279

It is not uncommon for people to think of time intwo conceptually different ways and yet consciously dis-tinguish between them only rarely. When inquiringabout the time of mail delivery, we may be told that mailcomes at 1:00 p.m., or that it arrives just after lunch.When we ask when a baby was born, the reply might bethat she was born November 21 of last year or that shewas born the week before Thanksgiving. In casual con-versation, we would probably consider the two answersto each question essentially equivalent, but there is a dif-ference. The first response in each example establishesthe absolute timing of an event—a specific time or date.The second response in each establishes only a sequenceof events; one identifiable event is followed by another,but the exact time or date of neither event is specified.

Despite the immense span of time over which geo-logic events have been going on, they can be dated inthe same two ways. We can say that a particular gran-ite body is 17.2 million years old, or we can observe thatit is younger than, say, some limestone layer in contactwith it. In the first case, we must have carried out someexperimental technique that has yielded a specific agefor the granite; in the second, we have made someobservations that confirm only that the limestone exist-ed before the granite, although we do not know wheneither was formed. The first kind of determination iscalled absolute dating, and the second is called rela-tive dating. Both are of great use in geology.

Absolute dating requires expensive and sophisticatedlaboratory equipment and painstaking effort, but the tech-niques of relative dating can often be applied successfullyby simple observation in the field. We shall begin our dis-cussion of geologic time with relative dating.

The Principles of Relative Dating

A prerequisite to successful relative dating of rocksand geologic events is that we understand, in a rudi-mentary way, how rocks originate. Recall from Chapter28 that igneous rocks are formed when molten materialcools and crystallizes; thus igneous rocks are those thathave been deposited on the surface of the earth by vol-canic activity or those that have crystallized beneath thesurface from molten magma that has been intruded intosurrounding rock. Sedimentary rocks consist either of

the erosional debris of older rocks, or of chemical pre-cipitates or organic material; they have been formed asthis sediment has been deposited in layers and buried.Metamorphic rocks have been significantly altered byintense heat and pressure; thus, they are modified pre-existing rocks that are in essentially the same locationsrelative to their surroundings as before metamorphism.

Given these basic definitions, most of the princi-ples upon which relative dating are based are almostintuitive:

(1) The Principle of Original Horizontality.Sediment is deposited in layers that are essen-tially horizontal (see Fig. 29.1), and thus thesedimentary rocks that form from them origi-nate as essentially horizontal strata. If sedi-mentary rocks are tilted, folded, or otherwisedeformed, we can assume with confidence thatthe deformation occurred after the depositionand lithification of the sediment.

Figure 29.1. Sediment tends to accumulate in horizon-tal layers, even if initial layers cover irregularities in thesurface. Recognition of this leads naturally to thePrinciple of Original Horizontality.

(2) The Principle of Superposition. ConsiderFigure 29.2, which shows a sequence of sedi-mentary rocks in cross section (that is, a “cut-

29. Geologic Time

280

away” view). If the rocks formed in the waythat has been indicated above, then it is obviousthat the layers at the bottom are older than theones at the top. It is not necessarily true thatdeposition has been uninterrupted, however.Most sediment is deposited in bodies of water,so if this section of sedimentary rocks was atsome time raised above sea level (by geologicforces that will be discussed in a later chapter),some layers that had previously existed mighthave been entirely stripped away by erosionbefore conditions allowed deposition toresume. We also assume in using this principlethat the rocks are right side up; the same forcesthat can raise the earth’s crust can severelydeform it, and if a sequence of rocks has beencompletely overturned, then failure to recog-nize that will lead to an erroneous result byapplication of the Principle of Superposition.

Figure 29.2. In this sequence of sedimentary strata, theoldest layers are those on the bottom, and the layers areprogressively younger toward the top. This is thePrinciple of Superposition.

(3) The Principle of Cross-Cutting Relations. Ifone rock unit cuts across or through another,then the one that is cut must have existed firstand must, therefore, be the older of the two.The same is true of a fault, which must beyounger than the rocks it cuts through. Figure29.3 illustrates the principle, and no specializedgeologic training is required to understand it.Even so, cross-cutting relationships can be verycomplex, and careful observation is oftenrequired to unravel a sequence of events. Theprinciple can be applied at all scales, fromfaults that have moved rocks on one side hun-

dreds of kilometers with respect to the rocks onthe other side, to small cracks in mineral grainsas seen under a microscope.

