Pronunciation: GAB•broe
Description*: Gabbro is an intrusive igneous rock composed primarily of calcium-rich plagioclase feldspar (which is commonly gray), olivine and various pyroxenes (dark minerals). Gabbro and basalt (its extrusive equivalent) are said to be mafic rocks because of their dark tone, in contrast to the light tone of the felsic/sialic rocks granite and rhyolite, and intermediate tone of diorite/andesite. The coarse size of the dark mineral grains in gabbro can be discerned by rotating a fresh specimen in the light to look for flashes of light reflecting off of cleavage faces in the plagioclase feldspar and pyroxene grains.
Basalt and gabbro form from the partial melting of the upper mantle, and constitute the dominant rock types in the oceanic crust.
*Description from Cronin, V.S., 2001, Geology laboratory projects for group learning: Primis McGraw-Hill, 156 pp., ISBN 0-07-252348-4
Used by permission of the author.
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