sraigrrahyandgeologicime地层与地质时间

StratigraphyandGeologicTimeStratigraphyBasicprinciplesofrelativeagedatingUnconformities:MarkersofmissingtimeCorrelationofrockunitsAbsolutedatingGeologicTimeHowoldistheEarth?Whendidvariousgeologiceventsoccur?InterpretingEarthhistoryisaprimegoalofgeology.SomeknowledgeofEarthhistoryandgeologictimeisalsorequiredforengineersinordertounderstandrelationshipsbetweengeologicunitsandtheirimpactonengineeringconstruction.Stratigraphy:Stratigraphyisthestudyofrocklayers(strata)andtheirrelationshipwitheachother.Stratigraphyprovidessimpleprinciplesusedtointerpretgeologicevents.TworockunitsatacliffinMissouri.(USGeologicalSurvey)Basicprinciplesofrelativeagedating

Relativedatingmeansthatrocksareplacedintheirpropersequenceofformation.Aformationisabasicunitofrocks.Belowaresomebasicprinciplesforestablishingrelativeagebetweenformations.PrincipleoforiginalhorizontalityPrincipleofsuperpositionPrincipleoffaunalsuccessionPrincipleofcross-cuttingrelationshipsPrincipleoforiginalhorizontality:Layersofsedimentaregenerallydepositedinahorizontalposition.Thusifweobservedrocklayersthatarefoldedorinclined,theymust,withexceptions,havebeenmovedintothatpositionbycrustaldisturbancessometimeaftertheirdeposition.

Mostlayersofsedimentaredepositedinanearlyhorizontalposition.Thus,whenweseeinclinedrocklayersasshown,wecanassumethattheymusthavebeenmovedintothatpositionafterdeposition.HartlandQuay,Devon,EnglandbyTomBean/DRKPhoto.Principleofsuperposition:

Inanundeformedsequenceofsedimentaryrocks,eachbedisolderthantheoneaboveandyoungerthantheonebelow.Therulealsoappliestoothersurface-depositedmaterialssuchaslavaflowsandvolcanicashes.Principleofsuperposition.(W.W.Norton)ApplyingthelawofsuperpositiontothelayersattheupperportionoftheGrandCanyon,theSupaiGroupistheoldestandtheKaibabLimestoneistheyoungest.(photobyTarbuck).Principleofcross-cuttingrelationships:

Whenafaultcutsthroughrocks,orwhenmagmaintrudesandcrystallizes,wecanassumethatthefaultorintrusionisyoungerthantherocksaffected.

