(n.) Geology. The mountainous, linear axis of ocean basins along which rifting occurs and new oceanic crust forms as magma wells up and solidifies. The most prominent midoceanic ridges are those of the Atlantic and Indian Oceans. The new crust is made of mafic igneous rock called basalt, commonly referred to as midocean ridge basalt, or MORB, whose composition reflects that of the deeper mantle of the Earth. The presence of the spreading plate boundaries of the midoceanic ridges; their symmetrically spreading, successively older crust outward from the ridge; and the lack of oceanic crust older than approximately 200 Ma support the theory of plate tectonicsand the recycling of oceanic crust through the process of subduction. See: Crust, Geomagnetic Polarity Reversal, Geomagnetic Polarity Time Scale, Igneous, Mafic, Mantle, Mohorovicic Discontinuity, Plate Tectonics, Rift, Subduction