Geologic TimeRock cycle diagramLeaves of HistoryModern geology Relative dating Principles of Relative DatingPrinciple of SuperpositionSuperposition is well illustrated by the strata in the Grand CanyonPrinciple of Original Horizontality Lateral ContinuityLateral ContinuityPrinciples of Relative DatingPrinciple of Cross-cutting RelationshipsCross-cutting relationshipsFolding occurred after depositionPrinciples of Relative DatingInclusionsHow inclusions formFormation of InclusionsThe fragments are included with the deposition of rock on top of the weathered surfaceTwo different modes of inclusionJames Hutton 1726-1797UnconformitiesAngular Unconformity Formation of an angular unconformitySimple angular unconformityFolding, erosion, deposition, foldingNonconformityNonconformityDisconformitySeveral unconformities are present in the Grand CanyonPrinciples of Relative DatingRelative Dating ExerciseRelative Dating ExercisePrinciples of Relative Dating Fossils: evidence of past life Remains or traces of prehistoric life Types of fossilsPetrifiedPetrifiedMoldCast CarbonizationImpressionPreservation in amberIndirect Evidence Includes TracksConditions favoring preservation Fossils and correlation Principle of faunal successionGeologic Time ScaleArchean through DevonianCarboniferous through QuaternaryCambrian Marine LifeTrilobiteCrinoid—380 maOrdovician sea floorOrdovician InvertebratesSilurian reefSilurian LandscapeDevonian SeaMid PaleozoicLate PaleozoicCarboniferous Fern ForestsPermian SeaPermian ReptilesPermian ExtinctionGeologic Time ScaleMesozoicMesozoicMesozoicMesozoicArcheopteryxMesozoic MammalMesozoic MammalJurassicCretaceousMesozoic SeaMesozoic seaCretaceous ExtinctionGeologic Time ScaleCenozoic mammalsCenozoicCenozoicCenozoicGeologic Time ScaleGeologic TimeChapter 21Rock cycle diagramLeaves of HistoryModern geology• Uniformitarianism• Fundamental principle of geology "The present is the key to the past“Relative dating Placing rocks and events in proper sequence of formationDeciphering Earth’s history from clues in the rocksPrinciples of Relative Dating• Principle of original horizontality• Principle of lateral continuity• Principle of superpositionNicolaus Steno 1636-1686 1669 work relates to depositionof sedimenthttp://www.ucmp.berkeley.edu/history/steno.html• Oldest rock A• Younger rocks above• E is the youngesthttp://cse.cosm.sc.edu/hses/RelatDat/pages/superpos.htmPrinciple of SuperpositionSuperposition is well illustrated by the strata in the Grand CanyonPrinciple of Original Horizontality • Sediment is deposited horizontally http://faculty.icc.edu/easc111lab/labs/labf/orig_horizontality.jpgLateral Continuityhttp://cse.cosm.sc.edu/hses/RelatDat/pages/lateral.htmLateral ContinuityPrinciples of Relative Dating• Principle of original horizontality• Principle of lateral continuity• Principle of superposition• 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 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 cross-cutting relationships• Principle of inclusionInclusions• One rock contained within another • Rock containing the inclusions is younger than the one the inclusions are derived fromHow inclusions formFormation of InclusionsThe 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 sedimentJames 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 unconformitySimple angular unconformityhttp://www.grisda.org/colorado/index.htmFolding, erosion, deposition, foldinghttp://rst.gsfc.nasa.gov/Sect2/Sect2_6.htmlNonconformity• Metamorphic or igneous rocks below • Younger sedimentary rocks aboveNonconformityDisconformity• 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• UnconformitiesRelative Dating Exercisehttp://www.winona.edu/geology/MRW/maps.htmRelative Dating Exercise• F—youngest: crosscutting relationships• D—crosscutting relationship and inclusions• A—superposition• B—superposition• C—superposition• E—oldest: superposition, on bottomhttp://www.winona.edu/geology/MRW/maps.htmPrinciples of Relative Dating• Principle of original horizontality• Principle of lateral continuity• Principle of superposition• Principle of cross-cutting relationships• Principle of inclusion• Principle of faunal successionFossils: evidence of past lifeRemains or traces of prehistoric lifeTypes of fossils• Petrified• Formed by replacement •Mold•Cast• Carbonization • Impression• Preservation in amber• Indirect evidencePetrifiedCavities 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 waterShape is preserved in the surrounding sedimenthttp://www.ammonoid.com/Manning.htmlCast Hollow space of a mold is filled with mineral
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