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Why do slow-spreading oceanic ridges have rift valleys?

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Earth's lithosphere and upper mantle — oceanic crust A cross section of Earth's outer layers, from the crust through the lower mantle.

Slow-spreading oceanic ridges, such as the Mid-Atlantic Ridge, feature prominent rift valleys at their crests due to the specific tectonic processes occurring at these divergent plate boundaries.12

These valleys are fault-controlled, elongated troughs formed by the subsidence of a segment of the oceanic crust between normal faults. At slow-spreading centers—defined as those spreading at rates up to 50 mm per year—the crust is under tension. As the lithospheric plates diverge, blocks of the crust drop down into the space created between them, similar to the keystone of an arch falling as its supports move apart.134

While faster-spreading ridges lack these features because they are characterized by an axial high—where the rapid upwelling of magma maintains a more elevated, smoother topography—slow-spreading ridges are defined by this rough, faulted landscape. These rift valleys typically measure about 1.4 km deep and 20–40 km wide, serving as the primary zone where new crust is formed through volcanic activity and tectonic rifting.12