Cross-cuttingrelationships:Anintrusiverockbodyisyoungerthantherocksitintrudes.Afaultisyoungerthantherocklayersitcuts.(TarbuckandLutgens)Unconformities:MarkersofmissingtimeWhenlayersofrockformedwithoutinterruption,wecallthemconformable.Anunconformityrepresentsalongperiodduringwhichdepositionceasedanderosionremovedpreviouslyformedrocksbeforedepositionresumed.AngularunconformitiesDisconformityNonconformityAngularunconformities:Anangularunconformityconsistsoftiltedorfoldedsedimentaryrocksthatareoverlainbyyounger,moreflat-lyingstrata.Itindicatesalongperiodofrockdeformationanderosion.Formationofanangularunconformity.Anangularunconformityrepresentsanextendedperiodduringwhichdeformationanderosionoccurred.(TarbuckandLutgents)AngularunconformityatSiccarPoint,southernScotland,thatwasfirstdescribedbyJamesHuttonmorethan200yearsago.(HamblinandChristiansenandW.W.Norton)Disconformity:Adisconformityisaminorirregularsurfaceseparatingparallelstrataonoppositesidesofthesurface.Itindicatesahistoryofupliftingabovesea(water)level,undergoingerosion,andloweringbelowthesealevelagain.Formationofdisconformity.(W.W.Norton)Disconformitiesdonotshowangulardiscordance,butanerosionsurfaceseparatesthetworockbodies.Thechannelinthecentralpartofthisoutcroprevealsthatthelowershaleunitsweredepositedandthenerodedbeforetheupperunitsweredeposited.(HamblinandChristiansen)NonconformityAnonconformityisabreaksurfacethatdevelopedwhenigneousormetamorphicrockswereexposedtoerosion,andyoungersedimentaryrocksweresubsequentlydepositedabovetheerosionsurface.(TarbuckandLutgens)AnonconformityattheGrandCanyon.Themetamorphicrocksandtheigneousdikesoftheinnergorgewereformedatgreatdepthsandsubsequentlyupliftedanderoded.Youngersedimentarylayerswerethendepositedontheerodedsurfaceoftheigneousandmetamorphicterrain.(HamblinandChristiansen)TypesofUnconformityThisanimationshowsthestagesinthedevelopmentofthreemaintypesofunconformityincross-section,andexplainshowanincompletesuccessionofstrataprovidesarecordofEarthhistory.View1showsadisconformity,View2showsanonconformityandView3showsanangularunconformity.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>DistinguishingnonconformityandintrusivecontactNonconformity:Thesedimentaryrockisyounger.Theerosionsurfaceisgenerallysmooth.Dikesmaycutthroughtheigneousbodybutstopatthenonconformity.Intrusivecontact:Intrusionisyoungerthanthesurroundingsedimentaryrocks.Thecontactsurfacemaybequiteirregular.Azoneofcontactmetamorphismmayformsurroundingtheigneousbody.Cross-cuttingdikesmaypenetrateboththeigneousbodyandthesedimentaryrocks.Contrastingfieldconditionsfor(a)anonconformityand(b)anigneousintrusion.(West,Fig9.4)Thethreebasictypesofunconformitiesillustratedbythiscross-sectionoftheGrandCanyon.(TarbuckandLutgents)GeologicHistoryAcross-sectionthroughtheearthrevealsthevarietyofgeologicfeatures.View1ofthisanimationidentifiesavarietyofgeologicfeatures;View2animatesthesequenceofeventsthatproducedthesefeatures,anddemonstrateshowgeologistsapplyestablishedprinciplestodeducegeologichistory.[byStephenMarshak]PlayAnimationWindowsversion>>PlayAnimationMacintoshversion>>Principleoffaunalsuccession:Groupsoffossilanimalsandplantsoccurthegeologichistoryinadefiniteanddeterminableorderandaperiodofgeologictimecanberecognizedbyitscharacteristicfossils.Fossilsaretheremainsofancientorganisms.Therearemanytypesoffossilization.(Top)naturalcastsofshelledinvertebrates.(Middle)Fishimpressions.(Bottom)Dinosaurfootprintinfine-grainedlimestonenearTuba,Az.Theprincipleoffossilsuccession.Notethateachspecieshasonlyalimitedrangeinasuccessionofstrata.(W.W.Norton)CorrelationofrockunitsThemethodofrelatingrockunitsfromonelocalitytoanotheriscalledcorrelation.Onewayofcorrelationistorecognizetherocktypeorrocksequenceattwolocations.Anotherwayofcorrelationistousefossils.Abasicunderstandingoffossilsisthatfossilorganismssucceededoneanotherinadefiniteanddeterminableorder,andthereforeatimeperiodcanberecognizedbyitsfossilcontent.Theprincipleofcorrelationofrockunits.Therockcolumnscanbecorrelatedbymatchingrocktypes.(W.W.Norton)WilliamSmith,acivilengineerandsurveyor,couldpiecetogetherthesequenceoflayersofdifferentagescontainingdifferentfossilsbycorrelatingoutcropsfoundinsouthernEnglandabout200yearsago.Inthisexample,FormationIIwasexposedatbothoutcropsAandB,thusFormationIandIIwereyoungerthanFormationIII.(PressandSiever).CorrelationofstrataatthreelocationsontheColoradoPlateaurevealsthetotalextentofsedimentaryrocksintheregion.Thegeologiccolumnwasconstructedbydeterminingtherelativeagesofrockunitsfromaroundtheworld.