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Why do slow-spreading oceanic ridges have deeper rift valleys?

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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.

Slow-spreading oceanic ridges are characterized by a deep, fault-controlled rift valley at their crests because of the tectonic processes occurring at these spreading centres.1

At these sites, the Earth's crust is under tension as lithospheric plates diverge. In slow-spreading environments—where plates move apart at rates of up to 50 mm per year—this extension causes the brittle crust to fracture. A central block of the crust then drops downward between normal faults, creating a tectonic depression known as a graben or rift valley.123

These valleys are typically 1.4 km deep and 20–40 km wide. In contrast, faster-spreading ridges do not develop these deep rift valleys; instead, they often feature an axial high at the crest because the rapid upwelling of magma and thermal expansion of the plates prevent the formation of such a deep, stable rift.12