What is Gabbro?
Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually black
or dark green in color and composed mainly of the minerals plagioclase and augite.
It is the most abundant rock in the deep oceanic crust. Gabbro has a variety of uses in the construcion industry. It is used for everything from crushed stone base materials at construction sites to polished stone counter tops and floor tiles.
What Minerals are in Gabbro?
Gabbro is composed mainly of calcium-rich plagioclase feldspar (usually labradorite or
bytownite) and clinopyroxene (augite). Minor amounts of olivine and orthopyroxene might
also be present in the rock. (see composition chart at right)
This mineral composition usually gives gabbro a black to
very dark green color. A minor amount of light-colored mineral grains may also be present.
Unlike many other igneous rocks, gabbro usually contains very little quartz. (see a
close-up view of gabbro at right)
Gabbro and Basalt are Related
Gabbros are equivalent in composition to basalts. The difference between the two
rock types is their grain size. Basalts are extrusive igneous rocks that cool quickly and have
fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly
and have coarse-grained crystals.
Gabbro in Oceanic Crust
It is often stated that Earth's oceanic crust is made-up of basalt. The word "basalt" is
used because the rocks of the oceanic crust have a "basaltic" composition. However;
only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust are generally
coarser-grained gabbro. Basalt occurs at the surface of the crust because the rocks there have cooled quickly. At greater depth the cooling rate is slower and large crystals have time to develop. (see illustration at right).
Gabbro in Continental Crust
On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows
large crystals to form. Gabbro will also be present in the deep plutons that form when
magma chambers that feed basaltic eruptions crystallize.
Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia
River flood basalts of Washington and Oregon and the Deccan Traps of India
Uses of Gabbro
Gabbro can be polished to a brilliant black luster. Brightly polish gabbro is used to
make cemetery markers, kitchen counter tops, floor tiles, facing stone and other dimension stone
products. It is a highly desirable rock that stands up to weathering and wear.
In the dimension stone industry gabbro is sold under the name "black granite".
Gabbro is also used to make a number of rough-cut products such as curbing, ashlars, paving
stones and other products.
The most common use of gabbro is as a crushed stone or aggregate. Crushed gabbro is used as
a base material in construction projects, as a crushed stone for road construction, as railroad ballast
and anywhere that a durable crushed stone is needed as fill.
Gabbro as an Ore
Gabbro sometimes contains economic amounts of some relatively rare metals.
Gabbros containing significant amounts of the mineral ilmenite are mined
for their titanium content. Other gabbros are mined to yield nickel, chromium
Contributor: Hobart King
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|Gabbro is a dark-colored coarse-grained intrusive igneous rock. The specimen shown above is about two inches (five centimeters) across.
|Magnified view of the gabbro shown in the photograph above. The area shown in this image is about 1/2 inch across.
|A chart that illustrates the generalized mineral composition of igneous rocks. By studying this chart you can see that gabbros and basalts are composed mainly of plagioclase feldspar, micas, amphiboles and olivine.
|In the oceanic crust basalt forms near the surface at a divergent boundary but gabbro forms at depth from slow crystallization. Learn about teaching plate tectonics.
|A view of polished gabbro (labradorite). Polished gabbro is sold under the name "black granite) and is used for cemetery markers, floor tile, kitchen counter tops, facing stone and other dimension stone uses.