(Next)Bycorrelation,thesecolumnswerestackedoneontopoftheothertogiverelativeagesofrockunits(W.W.Norton)AbsolutedatingThegeologictimebasedonstratigraphyandfossilsisarelativeone:wecanonlysaywhetheroneformationisolderthantheotherone.Absolutedatingwasmadepossibleonlyafterthediscoveryofradioactivity.RadioactivityAttheturnofthe20thcentury,nuclearphysicistsdiscoveredthatatomsofuranium,radium,andseveralotherelementsareunstable.Thenucleioftheseatomsspontaneouslybreakapartintootherelementsandemitradiationintheprocessknownasradioactivity.Wecalltheoriginalatomtheparentanditsdecayproductthedaughter.Forexample,aradioactive92U238atomdecaysintoastablenonradioactive82Pb206atom.exampletypesofradioactivedecayAlphadecay:anaparticle(composedof2protonsand2neutrons)isemittedfromanucleus.Theatomicnumberofthenucleusdecreasesby2andthemassnumberdecreasesby4.Betadecay:abparticle(electron)isemittedfromanucleus.Theatomicnumberofthenucleusincreasesby1butthemassnumberisunchanged.Illustrationofalphaandbetadecays.(adaptedfromTarbuckandLutgens)ThedecayofU238.Afteraseriesofradioactivedecays,thestableendproductPb206isreached.(TarbuckandLutgents)DecayconstantTherateofdecayofanunstableparentnuclideisproportionaltothenumberofatoms(N)remainingatthetimet.dN/dt=-l*NThereasonthatradioactivedecayoffersareliablemeansofkeepingtimeisthatthedecayconstantlofaparticularelementdoesnotvarywithtemperature,pressure,orchemistryofageologicenvironment.Half-lifeThehalf-lifeofanradioactiveelementisthetimerequiredforone-halfoftheoriginalnumberofradioactiveatomstodecay:T1/2=0.693/l.Thehalf-livesofgeologicallyusefulradioactiveelementsrangefromthousandstobillionsofyears.TheageoftheEarth(4.6billionyears)wasfirstobtainedusingU/Th/Pbradiometricdating.Thehalf-lifeofU238is4.5billionyears.Theradioactivedecayisexponential.Halfoftheradioactiveparentremainsafteronehalf-life,andone-quarteroftheparentremainsafterthesecondhalf-life.(TarbuckandLutgens)Theconceptofahalf-life.Theratioofparent-to-daughterchangeswiththepassageofeachsuccessivehalf-life.(W.W.Norton)GeologicTimeThegeologictimescalesubdividesthe4.6-billion-yearhistoryoftheEarthintomanydifferentunits,whicharelinkedwiththeeventsofthegeologicpast.Thetimescaleisdividedintoeons:PrecambrianandPhanerozoicanderas:Precambrian,Paleozoic("ancientlife"),Mesozoic("middlelife"),andCenozoic("recentlife").Theerasareboundedbyprofoundworldwidechangesinlife-forms.Theerasaredividedintoperiods.Theperiodsaredividedintoepochs.Thestandardgeologictimescalewasdevelopedusingrelativedatingtechniques.Radiometricdatinglaterprovidedabsolutetimesforthestandardgeologicperiods.(W.W.Norton)TheawesomespanofgeologictimeThegeologictimerepresentseventsofawesomespansoftime.Ifthe4.6-billion-yearEarthhistoryisrepresentedbya24-hourdaywiththebeginningat12midnight,thefirstindicationoflifewouldoccurat8:35am.Dinosaurswouldappearat10:48pmandbecomeextinctat11:40pm.Therecordedhistoryofmankindwouldrepresentonly0.2secbeforemidnight.TheKTextinctionAttheboundarybetweenCretaceous(thelastperiodofMesozoic)andTertiary(thefirstperiodOfCenozoic)about66millionyearsago,knownasKTboundary,morethanhalfofallplantandanimalspeciesdiedinamassextinction.Theboundarymarkstheendoftheerainwhichdinosaursandotherreptilesdominatedandthebeginningoftheerawhenmammalsbecameimportant.Thewidelyheldviewoftheextinctionistheimpacthypothesis.AlargeobjectcollidedwiththeEarth,producingadustcloudthatblockedthesunlightfrommuchoftheEarth’ssurface.Withoutsunlightforphotosynthesis,thefoodchainscollapsed,whichaffectedlargeanimalsmostseverely.9、静夜四无无邻，荒居居旧业贫。。。1月-231月-23Wednesday,January4,202310、雨中黄黄叶树，，灯下白白头人。。。23:06:3823:06:3823:061/4/202311:06:38PM11、以我独沈久久，愧君相见见频。。1月-2323:06:3823:06Jan-2304-Jan-2312、故故人人江江海海别别，，几几度度隔隔山山川川。。。。23:06:3823:06:3823:06Wednesday,January4,202313、乍见见翻疑疑梦，，相悲悲各问问年。。。1月-231月-2323:06:3823:06:38January4,202314、他乡乡生白白发，，旧国国见青青山。。。04一一月月202311:06:38下下午午23:06:381月-2315、比不了得就就不比，得不不到的就不要要。。。一月月2311:06下下午午1月月-2323:06January4,202316、行行动动出出成成果果，，工工作作出出财财富富。。。。2023/1/423:06:3823:06:3804January202317、做做前前，，能能够够环环视视四四周周；；做做时时，，你你只只能能或或者者最最好好沿沿着着以以脚脚为为起起点点的的射射线线向向前前。。。。11:06:38下下午午11:06下下午午23:06:381月月-239、没有失败败，只有暂暂时停止成成功！。1月-231月-23Wednesday,January4,202310、很多事情情努力了未未必有结果果，但是不不努力却什什么改变也也没有。。。23:06:3823:06:3823:061/4/202311:06:38PM11、成功功就是是日复复一日日那一一点点点小小小努力力的积积累。。。1月-2323:06:3823:06Jan-2304-Jan-2312、世间间成事事，不不求其其绝对对圆满满，留留一份份不足足，可可得无无限完完美。。。23:06:3823:06:3823:06Wednesday,January4,202313、不知