How does cooling cause oceanic crust to sink farther from an oceanic ridge?

As oceanic crust moves away from an oceanic ridge, it undergoes a process of thermal contraction as it cools. At the ridge crest, the crust is elevated because it is swollen with heat from upwelling mantle material.123

As the crust is carried away from the spreading centre, it gradually loses this heat. This cooling causes the oceanic crust and the underlying upper mantle to become slightly denser. Due to this increase in density, the crust undergoes isostatic adjustment, which causes it to subside or sink deeper into the mantle. Consequently, the depth of the ocean floor increases in proportion to the square root of the crust's age as it moves toward a steady thermal state.12