2/5/17
#7GeologicTime
GeologicTime
#7GeologicTime
GeologicTime
GeologicTime =alloftimesinceEarthformed(4.6billion
yearsago,b.y.a.).Geologistsstudyrocks,fossils, geologic
structures(faults,folds)tocharacterizeEarth'shistory.
Naturaldisastersinvolvepresent-day.Whyshouldwecare
aboutEarth'sgeologichistory?
GeologicTime
GeologicTime
StudyancientclimatesofEarthtounderstandpresentand
futureclimatechange
Naturaldisastersinvolvepresent-day.Whyshouldwecare
aboutEarth'sgeologichistory?
Studymassextinctionsingeologicpasttounderstand
presentandfuturemassextinctions
WhyshouldwecareaboutEarth'sgeologichistory?
Studyancientvolcanoes,faults,landslides,etc.to
understandpresentandfuturerisks
1
2/5/17
WebExercise1– Duetoday1:00PM
• 2-pointpenaltyforevery24-hourperiodthatassignmentislate.
NoWebExercise#1isacceptedafter1pmonFriday
• After~5PMFriday,WebExercise#1scoreswillbevisibleinIL
CompassGradebook;atthesametime,feedback,includingyour
answersandthecorrectanswers,willbeavailableinILCompass
Questions?ContacttheTA
GeologicTime- TypesofAges
AbsoluteAge =ageofrock(orgeologicevent)inyears.
Needconstantprocessaswellasrecordofprocess.
WebExercise#2- StartsFriday2/10;due2/17
• WebExercise#2(Earthquakes)willbeavailableontheGEOL118
ILCompassWebsiteonFriday
• WebExercise#2willincludequestionsbasedoninformationin
HazardCity software(MasteringGeology WebSite)andthe
Internet.
• YoumustbuyaccesstoMasteringGeologyandRegister(see
classwebpagesforinstructions)
GeologicTime- TypesofAges
I)AbsoluteAge =Constantprocessinrocksisradioactive
decay
Parent(radioactive)isotope→Daughter(stable)isotope+Energy
e.g.,Uranium→Lead
GeologicTime- TypesofAges
Rateofradioactivedecayisgivenbyhalf-life (timeneeded
forhalfofparentisotopestochangetodaughterisotopes).
Note:atomsmustnotmovearoundafter.
Isotope =differentformofatomwithsamenumberofprotons,
differentnumberofneutrons
GeologicTime- TypesofAges
Ageofrockisdeterminedbyknowinghalf-lifeofreaction
andamountofparentanddaughterisotopes(radiometric
agedating)
GeologicTime- TypesofAges
Igneousrocksyieldmostreliableradiometricagesof
formation
2
2/5/17
GeologicTime- TypesofAges
II)RelativeAge =ageofonerockwithrespecttoanother
(sequenceofeventsfromoldesttoyoungest)
GeologicTime- RelativeAgeDetermination
A)PrincipleofUniformitarianism ="Presentiskeytopast"
currentphysicalprocessesalsooperatedingeologicpast
(JamesHuttonin1700's).Geologistsusethisprincipleto
interpretEarthhistory. Examples?
Modernlavaflow
Ancientlavaflow
(Giant’sCauseway)
GeologicTime- RelativeAgeDetermination
GeologicTime- RelativeAgeDetermination
A)PrincipleofUniformitarianism =Examples?
A)PrincipleofUniformitarianism =Fornaturaldisasters,
uniformitarianismusedtopredictfutureevents.
Examples?
Modernmudcracks
Ancientmudcracks
GeologicTime- RelativeAgeDetermination
GeologicTime- RelativeAgeDetermination
Whichrocklayerisoldest(bottom,middleortop)?
C)PrincipleofCross-CuttingRelations =Ifigneousintrusion
(orfaultorrivervalley)cutsacrossrocklayers,intrusionis
youngerthanrocklayers
B)PrincipleofSuperposition =movingupwardthrough
sequenceofsedimentaryrocks,agebecomesyounger
3
2/5/17
GeologicTime- RelativeAgeDetermination
C)PrincipleofCross-CuttingRelations =Putintocorrect
sequence:darkhostrock,thickintrusion,thinintrusion
GeologicTime- RelativeAgeDetermination
D)PrincipleofFossilSuccession =Groupsoffossilsinrock
canrepresentintervalofgeologictime.
GeologicTime- RelativeAgeDetermination
D)PrincipleofFossilSuccession =Whenfossilsin
sedimentaryrocksarearrangedbyrelativeage,groupsof
fossilplants+animalshavechangedform(evolved)over
geologictimeindefinite+recognizableorder.
GeologicTimeScale
Usingfossils+radiometric
agesofrocksfromallover
worldresultedin
GeologicTimeScale - table
ofagesrepresentsEarth's
geologichistory
GeologicTimeScale- ImportantEvents
AgeofEarth=4.6b.y.(fromageofmeteorites,moonrocks)
Age-datedmeteorite
Age-datedmoonrock
4
2/5/17
GeologicTimeScale- ImportantEvents
OldestrocksfromEarth(continentalrock)=4- 4.2b.y.
Whynot4.6b.y.?
Earthisdynamic;meltingandmetamorphism“reset”
radiometricagesofrocksfromEarth
GeologicTimeScale- ImportantEvents
PreCambrian =Largestblockoftime(88%ofEarthhistory);
lifeevolvedat~3- 3.5b.y.a.(bacteria).Onlysoft-bodied
organismslived,sofewfossilsarepreserved.
Extremevolcanism- EarlyPrecambrian
3.5b.y.oldstromatolites (cyanobacteria)
GeologicTimeScale- ImportantEvents
PreCambrian/Paleozoicboundary =Appearanceofabundant
animalswithshells,fossilsnowabundant.
AbundantnewlifeinEarlyPaleozoic
GeologicTimeScale- ImportantEvents
Paleozoic("Oldlife")Era =CarboniferousPeriodinvolved
extensiveforests,swamps(laterbecamecoal).
IronFormation- MiddlePrecambrian
GeologicTimeScale- ImportantEvents
Paleozoic("Oldlife")Era =Trilobites,crinoids,coralreefs,
snails;Appearanceoffish,plants,amphibians,insects
AbundantoceanlifeinPaleozoic
GeologicTimeScale- ImportantEvents
Paleozoic/Mesozoicboundary =Massextinction.Halfofall
familiesoforganismssuddenlydisappeared.
CarboniferousForestandSwamp
5
2/5/17
GeologicTimeScale- ImportantEvents
Mesozoic ("Middlelife")Era =Dominanceofreptiles
(dinosaurs),Firstbirds.
DinosaursofMesozoicEra
GeologicTimeScale- ImportantEvents
Mesozoic/Cenozoicboundary =Massextinctionof1/4ofall
familiesoforganisms.Extinctionofalldinosaurs.
Mesozoic/CenozoicMassExtinction
GeologicTimeScale- ImportantEvents
Cenozoic("Recentlife")Era =Dominanceofmammals,
Appearanceofhumans,currentlyexperiencingmass
extinction.
MammalsofCenozoic
6