Why do oceanic ridges rise above the surrounding seafloor?

Oceanic ridges rise above the surrounding seafloor primarily because the lithospheric plates located near spreading centres are warm and thermally expanded.1

As new oceanic crust forms at these ridges, the material is hot. As the plates move away from the spreading centre, they gradually cool and undergo thermal contraction. This cooling process causes the crust and upper mantle to become denser and sink, which explains why the depth of the ocean floor is proportional to the square root of the crustal age. Consequently, the ridges appear as elevated features relative to the older, cooler, and more contracted seafloor found further away from the ridge crests.21