Figure 29.3. In this cross section the rock sequencelabeled A is truncated by the sequence labeled B, andfault C cuts through A but not B. The conclusion reachedby applying the Principle of Cross-Cutting Relations isthat the oldest feature is A because it is cut by both B andC; the next oldest is fault C, because it cuts A but not B;and sequence B cuts across both sequence A and fault C,so it must be the youngest of the three.

(4) The Principle of Inclusion. One kind of rockcannot contain inclusions (fragments) of anoth-er kind unless the inclusions existed first.Inclusions are therefore always older that therocks that contain them. Suppose that erosionexposed a sandstone that became weathered asit was exposed to the elements and then, subse-quently, limestone sediment was depositedover that. Pieces of the sandstone that hadbecome included in the limestone sedimentwould reveal that the sandstone was the olderrock. This is a particularly useful way to deter-mine if a sequence of sedimentary layers isright side up or overturned, as shown schemat-ically in Figure 29.4.

So far, we have determined that even quite complexgeologic sequences can be deciphered by applying a fewsimple ideas. The remaining technique of relative datinginvolves fossils, and its application does require special-ized training. Fossils will be discussed in some detail inChapter 33; for now, it is necessary only to understandthat they are the remains of once-living organisms andare found in many common sedimentary rocks.

For many years during the late 18th and early 19thcenturies, a British civil engineer named William Smith(1769-1839) worked around quarries and canals thathad been excavated in the rocks of southern England.

A

B

C

281

He observed that the rock layers occurred in a consistentvertical sequence, that they contained fossils, and thatthe fossils in the various layers were different. His care-ful study enabled him to characterize the rock layers bythe fossils found in them, so that if he were shown fos-sils obtained at the surface, he could predict not onlywhat kinds of rocks would lie beneath the surface, butalso how deep they would be. Over the subsequent cen-tury and a half, similar observations throughout much ofthe world have resulted in our fifth principle of relativedating:

(5) The Principle of Faunal Succession. A fauna(flora) is a group of animals (plants) that aresomehow associated, and the Principle ofFaunal Succession holds that the assemblagesof fossil animals (or plants) found in sedimen-tary rocks are diagnostic of the ages of therocks. The fossil assemblages typically varyfrom the bottom to the top of a sequence of sed-imentary rocks, because some of the organismsrepresented by the fossils became extinct whileothers evolved. Experience has shown thatassemblages that disappear never reappear inyounger (that is, more recent) rocks. Thus, forone trained in the science of paleontology (thestudy of fossils), it is possible to look at a groupof fossils from a rock and determine, from thefossils alone, the age of the rock relative torocks that contain different fossils. Figure 29.5illustrates this principle.

Fossils often reveal a great deal about the geologicenvironment in which a rock formed, and the paleontol-ogist must consider the kind of rock being examinedwhen searching for fossils. Failure to find marine fos-

sils in a freshwater limestone does not suggest that thosefossils are absent from rocks of the time period repre-sented, but only that they are not present in freshwaterlimestones. Again, we shall consider fossils as a spe-cialized topic in Chapter 33.

Examples of Relative Dating

With the exception of the Principle of FaunalSuccession, you can apply the principles of relative dat-ing to successfully determine a sequence of geologicevents, despite the fact that you may never have had anygeologic training beyond this sketchy introduction. Allyou must remember is that (1) sedimentary rock layersare essentially horizontal to begin with, (2) the ones atthe bottom of an undisturbed sequence are oldest, (3)rocks cut by other rocks are the older of the two, and (4)rocks containing inclusions of other rocks are youngerthan those from which the inclusions came.

Now consider Figure 29.6, which shows a hypo-thetical geologic cross section of a small part of theearth’s crust. Formations A and B consist of sedimenta-

Lim

esto

neS

ands

tone

Figure 29.4. Even though these beds have been over-turned by forces acting on the crust of the earth, theinclusions of sandstone in the limestone show the lime-stone to be the younger rock layer.

A B C D E

E

D+E

C+D+E

C+E

A+CA+B+C

A+B

A

Figure 29.5. A schematic illustration of the Principle ofFaunal Succession. Arrows indicate the sedimentarylayers in which each type of fossil is found and, there-fore, the lengths of time during which each type of ani-mal existed on the earth in this hypothetical location.By noting the types of fossils found at any given level,a relative age is determined. Depending on the lengthsof time represented by various fossils, this age may bequite precise or only approximate.

