In radiometric dating, scientists look at the ratios of naturally occuring radioactive isotopes (atoms that carry an electrical charge) found in igneous rocks. These unstable radioactive isotopes decay to non-radioactive elements at constant rates, known as the element's "half-life". This means that after the time of the half-life, half of the radioactive isotopes would have decayed to the non-radioactive form; so the ratio of radioactive to non-radioactive element would be 50:50. After another half-life, one-half of the remaining radioactive isotopes would have decayed to the non-radioactive element; so the ratio of radioactive to non-radioactive element would now be 25:75. (Half of the 50% remaining radioactive isotope decays, leaving half of it, or 25% of the original element, as radioactive, and the other half, 25% of the original element, as non-radioactive.)
A type of radiometric dating that you might have heard of before is Carbon 14 dating or Oxidizable Carbon Ratio Dating, often simply called "carbon dating". Naturally occurring Carbon 14 decays to Nitrogen 14, with a half-life of 5,730 years. Because Carbon 14 has such a short half-life, it is useful in archaeology for dating artifacts (man-made objects) and the bones of animals up to 50,000 to 60,000 years old. However, it cannot be used on anything older than Middle Pleistocene Epoch in age.
In order to date older fossils, scientists must use other radioactive isotopes. A commonly used technique is called Potassium-Argon dating. The element potassium is found in most rock-forming minerals, and the half-life of the radioactive isotope Potassium 40 is 1.25 billion years, allowing measurable quantities of Argon 40 (its decay element, known as the daughter element) to accumulate in potassium-bearing minerals of almost all ages. The amounts of potassium and argon isotopes can be measured accurately, even in very small quantities, making Potassium-Argon dating useful for both very young and very old rocks (and everything in between).
Although radiometric dating is much more precise than relative dating, it does have its drawbacks as well. With the exception of Carbon dating, radiometric dating can only be used on igneous rocks, not sedimentary rocks or the actual fossils. Because fossils are found in sedimentary rock, paleontologists have to use radiometric dating information on igneous rocks found below and above the fossils in order to determine an age range for the sedimentary rocks.
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