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Why are oceanic ridges higher than the surrounding seafloor?

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World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

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

As new oceanic crust forms at these ridges, the material is buoyant due to this high heat. As the plates move away from the ridge crests over millions of years, they gradually cool. This cooling causes the crust and upper mantle to undergo thermal contraction, increasing their density and causing the seafloor to subside to greater depths.123

Because this elevation is a direct function of the age of the crust, the height of the ridge is essentially a physical manifestation of the thermal state of the oceanic plate. This elevation difference also contributes to a gravitational force known as "ridge push," which helps drive the movement of tectonic plates away from the ridge axis.23