Rubidium (Rb) is the 16th most abundant element in the earth's crust, and is present in many different minerals. However, a fraction of the Rubidium in rocks (27.83%) is composed of 37 protons and 50 neutrons. Rubidium-87 is not stable and if given the chance, it would like to change to a stable state.
Most of the Rubidium in rocks (72.17%) is composed of 37 protons and 48 neutrons. This change to a stable state is what we call radioactive decay.
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Each of the different rock types in the breccia may have different ages, and the "welding" process itself may have even reset the age of some part of the rock.
Untangling this whole mess is what makes the science of determining rock ages fun.
The age of the Apollo 14 site (blue) is much less constrained that that of Apollo 11 and 12.
Geologist determine the age of rock using a technique called radioactive dating.
A more relevant example: Let us say that a rock is sitting on the surface of the Moon.