Many chemical elements in rock exist in a number of slightly different forms, known as isotopes. Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily ...

there is no argon from previous decay (argon, a gas, escapes into the atmosphere while the lava is still molten). When that mineral forms and the rock cools enough that argon can no longer escape ...

The boxes symbolize rocks containing those isotopes ... Share with students the following steps for simulating decay of their isotope. (You may want to model the procedure once.) ...