How can Age be Determined? Radioactive dating yields more precise estimates of age. It is based upon the decay of radioactive isotopes such as potassium, uranium, rubidium, and carbon. For the most part this can only be done with igneous rocks formed from molten lava. Table 2.1: Half-life of various parent isotopes. Isotope Parent Daughter Carbon 14 Nitrogen 14 Half-life of parent (years) Useful range (years) 5,720 100-30,000 100,000- 4.5 billion Potassium 40 Argon 40 Rubidium 87 Strontium 87 Uranium 238 Lead 206 Uranium 235 Lead 207 1.3 billion 47 billion 4.5 billion 710 million 10 million – 4.5 billion hundreds of millions to 15 billion years 10 million to 4.5 billion years In Table 2.1 the parent isotope is the original material and the daughter isotope is the material into which it will decay. The half-life is a measure of how long it takes one half of the material to change. By knowing the half-life, an estimate can be made as to the age of the material. For example, if one kilogram of Uranium-235 has gone through one half-life (710 million years), then there should be 0.5 kg of Uranium-235 and 0.5 kg of the daughter isotope, Lead 207. If the rock strata have both sedimentary layers and igneous features, a combination of both relative and radioactive dating can be used. A 10 kilogram meteorite is collected in space and brought back to Earth. It is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. 2. How many U-238 half lives have passed? What is the approximate age of the meteorite?