What type of rock primarily makes up much of the ocean floor?

The ocean floor is primarily composed of basalt, a volcanic rock that forms from the rapid cooling of magma at the mid-ocean ridges. This rock is predominantly found in oceanic crust, which is thinner and denser than continental crust. Basalt is rich in iron and magnesium and has a fine-grained texture due to its rapid solidification. This composition is essential for understanding plate tectonics, as new basalt is continually formed at divergent boundaries where tectonic plates pull apart.

Granite, on the other hand, is primarily found in continental crust and is not characteristic of oceanic environments. Bentonite and limestone also do not predominantly make up the ocean floor; bentonite is a type of clay formed from volcanic ash, and limestone is typically formed from the accumulation of marine organisms and sediment in shallow water settings. Thus, basalt is the defining rock type of the oceanic crust, underpinning much of the ocean floor structure.