Mid-Ocean Ridge Volcanism (Encyclopedia of Volcanoes, 3rd edition, book chapter)

The vast majority of the Earth’s volcanism takes place in the deep ocean along mid-ocean ridges (MORs), yet because it is difficult to detect and observe, it is also relatively poorly understood. MOR volcanism occurs where tectonic plates spread apart and mainly produces effusive basaltic fissure eruptions where dikes reach the surface. The character and frequency of volcanism varies greatly as a function of spreading rate and magma supply, as does the morphology of the ridge crest, the balance between volcanic construction and tectonic faulting, and the scales of ridge segmentation. The depth and continuity of magma storage in the crust beneath MORs also depends greatly on the local magma supply in space and time. The generation of MOR magmas is ultimately due to partial melting processes in the mantle where it rises beneath ridges due to plate spreading.