What accounts for the difference in density between oceanic and continental crust?

The continental crust is made mostly of rocks. Both oceanic crust and continental crust are less dense than the mantle, but oceanic crust is denser than continental crust.  The mantle, oceanic crust and continental crust have different densities because they are made of different kinds of rock with different densities. the oceanic crust is made mostly of rocks with a composition of basalt.