Volcanic settings of the ocean floor


Igneous basalt is generated on the ocean floor in this diagram at the mid-ocean ridge or the back-arc basin. A mid-ocean ridge forms where relative tectonic stress in the crust causes extension and the spreading apart of two tectonic plates. Magma is generated at the mid-ocean ridge due to decompression melting of the hot mantle as it experiences decreasing pressure as it rises to the surface where it erupts on the ocean floor as basaltic lava. The newly formed ocean floor will move away from a mid-ocean ridge as more lava is erupted and spreading of the two plates occurs. As the ocean floor ages, it cools until it becomes dense enough to sink at a convergent plate boundary to form a volcanic arc. This type of convergent plate boundary, called a subduction zone, is characterized by one plate sinking beneath an overriding plate. As the slab of subducted crust descends, melting occurs and rises to the surface of the overriding plate to form a volcanic arc. Although compressive tectonic forces dominate in a volcanic arc setting, extension and decompression melting to generate magma may occur in the back-arc basin as the front edge of the overriding plate moves faster due to drag force exerted by the convecting mantle. (Magma can also been generated at oceanic hot spots like Hawaii, but this type of volcanic center is not shown in the diagram above.)