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Acidic or Felsic Igneous Rocks
Felsic rocks contain high amounts of silicon and aluminum. They are considered acidic rocks and are usually light in color. The well-known granite is part of this classification. Granite is formed beneath the earth's crust, and comprises mainly of quartz, plagioclase feldspars, k-feldspars and hornblende. When the magma that forms granite erupts to the surface, it creates a different acidic rock called rhyolite. Rhyolite has much smaller crystals than granite, due to how quickly the lava cools on the surface.
Intermediate Igneous Rocks
As their name suggests, intermediate rock composition falls between felsic and mafic rocks. Andesite is an extrusive example of intermediate rock. It is formed of plagioclase fledspars, pyroxene, biotite, and/or hornblende, along with magnetite and quartz. Diorite is the related intrusive rock, formed of similar minerals, primarily the plagioclase feldspars, biotite, hornblende and/or pyroxene. Diorite is a hard stone that is commonly used for building materials, including cobblestone.
Mafic or Basic Igneous Rocks
Basic rocks contain magnesium and iron silicates in a high concentration, along with feldspar minerals, pyroxenes, and other accessory minerals. The most common igneous rock on the earth's crust is the extrusive, basic rock basalt. Basalt rocks are also commonly found on the moon, Mars, and Venus. These rocks are formed by hot spot volcanoes, such as those found in the Hawaiian and Galapagos Islands. The intrusive relative of basalt is gabbro. Gabbro is produced by mid-ocean ridges, and is extremely common in the earth's oceanic crust.
Ultramafic or Ultrabasic Igneous Rocks
These rocks are primarily made of magnesium and iron silicates. They are rarely found on the earth's crust, but are thought to be common in the mantle. Peridotite is a plutonic rock that is ultramafic. It contains pyroxene in addition to the two silicates, and is green in color. Kimberlite is another intrusive rock, found as a volcanic pipe. Diamonds are formed as this pipe erodes. Komatiite is an extremely rare extrusive, ultrabasic rock. It is only formed in condition found over 2 billion years ago, from extremely hot, watery lava.
Multiple Classifications
Igneous rocks can also be classified based on their texture and color. This subjective method of classification is not considered an official way to classify rocks, but it is utilized. Texture can be divided into groups from glassy (obsidian) and fine-grained or aphanitic (basalt and rhyolite) to course grained/phaneritic (granite, diorite, and gabbro). Texture can be described further by the size of crystals. Vesicular refers to rocks that have a lot of holes in them, such as certain basalts. Porphoritic describes rocks with a glassy or fine grain that have large crystals, such as granite or andesite. Pegmatitic means that all the crystals are large, such as with certain granites. Rocks are further divided by their color, from light to dark, and then by their primary mineral component.