The spontaneous decomposition is called radioactive decay.
Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed.
Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geological time scale.
By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.Using the decays of uranium and thorium, our galaxy has been found to be between 10 and 20 billion years old and the earth has been found to be 4.6 billion years old. Within experimental error, this estimate agrees with the 15 billion year estimate of the age of the Universe. Every month, half of X turns into Y (half life = 1 month). Lets say you have a rock with a radioactive mineral X which decays into mineral Y. thank you a Radioactive dating uses radioactive decay to find the exact age of something like a rock.