Notes - Geology Absolute Geochronology -Link index fossils...

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Geology 16:00 Absolute Geochronology -Link index fossils to absolute age -Radioactive decay of unstable elements to stable isotopes 1. Bible: In 1664, Archbishop Usher- the world began on Oct. 26, 4004 B.C. (Not  science) 2. Salt In The Ocean: (1899) Assume fresh water at start, current salinity requires  100 Million Years 3. Sediment Thickness: Assuming constant rate of deposition- 100 Million Years 4. Cooling of the Earth: (1870): Cooling from molten ball (30 degrees Celsius)  would take 30-100 Million Years BP. (Kelvin) Radioactive dating depends on isotopes Isotope: Same element: same number of protons, fewer neutrons Examples: 238U, 235U   87Sr, 86Sr   14C, 12C Exception when neutron decays to proton: Radioactive decay of rubidium to strontium: the extra proton transmutes the  element Half-Life= The time required for half of a parent isotope to decay to its daughter  isotope Isotopic Dating: -Radioactive parent elements decay to stable daughter elements -Steady rate of decay -Assume no daughter product initially -Measure ratio of parent/daughter isotope to determine elapsed time Geologically Useful Decay Schemes: Parent    Daughter   Half-Life (Years)    
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235U       207Pb         4.5x10,000,000,000 238U       206Pb         .71x10,000,000,000 40K         40Ar           1.25x10,000,000,000 87Rb       87Sr           47x10,000,000,000 14C         14N            5730 Fission Track Dating  1-100 Ma        238U Fissions into particles -Cut, polish and etch the surface of a zircon -Count # of track per sq. cm -Measure # of 238U atoms (usually done by irradiating the 235U in the sample) Useful for dating exhumation cooling of a rock.  Works above closure temp. Oldest Rocks On Earth Slave Province, North Canada -Zircons in a metamorphosed granite dated at 3.96 Ga by the U-Pb method Age Of The Earth -Oldest rocks are 3.96 Ga -Older rocks 4.6 to 4.0 Ga destroyed or remain to be found -Age of Earth based on age of moon (4.4 Ga), and meteorites (4.6 Ga) -Meteorites and moon rocks dated by up to 10 independent isotopic systems give  the same age -Most species of dinosaurs became extinct after the Cretaceous Period -Index Fossils are used to obtain the age of a rock relative to another rock -Correct sequence: shale, schist, migmatite
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-Highest metamorphic grade: Granulite Types Of Deformation Faults and Folds -Elastic -Ductile (plastic) -Brittle (rupture) Hooke’s Law: of elasticity is an approximation that states that the amount by  which a material body is deformed (the strain) is linearly related to the force  causing the deformation (the stress)
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This note was uploaded on 02/26/2008 for the course GEOL 1010 taught by Professor Pranter,ma during the Spring '07 term at Colorado.

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Notes - Geology Absolute Geochronology -Link index fossils...

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