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Absolute Dating of Rocks and Strata

Absolute Dating of Rocks and Strata. Absolute Dating. Calculating the age of rocks, fossils, or strata in years. Gives a numerical value. Example: A rock is found to have an absolute age of 300 million years. . Radiometric Dating.

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Absolute Dating of Rocks and Strata

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  1. Absolute Dating of Rocks and Strata

  2. Absolute Dating • Calculating the age of rocks, fossils, or strata in years. • Gives a numerical value. • Example: A rock is found to have an absolute age of 300 million years.

  3. Radiometric Dating • Radiometric dating uses radioactive decay of minerals in rocks and fossils to determine a rock or fossil’s absolute age. • Isotope: Element with the same number of protons and electrons but different number of neutrons. • Primary radioactive isotopes used in geology are: • Carbon 14 decays to Nitrogen 14 • Uranium 238 decays to Lead 206 • Uranium 235 decays to Lead 207 • Thorium 232 decays to Lead 208 • Rubidium 87 decays to Strontium 87 • Potassium 40 decays to Argon 40

  4. Absolute Age is Determined by Half-Life of Radioactive Isotopes • Half-Life: The time it takes for one half of the radioactive material to decay. • Parent Material (element): The original radioactive isotope before decay. • Daughter Material (element): The element the radioactive isotope decays to. • Example: Carbon 14 is the Parent. Nitrogen 14 is the Daughter because Carbon 14 decays to Nitrogen 14.

  5. Common Half-Lives • Carbon 14 decays to Nitrogen 14 in 5,730 Years • Primarily Used for dating organic objects. Limited to about 80,000 years old. • Uranium 238 decays to Lead 206 in 4.5 Billion years • Uranium 235 decays to Lead 207 in 713 Million years • Thorium 232 decays to Lead 208 in 1.4 Billion Years • Rubidium 87 decays to Strontium 87 in 48.8 Billion years • Potassium 40 decays to Argon 40 in 1.3 Billion Years

  6. Radiocarbon Dating • Can only be used for rocks containing organic material. • Carbon 14 produced in upper atmosphere and is incorporated into living matter through carbon dioxide. • As a result, all living things contain some Carbon 14. • Decaying Carbon 14 is continually replaced when organism is alive but stops being replaced at death. • We can measure ratio of Carbon 14 to nonradioactive Carbon 12 to determine a date of death. • Thus, the ratio of Carbon 14 to Carbon 12 tells us how old something is.

  7. Carbon 14 Decay Graph

  8. Half-Life Calculations

  9. Example Problems • What is the fraction of parent material remaining after 3 half-lives? • Answer: 1/(23) = 1/8 • What is the percentage of parent material remaining after 5 half lives? • Answer: 100/(25) = 3.125% • What is the percentage of daughter material after 4 half-lives? • Answer: Find Percent Parent first. 100/(24) = 6.25%. • Then find Percent Daughter by subtracting percent parent from 100. 100-6.25% = 93.75%

  10. Try These • What is the half-life of a 100.0 g sample of nitrogen-16 that decays to 12.5 g of nitrogen-16 in 21.6 seconds? • All isotopes of technetium are radioactive, but they have widely varying half-lives. If an 800.0 g sample of technetium-99 decays to 100.0 g of technetium-99 in 639,000 years, what is its half-life? • If a radioactive isotope has a half-life of 100 years, how many years would it take for a 20 gram sample to decay down to 5 grams? • Gold-198 has a half-life of 2.7 days. How much of a 96 g sample of gold-198 will be left after 8.1 days?

  11. Answers • 7.2 Seconds. 12.5% is left after 3 half lives. So 21.6 seconds/3 = 7.2 Seconds • 213,000 years. First: Figure out how many half lives: 800/2 =400, 400/2 = 200, 200/2 =100. So 3 half-lives have occurred. Then, 639,000 years/3= 213,000 years. • 200 years. Figure out how many half-lives: 2 half-lives. Then, 100 years x 2 = 200 years. • 12 grams. Figure out how many half lives: 8.1 Days/2.7 days = 3, so 3 half lives. 3 half lives = 1/8 of parent remaining. 96g x1/8 = 12.

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