Radioactivityad oact tyMeasuring Geologic TimeChapter 13pChapter 21click to get 9/page to printMeasuring RadioactivityMeasuring Radioactivity• Radioactive decay strips electrons from atoms• Ions created Gi thd•Geiger counter--charged wire, results in ‘clicks’ of counter•Others rely on visual•Others rely on visual reactions of ionsHalf LifeHalf LifeDecay is random for any radioactive atomBUT: Predictable for mass of materialBUT: Predictable for mass of material• One half of unstable parent material Ædaughter product: HALF LIFE: L1/2• Decay rate constant, unaffected by external conditionsParent to daughter ratios of H-3Half Life equationHalf Life equationAmount remaining can be calculated byIR1nIR2=2R remaining I initial n number of half-livesCalculate amount from halflifeCalculate amount from half-lifeSt t ith 400 f C601•Start with 400 mg of Co-60• Half life is 5.25 yearsnIR21=• How much is left after 15.75 years?• To use equation at right,g• First calculate how many half livesyryears25.57515=÷halflifeyears7515lihlf33 halfliveshalflifeyears75.15=÷=⋅yrffyears25.575.15liveshalf−33 half-livesCalculate amount from halflifeCalculate amount from half-life• 400 mg of Co-60• Half life is 5.25 yearsy• How much is left after 3 half-lives?1• Calculate amount with nIR21=1502mgR21)400(3=⋅=mg502Absolute Geologic TimeAbsolute Geologic Time• Radiometric Dating•Igneous rocks contain potassium uraniumIgneous rocks contain potassium, uranium thorium and rubidium that are radioactiveCfl tfti fth•Careful measurement of ratios of these and their daughter products, or of the isotopes of them that are not radioactive can be used to calculate absolute agescan be used to calculate absolute agesRadiometric datingRadiometric dating• Known Half-life•Closed systemClosed system• Cross-checked for accuracy • Yields numerical datesAbsolute AgesAbsolute Ages• Only possible for igneous rocks•Need to have crosscutting relationshipsNeed to have crosscutting relationships• Can bracket age of sediments, geologic t lik f lti f ldi ievents like faulting, folding, erosionImportance of radiometric datingImportance of radiometric dating•Confirms the idea that geologic time•Confirms the idea that geologic time is immense• Rocks from several localities have been dated at more than 3 billionbeen dated at more than 3 billion yearsRdi ti dti i l•Radiometric dating is a complex procedure that requires precise measurementRock cycle diagramRock cycle diagramLeaves of HistoryyLateral ContinuityLateral Continuityhttp://cse.cosm.sc.edu/hses/RelatDat/pages/lateral.htmPrinciple of Original Horizontality • Sediment is deposited phorizontally http://faculty.icc.edu/easc111lab/labs/labf/orig_horizontality.jpgPi i l fS itiPrinciple of Superposition• Oldest rock A•YoungerYounger rocks aboveEi th•E is the youngesthttp://cse.cosm.sc.edu/hses/RelatDat/pages/superpos.htmPrinciples of Relative DatingPrinciples of Relative DatingNicolaus Steno 1636-1686 1669 work relates to depositionof sedimentof sediment• Principle of original horizontalityhorizontality• Principle of lateral continuityPi i l f iti•Principle of superpositionhttp://www.ucmp.berkeley.edu/history/steno.htmlSuperposition is well illustratedSuperposition is well illustrated by the strata in the Grand CanyonLateral ContinuityyRelative datingRelative dating Pl i k d t iPlacing rocks and events in proper sequence of formationDeciphering Earth’s history from clues in the rocksthe rocksPrinciples of Relative DatingPrinciples of Relative Dating•Principle of original horizontality•Principle of original horizontality• Principle of lateral continuity• Principle of superposition•Principle of crosscutting relationships•Principle of cross-cutting relationshipsPrinciple of Cross-cutting Relationships• Younger feature cuts through an older feature – Something must exist first to be cut by another thinganother thing• The ‘things’ cutting may be ‘things’, such iitias igneous intrusions•Or they may be events, like fault breaks, yy , ,folding, or erosion periodsCross-cutting relationshipsgFolding occurred after depositionFolding occurred after depositionhttp://rst.gsfc.nasa.gov/Sect2/Sect2_6.htmlPrinciples of Relative DatingPrinciples of Relative Dating•Principle of original horizontality•Principle of original horizontality• Principle of lateral continuity• Principle of superposition•Principle of crosscutting relationships•Principle of cross-cutting relationships • Principle of inclusionInclusionsInclusions• One rock contained within another •Rock containing the inclusions isRock containing the inclusions is younger than the one the inclusions are derived fromderived fromHow inclusions formHow inclusions formFormation of InclusionsFormation of InclusionsThe fragments are includedwith the deposition of rock on top of the weathered surfacetop of the weathered surfaceTwo different modes of inclusionTwo different modes of inclusionhttp://www-rohan sdsu edu/~rhmiller/geologictime/GeologicTime htmInclusions of granite in sedimentaryhttp://wwwrohan.sdsu.edu/ rhmiller/geologictime/GeologicTime.htmInclusions of sediment in granite—granite formedin sedimentary deposit—formed later than granitegranitegranite formed later than sedimentlater than graniteModern geologyModern geology• Uniformitarianism •Fundamental principle of geologyFundamental principle of geology "The present is the key to the past“James Hutton 17261797James Hutton 1726-17971785 lectures Concerning the system of the Earth, its duration, and stability to the Royal Society of Edinburgh• Principle of crosscutting grelationships•Principle of inclusionsPrinciple of inclusionshttp://www-history.mcs.st-andrews.ac.uk/Mathematicians/Hutton_James.htmlGeologic time scaleGeologic time scale•Divides geologic history into units•Divides geologic history into units• Originally created using relative dates• Bracket events and arrive at agesGeologic Time ScaleGeologic Time ScaleSubdivisionsSubdivisionsE•Eons–ErasEras• Periods–EpochsEonEonGreatest expanse of timeGreatest expanse of time•Four eons ou eo s–Phanerozoic ("visible life") – the most recent eon: started 543 MaProterozoic: 2500543 Ma–Proterozoic: 2500 –543 Ma–Archean: 3800 –2500 Ma–Hadean – oldest eon 4500 – 3800 MaEras of the Phanerozoic eonEras of the Phanerozoic eon• Cenozoic ("recent life"): 65 Ma –now ()•
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