According to the theory of Plate Tectonics, Earth's surface is made up of thin, rigid plates composed of oceanic crust, continental crust, or, in transitional areas, both. These plates are always in motion.
Oceanic crust consists of basalt rock and it makes up the floor of the oceans and seas. The density of oceanic crust is approximately 3.0 g/cm^3.
Continental crust consists of granite rock and lies beneath the continents. It is less dense than oceanic crust - about 2.7 g/cm^3.
The difference in crust densities is responsible for the fact that the more dense oceanic crust sinks lower than the continental crust that sits at a higher elevation.
Also, oceanic crust is on average 5 km thick compared to continental crust which is from 20-50 km thick.