M_W_Chapter_17-4_Absolute_Time_short

M_W_Chapter_17-4_Absolute_Time_short - Our Plan for...

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Our Plan for Chapters 17 & 18 Chapter 17 Geologic Time and Change (File 17-A) Relative Geologic Time (File 17-B) Fossils (File 17-C) Absolute Geologic Time (File 17-D) Chapter 18 Evolution and Fossils
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Time stratigraphy Strata divided into units based upon their age. Major units established long before any realistic estimates of the age were available.
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Major Geologic Events 65 Ma: Mass Extinction 245 Ma: Mass Extinction 542 Ma: Marine invertebrate animals develop hard parts 2500 Ma: Lithosphere attains nearly its present-day thickness 3700 Ma: oldest known fossils 4000 Ma: Age of the oldest crustal rocks 4600 Ma: Age of the Earth
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Two basic problems How to correlate by age and recognize time-equivalent units? fossils How to measure the absolute age, in years?
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Early Quantitative Estimates of the Age of the Earth Archbishop Ussher (1658) Sunday, 23 October, 4004 B.C http://en.wikipedia.org/wiki/Ussher_chronology
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Early Quantitative Estimates of the Age of the Earth Lord Kelvin (1866) foe of Lyell and Darwin! based on cooling rates of the earth, estimated Earth to be first 100, then 30 then 10 million years old.
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Early Quantitative Estimates of the Age of the Earth John Joly (1899) based on ocean salinity, oceans 70 to 100 million years old
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Early Quantitative Estimates of the Age of the Earth Various authors up to about 1920, based on sedimentation rates, Earth 3 million years to 1.5 billion years old
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Quantitative Measurement of Geological Ages Tree rings (dendrochronology)
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Oldest living Bristlecone pine is 4,767 years old Dendrochronology has been extended back to 11,300 years BP (end of last ice age, roughly)
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Quantitative Measurement of Geological Ages Glacial varves Annual layers in periglacial lakes. Summer: melt waters bring in silt (light color) Winter: streams stop flowing (freeze) and fine mud (dark color) settles from suspension in lake waters
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In one of the first studies, Harold Urey showed that a belemnite had experienced cyclical water temperature changes during its lifetime. Winter/summer differences? Or seasonal migrations?
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Radiometric age dating Radiometric dating Unstable radioactive elements or isotopes (“parents”) change over time into stable products (“daughters”) PARENT DAUGHTER + heat + emissions Half-life: Time for 1/2 of parent to change to daughter Half-life is UNIQUE and CONSTANT
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Radioactive decay Alpha decay: emits α particle (helium nucleus – two protons, two neutrons) Beta decay: emits β particle (electron) Gamma decay: emits electromagnetic radiation (γ rays) Electron capture: proton + electron = neutron Figure 17-19. Uranium 238 decays to Lead 206
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M_W_Chapter_17-4_Absolute_Time_short - Our Plan for...

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