MORB (acronym of mid-ocean ridge basalt) is the rock (basaltic in composition) formed by eruption of magma at the oceanic spreading centers to form the oceanic crust. It is commonly porphyritic with plagioclase or more rarely olivine phenocrysts. Eruption under water produces pillow lava interspersed with rare sheet flows. The basalt is relatively magnesian (5–11 % MgO) and characterized by high Al2O3 content (12–18 %) and low concentrations of K2O. In N-MORB (normal MORB), the incompatible elements are depleted relative to more compatible elements, reflecting derivation from depleted (previously melted) upper mantle. MORB forms as the upper mantle passively upwells and partially melts where lithospheric plates diverge.