Granite Range:

What is Granite?

Granite is a light-colored plutonic rock found throughout the continental crust, most commonly in mountainous areas. It consists of coarse grains of quartz (10-50%), potassium feldspar, and sodium feldspar. These minerals make up more than 80% of the rock. Other common minerals include mica (muscovite and biotite) and hornblende (see amphibole). The chemical composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia and titania. Volcanic rock of equivalent chemical composition and mineralogy is called rhyolite.

Granite is the signature rock of the continents. More than that, granite is the signature rock of the planet Earth itself. The other rocky planets—Mercury, Venus and Mars—are covered with basalt, as is the ocean floor on Earth. But only Earth has this beautiful and interesting rock type in abundance.

Granite is found in large plutons on the continents, in areas where the Earth’s crust has been deeply eroded. This makes sense, because granite must solidify very slowly at deeply buried locations to make such large mineral grains. Plutons smaller than 100 square kilometres in area are called stocks, and larger ones are called batholiths.

How is it formed?

On the grandest scale, granite represents the way the continents maintain themselves. The minerals in granitic rocks break down into clay and sand and are carried to the sea. Plate tectonics returns these materials through seafloor spreading and subduction, sweeping them beneath the edges of the continents. There they are rendered back into feldspar and quartz, ready to rise again to form new granite when and where the conditions are right.

Granites are the most abundant plutonic rocks of mountain belts and continental shield areas. They occur in great batholiths that may occupy thousands of square kilometers and are usually closely associated with quartz monzonite, granodiorite, diorite, and gabbro.

Debate has long centered on whether granite is igneous or metamorphic in origin. Originally granite was thought to form mainly from magmatic differentiation of basaltic magma, but geologists now believe there is simply too much of it for it to have formed this way, except locally. Most granite seems to have formed either by melting, partial melting, or metamorphism of deeply buried shale and sandstone. Granite dikes are clearly igneous, and granite emplaced in the upper few kilometers of the Earth’s crust also often shows evidence of forceful intrusion into surrounding rocks, whereas some granites that formed deeper within the crust seem not to have been forcefully emplaced. Evidence of intrusion or great mobility is considered to indicate an igneous origin that stems from melting of sediments; but where no good evidence of either a magma chamber or of fluidity is observed, a metamorphic origin must be considered.

Granites are classified in three or four categories. I-type (igneous) granites appear to arise from the melting of preexisting igneous rocks, S-type (sedimentary) granites from melted sedimentary rocks (or their metamorphic equivalents in both cases). M-type (mantle) granites are rarer and are thought to have evolved directly from deeper melts in the mantle. A-type (anorogenic) granites now appear to be a special variety of I-type granites. The evidence is intricate and subtle, and the experts have been arguing for a long time, but that is the gist of where things stand now.

 

Almost all granite is igneous (it solidified from a magma) and plutonic (it did so in a large, deeply buried body orpluton). The random arrangement of grains in granite—its lack of fabric—is evidence of its plutonic origin. Rock with the same composition as granite can form through long and intense metamorphism of sedimentary rocks. But that kind of rock has a strong fabric and is usually called granite gneiss.

Granite is only one of the granitoids, a granitoid with quartz content between 20 and 60 percent and a feldspar content in which alkali feldspar rather than plagioclase feldspar predominates. 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. Granite takes a good polish and resists weathering and acid rain.

Lavas erupt all over the Earth, but lava with the same composition as granite (rhyolite) only erupts on the continents. That means that granite must form by the melting of continental rocks. That happens for two reasons: adding heat and adding volatiles (water or carbon dioxide or both). Continents are relatively hot because they contain most of the planet’s uranium and potassium, which heat up their surroundings through radioactive decay. Anywhere that the crust is thickened tends to get hot inside (for instance in the Tibetan Plateau).

And the processes of plate tectonics, mainly subduction, can cause basaltic magmas to rise underneath the continents. In addition to heat, these magmas release CO₂ and water, which helps rocks of all kinds melt at lower temperatures. It is thought that large amounts of basaltic magma can be plastered to the bottom of a continent in a process called underplating. With the slow release of heat and fluids from that basalt, a large amount of continental crust could turn to granite at the same time.

The immediate cause of granite collecting and rising in huge stocks and batholiths is thought to be the stretching apart, or extension, of a continent during plate tectonics. This explains how such large volumes of granite can enter the upper crust without exploding, shoving or melting their way upward. And it explains why the activity at the edges of plutons appears to be generally gentle and why their cooling is so slow.

What does it look like?

Granite is made of large mineral grains (which is where its name came from) that fit tightly together. It always consists of the minerals quartz and feldspar, with or without a wide variety of other minerals (accessory minerals). The quartz and feldspar generally give granite a light colour, ranging from pinkish to white. That light background colour is punctuated by the darker accessory minerals.

Thus classic granite has a “salt-and-pepper” look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende.