Gabbro facts for kids

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 formed when molten magma is trapped deep beneath the Earth's surface and cools into aholocrystalline mass.

Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Long belts of gabbro intrusions are typically formed at proto-rift zones and around ancient rift zone margins. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. It is chemically equivalent to rapid-cooling, fine-grained basalt.

Due to its variant nature, the term gabbro may be applied to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.

Etymology

The term "gabbro" was used in the 1760s to name a set of rock types that were found in the ophiolites of the Apennine Mountains in Italy. It was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany. Then, in 1809, the German geologist Christian Leopold von Buch used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" (metamorphosed gabbro).

Uses

Gabbro often contains valuable amounts of chromium, nickel, cobalt, gold, silver, platinum, and copper sulfides. For example, the Merensky Reef is the world's most important source of platinum.

Gabbro is known in the construction industry by the trade name of black granite. However, gabbro is hard and difficult to work, which limits its use.

Images for kids

• 

  Gabbro specimen

• 

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

• 

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

• 

  Cizlakite sample

• 

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

See also

In Spanish: Gabro para niños