Granite

Granite is the signature rock of the continents. Two things distinguish granite. First, granite has large crystalline grains that fit tightly together, which is where its name came from. Second, granite always consists of quartz plus feldspar, with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light color, ranging from pinkish to white. But that light background color is punctuated by the darker accessory minerals. Thus classic granite, like the specimen shown here, has a "salt-and-pepper" look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende.

Almost all granite is igneous and plutonic, that is, it solidified from a fluid state and in a large, deeply buried body or pluton. The random arrangement of grains in this granite—its lack of fabric—is evidence for a plutonic origin.

Geologists call any light-colored plutonic rock with large crystalline grains a granitoid. True granite is only one of the granitoids. Read more about classifying granitoids.

Granite is a strong stone because its mineral grains have grown tightly together during a very slow cooling period. And the quartz and feldspar that compose it are harder than steel. This makes granite a desirable stone for buildings and for ornamental purposes such as gravestones. But the commercial granite seen in buildings and sold in the stone dealer's showroom may not match the geologist's more precise definition of granite (which can be seen on the Igneous Rock Types Simplified page).

Rock with the same composition as granite can also form through long and intense metamorphism of sedimentary rocks. But that kind of granite has a strong fabric and is usually called granite gneiss.

This granite specimen comes from the Salinian block of central California, a chunk of ancient crust carried up from southern California along the San Andreas fault. Its composition is quartz (gray), plagioclase feldspar (white), alkali feldspar (beige), biotite and hornblende. It and other granite specimens appear in the granite photo gallery. Also see the granite landforms of Joshua Tree National Park. And big closeup pictures of granite are available in the closeup rock wallpaper photos.

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