The ocean floor is primarily made up of the igneous rock basalt. Basalt is the most common extrusive igneous rock and forms the volcanic layer of the oceanic crust, especially in the form of pillow lavas near mid-ocean ridges. It is a fine-grained, mafic rock rich in iron and magnesium, and it constitutes the uppermost volcanic layer of the oceanic crust
. The oceanic crust itself is composed of three main layers:
- The uppermost layer (Layer 1) consists mostly of sediments.
- Layer 2 includes a volcanic layer of glassy to finely crystalline basalt (pillow basalt) and underlying diabase dikes.
- Layer 3 is made up of coarse-grained gabbro and ultramafic cumulates formed by slow cooling of magma beneath the surface
Thus, basalt dominates the ocean floor as the primary igneous rock, making up the volcanic upper portion of the oceanic crust, while gabbro forms much of the lower oceanic crust
. This composition reflects the mafic nature of oceanic crust, which is denser and richer in iron and magnesium compared to continental crust
. In summary, basalt is the main igneous rock that makes up most of the ocean floor.
