrons together are not strong enough.As a result, the nuclei spontaneously break apart(decay), a process called radioactivity.

What happens when unstable nuclei break apart?Three common types of radioactive decay are illustrat-ed in Figure 10.12. and are summarized as follows:

1. Alpha particles ( ) may be emittedfrom the nucleus. An alpha particle consists of 2protons and 2 neutrons. Consequently, the emis-sion of an alpha particle means that the massnumber of the isotope is reduced by 4 and theatomic number is decreased by 2.

2. When a beta particle ( ), or electron, isgiven off from a nucleus, the mass number re-mains unchanged, because electrons have practi-cally no mass. However, because the electron hascome from a neutron (remember, a neutron is acombination of a proton and an electron), the nu-cleus contains one more proton than before. There-fore, the atomic number increases by 1.

3. Sometimes an electron is captured by the nu-cleus. The electron combines with a proton andforms an additional neutron. As in the last exam-ple, the mass number remains unchanged. How-ever, as the nucleus now contains one less pro-ton, the atomic number decreases by 1.

An unstable (radioactive) isotope of an element iscalled the parent. The isotopes resulting from the decayof the parent are the daughter products. Figure 10.13 pro-vides an example of radioactive decay. Here it can beseen that when the radioactive parent, uranium-238(atomic number 92, mass number 238), decays, it fol-lows a number of steps, emitting 8 alpha particles and6 beta particles before finally becoming the stabledaughter product lead-206 (atomic number 82, massnumber 206). One of the unstable daughter productsproduced during this decay series is radon. Box 10.2 ex-amines the hazards associated with this radioactive gas.

Certainly among the most important results of thediscovery of radioactivity is that it provided a reliablemeans of calculating the ages of rocks and minerals that

b particle

a particles

neutrons = 23421mass of protons +

contain particular radioactive isotopes. The procedureis called radiometric dating. Why is radiometric datingreliable? Because the rates of decay for many isotopeshave been precisely measured and do not vary underthe physical conditions that exist in Earth’s outer lay-ers. Therefore, each radioactive isotope used for datinghas been decaying at a fixed rate since the formation ofthe rocks in which it occurs, and the products of decayhave been accumulating at a corresponding rate. Forexample, when uranium is incorporated into a mineralthat crystallizes from magma, there is no lead (the sta-ble daughter product) from previous decay. The radio-metric “clock” starts at this point. As the uranium inthis newly formed mineral disintegrates, atoms of thedaughter product are trapped, and measurable amountsof lead eventually accumulate.

Half-LifeThe time required for one half of the nuclei in a sampleto decay is called the half-life of the isotope. Half-life isa common way of expressing the rate of radioactive dis-integration. Figure 10.14 illustrates what occurs when aradioactive parent decays directly into its stable daugh-ter product. When the quantities of parent and daugh-ter are equal (ratio 1:1), we know that one half-life hastranspired. When one-quarter of the original parent

Ato

mic

mas

s

238

236

234

232

230

228

226

224

222

220

218

216

214

212

210

208

206

92 9091 89 88 87 86 85 84 83 82

Atomic number

Alpha emissionBeta emission

U234

U238

Th234

Pa234

Th230

Ra226

Rn222

Po218

Bi214

Po214 Pb214

Bi210

Po210 Pb210

Pb206

Figure 10.13 The most common isotope of uranium (U-238) isan example of a radioactive decay series. Before the stable endproduct (Pb-206) is reached, many different isotopes areproduced as intermediate steps.


atoms remain and three-quarters have decayed to thedaughter product, the parent/daughter ratio is 1:3 andwe know that two half-lives have passed. After threehalf-lives, the ratio of parent atoms to daughter atomsis 1:7 (one parent for every seven daughter atoms).

If the half-life of a radioactive isotope is knownand the parent/daughter ratio can be measured, theage of the sample can be calculated. For example, as-sume that the half-life of a hypothetical unstable iso-tope is 1 million years and the parent/daughter ratioin a sample is 1:15. Such a ratio indicates that four half-lives have passed and that the sample must be 4 mil-lion years old.

Radiometric DatingNotice that the percentage of radioactive atoms that decayduring one half-life is always the same: 50 percent. How-ever, the actual number of atoms that decay with the pass-ing of each half-life continually decreases. Thus, as thepercentage of radioactive parent atoms declines, the pro-portion of stable daughter atoms rises, with the increasein daughter atoms just matching the drop in parentatoms. This fact is the key to radiometric dating.

