How does the density of oceanic crust differ that is continental crust why is this difference important

Oceanic thrust is thinner than continental crust, however it is denser, its density is about 2.9 g/cm^3 whereas the density of continental crust is about 2.7 g/cm^3. This density difference allows continents to float permanently on the upper mantle, persisting more or less intact for billions of years.