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Methods for Estimating the Age of the Earth

Methods for Estimating the Age of the Earth. Radiometric Dating Rates of deposition of sediments Cooling of the Earth from a molten state Fossils and evolution. Radiometric Dating of Rocks. 1896-1898 Radioactivity discovered by: Henri Becquerel and Marie and Pierre Curie.

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Methods for Estimating the Age of the Earth

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  1. Methods for Estimating the Age of the Earth • Radiometric Dating • Rates of deposition of sediments • Cooling of the Earth from a molten state • Fossils and evolution

  2. Radiometric Dating of Rocks 1896-1898 Radioactivity discovered by: Henri Becquerel and Marie and Pierre Curie.

  3. Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

  4. The Atom

  5. Alpha Particle emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

  6. Thorium - Protactinium Decay beta particle Th + Pa 234 234 90 91 beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron Protactinium: 91 protons + 143 neutrons

  7. U PbSeries The rate at which this process occurs is measured in terms of the “half life”.

  8. Half Life Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

  9. The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:   (1) As radioactive elements decay in the Earth, they heat up the surrounding rocks. (2) Radiometric Dating

  10. Radiometric Dating Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed. This is an example of uniformitarianism. The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

  11. To estimate the age of a rock: D = amount of daughter product. P = amount of parent.   For a particular radioactive element in a rock, determine the present ratio = D/P. D P

  12. Rate of decay (from theory and measurement) • Make assumptions about original ratios (from theory of geochemistry). As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified). D D P P

  13. Potassium - Argon Dating • K/Ar clock begins when K bearing minerals crystallize in a rock. • The new minerals will contain K40, but will not contain Ar40 (because Ar40 does not combine with other elements). • As time passes: K40  Ar40 (half-life = 1.3 billion years) K40/Ar40 ratio can be measured to determine age Uniformitarianism

  14. Geologic Time Scale

  15. Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

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