GEOLOGICAL TIME SCALE.pdf

GEOLOGICAL TIME SCALE

3. A record of the life forms and geologic events in Earth’s history.
TheGeologicTimeScale(GTS)isatimelinethatshowstheearth'shistorydividedintotimeunitsbasedonthesignificant
eventsoccurringatthattime.Scientistsusefossils,rocklayers,andtheirpositionsrelativetoeachothertoputthehistoryof
theearthintoorder.
Abbreviations:
❖Ka: "kilo anum," one-thousand years. Kya = thousand years ago.
❖Ma:"mega anum," one-million years. Mya = million years ago.
❖Ga:"giga anum," one-billion years. Bya= billion years ago.
Dr. Melvin A. Ekka

4. MainfeaturesofGTS-
•Thegeologicaltimescale(GTS)dividestheearth’sevolutionaryhistoryintovarioustimeperiodsfromthe
beginningtothepresentbasedondefiniteeventsthatmarkedamajorchangeinearth’sphysical,chemicaland
biologicalfeatures.
•SincethedivisionoftimeunitsintheGTSistypicallybasedontheoccurrenceofasignificantgeologicalevent
(e.g.massextinction,appearanceofnewlifeforms,orogeniceventsetc.)consequently,geologicaltime
categoriesarevariableinlengthanddonotusuallyconsistofauniformlengthoftime.
•Majorchangesinearth’sphysicalandbiologicalhistorystretchoverseveralmillionsofyearsandhenceinGTS
allthedivisionsareexpressedin‘millionyears(mya–millionyearsago).
•TheGeologicTimeScaleisdividedintofourmajorunits:Eons,Eras,PeriodsandEpochs.
•TheprimarilydefineddivisionsoftimeareEons.Eacheonissubsequentlydividedintoeras,whichinturnare
dividedintoperiods,whicharefurtherdividedintoepochs.
Dr. Melvin A. Ekka

5. EON > ERA > PERIOD > EPOCH>AGE
❖AnEonorEonothemisthelongestdivisionofgeologictime,solonginfactthattherehaveonlybeenfourEons-
TheHadean,theArchean,theProterozoicEon&PhanerozoicEon.Collectivelythefirstthreeeons[theHadean,the
Archean,theProterozoicEon]arecalledthePrecambrian-thatstretchofgeologicaltimefromtheformationofthe
EarthitselftothestartoftheCambrianperiod.ThisPrecambrianeoncoversalmost90%oftheentirehistoryof
Earth.
❖Theremainderofgeologictime,includingpresentday,belongstothePhanerozoicEon.
❖EraorErathem,arethesubdivisionsofeons.Theyaremoreonthescaleofhundredsofmillionsofyears.Thenames
oftheErasinthePhanerozoiceon(theeonofvisiblelife)aretheCenozoic("recentlife"),Mesozoic("middlelife")
andPaleozoic("ancientlife").
❖Periods,alsoreferredas"systems",arethesubdivisionsoferasandtypicallylasttensofmillionsofyears.Periods
ingeologicaltimescalereferstotheunitwhichissmallerthanerasbutbiggerthanepochs.Alltheerasaredivided
intosmallerunitscalledperiods.ThePalaeozoiceraisdividedintoperiods-Permian,Carboniferous,Devonian,
Silurian,Cambrian,Ordovician.Alltheotherperiodsaregiveninthetablebelow.
❖PeriodsarefurtherdividedintoEpochsalsoreferredas"series"liketheQuaternaryandTertiaryperiodof
CenozoicerawhichisfurtherdividedintoepochssuchasHolocene,Pleistocene,Plioceneetc.
Dr. Melvin A. Ekka

