Here's a breakdown:

* Basalt: A dark-colored, fine-grained volcanic rock rich in magnesium and iron. It's formed from the rapid cooling of lava erupted from underwater volcanoes.

* Gabbro: A coarse-grained, intrusive igneous rock that is chemically similar to basalt. It forms deep beneath the ocean floor where magma cools slowly.

While basalt is the dominant rock type, oceanic crust also contains other components, including:

* Peridotite: A dense, ultramafic rock found in the upper mantle. It's often found in small amounts within the oceanic crust.

* Sedimentary rocks: These are formed from the accumulation and cementation of sediments, often deposited on top of the basalt layer.

* Metamorphic rocks: These form when existing rocks are transformed by heat, pressure, or chemical reactions.

It's important to note that the composition of oceanic crust can vary depending on location and age. However, basalt remains the defining rock type.