The formation of the lowermost crust, made up of a dark, magnesium - rich rock called gabbro, is still largely a mystery, one that holds the key to the workings of the magma source that feeds the whole process. (discovermagazine.com)
This end - member process stands in contrast to the «gabbro glacier» hypothesis, in which all oceanic plutonic rocks crystallize in a single, shallow melt lens and undergo ductile flow downward and outward to «fill» the lower crust. (ldeo.columbia.edu)
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are generally referred to as diabase (also called dolerite) or, when more coarse - grained (crystals over 2 mm across), as gabbro. (en.wikipedia.org)