6. HadeanEon
TheearliesttimeoftheEarthiscalledtheHadeanandreferstoaperiodoftime~4.6-4.0billionyearsagoforwhich
wehavenorockrecord.ThistimerepresentsEarth’searliesthistory,duringwhichtheplanetwascharacterizedbya
partiallymoltensurface,volcanism,andasteroidimpacts.SeveralmechanismsmadethenewlyformingEarth
incrediblyhot:gravitationalcompression,radioactivedecay,andasteroidimpacts.Mostofthisinitialheatstillexists
insidetheEarth.IntheHadeanEon,weobtainaheatsource,magneticfield,andformationofamoon.TheHadean
wasoriginallydefinedasthebirthoftheplanetoccurring~4.6billionyearsagoandprecedingtheexistenceofmany
rocksandlifeforms.
IMPORTANT EVENTS IN THE GEOLOGICAL TIME SCALE
Dr. Melvin A. Ekka

7. ArcheanEon
TheArcheanEon,whichlastedfrom4.0–2.5billionyearsago,isnamedaftertheGreekwordforbeginning.Thiseon
representsthebeginningoftherockrecord.Evidenceforprokaryoticlifesuchasbacteriaandblue-greenalgaehas
beenfoundinsouthernAfrica,datedto3100Myr.Bandedironformationshavebeendatedto3700Myr,and
presumingthatthisrequiresoxygenandthattheonlysourceofmolecularoxygeninthiserawasphotosynthesis,this
makesacaseforlifeinthistimeperiod.Therearealsostromatolitesdatedto3500Myr.
Stromatolites formed by prokaryotic cyanobacteria
Dr. Melvin A. Ekka

8. ProterozoicEon
TheProterozoicEon,meaning“earlierlife,”istheeonoftimeaftertheArcheaneonandrangesfrom2.5billionyears
oldto541millionyearsold.NeartheendofthePrecambrian,thereisfossilevidenceofdiverseandcomplex
multicelledorganisms.Photosynthesis(inorganismslikestromatolites)hadalreadybeenaddingoxygenslowlytothe
atmosphere,butitwasquicklyabsorbedinminerals.Evolutionaryadvancementsinmulticellularcyanobacteria
completelytransformedtheatmospherebyaddingfreeoxygengas(O
2)andcausingthedecimationoftheanaerobic
(non-oxygen)bacteriathatexistedatthetime.ThisisknownastheGreatOxygenationEvent.Theformationofthe
bandedironlastedalongtimeandpreventedtheoxygenlevelfromincreasingsignificantlyintheoceanssincetherocks
literallytooktheoxygenoutofthewaterandformedalternatinglayersofiron-oxidemineralsandredchert.Eventually,
asoxygencontinuedtobemade,absorptionofoxygeninmineralprecipitationleveledoff,anddissolvedoxygengas
startedfillingtheoceansandeventuallybubblingoutintotheatmosphere.Oxygenationoftheatmosphereisthesingle
biggesteventthatdistinguishestheArcheanEarthandtheProterozoicEarth.Theformationofthebandediron
lastedalongtimeandpreventedtheoxygenlevelfromincreasingsignificantlyintheoceanssincetherocksliterally
tooktheoxygenoutofthewaterandformedalternatinglayersofiron-oxidemineralsandredchert.Eventually,as
oxygencontinuedtobemade,absorptionofoxygeninmineralprecipitationleveledoff,anddissolvedoxygengas
startedfillingtheoceansandeventuallybubblingoutintotheatmosphere.ThemostcommonPrecambrianfossilsare
stromatolites,whichbecomecommonabout2000Myrinthepast.Bacteriaandblue-greenalgaefossilshavebeen
foundinGunflintChertrocksatLakeSuperior,datingto1700Myr.Theseareprokaryoticlife.Eukaryoticlifehasbeen
foundatabout1000MyratBitterSprings,Australiaintheformofgreenalgae.
Dr. Melvin A. Ekka

9. ➢BytheProterozoiceon,lithosphericplateshadformedandstartedmovingaccordingtoplatetectonicmotionssimilar
totoday.Astheseplatesformedandstartedmoving,eventuallyasupercontinentformedfromcollisionsastheocean
basinsclosed.TheexactnumberofsupercontinentsduringtheProterozoic(orearlier)isunknown,butRodiniaisthe
bestunderstood.
Dr. Melvin A. Ekka