282

ry rocks, C is an igneous body consisting of solidifiedmagma, and D is the present land surface, including thecanyon. The geologic story told by the cross section isshown in the small illustrations on the right, each ofwhich may be thought of as a “snapshot” taken aftersome significant event in the geologic history of thisregion. The “snapshot” in frame 1 shows formation Aas originally deposited in essentially horizontal layers.This is followed in frames 2 and 3 by the folding anderosion of this formation. In frame 4, formation B isdeposited on the erosional surface; and then, in frame 5,the entire section is tilted and a new erosional surface iscarved. In frame 6 the magma is injected to form theigneous body, and finally the present erosional surfaceis developed in frame 7. The relative ages are clearly A,B, C, and D from oldest to youngest.

Even sequences of events that have occurred on themoon or other bodies in the solar system can be deci-

phered using the principles of relative dating. (Becausethe moon has never supported life, our lack of expertisewith the Principle of Faunal Succession is no disadvan-tage to us there.) Figure 29.7 is a photograph of a lunarterrain in the region of Mare Humorum, the darkish ovaljust above center. Mare Humorum is a huge basin some400 kilometers long made by meteor (planetesimal?)impact in the lunar southern highlands, the denselycratered lighter terrain surrounding the mare. Some ofthe larger craters cut across the margin of the basin, forinstance at locations marked A. At locations marked B,lava that later filled the basin invaded and partiallyfilled some of those craters. The fault marked C cutsthrough the lava flows. Small craters marked D are inthe solidified lavas; close examination will reveal dif-fuse white patches on the lavas around some of them—material ejected from the impact sites, like the rays seenin Color Plate 13. From these observations, we can say

1

2

3

4

5

6

7

A

B

C

D

Figure 29.6. Shown on the left is a hypothetical geologic cross section containing sedimentary formations A and B,igneous body C, and the erosional surface D. On the right is a series of “snapshots” depicting the geologic story toldby the cross section.

283

that the highlands must be oldest, followed by theHumorum basin. After creation of the basin, impactcraters of moderate size continued to be formed in andaround it, and then the mare lavas were erupted onto thefloor of the basin and into some of these craters. Last,the faulting occurred, along with impacts yielding smallcraters. Because the fault and the small craters both cutthe second-oldest feature, but do not come in contactwith each other, we cannot tell which of the two came

first.

Figure 29.7. The region of Mare Humorum in the lunarsouthern hemisphere. See the discussion in the text foran explanation.

The Geologic Column

As the science of geology developed in the early19th century, it became possible to define various arbi-trary divisions of rocks by the fossils they contained.Often these divisions were given names based on geo-graphic or historical features of the areas in which theywere described. Thus Cambrian, Ordovician, Silurian,and Devonian rocks were named after the Roman namefor Wales, two ancient British tribes, and the Englishcounty called Devonshire, respectively. Cambrian rockswere identified as those in which the earliest, very abun-dant fossils of marine invertebrates were found; the pres-ence of certain key fossils identified other rocks asOrdovician, or Silurian, or Devonian, and so forth. Thusevolved the Geologic Column, shown in Figure 29.8.

Note that the Geologic Column is divided intoperiods, which are grouped into four eras—theCenozoic, the Mesozoic, the Paleozoic, and thePrecambrian. The first three literally mean recent life,middle life, and ancient life, the names referring to the

similarity (or dissimilarity) between modern life formsand the fossils in rocks of those ages. The PrecambrianEra is characterized by very rare fossils—mostlyimpressions of soft-bodied creatures, single-celledorganisms, and other very primitive life forms. ThePaleozoic Era is characterized by abundant marineinvertebrates; but fish, amphibians, and reptiles, as wellas land plants, also appear in the fossil record of thatera. Reptiles flourished during the Mesozoic Era (it wasthe age of the dinosaurs), but it also saw the emergenceof birds, mammals, and flowering plants. We are nowin the Cenozoic Era, and the fossils of those rocks, aswell as presently living organisms, testify that it is theage of the mammals.

Because the Geologic Column evolved conceptual-ly over many years, before there were any techniquesfor determining the absolute ages of rocks, none of itsdevelopers knew how long any of the periods were oreven whether they were all of the same length. They areshown in Figure 29.8 as if they were of equal duration,but we shall see later that this is far from the case. Evenso, the use of fossils enabled geologists to identify rocksas, say, Mississippian in age, and know that they weretherefore younger than Devonian rocks, even if therewere no cross-cutting relationships or other relative-dat-

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Cenozoic

Mesozoic

Paleozoic

Precambrian

Worms andother soft-

bodied animals

Simple plants

Abundant marineinvertebrates (corals,sponges, molluscs,

etc.)

