Radiometric dating is finding the dates of objects using the radioactive decay of some of their materials. One compares the amounts of parent and descendant isotopes, using other isotopes of their elements as a reference. There are several isotope sets that have been used for radiometric dating, like uranium-lead, potassium-argon, samarium-neodymium, rubidium-strontium, and radiocarbon.
Young-earth creationists have a variety of objections to radiometric dating, all of which are successfully answered.
- It requires assumptions about initial composition. One can get around that problem in several ways.
- Known initial composition. Zircon is useful for uranium-lead dating because its crystal structure can incorporate uranium but not lead. So one can be confident that all a zircon's lead was produced by radioactive decay.
- Use of variations in initial composition. Different minerals have different affinities for the parent and descendant elements, and one can sort them out by grinding up the rock and dating each grain separately.
- Outside calibration. Radiocarbon dating is calibrated by dating wood from long-ago trees, wood that is dated using its annual growth rings. It has been possible to go back about 10,000 years, though doing so has required correlating the rings of a large number of trees over that time.
- It requires assuming that decay rates are constant. Here also, there are ways of getting around it.
- Most radioactive decay is not measurably altered by environmental conditions, as is evident from both theory and experiment. One needs extreme conditions, like inside the core of a massive star, to make much of a difference in most cases.
- Continental-drift rates estimated with radiometric dating agree with present-day drift rates found with quasars and GPS satellites and the like.
- Milankovitch astronomical cycles have the "right" periods over the parts of geological time where they have been compared to radiometric dates.
- Different radioisotopes would give different dates, and there is no evidence of such systematic discrepancies.
- Carbon-14 has been found in coal. However, it is a case of secondary radioactivity; radioactivity induced by other radioactivity.