Of the many radioactive isotopes that exist in na-ture, five have proved particularly useful in providingradiometric ages for ancient rocks (Table 10.1). Rubid-ium-87, thorium-232, and the two isotopes of uranium

*Dr. Hoffman is Professor of Chemistry, Emeritus,Illinois Central College.

PEOPLE AND THE ENVIRONMENT:Radon—A Threat to Human Health

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Richard L.Hoffman*

Radioactivity is de-fined as the spon-

taneous emission of atomic particlesand/or electromagnetic waves from un-stable atomic nuclei. For example, in asample of uranium-238, unstable nucleidecay and produce a variety of ra-dioactive progeny or “daughter” prod-ucts as well as energetic forms ofradiation (Table 10.A). One of its ra-dioactive decay products is radon—acolorless, odorless, invisible gas.

Radon gained public attention in1984 when a worker in a Pennsylvanianuclear power plant set off radiationalarms not when he left work, but whenhe first arrived. His clothing and hairwere contaminated with radon decayproducts. Investigation revealed thathis basement at home had a radon level2800 times the average level in indoorair. The home was located along a geo-logical formation known as the Read-ing Prong—a mass of uranium-bearingblack rock that runs from near Reading,Pennsylvania, to near Trenton, NewJersey.

Originating in the radio decay oftraces of uranium and thorium foundin almost all soils, radon isotopes (Rn-222 and Rn-220) are continually re-newed in an ongoing, natural process.Geologists estimate that the top six feetof soil from an average acre of land con-tains about 50 pounds of uranium(about 2 to 3 parts per million); sometypes of rocks contain more. Radon is

continually generated by the gradualdecay of this uranium. Because urani-um has a half-life of about 4.5 billionyears, radon will be with us forever.

Radon itself decays, having a half-life of only about four days. Its decayproducts (except lead-206) are all ra-dioactive solids that adhere to dust par-ticles, many of which we inhale. Duringprolonged exposure to a radon-conta-minated environment, some decay willoccur while the gas is in the lungs,thereby placing the radioactive radonprogeny in direct contact with delicatelung tissue. Steadily accumulating evi-dence indicates radon to be a significantcause of lung cancer second only tosmoking.

A house with a radon level of 4.0picocuries per liter of air has about eightto nine atoms of radon decaying everyminute in every liter of air. The EPAsuggests indoor radon levels be kept

below this level. EPA risk estimates areconservative; they are based on an as-sumption that one would spend 75 per-cent of a 70-year time span (about 52years) in the contaminated space, whichmost people would not.

Once radon is produced in the soil, itdiffuses throughout the tiny spaces be-tween soil particles. Some radon ulti-mately reaches the soil surface, whereit dissipates into the air. Radon entersbuildings and homes through holes andcracks in basement floors and walls.Radon’s density is greater than air, so ittends to remain in basements during itsshort decay cycle.

The source of radon is as enduringas its generation mechanism withinEarth; radon will never go away. How-ever, cost-effective mitigation strategiesare available to reduce radon to accept-able levels, generally without great ex-pense.

Some Decay Products Decay Particle of Uranium-238 Produced Half-Life

Uranium-238 alpha 4.5 billion yearsRadium-226 alpha 1600 yearsRadon-222 alpha 3.82 daysPolonium-218 alpha 3.1 minutesLead-214 beta 26.8 minutesBismuth-214 beta 19.7 minutesPolonium-214 alpha secondLead-210 beta 20.4 yearsBismuth-210 beta 5.0 daysPolonium-210 alpha 138 daysLead-206 none stable

1.6 * 10-4

Table 10.A Decay Products of Uranium-238


are used only for dating rocks that are millions of yearsold, but potassium-40 is more versatile. Although thehalf-life of potassium-40 is 1.3 billion years, analyticaltechniques make possible the detection of tiny amountsof its stable daughter product, argon-40, in some rocksthat are younger than 100,000 years.

It is important to realize that an accurate radiomet-ric date can be obtained only if the mineral remained aclosed system during the entire period since its forma-tion. A correct date is not possible unless there was nei-ther the addition nor loss of parent or daughter isotopes.This is not always the case. In fact, an important limi-tation of the potassium-argon method arises from thefact that argon is a gas, and it may leak from minerals,throwing off measurements. Cross-checking of samples,using two different radiometric methods, is done wherepossible to ensure accurate age determinations.

