How can radioactive decay be used to date rocks?
Radioactive decay can be used to date igneous rocks by assuming making some assumptions, finding the percentages of parent to daughter products and using experimentally determined half-lives.
Start with the assumptions that the rock 1. started with 100% of the parent element, and 0% of the daughter element. 2, there has been no loss of the parent element due to erosion. 3. there has been not gain of the daughter element due to intrusion. 4. That there have been no factors that have influenced the rate of radioactive decay during the "life" of the rock.
Second determine the percentages of the parent to the daughter elements in the rock. The percentages can be be used to calculate the number or fractions of the half lives that have occurred to create the percentages.
( note if there is 25% of the parent element that would be 2 half lives)
Third multiple the experimentally determined half live the parent to daughter transformation by the number of half lives determined by the percentages.
( Note igneous rocks can be dated using these assumptions and calculations, sedimentary fossil layers can not be dated using radioactive decay. There are no usually no radioactive material found in sedimentary layers and the assumptions of no erosion are clearly not valid as sedimentary layers are formed by erosion.