Land plants

Abundant fishesFirst land vertebrates

First birds

Dinosaurs

First snakes

Flying insects

Early mammals

Early reptiles

Flowering plants

Widespread forestsAbundant amphibians

Early primatesAbundant mammalsHorses, elephants

Abundant grasslands

Mammal-like reptiles

Starfish, sea urchins

Figure 29.8. The Geologic Column, showing life formstypical of each era, as determined from the fossil record.

284

ing indicators visible in the area.

Absolute Time

While the principles of relative dating yield impres-sive results, they fail to answer some of the questionsthat are scientifically and philosophically the mostinteresting: When was this sediment deposited? Whendid that volcano erupt? How old is the earth? Toanswer questions like these requires more than deter-mining a sequence of events, and we now proceed toconsider absolute dates for geologic phenomena.

People of virtually all civilizations have consideredthe earth to be ancient, but the precise meaning of thatimprecise word has proved elusive. In 1654 ArchbishopJames Ussher proclaimed that the world had been creat-ed in 4004 B.C., a date calculated by analyzing genealo-gies from the Old Testament. (A short time later aBiblical scholar at Cambridge, Dr. John Lightfoot, iden-tified the exact time of creation as 9:00 a.m. on October26 in that same year!) A 6000-year-old earth may soundancient, and in terms of human generations it is; buteven from a scriptural viewpoint Ussher’s method isfaulty because the measurement of Adam’s age presum-ably did not begin until after the creation was finished.Moreover, there is compelling evidence that the earth isvery much older than that. The hardy bristlecone pinesof Wheeler Peak, Nevada, show patterns in their annualgrowth rings that document over 9,000 years of contin-uous existence (although no single tree is that old). Inthe sedimentary Green River Formation of Utah andWyoming, over six million years of annual lake depositscan be counted. That is a thousand times longer than theage Bishop Ussher advocated, but even six millionyears appears to be a substantial underestimate becauserelative dating techniques reveal that the Green RiverFormation is near the top of the Geologic Column.

Uniformitarianism

Aside from the catastrophic events associated withvolcanoes, earthquakes, or floods, geologic processesare almost imperceptibly slow, and you have probablytaken little notice of the changes they bring. Yet thechanges come as even the most sluggish processesrelentlessly modify the earth. As James Hutton (1726-1797), a Scottish gentleman farmer with a geologic bent,contemplated erosion, deposition of sediment, and othernatural processes, he realized that given sufficient timeand the assumption that they also operated in the pastmuch as he then saw them, these processes couldaccount for all of the geology he saw about him. (Youmay recognize in this concept something reminiscent ofthe postulate of time symmetry introduced in Chapter 1,and, indeed, time symmetry is the basis for Hutton’s con-clusions.) Hutton’s conclusion was in stark contrast to

the common practice then of ascribing the earth’s surfacefeatures largely to one catastrophic event after another.

Hutton’s contention that geologic processes werepredictable consequences of immutable natural lawswas popularized and spread by other authors as thePrinciple of Uniformitarianism, and in a form not toodissimilar to the one accepted by most geologists today:Uniformitarianism postulates that (1) natural laws donot change with time; (2) there is a cause-and-effectrelationship between the laws and the geologic process-es that operate on the earth; and (3) the rates of process-es need not be considered constant because they aregoverned by physical conditions that may change, butunderstanding of the natural laws and the processes thatresult places realistic limits on those rates.

The Principle of Uniformitarianism is often sum-marized as “The present is the key to the past,” meaningthat geologic processes operating today have probablyalways operated in the same way; therefore, the study ofthose modern processes can lead to correct interpreta-tions of ancient geologic features. While this conveys agenerally correct impression of what uniformitarianismis all about, it is not entirely accurate. For instance,there is good reason to believe that the early atmosphereof the earth was quite unlike the atmosphere thatpresently surrounds us, and so some chemical reactionsbetween atmosphere and rocks that occur today may nothave occurred then and vice versa.

Early Estimates of the Age of the Earth

Based on the Principle of Uniformitarianism, someinteresting methods of estimating the age of the earthwere devised during the 19th century. One methodassumed the oceans were originally freshwater and thesalt now present was produced by erosion and carried inby rivers. It further assumed that erosion rates hadalways been about constant, resulting in a calculatedage of 90 million years for the earth. What was notappreciated at the time was that there are very thickbeds of rock salt beneath the surface in many areas ofthe world, and this is salt not accounted for by measur-ing the salinity of the oceans. Evidently, this methodproduced estimates that were too small.

