Geologic Time Rock cycle diagram Leaves of History Chapter 21 Lateral Continuity Principle of Original Horizontality Sediment is deposited horizontally http cse cosm sc edu hses RelatDat pages lateral htm http faculty icc edu easc111lab labs labf orig horizontality jpg Principles of Relative Dating Nicolaus Steno 1636 1686 1669 work relates to deposition of sediment Superposition is well illustrated by the strata in the Grand Canyon Principle of Superposition Oldest rock A Younger rocks above E is i th the youngest http cse cosm sc edu hses RelatDat pages superpos htm Lateral Continuity Principle of original horizontality Principle of lateral continuity Principle of superposition http www ucmp berkeley edu history steno html 1 Placing rocks and events in proper sequence of formation Deciphering Earth s history from clues in the rocks Principle of Cross cutting Relationships Principles of Relative Dating Relative dating Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cutting 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 periods Cross cutting relationships Principles of Relative Dating Folding occurred after deposition Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cutting cross cutting relationships Principle of inclusion http rst gsfc nasa gov Sect2 Sect2 6 html Inclusions How inclusions form Formation of Inclusions One rock contained within another Rock containing the inclusions is younger than the one the inclusions are derived from 2 The fragments are included with the deposition of rock on top of the weathered surface Two different modes of inclusion Modern geology Uniformitarianism Fundamental principle of geology The present is the key to the past http www rohan sdsu edu rhmiller geologictime GeologicTime htm Inclusions of granite in sedimentary deposit formed later than granite James Hutton 1726 1797 1785 lectures Concerning the system of the Earth its duration and stability to the Royal Society of Edinburgh Principle of crosscutting relationships Principle of inclusions Inclusions of sediment in granite granite formed later than sediment 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 http www history mcs st andrews ac uk Mathematicians Hutton James html Formation of an angular unconformity Simple angular unconformity http www grisda org colorado index htm Folding erosion deposition folding http rst gsfc nasa gov Sect2 Sect2 6 html 3 Nonconformity Disconformity Nonconformity Metamorphic or igneous rocks below Younger sedimentary rocks above 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 Principles of Relative Dating Superposition Original Horizontality and Lateral Continuity Inclusions Crosscutting Relationships Unconformities Absolute Geologic Time Radiometric Dating Igneous rocks contain 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 Principle of original horizontality Principle of lateral continuity Principle of superposition Principle of cross cutting cross cutting relationships Principle of inclusion Principle of faunal succession Radiometric dating Known Half life Closed system Cross checked for accuracy Yields numerical dates 4 Absolute Ages Only possible for igneous rocks Need to have crosscutting relationships Can bracket age of sediments geologic events t like lik ffaulting lti ffolding ldi erosion i Geologic time scale Importance of radiometric dating Stratigraphy of Bryce Canyon Confirms the idea that geologic time is immense Rocks from several localities have been dated at more than 3 billion years Radiometric dating is a complex procedure that requires precise measurement Geologic Time Scale Divides geologic history into units Originally created using relative dates Bracket events and arrive at ages Subdivisions Eons Eras Periods Epochs Eon Greatest 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 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 5 Petrified Petrified Cavities and pores are filled with precipitated mineral matter 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 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 fossil dung 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 6 Fossils and correlation Types of fossils Petrified Formed by replacement l t Mold Cast Carbonization Impression Preservation in amber Indirect evidence Principle of faunal succession Proposed by William Smith late 1700s Fossil organisms succeed one another in a definite and determinable order therefore any geologic time interval can be recognized by its fossil content Conditions favoring preservation Rapid burial Possession of hard parts Archean through Devonian Principle of faunal succession
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