Dr. Ryan Ickert, senior research scientist with the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University, describes how radioactive isotopes are used to determine the age of rocks and minerals in a field of study known as geochronology. When scientists knowing the rates of decay of elements such as uranium, thorium, and potassium, they can use this information as a type of clock to determine the age of materials as far back as the beginning of our Solar System!