Rocks in the group known to earth scientists as granite are an important and widespread component of the Earth’s crust, in particular the crust that underlies the continents. Granite (the word rhymes with “SPAN it”) is one of the igneous rocks, rocks that form by cooling from molten magma. Granite is perhaps the best-known of the intrusive igneous rocks, those which form deep below the surface. These rocks, which also include gabbro and diorite, all have a phaneritic texture: this means that the mineral grains comprising the rock are large enough that they can be distinguished by the unaided eye.
Granite is distinguished from other intrusive igneous rocks on the basis of the minerals it contains. The major component minerals of granite are quartz, potassium feldspar (orthoclase) and muscovite mica. Lesser quantities of sodium-rich feldspar (plagioclase) and iron- and magnesium-bearing (ferromagnesian) minerals, such as hornblende and biotite mica, are generally present. By definition, granite contains at least 10% free quartz (SiO2) by volume; silica in the quartz and other minerals typically exceeds 70% by volume. The extrusive igneous rock (formed at the surface from cooling lava) with the same mineralogy and chemistry as granite is called rhyolite.
To petrologists, earth scientists specializing in rocks; granite isn’t a single rock type, it’s a group of closely-related rocks. The granite group includes pegmatite, aplite, syenite, granodiorite, monzonite, latite and quartz monzonite, among others. While all members of the granite group have the same texture and fall within a set range of chemical compositions, they can be differentiated on the basis of the minerals present. The relative proportions of different feldspars (orthoclase vs. plagioclase; alkali vs. calcium) and quartz content are the most common criteria for differentiation. In common usage, however, granite simply refers to the rocks that can be identified as part of the granite group without chemical or mineralogical analysis.
Intrusive rocks, including granite, tend to form continuous bodies covering tens to hundreds of square miles. A small granite body (less than 100 square kilometers in area) is termed a stock; most stocks are believed to have formed close to the surface. If a deposit of granite is larger than 100 square kilometers in area, it is called a batholith. Most batholiths are much larger than the minimum size; often covering thousands of square miles. Many mountain ranges have a batholith at their core; for example, more than half of rocks comprising California’s Sierra Nevada are part of a granite batholith. These great granite deposits represent cooling and crystallization of the magma chamber deep below an extensive volcanic field. Famous and photogenic exposures of granite include Mt. Rushmore (South Dakota), Stone Mountain (Georgia), Pike’s Peak (Colorado) and El Capitan (California); however granite is widespread throughout the Rocky Mountains, Appalachians, and mountain ranges of the western USA.
Fresh samples of granite are generally pink or gray as a result of the large amounts of quartz and feldspar present. The color can range from red to white, generally with a “speckled” appearance due to the presence of dark mineral grains. True granite typically has an even texture of similar-sized mineral grains, though some bodies contain scattered larger crystals; a texture known as porphyritic. Porphyritic rocks are believed to represent two-stage cooling: first, slow cooling allowing large crystals to form, followed by more rapid cooling that creates the less-coarse matrix or groundmass. Orbicular granite is another, more unusual type oy porphyritic granite distinguished by large spherical inclusions. Orbicular granite is mined in Australia and Finland, as well as from a granite body near Albuquerque, New Mexico.
Granite’s hardness and even texture make it an attractive candidate for building stone. In recent years, so-called “granite” countertops have been popular options for new and remodeled kitchens. Not all granite used in building is actually granite, however; to building stone vendors the term simply means that the rock has visible mineral grains and is harder than marble. Consequently, “granite” used in countertops includes almost any intrusive igneous rock or even coarse-grained metamorphic rocks. Highly polished slabs of granite (both true and “granite”) are also used to face buildings and for decorative elements in construction. Rough-cut granite blocks have been used for road and building construction for thousands of years. The reddish-colored gravel used in many paths and walkway is often crushed, weathered granite.
The other intrusive rocks with a granular texture similar to granite’s range from the rare anorthosite (found on the moon and in the Earth’s mantle) though nearly black gabbro to diorite. These rocks are differentiated on the basis of the minerals they contain: for instance, gabbro (the intrusive equivalent of basalt) contains no quartz or orthoclase feldspar, instead it is almost entirely ferromagnesian minerals and plagioclase (sodium and calcium) feldspar. Closer to granite, diorite (intrusive equivalent of andesite) contains mainly feldspars plus less than 10% quartz.
In all intrusive igneous rocks such as granite, the individual mineral crystals create an interlocking texture, much like a three-dimensional jigsaw puzzle. The interlocking texture and random distribution of minerals is important, for this texture defines the difference between igneous granite and metamorphic gneiss, which may be composed of much the same group of minerals. The texture of gneiss is typically foliated, which gives a sample a layered appearance. In gneiss and other foliated rocks, minerals are concentrated in layers rather than randomly distributed.
