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Rocks » Igneous Rocks » Gabbro
Gabbro
What Is Gabbro, What Minerals Are In Gabbro and What Is It Used For?
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.
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 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
rocks is their grain size. Basalts are extrusive igneous rocks that cool quickly and have
fine-grained crystals, while 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,
a thin surface veneer of oceanic crust is basalt and the deeper rocks are generally
coarser-grained gabbro. These differential rates of cooling occur at divergent
plate boundaries in oceanic lithosphere. (see illustration at right).
Gabbro in Continental Crust
On the continents, gabbro can be found within thick lava flows 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.
Gabbro as an Ore
Gabbro often 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
or platinum.
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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. Click the image above to see the entire chart. |
| 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. |
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