Another approach was to estimate the total thick-ness of sedimentary rock that is present in the crust ofthe earth and then determine how long it would havetaken to deposit that much sediment. It was assumedthat rates of erosion and deposition had been relativelyconstant throughout the history of the earth, but there isno real basis for this assumption. Further, the estimatesof total thickness varied widely, and there was no wayto account for sedimentary rock that had been erodedand redeposited. Ages ranging from 3 million to 1500million years were calculated by this method.

Perhaps the most influential early estimate was

285

made by Lord Kelvin (1824-1907), a British physicist.After assuming that the earth had originated in a moltencondition, he did a rather straightforward calculation todetermine how long would be required for it to cool toits present state. He had to estimate the thermal proper-ties of the rocks in the interior of the earth (about whichvirtually nothing was known in those days), so his cal-culations were rough; but he concluded that the earthcould not be much older than 100 million years.Further, his estimates of the temperature of the sun lim-ited the duration of habitable conditions on earth tobetween 20 million and 40 million years. This baffledand worried geologists, who intuitively sensed that thisestimate had to be far too short; but in the face of soundmathematics produced by a renowned scientist, theirintuition was not convincing.

In fact, there were two serious errors in Kelvin’s cal-culations, but he was not responsible for either of them.In the first place, radioactivity had not yet been discov-ered, so he was unaware that there are radioactive ele-ments inside the earth that produce heat as they decay;thus, heat had not been lost as rapidly as he had sup-posed. Second, he assumed that the sun burned in a con-ventional way (that is, by combustion), because nuclearfusion had not yet been discovered either. Had heknown about fusion, he would not have determined sucha limited life span for the sun and the resulting short hab-itable time span on earth. The geologists were rightabout his results, but only later did they understand why.

Radiometric Dating

In 1896, Antoine-Henri Becquerel accidentallyexposed a photographic plate by placing a piece of ura-nium ore on top of it and thus introduced the scientificworld to radioactivity. Radioactivity, you will recallfrom Chapter 24, is the spontaneous decay of unstablenuclei by any of several processes. It takes place at arate that is unique but constant for each radioactive iso-tope. As experiment and observation revealed thenature of radioactive decay, it became apparent that itcould be used as a geologic clock—a means of deter-mining the age of a rock containing radioactive iso-topes. Here is how.

Consider the uranium isotope 238U, often written asuranium-238. It decays to a daughter product that isalso radioactive, and which then decays to yet anotherradioactive isotope, and so on through a complex seriesof alpha and beta decays that finally arrive at lead-206,which is a stable isotope that undergoes no furtherdecay. The entire 238U to 206Pb decay sequence has ahalf-life of 4.5 billion years. Given that a half-life is thetime required for one-half of the amount of parent iso-tope present to decay to the daughter product, it is asimple exercise to construct the decay curve of Figure29.9. This graph would work for any radioactive iso-

tope by changing only the time period corresponding toa half-life.

The vertical axis of the decay curve shows the frac-tion of original parent isotope remaining. At “timezero” there is no daughter product yet, so the fraction is1. After one half-life, the fraction is 1/2 because half ofthe original parent is left. Likewise, after two half-lives,the fraction is 1/4, and so on. If we had some way ofdetermining how much uranium-238 there was in a rockoriginally and how much is left at present, we coulddivide the latter by the former to obtain the fraction. Wecould then draw a horizontal line from that value on thevertical axis, intersect the decay curve, and draw a ver-tical line from that intersection to the horizontal axis,where we would read the age of the rock.

All of the information needed to perform this taskcan be obtained by experiment. The various isotopes ofan element all behave identically in chemical reactions,so a chemical analysis for uranium and for lead wouldbe insufficient. But we could analyze for each isotopeby using a mass spectrometer (see Chapter 15), and thatis exactly what is done. The total amount of parent iso-tope (uranium-238 in this example) originally present isdetermined by adding the amount of uranium-238 rep-resented by its daughter product (lead-206) to theamount of uranium-238 currently in the rock.

You may have recognized that this procedurerequires the assumption that the rock, as originallyformed, had no lead-206, so that all of that isotope nowpresent can be ascribed to uranium decay. This assump-tion is not always valid, but there are ways (which arebeyond the scope of this book) to determine how much

Number of billions of years elapsed

Per

cent

of p

aren

t rem

aini

ng

4.5 9.0 13.5 18.0

100

80

60

40

20

0

50%

25%

12.5%6.25%

1.35

1 2 3 4Number of half-lives elapsed

Figure 29.9. The decay curve for 238U to 206Pb.Actually, by changing the number of years per half-life,the same curve could be used for the decay of anyradioactive isotope. If we could determine that the frac-tion of parent remaining in a rock was 0.80, then the ageof the rock would be 0.30 half-lives, or 1.35 billionyears for the 206Pb clock.

