Question

why the thickness of the crust below the continent is less as compared to ocean?

Answer 1

because it is non stop movement

Answer 2

Both oceanic crust and continentalcrust are less dense than the mantle, but oceanic crust is denser than continental crust. This is partly why the continents are at a higher elevation than the ocean floor. Because continental crust is less dense thanoceanic crust it floats higher on themantle .