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

Slow-spreading oceanic ridges, such as the Mid-Atlantic Ridge, are characterized by a central rift valley because they are formed by fault-controlled processes at divergence zones.

At these spreading centres, the oceanic crust is under tension as lithospheric plates move apart. In slow-spreading environments (up to 50 mm per year), this tension leads to the subsidence of a crustal segment between normal faults, creating an elongated trough known as a rift valley. These valleys are typically 1.4 km deep and 20–40 km wide.12

In contrast, faster-spreading ridges lack these rift valleys and instead feature an axial high at the crest. This difference occurs because the rate of magma upwelling and the thermal state of the crust at faster-spreading centres prevent the formation of the deep, fault-bounded depressions seen at slower rates.13