What principal involves an intrusive rock body younger than the one in which it intrudes?

The principle that involves an intrusive rock body being younger than the rock it intrudes is known as the principle of cross-cutting relationships. This geological principle states that if a rock layer is cut by another rock body, the intruding rock must be younger than the layers it disrupts. This is fundamental in relative dating of geological formations, as it helps establish the chronological order of rock layers and geological events.