Radioactivity ad oact ty Measuring Geologic Time Chapter p 13 Chapter 21 click to get 9 page to print Measuring Radioactivity Radioactive decay strips electrons from atoms Ions created Geiger G i counter charged t h d wire results in clicks of counter Others rely on visual reactions of ions Half Life Decay is random for any radioactive atom BUT Predictable for mass of material One half of unstable parent material daughter product HALF LIFE L1 2 Decay rate constant unaffected by external conditions Parent to daughter ratios of H 3 Half Life equation Amount remaining can be calculated by 1 R I n 2 R remaining I initial n number of half lives Calculate amount from half half life life Startt with St ith 400 mg off Co 60 C 60 Half life is 5 25 years How much is left after 15 75 years g To use equation at right First calculate how many half lives 1 R I n 2 5 25 yr halflife f f 15 75 years 15 75 years 3half h lf lives li halflife 5 25 yr 3 half half lives lives Calculate amount from half half life life 400 mg of Co 60 Half life is 5 25 years y How much is left after 3 half lives Calculate amount with 1 R I n 2 1 R 400 mg 3 2 50mg 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 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 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
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