Here's why:

* Basalt: Oceanic crust is primarily composed of basalt, a dark-colored volcanic rock.

* Density: Basalt is denser than the granite that makes up continental crust. This is why oceanic crust sits lower than continental crust, forming the ocean basins.

* Thickness: Oceanic crust is also thinner than continental crust, typically ranging from 5 to 10 kilometers thick.

Let me know if you have any other questions about Earth's crust!