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Fine-grained, dark-colored extrusive igneous rock with less than about 52 weight percent silica (SiO₂). Because of its low silica content, basalt has a low viscosity (resistance to flow. ) Basalt is composed primarily of plagioclase feldspar and pyroxene; other minerals such as olivine and ilmenite are usually present. Basalt is the most common volcanic rock on the terrestrial planets.