Geologic TimegChapter 21Rock 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.htmlUnconformitiesUnconformitiesA break in the rock record •Three types of unconformities•Three types of unconformities • Angular unconformity –• Disconformity – strata on either side are parallelparallel • NonconformityAngular UnconformityAngular Unconformity• Tilted rocks are overlain by flat-lying rocksrocks• Remember the principle of original horizontality?Formation of an ang larangular unconformityunconformitySimple angular unconformitySimple angular unconformityhttp://www.grisda.org/colorado/index.htmFolding erosion deposition foldingFolding, erosion, deposition, foldinghttp://rst.gsfc.nasa.gov/Sect2/Sect2_6.htmlNonconformityNonconformity•Metamorphic or igneous rocksMetamorphic or igneous rocks below • Younger sedimentary rocks aboveNonconformityNonconformityDisconformityDisconformity• Gap in sedimentation that may have erosion alsoerosion also• Represents an interval in geologic time without rock depositedMost difficult of the three•Most difficult of the three unconformities to detect• Strata on either side are parallelSeveral unconformities are present in the Grand CanyonPrinciples of Relative DatingPrinciples of Relative Dating• Superposition•Original Horizontality and Lateral ContinuityOriginal Horizontality and Lateral Continuity• Inclusions• Crosscutting Relationships•Unconformities•UnconformitiesPrinciples 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 inclusion• Principle of faunal successionAbsolute 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 measurementStratigraphy of Bryce CanyonGeologic 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
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