Gabbro refers to a large group of dark, often phaneritic (coarse-grained), mafic intrusive igneous rocks chemically equivalent to basalt, being its coarse-grained analogue. It forms when molten magma is trapped beneath the Earth’s surface and slowly cools into a holocrystalline mass.
Much of the Earth’s oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term “gabbro” may be applied loosely to a wide range of intrusive rocks, many of which are merely “gabbroic”.