Felsic, intermediate and mafic igneous rocks differ in their mineral composition. Felsic rocks are light in color and are composed of feldspars and silicates. Mafic rocks are darker and are composed of magnesium and iron. Intermediate rocks are composed of the minerals amphibole and feldspar and contain a combination of light and dark minerals.
Igneous rocks are formed by the cooling and crystallization of magma. The temperature of cooling determines which types of minerals are found in the rock and helps scientists classify the rock as felsic, intermediate or mafic. Rocks that begin their cooling at low temperatures are rich in minerals containing silicon, potassium and aluminum. High temperature igneous rocks contain minerals higher in calcium, sodium, iron and magnesium. Ultramafic rocks are a fourth classification of igneous rock. These rocks are rare on the Earth's surface and contain low amounts of silica the minerals olivine, plagioclase feldspars and pyroxene.
The temperature of cooling igneous rock creates a difference in the texture of the rock. A gradual cooling of magma creates large crystals. Magma that cools fast produces smaller crystals. If volcanic magma cools very quickly on the Earth's surface, obsidian glass is the result. Obsidian glass contains no crystalline structures.