One way is to look at any fossils the rock may contain. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period.
Another way is to use the "What's on top?" rule.
When you find layers of rocks in a cliff or hillside, younger rocks are on top of older rocks.(But these two methods only give the relative age of rocks--which are younger and which are older.)
The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. When rocks are formed, small amounts of radioactive elements usually get included. As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements.
Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements are found in the rock.
A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.
The age of the rock in years can be found by measuring the rate at which a parent element decays and then measuring the ratio of parent element to daughter element in the rock.
The ages in years of the different geological time periods are found by measuring the absolute ages of many rocks from all of the different periods.