286

of the lead-206 is “original” and to correct for that. In order to be useful as a geologic clock, a radioac-

tive isotope must meet certain requirements. First, itmust have a sufficiently long half-life that the rocks stillhave a measurable amount of parent isotope in them.Second, the parent isotope must be relatively abundantin common minerals so that the rocks will containenough of it to be useful. Several qualifying isotopesexist, and the most commonly used parents and daugh-ters are listed in Table 29.1, along with their half-lives.The experimental procedures required to measureminute quantities of isotopes are demanding and diffi-cult, and whenever possible, two or more “clocks” areused on the same rock to verify the results.

You will observe that carbon-14 has a very shorthalf-life compared to the other parent isotopes. It is dif-ferent from the other clocks in other ways, too. It onlyworks for dating objects that have once been alive.Carbon-14 is produced naturally in the atmosphere, anda living organism continuously replaces what decays.When the organism dies, however, the carbon-14 is nolonger replaced, and the amount remaining reveals howlong ago the organism died. After a few half-lives, thereis not enough 14C left to measure accurately, so themethod is limited to organisms that died within the last70,000 years or so. It is thus not universally useful ingeology but is well suited to archaeology and recentCenozoic geology.

The Geologic Column Revisited

With the ability to determine absolute ages ofrocks, we are now almost ready to place ages on theGeologic Column and turn it into a quantitative tool.But first we have to ask ourselves the question, “Whatare we actually dating when we determine a radiometricage?” For igneous rocks, we are ideally determiningtime since crystallization from the magma or lava, andthis corresponds to what we would likely have meantintuitively by “the age of the rock.” Some radiometricclocks are more sensitive to heat than others, though, sosome will give us the time since the igneous bodycooled below a certain temperature. This can be very

Parent Daughter Half-lifeisotope isotope (years)

rubidium-87 strontium-87 47.0 billionthorium-232 lead-208 14.1 billionuranium-238 lead-206 4.5 billionpotassium-40 argon-40 1.3 billionuranium-235 lead-207 713 millioncarbon-14 nitrogen-14 5730

Table 29.1. Radioactive isotopes used in absolute dating of rocks.

Cambrian Period

Ordovician Period

Silurian Period

Devonian Period

Mississippian Period

Triassic Period

Jurassic Period

Cretaceous Period

Tertiary Period

Quaternary Period

Permian Period

Pennsylvanian Period

02

66

144

208

245

286

320

360

408

438

505

570

CenozoicEra

PaleozoicEra

MesozoicEra

Precambrian

M i l l i o n s o f y e a r s a g o

Figure 29.10. The modern geologic timescale.Absolute ages are obtained from radiometric dating, andthe periods are shown with correct relative durations.

287

useful information, but we do have to understand theclocks well enough to properly interpret the “age.”Thus, the “age” of a metamorphic rock may be the timesince the metamorphic event, not the time since the ori-gin of the premetamorphic rock. A sedimentary rockgenerally consists of sediment from several sources, andso an “age” determined from such a rock would notmean very much.

For these reasons, the types of rocks on which mostof the Geologic Column is based—sedimentary rocks—are not the sort that can usually be used to find absolutedates for the time scale. However, by dating igneousrocks and noting their relative-dating relations to fossil-bearing sedimentary rocks, it is possible to bracket theabsolute ages within which the fossils lived. By contin-ually refining the measurements, we can “zero in” onthe absolute ages of key fossils and establish dates forthe beginning and end of each of the geologic periods.Figure 29.10 shows the modern geologic timescale,with absolute dates and the lengths of the periods shownto scale.

The Age of the Earth

We return now to a consideration of the age of theearth. The oldest terrestrial minerals that have so farbeen radiometrically dated come from Australia andyield an age of 4.2 billion years. These minerals arepart of younger sedimentary rocks now, but their pres-ence testifies of rocks that existed on the earth at least4.2 billion years ago. This establishes a lower limit onthe age of the earth, but the planet could be much olderthan that because (1) erosion or other processes thatmodify the surface of the earth could have obliteratedrocks older than that, and (2) there might be older rockssomewhere that have simply not yet been radiometri-cally analyzed.

