How do scientists use radioactivity to determine the age of a rock?

They combine several well-tasted techniques to find out the ages of fossils. But the most important are Relative Dating, in which fossils and layers of rock are placed in order from older to younger, and Radiometric Dating, which allows the actual ages of certain types of rock to be calculated. Relative Dating.

Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. Radiometric clocks are "set" when each rock forms.