Here's why:

* Oceanic crust: The majority of the Earth's crust is oceanic crust, which is found beneath the oceans. Oceanic crust is primarily composed of basalt, a dark-colored volcanic rock.

* Continental crust: The remaining part of the crust is continental crust, which makes up the landmasses. Continental crust is more complex, with a variety of rock types, but it generally contains more granite than basalt.

So, while continental crust has a wider variety of rocks, the vast majority of Earth's crust, due to the sheer size of the ocean floor, is made of basalt.