Igneous Rock Composition

Igneous rocks are commonly classified by their composition and texture. Most are composed of the eight most abundant elements in the Earth's crust. Because of the dominance of oxygen and silicon in the crust, igneous rocks are mostly made up of silicate minerals. These silicates can be generally divided into light and dark silicates. The dark silicates are also called ferromagnesian because of the presence of iron and magnesium in them. They include olivine, pyroxene, amphibole and biotite. The light-colored silicates include quartz, muscovite and feldspar.

Solidification from magma produces great diversity in the mineral compositions which make up the rocks. There are general catagories which are keyed to the amounts of light and dark silicates in the rocks. At the light-colored extreme are rocks made up mainly of quartz and the feldspars, with about 70% silica. Such rocks are called granitic rock. Rocks which contain large amounts of the ferromagnesian dark matter and about 50% silica are said to have basaltic composition. Some organization was brought to the continuous variation between these extremes by the Bowen reactions. This model of the process of solidification from magma pictures the processes which causes the composition of the magma and the subsequent rocks to change.

Igneous Rock Texture

Igneous rocks are commonly classified by their composition and texture. Texture is the term applied to the overall appearance of a rock based on the size, shape, and arrangement of the interlocking mineral crystals which form it. The table below summarizes the common classifications.

Elements Abundant in Igneous Rock

Eight elements make up about 98% by weight of most magmas from which igneous rocks are made. Click on any element for further details.

The dominance of oxygen and silicon in the Earth's crust gaurantees that most igneous rocks are made up of silicate minerals. The main differences in the composition of igneous rocks are the variations in the other six elements.

Beyond the "big 8", manganese and titanium are present in small concentrations in magma and therefore appear in a number of minerals.