1

1/2

1/4

1/8

1/161/32

Frac

tion

of e

lem

ents

pre

sent

0 1 2 3 4 5

Number of half-lives

Parent element (isotope) Daughter product

Figure 10.14 The radioactive decaycurve shows change that is exponential.Half of the radioactive parent remainsafter one half-life. After a second half-lifeone-quarter of the parent remains, and soforth.

Stable Currently Radioactive Daughter Accepted

Parent Product Half-Life Values

Uranium-238 Lead-206 4.5 billion yearsUranium-235 Lead-207 713 million yearsThorium-232 Lead-208 14.1 billion yearsRubidium-87 Strontium-87 47.0 billion yearsPotassium-40 Argon-40 1.3 billion years

Table 10.1 Radioactive isotopes frequently used in radiometric dating.?S T U D E N T S S O M E T I M E S A S K . . .

With radioactive decay, is there ever a time that all of the parent material

is converted to the daughter product?

Theoretically, no. During each half-life, half of the parent ma-terial is converted to daughter product. Then half again is con-verted after another half-life, and so on. (Figure 10.14 showshow this logarithmic relationship works—notice that the redline becomes nearly parallel to the horizontal axis after severalhalf-lives.) By converting only half of the remaining parentmaterial to daughter product, there is never a time when allthe parent material would be converted. Think about it this

way. If you kept cutting a cake in half and eating only half,would you ever eat all of it? (The answer is no, assuming youhad a sharp enough knife to slice the cake at an atomic scale!)However, after many half-lives, the parent material can existin such small amounts that it is essentially undetectable.

Dating with Carbon-14To date very recent events, carbon-14 is used. Carbon-14 is the radioactive isotope of carbon. The processis often called radiocarbon dating. Because the half-lifeof carbon-14 is only 5730 years, it can be used for dat-ing events from the historic past as well as those fromvery recent geologic history. In some cases carbon-14can be used to date events as far back as 75,000 years.

Carbon-14 is continuously produced in the upper at-mosphere as a consequence of cosmic-ray bombardment.Cosmic rays, which are high-energy particles, shatter the


A.

B.

Nitrogen-14atomic number 7atomic mass 14 Carbon-14

atomic number 6atomic mass 14

+–

+– +

+

Neutron capture

Protonemission

Neutron

Beta(electron)emission

Carbon-14 Nitrogen-14

Proton

(–)

Figure 10.15 A. Production and B. decay of carbon-14. Thesesketches represent the nuclei of the respective atoms.

nuclei of gas atoms, releasing neutrons. Some of the neu-trons are absorbed by nitrogen atoms (atomic number7), causing their nuclei to emit a proton. As a result, theatomic number decreases by 1 (to 6), and a different ele-ment, carbon-14, is created (Figure 10.15A). This isotopeof carbon quickly becomes incorporated into carbon diox-ide, which circulates in the atmosphere and is absorbedby living matter. As a result, all organisms contain a smallamount of carbon-14, including yourself.

While an organism is alive, the decaying radiocar-bon is continually replaced, and the proportions of car-bon-14 and carbon-12 remain constant. Carbon-12 is thestable and most common isotope of carbon. However,when any plant or animal dies, the amount of carbon-14 gradually decreases as it decays to nitrogen-14 bybeta emission (Figure 10.15B). By comparing the pro-portions of carbon-14 and carbon-12 in a sample, ra-diocarbon dates can be determined.

Although carbon-14 is useful in dating only the lastsmall fraction of geologic time, it has become a veryvaluable tool for anthropologists, archaeologists, andhistorians, as well as for geologists who study very re-cent Earth history. (Box 10.3 explores another method ofstudying and dating recent events.) In fact, the de-

UNDERSTANDING EARTH:Using Tree Rings to Date and Study the Recent Past

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If you look at thetop of a tree stump

or at the end of a log, you will see thatit is composed of a series of concentricrings. Each of these tree rings becomeslarger in diameter outward from thecenter (Figure 10.B). Every year in tem-perate regions trees add a layer of newwood under the bark. Characteristics of

each tree ring, such as size and density,reflect the environmental conditions(especially climate) that prevailed dur-ing the year when the ring formed. Fa-vorable growth conditions produce awide ring; unfavorable ones produce anarrow ring. Trees growing at the sametime in the same region show similartree-ring patterns.