10. Dr. Melvin A. Ekka
PaleozoicEra
ThebeginningoftheCambrianisthetimeofthefirstorganismswithshells.Trilobites
weredominanttowardtheendoftheCambrianPeriod.TheCambrianPeriodmarksthe
timeofemergenceofavastnumberoffossilsofmulticellularanimals,andthis
proliferationoftheevidenceforcomplexlifeisoftencalledthe"Cambrian
Explosion".IntheOrdovician,largecephalopodsdevelopedaspredatorsofsizeupto
10meters.Theyareconsideredtobethefirstlargeorganisms.Thelaterpartofthe
Ordoviciansawtheappearanceofthefirstfishes.
TheOrdovicianandCambrianPeriodsarereferredtoasthe"ageofinvertebrates".
TheSilurianPeriodmarkedtheemergenceofthefirstlandplants.IntheDevonian
Periodfishesweredominant.Primitivesharksdeveloped.Towardtheendofthe
Devonianthereisevidenceofinsectswiththefirstinsectfossils.Fromfinger-sized
earliercoastalplants,landplantsdevelopedandmovedawayfromthecoasts.Bythe
endoftheDevonian,fossilevidencesuggestsforestswithtreestensofmetershigh.By
lateDevonian,twogroupsofbonyfishes,thelungfishandthelobe-finnedfishhad
adaptedtolandenvironments,andtrueair-breathingamphibiansdeveloped.The
amphibianscontinuedtodiversifywithabundantfoodandminimalcompetitionand
becamemorelikemodernreptiles.TheDevonianperiodisnamedafterDevoninthe
westofEngland.
TheDevonianandSilurianPeriodsarereferredtoasthe"ageoffishes".

11. Dr. Melvin A. Ekka BytheendofthePermianPeriodtheoncedominanttrilobitesareextinctalong
withmanyothermarineanimalsreferredtoas"TheGreatPaleozoicExtinction"
anditwasthegreatestofatleastfivemajorextinctionsoverthepast600million
years.ThemodellingofplatetectonicssuggeststhatattheendofthePermian
PeriodthecontinentswerealltogetherintheformcalledPangaea,andthatthe
separationsthathavecreatedtoday'salignmentofcontinentshavealloccurred
sincethattime.
PermianPeriodisnamedafterthePermregionofRussia,wherethetypesoffossilscharacteristicofthatperiodwerefirst
discoveredbygeologistRoderickMurchisonin1841.ThePermian,PennsylvanianandMississippianPeriodsare
collectivelyreferredtoasthe"ageofamphibians".
CarboniferousPeriod-[MississippianPeriodPennsylvanianPeriod].MississippianPeriod-Amphibiansbecameabundantin
thisperiod,andtowardtheendofitthereisevidenceoflargecoalswamps.ThePennsylvanianPeriodsawtheemergence
ofthefirstreptiles.ThisperiodsawthedevelopmentoflargetropicalswampsacrossNorthAmerica,EuropeandSiberia
whicharethesourceofgreatcoaldeposits.NamedaftertheareaoffinecoaldepositsinPennsylvania.

12. Dr. Melvin A. Ekka
MesozoicEra
FollowingthePermianMassExtinction,theMesozoic(“middlelife”)wasfrom252millionyearsagoto66million
yearsago.AsPangeastartedtobreakapart,mammals,birds,andfloweringplantsdeveloped.TheMesozoicisprobably
bestknownastheageofreptiles,mostnotably,thedinosaurs.
TriassicPeriod-Thefirstappearanceofmanymoderngroupsofanimalsthatwouldlaterflourishoccurredatthistime.
Thisincludesfrogs(amphibians),turtles(reptiles),marineichthyosaursandplesiosaurs(marinereptiles),mammals,
andthearchosaurs.
JurassicPeriod-ThedistinctivefossilprogressioncharacteristicofthisperiodwasfirstfoundintheJuraMountainsof
Russia-Dinosaursandotherreptileswerethedominantspecies.TheJurassicPeriodsawthefirstappearanceofbirds.
TheearlyJurassicPeriodatabout200MyrsawthebeginningofthebreakupofPangaeaandariftdevelopedbetween
whatisnowtheUnitedStatesandwesternAfrica,givingbirthtotheAtlanticOcean.ThewestwardmovingAtlantic
platebegantooverridethePacificplate.ThecontinuingsubductionofthePacificplatecontributedtothewestern
mountainsandtotheigneousactivitythatresultedintheRockyMountains.
TheCretaceousPeriod-ThefirstfloweringplantsappearednearthebeginningoftheCretaceousPeriod.The
CretaceousPeriodisperhapsmostfamiliarbecauseofthemajorextinctioneventwhichmarkstheCretaceous-Tertiary
boundary.ItistypicallycalledtheK-Textinction,usingthefirstletteroftheGermanspellingofCretaceous,andit
markedtheendofthedinosaurs.Thereislargebodyofevidenceassociatingthisextinctionwiththelargeimpact
crateratChicxulub,YucatanPeninsula,Mexico.
TheCretaceous,JurassicandTriassicPeriodsarecollectivelyreferredtoasthe"ageofreptiles".

