Here's why:

* Dikes are intrusions: They form when molten rock (magma) forces its way into existing rock layers.

* Cross-cutting relationships: In geology, the principle of cross-cutting relationships states that a geologic feature that cuts across another feature must be younger.

* Therefore, the dike, which cuts across the rock layers, must have formed after the rock layers were already in place.

Think of it like a knife cutting through a loaf of bread. The knife (dike) is clearly younger than the bread (rock layers).