Radiometric dating assigns numeric ages to rocks using the predictable decay of radioactive isotopes. The method became possible after the discovery of radioactivity in the late 1800s. Unstable isotopes, called radioactive isotopes, decay spontaneously into stable daughter products at a constant rate. The half-life is the time required for half of the parent isotopes to decay. Each radioactive isotope has a unique half-life, ranging from fractions of a second to billions of years. While the decay of a single atom is random, a large group decays at a predictable rate, making half-life a reliable natural clock. By measuring the ratio of parent isotope to daughter product, scientists can calculate how long decay has been occurring. The age is determined by comparing the measured ratio to the known decay constant. Radiometric dating is an absolute dating method because it provides numeric ages, not just relative order. Geologists use it to estimate how long ago rocks formed and to infer the ages of fossils within them.
Different isotope pairs are used based on the material and its expected age. Carbon-14 dating measures the decay of carbon-14 to nitrogen-14. It is used to date organic remains such as wood, bone, and shell that are up to about 50,000 years old. Because carbon-14 decays relatively quickly, it cannot date older materials.
Potassium-argon dating measures the decay of potassium-40 to argon-40. It is ideal for dating volcanic rocks and ash layers. When volcanic rock forms, any argon gas that was present is driven off by the heat. After the rock solidifies, argon-40 begins to accumulate from the decay of potassium-40. The amount of argon-40 tells when the rock cooled. This method can date rocks from hundreds of thousands to billions of years old.
Uranium-lead dating measures the decay of uranium to lead. It is applied to the mineral zircon, which incorporates uranium when it crystallizes but excludes lead. Thus, any lead found in zircon is due to uranium decay. Zircon crystals are extremely durable and can survive geological processes that destroy other minerals. Uranium-lead dating is used to date the oldest rocks on Earth, including those in Canada's Nuvvuagittuq Greenstone Belt.
Radiometric dating relies on the assumption that the rock has remained a closed system, with no gain or loss of parent or daughter isotopes after formation. The initial amount of the daughter isotope must also be known or negligible. For example, when a zircon crystal forms, it contains no lead, so any lead found is from uranium decay. Similarly, when volcanic rock cools, it contains no argon, so argon accumulation is entirely from potassium decay.
Geologists test these assumptions by cross-checking ages using different isotope pairs or different minerals from the same rock. If two different dating methods on the same sample give the same age, the result is considered robust. For instance, a rock layer can be dated using both potassium-argon and uranium-lead methods. Consistent results from multiple independent measurements build confidence in the age. Additionally, scientists compare results from different laboratories to minimize errors.
The technique has been validated by dating rocks of known age, such as volcanic rocks from eruptions that occurred in recorded human history. These checks confirm that radiometric dating is accurate when the assumptions are satisfied.
Radiometric dating has been applied to Earth's oldest known rocks to determine the planet's age. The Nuvvuagittuq Greenstone Belt in Canada contains rocks that may be among the oldest preserved on Earth. Analysis using multiple dating methods gives ages of several billion years. By combining ages from Earth's oldest rocks with data from meteorites, which are thought to have formed at the same time as the solar system, scientists arrive at an age for Earth of about 4.5 billion years. The convergence of many independent radiometric measurements on this age makes it one of the most robust inferences in geology.
These same methods have been used to date the Moon rocks returned by the Apollo missions and Martian meteorites, giving consistent ages for the solar system. This consistency further supports the overall framework of radiometric dating.
