Common extrusive igneous rocks are rhyolite, andesite, and basalt. Rhyolite is high in silica; it contains many of the same minerals as granites, but has been extruded from a volcano. Andesite mostly contains plagioclase feldspar with some other minerals, and is generally fine grained. Basalt is generally dark in color, with plagioclase feldspar and pyroxene predominant minerals and is fine grained.
Common intrusive igneous rocks include granite, diorite, gabbro, and peridotite. Granite is similar to rhyolite, and is composed of alkali feldspar, quartz, and plagioclase feldspar. Granite cools slowly as it is intruded into the country rock. Diorite is an intermediate rock similar to andesite. When basalt solidifies before it reaches the surface it is called gabbro. Peridotite is a course grained ultrabasic rock, composed of mostly olivine and pyroxene.
A few types of igneous rocks are classified primarily based on their texture. A pegmatite is a very coarse-grained rock, similar to granite in composition. They occasionally contain very rare minerals such as diamonds. Obsidian is a glassy rock that forms when magma cools so rapidly that there is almost no crystals formed. Obsidian is generally black, but different mineral impurities can alter the standard black color. After volcanic ash has cooled and solidifies it is called a tuff. Similarly, breccia forms as tuffs do, but contain larger fragments of rock in a finer matrix. Igneous rocks are classified in two simple manners, based on texture and chemical composition. The classification depends on whether the rock is intrusive or extrusive in nature.
Although I’ve presented a very simple classification here, the reality of igneous rocks is far more complex. Rocks with names like quartz monzodiorite, hornblende syenite, or diabase porphyry are common. Even through the classification I’ve presented is simplified; it serves as a good beginning for identifying igneous rocks in the field.
Glossary
Acidic: Classification of igneous rock that contains greater than 66 percent silica.