Meteors, you will recall from the last chapter, arepieces of rock that orbit the sun and frequently strike theearth. If they survive their fiery trip through the atmos-phere and land on the surface, we call them “mete-orites.” They are thought to be remnants from the for-mation of the solar system and to be of essentially thesame age as the sun and planets. There are several typesof meteorites, but the oldest ones turn out to be uni-formly about 4.6 billion years old, as determined byradiometric dating.

During the Apollo space program of the late 1960sand 1970s, men traveled to the moon and returned withsamples of rock from its surface. Like the meteorites,there are differences between one lunar rock and anoth-er, and all are not the same age. As implied by Figure29.7, the moon, like the earth, has a long and complexhistory; and we are not surprised that lunar rocks vary inage. The oldest ones, however, also yield radiometricdates of about 4.6 billion years.

Considered together, all of this suggests that thesolar system came into being about 4.6 billion yearsago, and that is the current best estimate of the age ofthe earth.

Summary

Geologic time is measured in both relative andabsolute terms. Absolute dating is more informative,but also more expensive, time consuming, and techni-cally demanding. Relative dating is accomplished inessentially two ways: Either the principles of originalhorizontality, superposition, cross-cutting relationships,and inclusions are applied to determine a sequence ofevents, or the law of faunal succession is used byexperts in paleontology. Absolute dating techniqueshave provided estimates for the ages of subdivisions ofthe Geologic Column, so that fossils may now be usedto yield approximate absolute dates for the rocks inwhich they occur.

Absolute (radiometric) dating, most often appliedto igneous rocks, depends on the radioactivity of certainnatural isotopes that occur in rocks. In order to be use-ful as a geologic clock, an unstable isotope must have ahalf-life sufficiently long that after substantial periodsof time there is still enough left to measure accurately,and it must be abundant enough in rocks to allow accu-rate analysis of both parent and daughter product. Adecay curve, in which the fraction of remaining parentisotope is plotted against the number of half-liveselapsed, can be constructed without reference to anyexperimental data at all. Once this is done and the frac-tion of parent material remaining in a given specimen isdetermined by experiment, the curve will yield the num-ber of half-lives elapsed. Multiplying by the number ofyears per half-life for the given isotope (also determinedby experiment) yields the age of the rock in years.There are various corrections that must be applied inpractice, depending on which isotopes are being used,and usually agreement between dates obtained by twodifferent methods is sought.

Radiometric dating of terrestrial materials yields 3.8billion years as the age of the oldest rocks and 4.2 billionyears as the age of the oldest minerals (which now hap-pen to be part of a younger sedimentary formation).Meteorites of the oldest kind consistently turn out to be4.6 billion years old, as do the oldest rocks from themoon. Because it is likely that erosion and other process-es that operate on earth have destroyed the earliest rocksof our planet, we take the age of the earth to be 4.6 bil-lion years, the same as the moon and the meteorites.

288

STUDY GUIDEChapter 29: Geologic Time

A. FUNDAMENTAL PRINCIPLES1. Time Symmetry and Causality: See Chapter 1.

Time symmetry and causality become the essentialfeatures of Uniformitarianism in this chapter.

2. Uniformitarianism: The idea that (1) natural lawsdo not change with time; (2) there is a cause-and-effect relationship between the laws and the geo-logic precesses that operate on the Earth; and (3)the rates of processes need not be considered con-stant because they are governed by physical condi-tions which may change, but understanding of thenatural laws and the processes that result placesrealistic limits on those rates.

B. MODELS, IDEAS, QUESTIONS, OR APPLICA-TIONS

1. How do many geologists view the importance ofcatastrophism and uniformitarianism in under-standing the history of the earth?

2. What principles make it possible to determine therelative ages of geologic events?

3. What principle makes it possible to estimate theabsolute age of a geological event?

4. What are the main conclusions reached about thegeological and biological history of the earth afterapplying the principles of both relative andabsolute dating?

5. When did the Precambrian Era begin and end?Paleozoic Era? Mesozoic Era? Cenozoic Era?What events mark these boundaries?

6. Are fossils rare? How are fossils preserved? Whatdo fossils teach us?

C. GLOSSARY1. Absolute Dating: The determination of a specific

time, age, or date in geologic history.2. Cambrian Period: The beginning subdivision

(period) of the Paleozoic Era. During the Cambrianperiod, life-forms began to flourish. Trilobitesappear.

