Oceanic crust is much thinner than continental crust.
Recall that mafic rocks are more dense than sialic rocks. As a result of this density difference, the regions of oceanic crust are at lower elevations than the continental crust regions and fill with water, thus forming the oceans. The density difference will also be important when we consider interactions of continent-sized slabs of rock. This region is the boundary separating the crust from the mantle. The intermediate region of the earth is composed of ultramafic rock.