Here's why:

* Formation: Oceanic crust is created at mid-ocean ridges where magma rises from the Earth's mantle and cools.

* Composition: Basalt is a dark-colored, fine-grained igneous rock rich in magnesium and iron, which are common elements in the Earth's mantle.

* Density: Basalt is denser than continental crust, which is why it sits lower in the Earth's mantle.

While basalt is the dominant rock type in oceanic crust, there are also smaller amounts of other igneous rocks like gabbro, which is a coarser-grained version of basalt.