measuring the proportion of radioactive carbon-14 to stable carbon-12. As a substance ages,
the relative amount of carbon-14 decreases.
Carbon is removed from the atmosphere by plants during the process of photosynthesis.
Animals consume this carbon when they eat plants or other animals that have eaten plants.
Therefore carbon-14 dating can be used to date plant and animal remains. Examples include
timbersfrom an old building, bones, or ashes froma fire pit. Carbon dating can be effectively
used to find the age of materials between 100 and 50,000 years old.
Potassium-Argon Dating
Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Because argon is a
gas, it can escape from molten magma or lava. Therefore any argon that is found in a
crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of
potassium-40 to argon-40 will yield a good estimate of the age of the sample.
Potassium is a common element found in many minerals such as feldspar, mica, and amphi-
bole. The technique can be used to date igneous rocks from 100,000 years to over a billion
years old. Because it can be used to date geologically young materials, the technique has
been useful in estimating the age of deposits containing the bones of human ancestors.
Uranium-Lead Dating
Two isotopes of uranium are used for radiometric dating. Uranium-238 decays to form lead-
206 with a half-life of 4.47 billion years. Uranium-235 decays to form lead-207 with a half-life
of 704 million years.
Uranium-lead dating is usually performed on crystals of the mineral zircon (Figure11.26).
When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but
reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it can be assumed
that it was produced from the decay of uranium.
Uranium-lead dating can be used to date igneous rocks from 1 million years to around 4.5
billion years old. Some of the oldest rocks on Earth have been dated using this method,
including zircon crystals from Australia that are 4.4 billion years old.
Limitations of Radiometric Dating
Radiometric dating can only be used on materials that contain measurable amounts of
radioactive materials and their daughter products. This includes organic remains (which
compared to rocks are relatively young, less than 100,000 years old) and older rocks. Ide-
ally, several different radiometric techniques will be used to date the same rock. Agreement
between these values indicates that the calculated age is accurate.