Final answer:
Igneous rocks are classified based on their mineral content and texture, with felsic rocks being silica-rich and light-colored, mafic and ultramafic rocks being iron and magnesium-rich with darker minerals, and intermediate rocks having a combination of these minerals. The rate at which the rocks cool, whether it's slowly underground as intrusive rocks or rapidly on the surface as extrusive rocks, also affects their texture and composition.
Step-by-step explanation:
Differences in Composition of Igneous Rocks
The composition of igneous rocks varies based on the mineral content and the tectonic environment in which they form. Felsic rocks are rich in silica and light-colored minerals such as quartz and potassium feldspar. Intermediate rocks contain moderate amounts of both light and dark minerals. Mafic rocks are darker, containing minerals like pyroxene and plagioclase, and have a higher iron and magnesium content. Ultramafic rocks, which are highest in magnesium and iron, typically contain abundant olivine and some pyroxene and Ca-rich plagioclase.
The initial composition of the magma, combined with the cooling process, are integral to the formation of these rock types. Intrusive igneous rocks cool slowly beneath the Earth's surface, often forming larger crystals, while extrusive rocks cool rapidly at or near the surface, resulting in finer textures or even a glassy appearance. Geologists use the relative amounts of light-colored (felsic) and dark-colored (mafic) minerals as a guide to classify the rocks. The mineral color is indicative of the rock type and can aid in the determination of whether a rock is mafic, felsic, intermediate, or ultramafic. The texture provides clues as to whether the rock formed as an intrusion or an extrusion.