Because a single growth ring is usu-ally added each year, the age of the treewhen it was cut can be determined bycounting the rings. If the year of cuttingis known, the age of the tree and theyear in which each ring formed can bedetermined by counting back from theoutside ring.* This procedure can beused to determine the dates of recentgeologic events. For example, the min-imum number of years since a new landsurface was created by a landslide or aflood. The dating and study of annualrings in trees is called dendrochronology.

To make the most effective use oftree rings, extended patterns known asring chronologies are established. Theyare produced by comparing the pat-terns of rings among trees in an area. Ifthe same pattern can be identified intwo samples, one of which has beendated, the second sample can be datedfrom the first by matching the ring pat-tern common to both. This technique,called cross dating, is illustrated in Fig-ure 10.C. Tree-ring chronologies ex-tending back for thousands of years

Figure 10.B Each year a growing tree produces a layer of new cells beneath the bark.If the tree is felled and the trunk examined (or if a core is taken, to avoid cutting thetree), each year’s growth can be seen as a ring. Because the amount of growth(thickness of a ring) depends upon precipitation and temperature, tree rings areuseful records of past climates. (Photo by Stephen J. Krasemann/DRK Photo)

*Scientists are not limited to working with treesthat have been cut down. Small, nondestructivecore samples can be taken from living trees.


velopment of radiocarbon dating was considered soimportant that the chemist who discovered this appli-cation, Willard F. Libby, received a Nobel prize.

Importance of Radiometric DatingBear in mind that although the basic principle of radio-metric dating is simple, the actual procedure is quitecomplex. The analysis that determines the quantities ofparent and daughter must be painstakingly precise. Inaddition, some radioactive materials do not decay directlyinto the stable daughter product. As you saw in Figure10.13, uranium-238 produces 13 intermediate unstabledaughter products before the fourteenth and final daugh-ter product, the stable isotope lead-206, is produced.

Radiometric dating methods have produced liter-ally thousands of dates for events in Earth history.Rocks from several localities have been dated at morethan 3 billion years, and geologists realize that still olderrocks exist. For example, a granite from South Africahas been dated at 3.2 billion years, and it contains in-clusions of quartzite. (Remember that inclusions areolder than the rock containing them.) Quartzite itself isa metamorphic rock that originally was the sedimenta-ry rock sandstone. Sandstone, in turn, is the product of

the lithification of sediments produced by the weather-ing of existing rocks. Thus, we have a positive indicationthat much older rocks existed.

Radiometric dating has vindicated the ideas ofJames Hutton, Charles Darwin, and others who inferredthat geologic time must be immense. Indeed, moderndating methods have proved that there has beenenough time for the processes we observe to have ac-complished tremendous tasks.

The Geologic Time Scale

Geologists have divided the whole of geologic historyinto units of varying magnitude. Together they com-prise the geologic time scale of Earth history (Figure10.16). The major units of the time scale were delineat-ed during the nineteenth century, principally by scien-tists working in Western Europe and Great Britain.Because radiometric dating was unavailable at that time,the entire time scale was created using methods of rela-

Geologic TimeGeologic Time ScaleEE

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have been established for some regions.To date a timber sample of unknownage, its ring pattern is matched againstthe reference chronology.

Tree-ring chronologies are uniquearchives of environmental history andhave important applications in such dis-ciplines as climate, geology, ecology,

and archaeology. For example, treerings are used to reconstruct climatevariations within a region for spans ofthousands of years prior to human his-torical records. Knowledge of suchlong-term variations is of great value inmaking judgments regarding the recentrecord of climate change.

In summary, dendrochronologyprovides useful numerical dates forevents in the historic and recent prehis-toric past. Moreover, because tree ringsare a storehouse of data, they are a valu-able tool in the reconstruction of pastenvironments.

Figure 10.C Cross dating is a basic principle in dendrochronology. Here it was used to date an archaeological site by correlatingtree-ring patterns for wood from trees of three different ages. First, a tree-ring chronology for the area is established usingcores extracted from living trees. This chronology is extended further back in time by matching overlapping patterns from older,dead trees. Finally, cores taken from beams inside the ruin are dated using the chronology established from the other two sites.

Live tree

Dead tree

Log from ruins


tive dating. It was only in the twentieth century that ra-diometric dating permitted numerical dates to be added.

