<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2007) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (October 2007) (Learn how and when to remove this template message) </Td> </Tr> <P> A mid-ocean ridge (MOR) is an underwater mountain system formed by plate tectonics . It consists of various mountains linked in chains, typically having a valley known as a rift running along its spine . This type of oceanic mountain ridge is characteristic of what is known as an' oceanic spreading center', which is responsible for seafloor spreading . The production of new seafloor results from mantle upwelling in response to plate spreading; this isentropic upwelling solid mantle material eventually exceeds the solidus and melts . The buoyant melt rises as magma at a linear weakness in the oceanic crust, and emerges as lava, creating new crust upon cooling . A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary . </P> <P> Mid-ocean ridges are geologically active, with continuing volcanism and seismicity . New magma steadily emerges onto the ocean floor and intrudes into the ocean crust at and near rifts along the ridge axes . The crystallized magma forms new crust of basalt (known as MORB for mid-ocean ridge basalt) and below it (in the lower oceanic crust), gabbro . MORs are formed by two oceanic plates moving away from each other . Hydrothermal vents are a common feature at oceanic spreading centers . </P>

Mid-ocean ridges form at which type of boundary