Composition is the rocks chemical and mineral make up. The exact composition of a rock can only be determined by advanced geologic equipment. Luckily, however, geologists without those resources can determine the composition of a rock (felsic, intermediate, mafic, or ultramafic) through color identification. Generally, felsic and intermediate rocks will have much lighter colors while mafic and ultramafic have very dark shades. This is caused by the amount of silicates in the rock. Mafic and ultramafic rocks have less silicates while felsic rocks are composed primarily of silicates. Intermediate rocks are a midpoint between felsic and mafic, meaning they have more silicates than mafic and ultramafic rocks, but not as much as felsic rocks (Reynolds 113). One exception to these guidelines is obsidian: a dark colored felsic rock
Composition is the rocks chemical and mineral make up. The exact composition of a rock can only be determined by advanced geologic equipment. Luckily, however, geologists without those resources can determine the composition of a rock (felsic, intermediate, mafic, or ultramafic) through color identification. Generally, felsic and intermediate rocks will have much lighter colors while mafic and ultramafic have very dark shades. This is caused by the amount of silicates in the rock. Mafic and ultramafic rocks have less silicates while felsic rocks are composed primarily of silicates. Intermediate rocks are a midpoint between felsic and mafic, meaning they have more silicates than mafic and ultramafic rocks, but not as much as felsic rocks (Reynolds 113). One exception to these guidelines is obsidian: a dark colored felsic rock