第四章地球的年龄和地质年代学TwoConceptionsofEarthHistoryCatastrophism(灾变说)Assumption:GreatEffectsRequireGreatCausesEarthHistoryDominatedbyViolentEventsUniformitarianism(均变论)Assumption:WeCanUseCauseAndEffecttoDetermineCausesofPastEventsFinding:EarthHistoryDominatedbySmall-scaleEventsTypicalofthePresent.CatastrophesDoHappenButAreUncommon4.1相对地质年代N.Steno地层层序律层序地层学方法W.Smith生物地层学方法标准化石时间短、演化快、分布广地层的缺失-不整合(角度不整合、假整合)相互切割关系地质年代表relativeageandabsoluteageRelativeage(相对年龄)-Relativemeansthatwecandetermineifsomethingisyoungerthanorolderthansomethingelse.Relativetimedoesnottellhowoldsomethingis,allweknowisthesequenceofevents.Forexample:thesandstoneinthisareaisolderthanthelimestone.Absoluteage(绝对年龄)-Absoluteagemeansthatwecanmoreorlesspreciselyassignanumber(inyears,minutes,seconds,orsomeotherunitsoftime)totheamountoftimethathaspassed.Thuswecansayhowoldsomethingis.Forexample:Thesandstoneis300millionyearsold.PKUTrashPitTHUTrashPitWhatcanwesayandlearnfromtheseexcavations?•Relativeageoftrashlayers•Similarculturesmusthaveexistedinbothareasandlivedatthesametime•Thetrashpitscontainsomecluestoabsoluteage•Recognizeahiatus,orbreakinthedepositionalsequenceattheTHUsiteIngeology,weusesimilarprinciplestodeterminerelativeages,correlations,andabsoluteages.Relativeages-PrinciplesofStratigraphyCorrelations-Fossils,keybeds,physicalcriteriaAbsoluteages-Radiometricdating.PrinciplesofStratigraphyStratigraphy(地层学)=thestudyofstrata(layers)intheEarth'scrust.LawsofStratigraphy(地层学定律):1.OriginalHorizontality-sedimentarystrataaredepositedinlayersthatarehorizontalornearlyhorizontal,paralleltoornearlyparalleltotheEarth'ssurface.2.StratigraphicSuperposition-BecauseofEarth'sgravity,depositionofsedimentwilloccurdepositingolderlayersfirstfollowedbysuccessivelyyoungerlayers.水平地层相对地质年代BreaksintheStratigraphicRecord---unconformity(不整合)AngularUnconformity(角度不整合)•Disconformity(平行不整合、假整合)•Nonconformity(非整合、不整合)不整合关系示意图VariationinUnconformitiesStratigraphicClassificationTwotypesofstratigraphicclassificationareused,onebasedonphysicalcharacteristicsormaterialpropertiesoftherocks-RockStratigraphicUnits,andtheotherbasedonthetimeoverwhichthematerialwasformed-timestratigraphicunits.RockStratigraphicUnits(岩石地层单位):Distinctivebodiesofrocksthatdifferfromtherocksaboveandbelowinthegeneralcharacteristics.Thebasicunitisaformation.TimeStratigraphicUnits(年代地层单位):Abodiesofrocksthatweredepositedduringthesamegeologictimeinterval.Thebasicunitisasystem.CorrelationofRockUnitsInorderforrockunitstobecorrelatedoverwideareas,theymustbedeterminedtobeequivalent.Determinationofequivalenceisbasedon:RelativeAge-iftworockunitsareequivalenttheymusthavethesameagerelativetorocksthatoccurbelowandabove.Thelawsofstratigraphyenablethisdetermination.PhysicalCriteria-Similarityinrocktype,butonlyifrelativeageisequivalent.Keybeds,likewidespreadvolcanicashlayersthatarethesameoverwideareasareoftenusedtoestablishequivalence.fossilspresent-Fossilsarekeyindicatorsofrelativeage(lifehasevolvedthroughtime)andenvironmentsofdepositionLawofCross-cuttingrelationships(切割关系定律)-thezucchiniconceptifonebodyofrockcutsacrossanotherbodyofrock,thelattermustbeolderthantheformer.Therefore,thethingbeingacteduponhastobeolderthanthethingdoingtheacting4.2绝对地质年代放射性同位素法-同位素年龄1Ntt=—ln(—+1)N0t为衰变经历的时间(即岩石形成年龄),为衰变常数,N0为母体原始数量(按原子数计),Nt为N0经时间t后剩下的数量AbsoluteGeologicTimeAlthoughgeologistscaneasilyestablishrelativeagesofrocksbasedontheprinciplesofstratigraphy,knowinghowmuchtimeageologicEon,Era,Period,orEpochrepresentsisamoredifficultproblemwithouthavingknowledgeofabsoluteagesofrocks.Intheearlyyearsofgeology,manyattemptsweremadetoestablishsomemeasureofabsolutegeologictimeAbsoluteAges:EarlyAttemptsTheBibleAddupDatesinBibleGetanAgeof4000-6000B.C.ForEarthJohnLightfootandBishopUssher-4004B.C.(1584)TooShortAbsoluteAges:EarlyAttemptsSaltinOceanRiversbringdissolvedsolidstooceanIfweknowratesaltisadded,andhowmuchsaltisinocean,canfindageofoceans.Gaveageofabout100millionyears.Problems–Israteatwhichsaltisaddedconstant?–Howmuchsaltleavesocean?AbsoluteAges:EarlyAttemptsSedimentThicknessAddupThickestsedimentsforeachperiodEstimaterateofdepositiontofindageProblem:ratesofdepositionveryvariable!Indicatedagesofatleast100millionyearsRadiometricDatingRadiometricdatingreliesonthefactthattherearedifferenttypesofisotopes.RadioactiveIsotopes-isotopes(parentisotopes)thatspontaneouslydecayataconstantratetoanotherisotope.RadiogenicIsotopes-isotopesthatareformedbyradioactivedecay(daughterisotopes).RadiometricDating:Half-LifePresentRadiometricDatingMethodsCosmogenicC-145700Yr.Be-102.5M.Y.PrimordialK-Ar(K-40)1.25B.Y.Rb-Sr(Rb-87)48.8byU-235704M.Y.Th-23214B.Y.U-2384.5B.Y.PrimordialNd-Sm(Sm-147-Nd-143)106B.Y.Re-18743B.Y.Lu-Hf(Lu-176)36B.YFissionTrackHowDoWeDeterminetheageofRocks?N=N0exp(-kt)1ln10NNttTheGeologicTimeScaleAbsoluteDatingandGeologicTimeScale•TheOligocenerocksareyoungerthanthe30m.yoldlavaflowandolderthanthe20m.y.oldlavaflow.•TheEocenerocksareolderthanthe57m.y.olddikeandyoungerthanthe36m.y.olddikethatcutsthr