The relative densities of crust and mantle determine the amount of material that must be displaced by the mantle to support the crust's weight. The thickness of the crust determines how high it will rise out of the mantle. Thick crust is found in mountainous areas, thin crust is found in ocean basins and lowlands. As streams erode mountains the crustal root rises as it is unloaded just like an iceberg would rise as you chooped its top off. Loading of the crust causes it to sink into the mantle, just as an iceberg would sink if you loaded it with many people.
Oceans are low and continents are high relative to one another.
Plate Tectonics Theory Earth's lithosphere is divided into about 2 dozen plates. These plates move in relation to each other, driven by convection cycles in the mantle. The weak and plastic asthenosphere allows the plates to move. Lithosphere may be carrying either ocean or continental crust, or both. Interactions between plates cause earthquakes and volcanoes.