READ: Carbon Dating

Different radioisotopes decay at different rates. You can see some examples in the Table below. Radioisotopes with longer half-lives are used to date older rocks or other specimens, and those with shorter half-lives are used to date younger ones. For example, the oldest rocks at the bottom of the Grand Canyon were dated by measuring the amounts of potassium-40 in the rocks. Carbon-14 dating, in contrast, is used to date specimens that are much younger than the rocks in the Grand Canyon.

Parent Isotope

Daughter Isotope

Half-Life

potassium-40

argon-40

1.3 billion years

uranium-235

lead-207

700 million years

uranium-234

thorium-230

80,000 years

carbon-14

nitrogen-14

5,700 years

Focus on Carbon-14 Dating

One of the most familiar types of radioactive dating is carbon-14 dating. Carbon-14 forms naturally in Earth’s atmosphere when cosmic rays strike atoms of nitrogen-14. Living things take in and use carbon-14, just as they do carbon-12. The carbon-14 in living things gradually decays to nitrogen-14. However, as it decays, it is constantly replaced because living things keep taking in carbon-14. As a result, there is a constant ratio of carbon-14 to carbon-12 in organisms as long as they are alive. This is illustrated in the top part of the Figure below.

After organisms die, the carbon-14 they already contain continues to decay, but it is no longer replaced (see the bottom part of the figure below). Therefore, the carbon-14 in a dead organism constantly declines at a fixed rate equal to the half-life of carbon-14. Half of the remaining carbon-14 decays every 5,700 years. If you measure how much carbon-14 is left in a fossil, you can determine how many half-lives (and how many years) have passed since the organism died.


Q: Why can’t carbon-14 dating be used to date specimens older than about 60,000 years?

A: Carbon-14 has a half-life of 5700 years. After about 60,000 years, too little carbon-14 is left in a specimen to be measured.

Summary

  • A radioisotope decays and changes to a different element at a certain constant rate called the half-life. This is the length of time it takes for half of a given amount of the radioisotope to decay.
  • Different radioisotopes may vary greatly in their rate of decay. The more unstable their nuclei are, the faster they decay.
  • The age of a rock or other specimen can be estimated from the remaining amount of a radioisotope it contains and the radioisotope’s known rate of decay, or half-life. This method of dating specimens is called radioactive dating.
  • Radioisotopes with longer half-lives are used to date older specimens, and those with shorter half-lives are used to date younger ones.
  • Carbon-14 dating is used to date specimens younger than about 60,000 years old. It is commonly used to date fossils of living things and human artifacts.
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Last modified: Tuesday, 26 July 2016, 1:21 PM