Pronunciation: bah•SALT
Description*: Basalt is an extrusive igneous rock composed primarily of calcium-rich plagioclase feldspar (which is commonly gray), olivine and various pyroxenes (dark minerals). Basalts can have a vesicular (with gas bubbles), scoriaceous (with LOTS of small gas bubbles), porphyritic (with included large mineral grains called phenocrysts) or massively aphanitic (fine-grained) texture.
Basalt and gabbro (its intrusive 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.
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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