High School Earth Science: Evidence About Earth’s Past - Absolute Ages of Rocks — Flashcards

Radiometric dating is the process of using concentrations of radioactive isotopes and their daughter products to estimate the ages of materials. It is a very useful tool for dating fossils and rocks. Different isotopes are used to date materials of different ages because different isotopes have different rates of decay. Using more than one isotope also helps scientists check the accuracy of the ages they calculate.    
Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining the absolute ages of recent human fossils and living sites. Carbon-14 is the only radioactive isotope of carbon, and it has a half-life of 5,730 years. A tiny and constant percentage of carbon in the atmosphere is carbon-14. Plants take in carbon dioxide containing carbon-14—along with nonradioactive isotopes of carbon—during photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. The carbon-14 in organisms constantly decays, but it is continuously replaced as long as an organism is alive. After an organism dies, the carbon-14 it contains continues to decay, but no new carbon-14 is taken in to replace it. Therefore, the carbon-14 content of the remains constantly declines. The remaining carbon-14 in organic materials can be measured and used to estimate the amount of time that has passed since the organism died.    
Different radioactive isotopes are more useful for estimating the ages of older materials. For example, potassium- decays to argon-40 with a half-life of 1.26 billion years. Potassium-argon dating can be used to date materials between 100,000 and over a billion years old. Two uranium isotopes are also used for radiometric dating. Uranium-  decays to 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 can be used to date materials between 1 million and 4.6 billion years old.