Geologic g Time Chapter 21 Rock cycle diagram Leaves of History y Lateral Continuity http cse cosm sc edu hses RelatDat pages lateral htm Principle of Original Horizontality Sediment is deposited p horizontally http faculty icc edu easc111lab labs labf orig horizontality jpg P i i l off Superposition Principle S iti Oldest rock A Younger rocks above E is i th the youngest http cse cosm sc edu hses RelatDat pages superpos htm Principles of Relative Dating Nicolaus Steno 1636 1686 1669 work relates to deposition of sediment Principle of original horizontality Principle of lateral continuity Principle P i i l off superposition iti http www ucmp berkeley edu history steno html Superposition is well illustrated by the strata in the Grand Canyon Lateral Continuity y Relative dating Placing Pl i rocks k and d events t iin proper sequence of formation Deciphering Earth s history from clues in the rocks Principles of Relative Dating Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cross cutting cutting relationships Principle 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 i iintrusions t i Or they y may y be events like fault breaks folding or erosion periods Cross cutting g relationships Folding occurred after deposition http rst gsfc nasa gov Sect2 Sect2 6 html Principles of Relative Dating Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cross cutting cutting relationships Principle of inclusion Inclusions One rock contained within another Rock containing the inclusions is younger than the one the inclusions are derived from How inclusions form Formation of Inclusions The fragments are included with the deposition of rock on top of the weathered surface Two different modes of inclusion http www rohan http www rohan sdsu edu sdsu edu rhmiller geologictime GeologicTime rhmiller geologictime GeologicTime htm htm Inclusions of granite in sedimentary deposit formed later than granite Inclusions of sediment in granite granite granite granite formed later than sediment Modern geology Uniformitarianism Fundamental principle of geology The present is the key to the past James Hutton 1726 1726 1797 1797 1785 lectures Concerning the system of the Earth its duration and stability to the Royal Society of Edinburgh Principle of crosscutting g relationships Principle of inclusions http www history mcs st andrews ac uk Mathematicians Hutton James html Unconformities A break in the rock record Three types of unconformities Angular unconformity Disconformity strata on either side are parallel Nonconformity Angular Unconformity Tilted rocks are overlain by flat lying rocks Remember the principle of original horizontality Formation of an ang lar angular unconformity Simple angular unconformity http www grisda org colorado index htm Folding erosion Folding erosion deposition deposition folding http rst gsfc nasa gov Sect2 Sect2 6 html Nonconformity Metamorphic or igneous rocks below Younger sedimentary rocks above Nonconformity Disconformity 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 parallel Several unconformities are present in the Grand Canyon Principles of Relative Dating Superposition Original Horizontality and Lateral Continuity Inclusions Crosscutting Relationships Unconformities Principles of Relative Dating Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cross cutting cutting relationships Principle of inclusion Principle of faunal succession Absolute Geologic Time Radiometric Dating Igneous rocks contain potassium potassium uranium thorium and rubidium that are radioactive Careful C f l measurementt off ratios ti off these th and their daughter products or of the isotopes of them that are not radioactive can be used to calculate absolute ages Radiometric dating Known Half life Closed system Cross checked for accuracy Yields numerical dates Absolute Ages Only possible for igneous rocks Need to have crosscutting relationships Can bracket age of sediments geologic events t like lik faulting f lti ffolding ldi erosion i Importance of radiometric dating Confirms the idea that geologic time is immense Rocks from several localities have been dated at more than 3 billion years Radiometric R di ti d dating ti iis a complex l procedure that requires precise measurement Stratigraphy of Bryce Canyon Geologic time scale Divides geologic history into units Originally created using relative dates Bracket events and arrive at ages Geologic Time Scale Subdivisions Eons E Eras Eras Periods Epochs Eon Greatest expanse of time Four ou eo eons s Phanerozoic visible life the most recent eon started 543 Ma Proterozoic Proterozoic 2500 543 Ma Archean 3800 2500 Ma Hadean oldest eon 4500 3800 Ma Eras of the Phanerozoic eon Cenozoic recent life 65 Ma now Mesozoic middle life 248 65 Ma Paleozoic ancient life 543 248 Ma Fossils evidence of past life Remains or traces of prehistoric life Petrified Cavities and pores are filled with precipitated p p mineral matter Petrified Formed by replacement Cell material is removed and replaced with mineral matter Mold Shell or other structure is buried and then dissolved by underground water http www ammonoid com Manning html http www ammonoid com Manning html Shape is preserved in the surrounding sediment Cast Hollow space of a mold is filled with mineral matter Carbonization Organic matter becomes a thin residue of carbon This is a compression of the original organism Impression Replica of the fossil s surface preserved in fine grained sediment http www lfbuffalo org exhibitions map t Preservation in amber Hardened resin of ancient trees surrounds an organism Indirect Evidence Includes Tracks Burrows Coprolites C lit fossil dung g and stomach contents Gastroliths stomach stones used to grind food by some extinct reptiles Tracks Dinosaur footprint in fine grained limestone near Tuba City Arizona Types of fossils Petrified Formed by replacement l t Mold Cast Carbonization Impression Preservation in amber Indirect evidence Conditions favoring preservation Rapid burial Possession of hard parts Fossils and correlation
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