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Geochronology

Geochronology. Methods for Determining the Relative and Absolute Age of Rocks. Relative vs. Absolute Ages. Relative - Know Order But Not Dates Civil War Happened Before W.W.II Bedrock in Wisconsin Formed Before The Glaciers Came Absolute - Know Dates Civil War 1861-1865

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Geochronology

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  1. Geochronology Methods for Determining the Relative and Absolute Age of Rocks

  2. Relative vs. Absolute Ages Relative - Know Order But Not Dates • Civil War Happened Before W.W.II • Bedrock in Wisconsin Formed Before The Glaciers Came Absolute - Know Dates • Civil War 1861-1865 • World War II 1939-1945 • 7,700 years ago Mt. Mazama erupted

  3. Absolute Ages: Early Attempts The Bible • Add up Dates in Bible • Get an Age of 4000-6000 B.C. For Earth • John Lightfoot and Bishop Ussher - 4004 B.C. (1584) • Too Short !!!

  4. Absolute Ages: Early Attempts Salt in Ocean • If we know rate salt is added, and how much salt is in ocean, can find age of oceans. Sediment Thickness • Add up thickest sediments for each period, estimate rate. Both methods gave age of about 100 million years • Problem: Rates Variable

  5. Age of The Sun • If sun gets its heat from burning or other chemical reactions, could only last 10,000 years or so. • Best 19th century guess: sun was slowly contracting. • Problem: only 30 million years ago, sun would have extended out to earth's orbit! • Geologists wanted more time, but you can't fight the laws of physics... • Sun actually gets its energy from nuclear reactions and can keep going for billions of years • The Geologists were right after all. Go Team.

  6. The Fundamental Rule of Absolute Ages The Earth is older than everything on or in it -Except its atoms -All ages are minimum ages

  7. Absolute Age Determinations

  8. Radiometric Dating: Half-Life By measuring the amount of radioactive decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material.

  9. Radioactive Decay

  10. Radioactive Decay Half-life Zircon ZrSiO4 U substitutes for Zr, but Pb does not.

  11. Rb-Sr Dating of a Granite 87Rb 87Sr; 86Sr is stable Rb – chemically substitutes for K Sr – chemically substitutes for Ca Isochron t=2 t=1 87Sr/86Sr Metamorphism at t=1 87Sr/86Sr°(m) t=0 87Sr/86Sr° 87Rb/86Sr Ca-plagioclase Granite whole rock K-feldspar Biotite (K-rich)

  12. Carbon-14 • Carbon exists in all biotic (organic) materials. • Carbon-14 or 14C is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. • Used to estimate the age up to about 58,000 to 62,000 years • Carbon is found in various forms, including the main stable isotope (12C) and an unstable isotope (14C). • Through photosynthesis, plants absorb both forms from carbon dioxide in the atmosphere. • When an organism dies, it contains a ratio of 14C to 12C, but, as the 14C decays with no possibility of replenishment, the ratio decreases at a regular rate (the half-life of 14C). • The measurement of 14C decay provides an indication of the age of any carbon-based material.

  13. Absolute Age Determinations

  14. Present Radiometric Dating Methods Cosmogenic • C-14 5700 Yr. Primordial • K-Ar (K-40) 1.25 • Rb-Sr (Rb-87) 48.8 B.Y. • U-235 704 M.Y.

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