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Gabbro

Pictures of Igneous Rock Types

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Gabbro is a dark plutonic rock that is considered to be the plutonic equivalent of basalt. (more below)
A plutonic basalt
Photo courtesy State of New South Wales through the Department of Education and Training
Unlike granite, gabbro is low in silica and has no quartz; also gabbro has no alkali feldspar, only plagioclase that has a high calcium content. The other dark minerals may include amphibole, pyroxene and sometimes biotite, olivine, magnetite, ilmenite and apatite.

Gabbro is named after a town in Tuscany, Italy. You can get away with calling almost any dark, coarse-grained igneous rock gabbro, but true gabbro is a narrowly defined subset of dark plutonic rocks.

Gabbro makes up most of the deep part of the oceanic crust, where melts of basaltic composition cool very slowly to create large mineral grains. That makes gabbro a key sign of an ophiolite, a large body of oceanic crust that ends up on land. Gabbro is also found with other plutonic rocks in batholiths when bodies of rising magma are low in silica.

Igneous petrologists are careful about their terminology for gabbro and similar rocks, in which "gabbroid," "gabbroic" and "gabbro" have distinct meanings.

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