3. Catastrophism: The idea that geologic featuresare caused by unique cataclysmic events that arenoncyclical and of relatively short duration in time.

4. Cenozoic Era: The era of the Geologic Columnwe are now in, characterized by mammals andflowering plants. The era began 66 million yearsago.

5. Cross-Cutting Relations (Principle of ): A prin-ciple of relative dating which observes that if onerock unit cuts across or through another, then theone that is cut must have existed first and musttherefore be the older of the two rock units.

6. Daughter Product: The isotope to which the

radioactive parent isotope decays.7. Era: The Geologic Column is subdivided into

eras; eras are subdivided into periods; periods aresubdivided into epochs.

8. Faunal Succession (Principle of): A principle ofrelative dating which observes that assemblages offossil animals (fauna) or plants (flora) found insedimentary rocks are diagnostic of the ages of therocks. Extinct assemblages never reappear inyounger, more recent rocks.

9. Geologic Column: A division of rocks in theearth’s crust by age according to the fossils theycontain.

10. Igneous Rock: See Chapter 28.11. Inclusion (Principle of): A principle of relative

dating which observes that inclusions (rock frag-ments) are always older than the rocks that containthem.

12. Isotope: See Chapter 24.13. Mesozoic Era: The second-youngest era of the

Geologic Column, characterized by reptiles anddinosaurs. This era lasted from 245 million yearsago to 66 million years ago.

14. Metamorphic Rock: See Chapter 28.15. Original Horizontality (Principle of): A principle

of relative dating which observes that sediment isdeposited in layers that are essentially horizontalwhen originally formed.

16. Paleontology: The study of fossils.17. Paleozoic Era: The second-oldest era of the four

eras in the Geologic Column, characterized byabundant marine invertebrates and fish. This eralasted from 570 million years ago to 245 millionyears ago.

18. Parent Isotope: The original, undecayed radioac-tive isotope used in radiometric dating.

19. Precambrian Era: The oldest era of the GeologicColumn, characterized by very rare fossils, mostlyimpressions of soft-bodied creatures, single-celledorganisms, and other very primitive life. This erabegan with the formation of the earth and ended570 million years ago.

20. Radioactive Half-Life: See Chapter 24.21. Radiometric Dating: Use of radioactive isotopes

to determine the age of a rock by determining theratio of parent isotope to daughter product and cal-culating, based on the parent isotope half-life, thetime needed to obtain this ratio.

22. Relative Dating: The establishment of a sequenceof events without specifying an exact time or spanof time in geologic history.

23. Sedimentary Rock: See Chapter 28.24. Superposition (Principle of): A principle of rela-

tive dating which observes that, for rocks formed inlayers, the bottom layers are older than the top lay-ers, unless the sequence of rocks has been com-

289

pletely overturned.25. Uniformitarianism: See “Fundamental

Principles” above.

D. FOCUS QUESTIONS1. Consider the Geologic Column:

a. Name and state in your own words five princi-ples of relative dating.b. Sketch the Geologic Column showing the foureras and representative life forms found in therocks of each era.c. Describe the general process of radiometricdating.d. Indicate on your sketch of the GeologicColumn the absolute dates of the beginning of eachera as determined from radiometric dating.

E. EXERCISES29.1. How are relative dating and absolute dating

different in concept?

29.2. Which of the following choices is not used inrelative dating?

(a) radioactive isotopes(b) the Principle of Superposition(c) the Principle of Cross-Cutting relations(d) the Principle of Inclusion

29.3 If a sedimentary rock layer lies on top of alava flow dated radiometrically at 44 million years, butis itself cut by an igneous body dated at 27 millionyears, its age

(a) is less than 27 million years.(b) is more than 44 million years.(c) is the average of 27 and 44 million years, or35.5 million years.(d) is between 27 and 44 million years.

29.4. Why are the geologic time periods not ofequal length?

29.5. Explain how the same decay curve can beused for any radioactive isotope, regardless of thelength of its half-life.

29.6. To be useful as geologic clocks, radioactiveisotopes must be

(a) extremely rare and have long half-lives.(b) extremely rare and have short half-lives.(c) fairly common and have long half-lives.(d) fairly common and have short half-lives.

29.7. What two factors limit the usefulness of car-bon-14 dating in geology?

29.8. How do the radiometric ages of meteorites

and lunar rocks affect our thinking about the age of theearth?

29.9. A fault that cuts through a sedimentary rocklayer is

(a) younger than that layer.(b) older than that layer.(c) either (a) or (b), depending on how theabsolute dating turns out.

290