The half-life of an isotope helps scientists determine a rock’s
radiometric age. So how do we take our knowledge of half-lives and use it to date rocks? The process of determining radiometric age is called radiometric dating. To do this, scientists measure the ratio of parent to daughter isotopes in samples collected in the field. Using this ratio, they can determine how many half-lives have occurred since the sample formed. They can then multiply the number of half-lives by the length of time it takes for one half-life to occur.
Both relative and radiometric dating are used by geologists. Relative dating allows geologists in the field to quickly determine when specific layers of rock formed relative to each other. Radiometric dating requires more time and equipment than relative dating. However, this dating technique is more accurate. By examining specific ratios of isotopes in the lab, geologists can estimate the actual age of mountains, fossils, volcanoes, and even Earth!
Uranium Dating
Zircon is a mineral that many geologists use to date rocks. The crystal structure of this mineral is especially useful because it contains uranium-235. When zircon originally forms from molten rock, it contains only the parent isotope, uranium-235, and none of the daughter isotope, lead-207. Over time, however, the parent isotope decays into the daughter isotope.
Carbon Dating
Carbon atoms can be found in the atmosphere as a part of the gas carbon dioxide. Every time you inhale, you take in a small portion of carbon dioxide into your body. A small percentage of this carbon dioxide is carbon-14. Although carbon-14 undergoes radioactive decay to form carbon-12, the amount of carbon-14 atoms within an organism’s body remains the same as long as an organism is living.
When an organism dies, it stops taking in new carbon atoms, so the amount of carbon-14 is no longer constant. As time passes, carbon-14 decays to become its daughter isotope, carbon-12, and the ratio between carbon-14 and carbon-12 changes. When scientists discover samples of once living organisms, they can analyze this ratio and calculate the number of carbon-14 half-lives that have passed since its death. Multiplying this number by 5,730 (half-life of carbon-14) will yield the approximate amount of time that has passed since the organism died.
Using carbon to date organisms does have its limitations. Because very little carbon-14 is left over after about nine half-lives, the practical upper limit for carbon dating is about 50,000 years.
