地质中的时间尺度

UnderstandingEarthChapter8:TheRockRecordandtheGeologicTimeScaleCopyright©2007byW.H.Freeman&CompanyJohnGrotzinger,ThomasH.JordanFrankPressandRaymondSieverFifthEditionConceptsyouwillneedtoknowfortheexamsCHAPTER8GeologicTimeScaleRelativeageAbsoluteagePrincipleoforiginalhorizontalityPrincipleofsuperpositionPrincipleofcross-cuttingrelationahipsUnconformityAngularunconformityDisconformityHalf-lifeGeologicTimeScaleGeologistsdifferfromotherprofessionalscientistsintheirviewoftime.Geologistsdealwithperiodsoftimethatgoasfarbackastheoriginoftheuniverse.Beforewehadtheabilitytoreadthe“atomicclocks”ofnature,geologistsusedtheconceptofrelativeages.Wecanseechangesinthelandscapeovertime.Wecanseeonetypeoforganismreplaceanothertypeoforganismthroughthefossilizedremainsofthesechangesovertime.Withtheadventofatomic/radioactivedecaytechniqueswebegantoobtainabsolutedates.AbsolutedatesorageshavespecificnumbersRelativedatinghasbeendoneinthepastthroughthestudyoflayersofsedimentaryrock(aka“thefieldofstratigraphy”)PrincipleofOriginalHorizontalitysedimentsdepositedinnearlyflatlayers.Cross-beddedlayersarecontainedwithinoverallflatbeds.Iflayersarefaulted(broken)orfoldeditmeansthattheyweredeformedAFTERsedimentsweredeposited.PrincipleofSuperpositionLowestlayersareolderIfthisisnotso,thenthelayershavebeenoverturnedbytectonicactivity:Therearelimitationsforusingstratigraphytokeeptimebecause,(1)sedimentationratesarevariablee.g.Mississippicandeposit1mofsedimentin1000yThedeepoceanmaydeposit1mm/1000yearsWeneedtoestimatetimeinanotherwayandbeabletorecognizewhentherepresentationoftimeviasedimentsisincomplete.Paleontologyisthestudyofancientlifefromfossilizedremains.WilliamSmithinthe19thcenturywasanengineerwhonotedthatsimilarrocksindifferentareashadsimilarfossilsandthatdifferentrocktypeshaddifferentfossils.Hemadeacomposite(ideal,pieced-together)stratigraphicsuccessionasinthefollowingfigureMostofgeologictimegoesunrecordedSometimesNOSEDIMENTisDEPOSITEDoveranintervaloftime(hiatus)Othertimessedimentiserodedaway.Thisishappeninghere:MississippiValleyisfillingwhereasthehighergroundisbeingerodedaway.AnunconformityinwhichthelayersaboveanerosionalsurfaceareparalleltothelayersbelowisknownasaDISCONFORMITYOnewayofproducingadisconformityistochangeglobalsea-levelbyglaciationandglacialmelting.Cross-cuttingrelationshipsinsedimentaryrocksallowistodatetherelativetimingofeventssuchasfaultingandigneousintrusionTheGeologicTimeScale…..isstillunderconstructionLifebeganonly2500MaRadioactiveDatingMethodsMiningUraniumOxideU3O8U-23899.3%,U-2350.7%MillingcrushedsoakedinsulfuricacidYellowCakeU3O8CONVERSION:yellowcakeisheatedtoabout147F,convertedintouraniumhexafluoridegas(UF6).ENRICHMENT:TheUF6gascentrifugedheavierU-238movesoutsideU-235concentratesinwardConverttoUO2powderNuclearReactors:enriched3-5%U-235NuclearWeapons:enriched90%U-235RubidiumexistRb87isunstableRbhasanaverageatomicweightof~85.5Rb87hasanatomicmass~87Rb87has37protonsand50neutronsinitsnucleusSr87isstableSrhasanaverageatomicweightof~87.62Sr87hasanatomicmass~87Sr87has38protonsand49neutronsinitsnucleusSource:wikipedia11-5-2012Rb37p+50nSr38p+49n+electronNeutron-------proton+electronAlllead,except204Pb(1.4%),istheendproductofacomplexradioactivedecay.ThemostcommonisotopeofLeadis:208Pb52.4%(stable)TheEND…..Fornow