Structure of the Time ScaleThe geologic time scale subdivides the 4.5-billion-yearhistory of Earth into many different units and providesa meaningful time frame within which the events of thegeologic past are arranged. As shown in Figure 10.16,eons represent the greatest expanses of time. The eon

that began about 540 million years ago is the Phanero-zoic, a term derived from Greek words meaning visiblelife. It is an appropriate description because the rocksand deposits of the Phanerozoic eon contain abundantfossils that document major evolutionary trends.

Another glance at the time scale reveals that thePhanerozoic eon is divided into eras. The three eraswithin the Phanerozoic are the Paleozoic (

, ), the Mesozoic ( ,meso = middlezoe = lifeancientpaleo =

Eon

Era Period Epoch

Phanerozoic

Pro

tero

zoic

Arc

hean

Had

ean

Cenozoic

Mesozoic

Paleozoic

2500

3800

4500

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Car

bon

ifero

us Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Precambrian

144

206

248

290

323

354

417

443

490

540

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

0.01

1.8

5.3

23.8

33.7

54.8

65.0

Millionsof years

ago

Pre

cam

bria

n

Cenozoic

Mesozoic

Paleozoic

Millionsof years

agoEra

65

248

540

900

1600

Late

Middle

Early

Late

Middle

Early

3000

3400

Figure 10.16 The geologic time scale. The numerical dates were added long after the time scale had been established usingrelative dating techniques. (Data from Geological Society of America)

), and the Cenozoic ( , ).As the names imply, the eras are bounded by profoundworldwide changes in life forms. Each era is subdivid-ed into periods. The Paleozoic has seven, the Mesozoicthree, and the Cenozoic two. Each of these 12 periods ischaracterized by a somewhat less profound change inlife forms as compared with the eras.

Finally, periods are divided into still smaller unitscalled epochs. As you can see in Figure 10.16, sevenepochs have been named for the periods of the Ceno-zoic. The epochs of other periods, however, are not usu-ally referred to by specific names. Instead, the termsearly, middle, and late are generally applied to the epochsof these earlier periods.

Precambrian TimeNotice that the detail of the geologic time scale does notbegin until about 540 million years ago, the date for thebeginning of the Cambrian period. The more than 4 bil-lion years prior to the Cambrian is divided into threeeons, the Hadean, the Archean, and the Proterozoic. It isalso common for this vast expanse of time to simply bereferred to as the Precambrian. Although it representsabout 88 percent of Earth history, the Precambrian isnot divided into nearly as many smaller time units as isthe Phanerozoic eon.

The quantity of information geologists have deci-phered about Earth’s past is somewhat analogous to thedetail of human history. The further back we go, the lesswe know. Certainly more data and information exist aboutthe past 10 years than for the first decade of the twentiethcentury; the events of the nineteenth century have beendocumented much better than the events of the first cen-tury A.D., and so on. Thus it is with Earth history. The morerecent past has the freshest, least disturbed, and most ob-

zoe = lifeceno = recentzoe = life servable record. The further back in time the geologistgoes, the more fragmented the record and clues become.

Difficulties in Dating the Geologic Time ScaleAlthough reasonably accurate numerical dates havebeen worked out for the periods of the geologic timescale (see Figure 10.16), the task is not without difficul-ty. The primary problem in assigning numerical datesto units of time is the fact that not all rocks can be datedby radiometric methods. Recall that for a radiometricdate to be useful, all minerals in the rock must haveformed at about the same time. For this reason, ra-dioactive isotopes can be used to determine when min-erals in an igneous rock crystallized and when pressureand heat created new minerals in a metamorphic rock.

However, samples of sedimentary rock can onlyrarely be dated directly by radiometric means. A sedi-mentary rock may include particles that contain ra-dioactive isotopes, but the rock’s age cannot beaccurately determined because the grains making upthe rock are not the same age as the rock in which theyoccur. Rather, the sediments have been weathered fromrocks of diverse ages.

Radiometric dates obtained from metamorphicrocks may also be difficult to interpret, because the ageof a particular mineral in a metamorphic rock does notnecessarily represent the time when the rock initiallyformed. Instead, the date may indicate any one of anumber of subsequent metamorphic phases.

