granite

granite

[gran-it]
granite, coarse-grained igneous rock of even texture and light color, composed chiefly of quartz and feldspars. It usually contains small quantities of mica or hornblende, and minor accessory minerals may be present. Depending on the feldspar present, granite may be pink, dark gray, or light gray. It is commonly believed to have solidified from molten rock (called magma) under pressure. However, some granites show no contacts with surrounding wall rock, but instead gradually grade into metamorphic rock. Others show relic features found in sediments. This evidence suggests that some granites are not igneous in origin, but metamorphic. Some granites are the oldest known rocks on earth; others were formed during younger geologic periods. Crystallized at depth, granite masses are exposed at the earth's surface by crustal movement or by the erosion of overlying rocks. Very coarse-grained granite, called pegmatite, may contain minerals and gemstones of economic value. Such pegmatites are found in the Black Hills of South Dakota. Granite has been used since ancient times as a building material.

Coarse- or medium-grained intrusive rock that is rich in quartz and alkali feldspar. One of the most common rocks of the Earth's crust, it is formed by the cooling of magma. Granite was once used extensively as paving blocks and building stone, but today its principal uses are as roadway curbing, veneer for building faces, and tombstones. Granite characteristically forms irregular masses of extremely variable size, ranging from less than 5 mi (8 km) in maximum dimension to larger masses (batholiths) that are often hundreds of square miles in area.

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Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is 2.75 g/cm3 and its viscosity at standard temperature and pressure is ~4.5 • 1019 Pa·s . The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Mineralogy

Granite is classified according to the QAPF diagram for coarse grained plutonic rocks (granitoids) and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite; pyroxene and amphibole are common in tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite. Granite has poor primary permeability but strong secondary permeability.

Chemical composition

A worldwide average of the average proportion of the different chemical components in granites, in descending order by weight percent, is:

Based on 2485 analyses

Occurrence

Granite is currently known only on Earth where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.

Despite being fairly common throughout the world, the areas with the most commercial granite quarries are located in Finland, Norway and Sweden (Bohuslän), northern Portugal in Chaves and Vila Pouca de Aguiar, Spain (mostly Galicia and Extremadura), Brazil, India and several countries in southern Africa, namely Angola, Namibia, Zimbabwe and South Africa.

Origin

Granite is an igneous rock and is formed from magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional; there is a French scheme, a British scheme and an American scheme. This confusion arises because the classification schemes define granite by different means. Generally the 'alphabet-soup' classification is used because it classifies based on genesis or origin of the magma.

Geochemical origins

Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a eutectic point (or a temperature minimum on a cotectic curve). Magmas will evolve to the eutectic because of igneous differentiation, or because they represent low degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkali feldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite.

This process operates regardless of the origin of the parental magma to the granite, and regardless of its chemistry. However, the composition and origin of the magma which differentiates into granite, leaves certain geochemical and mineralogical evidence as to what the granite's parental rock was. The final mineralogy, texture and chemical composition of a granite is often distinctive as to its origin. For instance, a granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are based.

Alphabet soup classification

The 'alphabet soup' scheme of Chappell & White was proposed initially to divide granites into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite. Both of these types of granite are formed by melting of high grade metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.

M-type or mantle derived granite was proposed later, to cover those granites which were clearly sourced from crystallised mafic magmas, generally sourced from the mantle. These are rare, because it is difficult to turn basalt into granite via fractional crystallisation.

A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar mineralogy and geochemistry. These granites are formed by melting of the lower crust under conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.

Granitization

An old, and largely discounted theory, granitization states that granite is formed in place by extreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g. calcium to transform the metamorphic rock into a granite. This was supposed to occur across a migrating front. The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where leucosome and melanosome textures are present in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires high temperature, and also water or other volatiles which act as a catalyst by lowering the solidus temperature of the rock.

