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 half-life useful range parent daughter (years) (ears) carbon 14nitrogen 14 5,720 100 -30,000 100,000- 1.3 billion4.5 billion potassium 40 argon 40 10 million - 4.5 billion strontium 8747 billion lead 206 lead 207 rubidium 87 hundreds of millions uranium 238 4.5 billion to 15 billion years 10 million to 4.5 billion years uranium 235 710 million 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. procedure use table 2.1 to answer the following questions. i. a sample contains 25% potassium 40 and 75% argon 40. how old is this sample? 19 exercise 2 evolutionary observations