If samples of sedimentary rocks rarely yield reliableradiometric ages, how can numerical dates be assignedto sedimentary layers? Usually the geologist must relatethem to datable igneous masses, as in Figure 10.17. In


Wasatch FormationMesaverde Formation

Mancos Shale

Dakota Sandstone

Volcanic ash bed dated at 160 million years

Summerville Foundation

Morrison Formation

Igneous dike datedat 66 million years

Rocks ofTertiary age

Rocks ofCretaceous

age

Rocks ofJurassic

age

Figure 10.17 Numerical dates forsedimentary layers are usually determinedby examining their relationship to igneousrocks. (After U.S. Geological Survey)


Chapter Summary• During the seventeenth and eighteenth centuries, catas-trophism influenced the formulation of explanations aboutEarth. Catastrophism states that Earth’s landscapes have beendeveloped primarily by great catastrophes. By contrast, uni-formitarianism, one of the fundamental principles of moderngeology advanced by James Hutton in the late 1700s, states thatthe physical, chemical, and biological laws that operate todayhave also operated in the geologic past. The idea is oftensummarized as “the present is the key to the past.” Huttonargued that processes that appear to be slow-acting could,over long spans of time, produce effects that were just as greatas those resulting from sudden catastrophic events.

• The two types of dates used by geologists to interpret Earthhistory are (1) relative dates, which put events in their proper se-quence of formation, and (2) numerical dates, which pinpoint thetime in years when an event took place.

• Relative dates can be established using the law of superposi-tion, principle of original horizontality, principle of cross-cuttingrelationships, inclusions, and unconformities.

• Correlation, the matching up of two or more geologic phe-nomena in different areas, is used to develop a geologic timescale that applies to the entire Earth.

• Fossils are the remains or traces of prehistoric life. The spe-cial conditions that favor preservation are rapid burial and thepossession of hard parts such as shells, bones, or teeth.

• Fossils are used to correlate sedimentary rocks from differentregions by using the rocks’ distinctive fossil content and apply-ing the principle of fossil succession. It states that fossil organismssucceed one another in a definite and determinable order, andtherefore any time period can be recognized by its fossil content.

• Each atom has a nucleus containing protons (positively chargedparticles) and neutrons (neutral particles). Orbiting the nucleusare negatively charged electrons. The atomic number of an atom is

the number of protons in the nucleus. The mass number is thenumber of protons plus the number of neutrons in an atom’snucleus. Isotopes are variants of the same atom, but with a dif-ferent number of neutrons and hence a different mass number.

• Radioactivity is the spontaneous breaking apart (decay) ofcertain unstable atomic nuclei. Three common types of ra-dioactive decay are (1) emission of alpha particles from thenucleus, (2) emission of beta particles (electrons) from the nu-cleus, and (3) capture of electrons by the nucleus.

• An unstable radioactive isotope, called the parent, will decayand form stable daughter products. The length of time for halfof the nuclei of a radioactive isotope to decay is called the half-life of the isotope. If the half-life of the isotope is known andthe parent/daugher ratio can be measured, the age of a sam-ple can be calculated.

• The geologic time scale divides Earth’s history into units ofvarying magnitude. It is commonly presented in chart form,with the oldest time and event at the bottom and the youngestat the top. The principal subdivisions of the geologic timescale, called eons, include the Hadean, Archean, Proterozoic (to-gether, these three eons are commonly referred to as the Pre-cambrian), and, beginning about 540 million years ago, thePhanerozoic. The Phanerozoic (meaning “visible life”) eon isdivided into the following eras: Paleozoic (“ancient life”), Meso-zoic (“middle life”), and Cenozoic (“recent life”).

• A significant problem in assigning numerical dates to unitsof time is that not all rocks can be dated radiometrically. A sedi-mentary rock may contain particles of many ages that havebeen weathered from different rocks that formed at varioustimes. One way geologists assign numerical dates to sedi-mentary rocks is to relate them to datable igneous masses,such as dikes and volcanic ash beds.

this example, radiometric dating has determined theages of the volcanic ash bed within the Morrison For-mation and the dike cutting the Mancos Shale andMesaverde Formation. The sedimentary beds below theash are obviously older than the ash, and all the layersabove the ash are younger (principle of superposition).The dike is younger than the Mancos Shale and theMesaverde Formation but older than the Wasatch For-mation because the dike does not intrude the Tertiaryrocks (cross-cutting relationships).

From this kind of evidence, geologists estimate thata part of the Morrison Formation was deposited about160 million years ago, as indicated by the ash bed. Fur-ther, they conclude that the Tertiary period began afterthe intrusion of the dike, 66 million years ago. This isone example of literally thousands that illustrates howdatable materials are used to bracket the variousepisodes in Earth history within specific time periods.It shows the necessity of combining laboratory datingmethods with field observations of rocks.