13. Dr. Melvin A. Ekka
CenozoicEra
TheCenozoic,meaning“newlife”isEarth'scurrentgeologicalera,representingthelast66millionyearsof
Earth'shistory.Itisknownasthe“ageofmammals”becauseitisinthiserathatmammalscametobea
dominantandlargelifeform,includinghumanancestors.Birds,aswell,flourishedintheopennichesleftby
thedinosaur’sdemise.MostoftheCenozoichasbeenrelativelywarm,withthemainexceptionbeingtheice
agethatstartedabout2.558millionyearsagoand(despiterecentwarming)continuestoday.
DuringtheCenozoicEra,thecontinentsmovedtotheirpresentpositions,andEarth'sclimatebecamecooler
anddrier.Thesechangeshadamajorimpactontheevolutionoflifeduringtheera.
Today,theCenozoicEraisgenerallysubdividedintothreeperiods:thePaleogene,Neogene[togetherknown
asTertiary]andQuaternary(orAnthropogene)Periods.
ThethreeperiodsoftheCenozoicEraaresubdividedintosevenepochs.ThePaleogenePeriodincludesthe
Paleocene,Eocene,andOligoceneEpochs.TheNeogenePeriodincludestheMiocenceandPlioceneEpochs.
TheQuaternaryPeriodincludesthePleistoceneandHolocene(recent)Epochs.
TheearliestprimatesevolvedinthePaleoceneEpoch;apesdiversifiedduringtheMioceneEpoch;andclose
humanrelativescalledAustralopithecinesevolvedduringthePlioceneEpoch.TheCenozoiccoolingtrend
culminatedinaniceageduringthePleistoceneEpoch,whichisalsotheepochduringwhichthehuman
genus(Homo)andhumanspecies(Homosapiens)evolved.Humanbeingsbecameascendant,andallof
recordedhumanhistoryoccurredduringtheHoloceneEpoch.

14. Dr. Melvin A. Ekka
TheHoloceneisthecurrentgeologicalepoch.Itbeganapproximatelyapprox.11,650yearsBP(BeforePresent).
TheInternationalCommissiononStratigraphy,considerstheHolocenetobeanepochfollowingthePleistoceneand
specificallyfollowingthelastglacialperiod(iceage).
❖TheMeghalayanAgeisthenamegivenin2018bytheInternational
CommissiononStratigraphy,tothecurrentageorlatestgeologic
age–oruppermoststageoftheQuaternary.Itisalsotheupper,or
latest,ofthreesubdivisionsoftheHoloceneepoch.
❖TheMeghalayanbegins4,200yearsBPleavingroomforthepossible
introductionoftheAnthropocene.TheInternationalCommissionon
StratigraphyofficiallyratifiedthisageinJune2018,alongwiththe
earlierGreenlandianandNorthgrippianages/stages.
❖ItsGlobalBoundaryStratotypeSectionandPoint(GSSP)isaKrem
MawmluhCaveformationinMeghalaya.
❖Theagebeganwitha200-yeardroughtthatimpactedhuman
civilizationsintheEasternMediterranean,Mesopotamia,theIndus
ValleyandtheYangtzeRiverValley.
❖"Thefactthatthebeginningofthisagecoincideswithaculturalshift
causedbyaglobalclimateeventmakesitunique,"