Geologists can estimate the age of rocks by their uranium-238 content. the uranium is incorporated in the rock as it hardens and then decays with first-order kinetics and a half-life of 4.5 billion years. a rock is found to contain 82.6% of the amount of uranium-238 that it contained when it was formed. (the amount that the rock contained when it was formed can be deduced from the presence of the decay products of u−238.)