Key Termsangular unconformity (p. 291)catastrophism (p. 287)Cenozoic era (p. 305)conformable (p. 290)correlation (p. 291)cross-cutting relationships, principle of

(p. 289)disconformity (p. 291)

eon (p. 304)epoch (p. 305)era (p. 304)fossil (p. 292)fossil succession, principle of (p. 297)geologic time scale (p. 303)half-life (p. 299)inclusions (p. 289)

index fossil (p. 297)Mesozoic era (p. 304)nonconformity (p. 291)numerical date (p. 288)original horizontality, principle of

(p. 289)Paleozoic era (p. 304)period (p. 305)


Review Questions1. Contrast catastrophism and uniformitarianism. How did

the proponents of each perceive the age of Earth?

2. Distinguish between numerical and relative dating.

3. What is the law of superposition? How are cross-cuttingrelationships used in relative dating?

4. When you observe an outcrop of steeply inclined sedi-mentary layers, what principle allows you to assume thatthe beds became tilted after they were deposited?

5. Refer to Figure 10.4 to answer the following questions:(a) Is fault A older or younger than the sandstone layer?(b) Is dike A older or younger than the sandstone layer?(c) Was the conglomerate deposited before or after fault A?(d) Was the conglomerate deposited before or after fault B?(e) Which fault is older, A or B?(f) Is dike A older or younger than the batholith?

6. A mass of granite is in contact with a layer of sandstone.Using a principle described in this chapter, explain howyou might determine whether the sandstone was de-posited on top of the granite or the granite was intrudedfrom below after the sandstone was deposited.

7. Distinguish among angular unconformity, disconformi-ty, and nonconformity.

8. What is meant by the term correlation?

9. List and briefly describe at least five different types offossils.

10. List two conditions that improve an organism’s chancesof being preserved as a fossil.

11. Why are fossils such useful tools in correlation?

12. In addition to being important aids in dating and corre-lating rocks, how else are fossils helpful in geologic in-vestigations?

13. If a radioactive isotope of thorium (atomic number 90,mass number 232) emits 6 alpha particles and 4 beta par-ticles during the course of radioactive decay, what arethe atomic number and mass number of the stable daugh-ter product?

14. Why is radiometric dating the most reliable method ofdating the geologic past?

15. Assume that a hypothetical radioactive isotope has a half-life of 10,000 years. If the ratio of radioactive parent to sta-ble daughter product is 1:3, how old is the rock containingthe radioactive material? What if the ratio were 1:15?

16. To make calculations easier, let us round the age of Earthto 5 billion years.(a) What fraction of geologic time is represented by

recorded history (assume 5000 years for the lengthof recorded history)?

(b) The first abundant fossil evidence does not appear untilthe beginning of the Cambrian period (approximately550 million years ago). What percentage of geologictime is represented by abundant fossil evidence?

17. What subdivisions make up the geologic time scale?What is the primary basis for differentiating the eras?

18. Briefly describe the difficulties in assigning numericaldates to layers of sedimentary rock.

Examining the Earth System1. Figure 10.10A is a large petrified log in Arizona’s Petri-

fied Forest National Park. Describe the transition of thistree from being part of the biosphere to being a compo-nent of the solid Earth. How might the hydrosphereand/or atmosphere have played a role in the transition?

2. The famous angular unconformity at Scotland’s SiccarPoint was originally studied by James Hutton in the late

1700s (see Figure 10.7E, p. 293). Can you describe in ageneral way what occurred to produce this feature?Could all of the spheres of the Earth system have beeninvolved? The Earth system is powered by energy fromtwo sources. How are both sources represented here?

Phanerozoic eon (p. 304)Precambrian (p. 305)radioactivity (p. 299)

radiocarbon dating (p. 301)radiometric dating (p. 299)relative dating (p. 288)

superposition, law of (p. 289)unconformity (p. 290)uniformitarianism (p. 287)

Web ResourcesThe Earth Science Website uses the resourcesand flexibility of the Internet to aid in yourstudy of the topics in this chapter. Writtenand developed by Earth science instructors,

this site will help improve your understanding of Earth sci-ence. Visit http://www.prenhall.com/tarbuck and click onthe cover of Earth Science 10e to find:

• Online review quizzes.• Web-based critical thinking and writing exercises.• Links to chapter-specific Web resources.• Internet-wide key term searches.

http://www.prenhall.com/tarbuck

geology needs a time scale a brief history of geology relative

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