Here's why:
* Intrusions are formed when molten rock (magma) pushes its way into existing rock layers.
* The existing rock layers are already solidified and have been around longer.
* The magma cools and solidifies within the existing rock, becoming an igneous intrusion.
Think of it like this:
Imagine a cake with layers of different colored batter. If you pour a hot liquid into the cake, it would cut through the existing layers and solidify, forming a new layer. The new layer is clearly younger than the existing layers of the cake.
The same principle applies to intrusions. The intrusion cuts through and is younger than the surrounding layers of rock. This is a key principle in geology called cross-cutting relationships.
