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Radiometric Dating

Radioactive decay is a tool for determining the ages of certain minerals and other materials, including the limits of the technique.

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Discovering Earth's Age

Discovering Earth’s Age

Credit: USGS
Source: http://en.wikipedia.org/wiki/File:Geological_time_spiral.png
License: CC BY-NC 3.0

Bertram Boltwood calculated Earth’s age as 2 billion years using radiometric dating. Although less than half of the true age, this number was much closer than anyone had ever gotten before.

Why It Matters

  • Lord Kelvin calculated the age of the Earth based the rate he thought the planet was cooling. He got 100 million years, much older than previous estimates.
  • Scientists were working on radioactive decay. They discovered that one isotope decays to another isotope at a fixed rate. Decay might be in a chain of isotopes from the original radioactive parent to the final stable daughter.
  • Bertram B. Boltwood suggested that the final stable daughter of radium was lead (radium was already known to be a radioactive daughter of uranium).
  • Boltwood used the radium-lead decay chain to date rock samples. Although his measurements were not accurate, he started radiometric dating of rocks.
  • There are many different modern methods of radioactive (or radiometric) dating. 
  • Credit: Radiogenic
    Source: http://en.wikipedia.org/wiki/File:Thermal_ionization_mass_spectrometer.jpg
    License: CC BY-NC 3.0

    Today, one technique of radiometric dating involves the use of thermal ionization mass spectrometers [Figure2]

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With the link below, learn more about using radiometric dating to calculate Earth’s age. Then answer the following questions.

  1. What did Boltwood say was the final stable daughter product of uranium?
  2. What did Boltwood say could be done if you had the rate of decay of an isotope and the amount of the daughter isotope in a sample?
  3. Why was radiometric dating better than any other dating schemes that had been developed before?
  4. What information has radiometric dating given geologists in the Grand Canyon?
  5. What did Boltwood calculate the age of Earth as? Was this number a triumph or a failure for Boltwood?
  6. How did geologists use radiometric dating to determine the age as 4.5 billion years.

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