1Geologic TimeChapter 21Rock cycle diagramLeaves of HistoryLateral Continuityhttp://cse.cosm.sc.edu/hses/RelatDat/pages/lateral.htmPrinciple of Original Horizontality • Sediment is deposited horizontally http://faculty.icc.edu/easc111lab/labs/labf/orig_horizontality.jpg• Oldest rock A• Younger rocks aboveEi thPrinciple of Superposition•E is the youngesthttp://cse.cosm.sc.edu/hses/RelatDat/pages/superpos.htmPrinciples of Relative DatingNicolaus Steno 1636-1686 1669 work relates to depositionof sediment• Principle of original horizontality• Principle of lateral continuity• Principle of superpositionhttp://www.ucmp.berkeley.edu/history/steno.htmlSuperposition is well illustrated by the strata in the Grand CanyonLateral Continuity2Relative dating Placing rocks and events in proper sequence of formationDeciphering Earth’s history from clues in the rocksthe rocksPrinciples of Relative Dating• Principle of original horizontality• Principle of lateral continuity• Principle of superposition•Principle of crosscutting relationships•Principle of cross-cutting relationships Principle 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 as igneous intrusions• Or they may be events, like fault breaks, folding, or erosion periodsCross-cutting relationshipsFolding occurred after depositionhttp://rst.gsfc.nasa.gov/Sect2/Sect2_6.htmlPrinciples of Relative Dating• Principle of original horizontality• Principle of lateral continuity• Principle of superposition•Principle of crosscutting relationships•Principle of cross-cutting relationships • Principle of inclusionInclusions• One rock contained within another • Rock containing the inclusions is younger than the one the inclusions are derived fromderived fromHow inclusions form Formation of Inclusions3The fragments are includedwith the deposition of rock on top of the weathered surfaceTwo different modes of inclusionInclusions of granite in sedimentary deposit—formed later than granitehttp://www-rohan.sdsu.edu/~rhmiller/geologictime/GeologicTime.htmInclusions of sediment in granite—granite formed later than sedimentModern geology• Uniformitarianism • Fundamental principle of geology "The present is the key to the past“James Hutton 1726-17971785 lectures Concerning the system of the Earth, its duration, and stability to the Royal Society of Edinburgh• Principle of crosscutting relationships• Principle of inclusionshttp://www-history.mcs.st-andrews.ac.uk/Mathematicians/Hutton_James.htmlUnconformitiesA break in the rock record • Three types of unconformities • Angular unconformity –• Disconformity – strata on either side are parallel • NonconformityAngular Unconformity• Tilted rocks are overlain by flat-lying rocks• Remember the principle of original horizontality?Formation of an angular unconformityunconformitySimple angular unconformityhttp://www.grisda.org/colorado/index.htmFolding, erosion, deposition, foldinghttp://rst.gsfc.nasa.gov/Sect2/Sect2_6.html4Nonconformity• Metamorphic or igneous rocks below • Younger sedimentary rocks above NonconformityDisconformity• Gap in sedimentation that may have erosion also• Represents an interval in geologic time without rock deposited• Most difficult of the three unconformities to detect• Strata on either side are parallelSeveral unconformities are present in the Grand CanyonPrinciples of Relative Dating• Superposition• Original Horizontality and Lateral Continuity• Inclusions• Crosscutting Relationships• UnconformitiesPrinciples of Relative Dating• 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 Time• Radiometric Dating• Igneous 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 agesRadiometric dating• Known Half-life• Closed system• Cross-checked for accuracy • Yields numerical dates5Absolute Ages• Only possible for igneous rocks• Need to have crosscutting relationships• Can bracket age of sediments, geologic t lik f lti f ldi ievents like faulting, folding, erosionImportance of radiometric dating• 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 years• Radiometric dating is a complex procedure that requires precise measurementStratigraphy of Bryce CanyonGeologic time scale• Divides geologic history into units• Originally created using relative dates• Bracket events and arrive at agesGeologic Time ScaleSubdivisions• Eons–Eras• Periods–EpochsEonGreatest expanse of time• Four eons –Phanerozoic ("visible life") – the most recent eon: started 543 Ma–Proterozoic: 2500 – 543 Ma–Archean: 3800 – 2500 Ma–Hadean – oldest eon 4500 – 3800 MaEras of the Phanerozoic eon• Cenozoic ("recent life"): 65 Ma – now • Mesozoic ("middle life"): 248 – 65 Ma• Paleozoic ("ancient life"): 543 – 248 MaFossils: evidence of past lifeRemains or traces of prehistoric life6PetrifiedCavities and pores are filled with precipitated mineral matterPetrifiedFormed by replacementCell material is removed and replaced with mineral matterMoldShell or other structure is buried and then dissolved by underground waterunderground waterShape is preserved in the surrounding sedimenthttp://www.ammonoid.com/Manning.htmlCast Hollow space of a mold is filled with mineral matter CarbonizationOrganic matter becomes a thin residue of carbon. This is a ‘compression’ of the original organismImpressionReplica of the fossil's surface preserved in fine-grained sedimenthttp://www.lfbuffalo.org/exhibitions/map/t/Preservation in amberHardened resin of ancient trees surrounds an organismIndirect Evidence Includes• Tracks• Burrows• Coprolites – fossil dung and stomach contents• Gastroliths – stomach stones used to grind food by some extinct reptilesTracksDinosaur footprint in fine-grained limestone near Tuba City, Arizona.7Types of
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