Gabbro

Gabbro is a dark, coarse-grained intrusive rock with a mafic mineral composition. Its crystals grow slowly from magma that remains underground, producing an interlocking texture visible in hand specimens and unlike the usually fine-grained texture of rapidly cooled volcanic rocks.¹

The principal minerals are calcium-rich plagioclase feldspar and clinopyroxene or orthopyroxene; olivine, amphibole, biotite, and iron-titanium oxides may occur in subordinate amounts. Mineral proportions create named varieties, including olivine gabbro and hornblende gabbro. Gabbro is broadly equivalent in chemical composition to basalt, but the two names distinguish their cooling environments and textures.

Gabbro forms when mafic magma crystallizes slowly in the lower parts of magma chambers, plutons, dikes, or oceanic crust. As crystals grow, some settle or float, separating minerals from the remaining melt and producing cumulate layers with distinctive compositions.

Much of the gabbroic rock in Earth’s crust is associated with oceanic spreading centers, where it makes up a substantial part of the lower oceanic crust beneath basaltic lavas and sheeted dikes.² Gabbro is also exposed in continental layered intrusions and in ophiolites, fragments of oceanic lithosphere tectonically placed on land. Uplift, erosion, faulting, and plate collision can therefore expose rocks that originally crystallized several kilometers below the seafloor.

Gabbro is identified chiefly by its coarse crystals, dark overall color, and abundance of mafic minerals, although color alone is not diagnostic. Plagioclase commonly appears white or gray, while pyroxene and olivine are commonly dark green, brown, or black; polished surfaces can reveal a mottled pattern of interlocking grains.

Geologists distinguish gabbro from related rocks by texture and mineral proportions. Diabase, also called dolerite, has a similar composition but typically forms in shallower intrusions and is finer grained, whereas basalt is volcanic and usually much finer grained. Anorthosite is dominated by plagioclase and contains far less mafic material. In thin section, crystal boundaries, alteration minerals, and textures such as ophitic intergrowths provide more reliable evidence than field color.

Gabbro records the chemical differentiation of magma rather than representing a single uniform rock type. In a layered intrusion, repeated bands of pyroxene-rich, olivine-rich, and plagioclase-rich material can preserve successive stages of crystal accumulation; some such bodies are economically important sources of chromium, platinum-group elements, nickel, or copper.³

Ocean-floor gabbros may also preserve evidence of hydrothermal alteration, melt migration, and the construction of the crust at fast-spreading ridges. Their chemistry can differ substantially from that of surface basalt because crystals are removed from, or added to, the melt during storage and transport.² Gabbro is used as a durable crushed stone and dimension stone, but its geological value often lies in revealing processes hidden beneath active volcanic systems and the seafloor.

Mafic

Describing igneous rocks or magmas relatively rich in magnesium and iron and generally lower in silica than felsic rocks.

Intrusive

Formed when magma crystallizes below Earth’s surface.

Cumulate

An igneous rock formed largely by crystals that accumulated as they separated from a magma.

Ophiolite

A tectonic slice of oceanic crust and upper mantle exposed on land.