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Following from online notes: Chapter 1Document created by: Pamela J. W. Gore Georgia Perimeter College, Clarkston, GA September 12, 2002 A. Quantitative scientific methods for dating the Earth In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation. Age of Earth was calculated to be about 24-40 million years. 1. In 1899 - 1901, John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea. Evaporative concentration of salts. 2. Age of Ocean = Total salt in oceans (in grams) divided by rate of salt added (grams per year) 3. Age of Earth was calculated to be 90-100 million years. 4. Thickness of total sedimentary record divided by average sedimentation rates (in mm/yr). In 1860, calculated to be about 3 million years old. In 1910, calculated to be about 1.6 billion years old. 5. Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m. 6. Sedimentation rates average about 0.3 m/1000 years. Problem: Earth has an internal heat source (radioactive decay) Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits. Problems: did not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted. At this rate, the age of the first fossiliferous rocks is about 500 million years. Charles Lyell 1800's compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. Estimated 80 million years for the Cenozoic alone. 7. Discovery of radioactivity by Henri Becquerel in 1896. In 1905, Rutherford and Boltwood used radioactive decay to measure the age of rocks and minerals. Uranium decay produces helium (He), leading to a date of 500 million years. In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium. Published the age of a sample of urananite based on Uranium-Lead dating. Date was 1.64 billion years. (our text p. 657: radioactive decay) There are three types of subatomic particles involved: 8. Alpha particles large, easily stopped by paper charge = +2 mass = 4 9. Beta particles penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays. charge = -1 mass = negligible 10. Neutrons highly penetrating no charge mass = 1