stratigrraphyandgeologictime地層和地質時間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:Markersofmissingtime

Whenlayersofrockformedwithoutinterruption,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>>Distinguishingnonconformityandintrusivecontact

Nonconformity:

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)Correlationofrockunits

Themethodofrelatingrockunitsfromonelocalitytoanotheriscalledcorrelation.Onewayofcorrelationistorecognizetherocktypeorrocksequenceattwolocations.Anotherwayofcorrelationistousefossils.Abasicunderstandingoffossilsisthatfossilorganismssucceededoneanotherinadefiniteanddeterminableorder,andthereforeatimeperiodcanberecognizedbyitsfossilcontent.Theprincipleofcorrelationofrockunits.Therockcolumnscanbecorrelatedbymatchingrocktypes.(W.W.Norton)WilliamSmith,acivilengineerandsurveyor,couldpiecetogetherthesequenceoflayersofdifferentagescontainingdifferentfossilsbycorrelatingoutcropsfoundinsouthernEnglandabout200yearsago.Inthisexle,FormationIIwasexposedatbothoutcropsAandB,thusFormationIandIIwereyoungerthanFormationIII.(PressandSiever).CorrelationofstrataatthreelocationsontheColoradoPlateaurevealsthetotalextentofsedimentaryrocksintheregion.Thegeologiccolumnwasconstructedbydeterminingtherelativeagesofrockunitsfromaroundtheworld.(Next)Bycorrelation,thesecolumnswerestackedoneontopoftheothertogiverelativeagesofrockunits(W.W.Norton)Absolutedating

Thegeologictimebasedonstratigraphyandfossilsisarelativeone:wecanonlysaywhetheroneformationisolderthantheotherone.Absolutedatingwasmadepossibleonlyafterthediscoveryofradioactivity.RadioactivityAttheturnofthe20thcentury,nuclearphysicistsdiscoveredthatatomsofuranium,radium,andseveralotherelementsareunstable.Thenucleioftheseatomsspontaneouslybreakapartintootherelementsandemitradiationintheprocessknownasradioactivity.Wecalltheoriginalatomtheparentanditsdecayproductthedaughter.Forexle,aradioactive92U238atomdecaysintoastablenonradioactive82Pb206atom.exletypesofradioactivedecayAlphadecay:anaparticle(composedof2protonsand2neutrons)isemittedfromanucleus.Theatomicnumberofthenucleusdecreasesby2andthemassnumberdecreasesby4.Betadecay:abparticle(electron)isemittedfromanucleus.Theatomicnumberofthenucleusincreasesby1butthemassnumberisunchanged.Illustrationofalphaandbetadecays.(adaptedfromTarbuckandLutgens)ThedecayofU238.Afteraseriesofradioactivedecays,thestableendproductPb206isreached.(TarbuckandLutgents)Decayconstant

Therateofdecayofanunstableparentnuclideisproportionaltothenumberofatoms(N)remainingatthetimet.dN/dt=-l*NThereasonthatradioactivedecayoffersareliablemeansofkeepingtimeisthatthedecayconstantlofaparticularelementdoesnotvarywithtemperature,pressure,orchemistryofageologicenvironment.

Half-life

Thehalf-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)GeologicTime

Thegeologictimescalesubdividesthe4.6-billion-yearhistoryoftheEarthintomanydifferentunits,whicharelinkedwiththeeventsofthegeologicpast.Thetimescaleisdividedintoeons:PrecambrianandPhanerozoicanderas:Precambrian,Paleozoic("ancientlife"),Mesozoic("middlelife"),andCenozoic("recentlife").Theerasareboundedbyprofoundworldwidechangesinlife-forms.Theerasaredividedintoperiods.Theperiodsaredividedintoepochs.Thestandardgeologictimescalewasdevelopedusingrelativedatingtechniques.Radiometricdatinglaterprovidedabsolutetime