Gabbro

Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

A thin section of gabbro under the microscope shows the crystals of which it is made.

A gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.

Gabbro as a xenolith ('foreign' rock) in a granite, Sierra Nevada, Rock Creek Canyon, California.

Gabbro is a large group of dark, coarse-grained, igneous rocks. They are chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped under the Earth's surface and cools into a crystalline mass.

Most of the Earth's surface has gabbro in the oceanic crust, produced by basalt magmatism at mid-ocean ridges. In general, when the gabbro is formed, it lies above the Earth's mantle, and below the basalt.

Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. It is an essential part of the oceanic crust. Long belts of gabbro intrusions are typically formed at proto-rift zones and around ancient rift zone margins.

Gabbro Media

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  gabbro

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  Mineral assemblage of igneous rocks

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  QAPF diagram with the gabbroid/dioritoid fields highlighted in yellow. Gabbroids are distinguished from dioritoids by an anorthite content of greater than 50% of their plagioclase.

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  QAPF diagram with the gabbro field highlighted in yellow. Gabbro is distinguished from diorite by an anorthite content of greater than 50% of its plagioclase and from anorthosite by a mafic mineral content greater than 10%.

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  Cizlakite stoneː a melanocratic variety of quartz monzodiorite or monzogabbro consisting of abundant augite with smaller amounts of hornblende and plagioclase with minor alkali feldspar, quartz and biotite. Sample from Cezlak, Slovenia.

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  Zuma Rock, Nigeria, a massive, nearly uniform, intrusion of gabbro and granodiorite.