Relative Geologic Time ScaleRelative-Dating PrinciplesCross-cutting RelationshipsCross-cutting RelationshipsUnconformitiesUnconformitiesExample of an UnconformityBack to StenoWhy are layers tilted?FoldingFaultingRelative DatingRelative Dating – Cross-sectionRelative Dating – Cross-sectionRelative Dating – Cross-sectionRelative Dating – Cross-sectionGrand Canyon: history revealedGrand CanyonGeologic Time ScaleChanges in the Concept of Geologic TimeChanges in the Concept of Geologic TimeChanges in the Concept of Geologic TimeHistory of Historical GeologyHistory of Historical GeologyUniformitarianismUnconformity at Siccar PointUniformitarianismCrisis in GeologyCrisis in GeologyAbsolute-Dating MethodsAbsolute-Dating MethodsRadioactive DecayRadioactive DecayRadioactive DecayRadioactive DecayUranium 238 decayHalf-LivesHalf-LivesGeometric Radioactive DecayDetermining AgeDetermining AgeWhat Materials Can Be Dated?Igneous Crystallization Sources of UncertaintySources of UncertaintyDating MetamorphismLong-Lived Radioactive Isotope Pairs Used in DatingMass SpectrometerHow do we know the Earth can’t be older than about 6-7 b.y.?Radiocarbon Dating MethodCarbon 14Carbon 14Tree-Ring Dating MethodTree-Ring Dating MethodSummarySummarySummarySummary• The Map That Changed the World– William Smith, 1815– Geologic map of England, Wales, part of ScotlandDating the EarthThe Map That Changed the World• The relative geologic time scale has a sequence of – eons –eras – periods – epochs– but no numbers indicating how long ago each of these times occurredRelative Geologic Time Scale• Large divisions based on…?• Paleozoic Era –• Mesozoic Era –• Cenozoic Era –Geologic Time Scale• Large divisions based on characteristics of fossils• Paleozoic Era – early life dominated by invertebrate animals• Mesozoic Era – middle life• Cenozoic Era – recent lifeGeologic Time ScaleHow was the timescale created?• Mapping in 1800s using the principles of – Superposition– Original Horizontality– Original Lateral Continuity– Cross-cutting relationships– Also Fossil CorrelationHow was the timescale created?• Superposition– Oldest on bottom, youngest on topRelative-Dating PrinciplesChattanooga Shale, TN• Original Horizontality– Sediment originally deposited in flat parallel layers• Lateral continuity– sediment extends laterally in all directions until it thins and pinches out or terminates against the edges of the depositional basin• Cross-cutting relationships– an igneous intrusion or a fault must be younger than the rocks it intrudes or displacesRelative-Dating Principles• A dark-colored dike has intruded into older light colored granite: the dike is younger than the graniteCross-cutting RelationshipsNorth shore of Lake Superior, Ontario Canada• A small fault displaces tilted beds: the fault is younger than the bedsCross-cutting RelationshipsTemplin Highway, Castaic, CaliforniaUnconformities• What is an unconformity?Unconformities• What is an unconformity?– A surface of erosion or non-deposition– Recognizable surface in the rock recordExample of an Unconformity• Tilted sandstone and siltstone below, conglomerate abovewww.geology.sdsu.edu/visualgeology/geology101/erosion6.htmBack to Stenowww.gly.uga.edu/railsback/1121Steno.jpgWhy are layers tilted?• Deformation of rocks– Occurs after they are deposited– Important factor in relative dating• Folding– Anticlines, synclines– Rock bends, but does not break• Faulting– Normal, reverse, transform– Rock breaksFoldingwww.hill.anorak.org.uk/dhtml/glgchap5.htmlFaultingwww.stmarys.ca/academic/science/geology/structural/faults.htmlRelative Dating• Principles• Unconformities• DeformationRelative Dating – Cross-sectionhttp://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/Relative Dating – Cross-sectionhttp://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/dating/Key:EErosionGLCHTiltingErosionMDJAErosionNKBTiltingErosionFErosionRelative Dating – Cross-sectionhttp://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htmRelative Dating – Cross-sectionhttp://facweb.bhc.edu/academics/science/harwoodr/Geol101/labs/Dating2/Index.htmKey:HFDGIErosionCErosionABMLKErosionJGrand Canyon: history revealed• More than 1 billion years of history are preserved in the rock layers of the Grand Canyon• Reading this rock book shows: – periods of mountain building– advancing and retreating shallow seas– evolution of faunas• Determine these things by: – applying the principles of relative dating to the rocks– and recognizing that present-day processes have operated throughout Earth history - UniformitarianismGrand Canyon• Radiometric dating is the most common method of obtaining absolute ages– calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks• Other methods?– Tree ring counting–Varves– Ice coresAbsolute Dating• The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale• The geologic time scale is a dual scale– a relative scale– and an absolute scaleGeologic Time Scale• The concept and measurement of geologic time has changed through human history• James Ussher (1581-1665) in Ireland – calculated the age of Earth based on recorded history and genealogies in Genesis• he announced that Earth was created on October 22, 4004 B.C.• a century later it was considered heresy to say Earth was more than about 6000 years oldChanges in the Concept of Geologic Time• During the 1700s and 1800s Earth’s age was estimated scientifically– Georges Louis de Buffon (1707-1788) calculated how long Earth took to cool gradually from a molten beginning • used melted iron balls of various diameters• he estimated Earth was 75,000 years oldChanges in the Concept of Geologic Time– Others used rates of deposition of various sediments and thickness of sedimentary rock in the crust • gave estimates of <1 million • to more than 2 billion years– Or the amount of salt carried by rivers to the ocean and the salinity of seawater • John Joly in 1899 obtained a minimum age of 90 million yearsChanges in the Concept of Geologic Time• Neptunism– proposed in 1787 by Abraham Werner (1749-1817)– all rocks, including granite and basalt, were precipitated in an orderly