Ascent and emplacement

The ascent and emplacement of large volumes of granite within the upper continental crust is a source of much debate amongst geologists. There is a lack of field evidence for any proposed mechanisms, so hypotheses are predominantly based upon experimental data. There are two major hypotheses for the ascent of magma through the crust:

  • Stokes Diapir
  • Fracture Propagation

Of these two mechanisms, Stokes diapir was favoured for many years in the absence of a reasonable alternative. The basic idea is that magma will rise through the crust as a single mass through buoyancy. As it rises it heats the wall rocks, causing them to behave as a power-law fluid and thus flow around the pluton allowing it to pass rapidly and without major heat loss (Weinberg, 1994). This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, but runs into problems in the upper crust which is far colder and more brittle. Rocks there do not deform so easily: for magma to rise as a pluton it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust.

Nowadays fracture propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust. Magma rises instead in small channels along self-propagating dykes which form along new or pre-existing fault systems and networks of active shear zones (Clemens, 1998). As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.

Granitic magma must make room for itself or be intruded into other rocks in order to form an intrusion, and several mechanisms have been proposed to explain how large batholiths have been emplaced:

  • Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
  • Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
  • Inflation, where the granite body inflates under pressure and is injected into position

Most geologists today accept that a combination of these phenomena can be used to explain granite intrusions, and that not all granites can be explained entirely by one or another mechanism.

Natural Radiation

Granite is a natural source of radiation. Granite contains around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.

Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites.

Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils formed over or from particularly uraniferous granites can become a trap for radon gas, which is heavier than air.

However, in the majority of cases, although granite is a significant source of natural radiation as compared to other rocks it is not thought an acute health threat or significant risk factor. Various resources from national geological survey organisations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.

Uses

Antiquity

The Red Pyramid of Egypt (c.26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of Egyptian pyramids. Menkaure's Pyramid, likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c.2580 BC) contains a huge granite sarcophagus fashioned of "Red Aswan Granite." The mostly ruined Black Pyramid dating from the reign of Amenemhat III once had a polished granite pyramidion or capstone, now on display in the main hall of the Egyptian Museum in Cairo (see Dahshur). Other uses in Ancient Egypt, include columns, door lintels, sills, jambs, and wall and floor veneer. How the Egyptians worked the solid granite is still a matter of debate. Dr. Patrick Hunt has postulated that the Egyptians used emery shown to have higher hardness on the Mohs scale.

Many large Hindu temples in southern India, particularly those built by the 11th century king Rajaraja Chola I, were made of granite. There is a large amount of granite in these structures. They are comparable to the Great Pyramid of Giza.

Modern

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Because of its abundance granite was commonly used to build foundations for homes in New England. The Granite Railway, America's first railroad, was built to haul granite from the quarries in Quincy, Massachusetts, to the Neponset River in the 1820s. With increasing amounts of acid rain in parts of the world, granite has begun to supplant marble as a monument material, since it is much more durable. Polished granite is also a popular choice for kitchen countertops due to its high durability and aesthetic qualities.
Engineers have traditionally used polished granite surfaces to establish a plane of reference, since they are relatively impervious and inflexible. Sandblasted concrete with a heavy aggregate content has an appearance similar to rough granite, and is often used as a substitute when use of real granite is impractical. A most unusual use of granite was in the construction of the rails for the Haytor Granite Tramway, Devon, England, in 1820. Curling stones are traditionally fashioned of Ailsa Craig granite. The first stones were made in the 1750s, the original source being Ailsa Craig in Scotland. Because of the particular rarity of the granite, the best stones can cost as much as US$1,500. Between 60–70 percent of the stones used today are made from Ailsa Craig granite, although the island is now a wildlife reserve and is no longer used for quarrying.

Rock climbing

Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction. Well-known venues for granite climbing include Yosemite, the Bugaboos, the Mont Blanc massif (and peaks such as the Aiguille du Dru, the Aiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram, the Fitzroy Massif, Patagonia, Baffin Island, the Cornish coast and the Cairngorms.

Granite rock climbing is so popular that many of the artificial rock climbing walls found in gyms and theme parks are made to look and feel like granite. Most, however, are made from manufactured materials, given the fact that granite is generally too heavy for portable rock climbing walls, as well as the buildings in which stationary walls are located.

See also

References

External links

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