Cumulate is a name for plutonic rocks which were formed either by the process of accumulation of crystals (crystal settling) or by direct crystallization (in situ crystallization) in a magma chamber. Since “cumulate” is clearly a genetic term, it should not be used as rock name (qv. “gabbro,” and the chapter on rock classification of gabbroic rocks therein), but can be used as additional qualifier, in order to express that rocks were formed by a distinct process within a magma chamber.

For the oceanic crust, this term is directly related to the accretion of oceanic gabbros, forming the lower part of the oceanic crust (layer 3; qv. “seismic layers”). The corresponding magma chambers are located directly under the spreading centers of mid-ocean ridges or back-arc systems (qv. “axial magma chamber”). Here, primitive MORB (mid-oceanic ridge basalt; qv. “mid-ocean ridge magmatism”) melts differentiate by fractionation of mainly olivine, plagioclase, and clinopyroxene, which may accumulate...