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Granite
Igneous rock
Granite containing plagioclase feldspar, potassium feldspar, quartz, biotite, and/or amphibole
Composition
ClassificationFelsic
Primarypotassium feldspar, plagioclase feldspar, and quartz
SecondaryDiffering amounts of muscovite, biotite, and hornblende-type amphiboles

Granite (/ˈɡræ.nɪt/, GRAN-it) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.

Granite is nearly always massive (lacking any internal structures), hard (falling between 6 and 7 on the Mohs hardness scale),[1] and tough. These properties have made granite a widespread construction stone throughout human history.

Description

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Granite with a moderate amount of plagioclase, alkali feldspar, and quartz
Granite with a composition of alkali feldspar and quartz

The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a completely crystalline rock.[2] Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Granitic rocks mainly consist of feldspar, quartz, mica, and amphibole minerals, which form an interlocking, somewhat equigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks. Petrographic examination is required for identification of specific types of granitoids.[3] The alkali feldspar in granites is typically orthoclase or microcline and is often perthitic. The plagioclase is typically sodium-rich oligoclase. Phenocrysts are usually alkali feldspar.[4]

Granitic rocks are classified according to the QAPF diagram for coarse grained plutonic rocks and are 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 between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of alkali feldspar. Granitic rocks poorer in quartz are classified as syenites or monzonites, while granitic rocks dominated by plagioclase are classified as granodiorites or tonalites. Granitic rocks with over 90% alkali feldspar are classified as alkali feldspar granites. Granitic rock with more than 60% quartz, which is uncommon, is classified simply as quartz-rich granitoid or, if composed almost entirely of quartz, as quartzolite.[5][6][7]

Granite in thin section, under cross-polarized light

True granites are further classified by the percentage of their total feldspar that is alkali feldspar. Granites whose feldspar is 65% to 90% alkali feldspar are syenogranites, while the feldspar in monzogranite is 35% to 65% alkali feldspar.[6][7] 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, as described below.[8][9]

Another aspect of granite classification is the ratios of metals that potentially form feldspars. Most granites have a composition such that almost all their aluminum and alkali metals (sodium and potassium) are combined as feldspar. This is the case when K2O + Na2O + CaO > Al2O3 > K2O + Na2O. Such granites are described as normal or metaluminous. Granites in which there is not enough aluminum to combine with all the alkali oxides as feldspar (Al2O3 < K2O + Na2O) are described as peralkaline, and they contain unusual sodium amphiboles such as riebeckite. Granites in which there is an excess of aluminum beyond what can be taken up in feldspars (Al2O3 > CaO + K2O + Na2O) are described as peraluminous, and they contain aluminum-rich minerals such as muscovite.[10]

Physical properties

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The average density of granite is between 2.65 and 2.75 g/cm3 (165 and 172 lb/cu ft),[11] its compressive strength usually lies above 200 MPa (29,000 psi), and its viscosity near STP is 3–6·1020 Pa·s.[12]

The melting temperature of dry granite at ambient pressure is 1215–1260 °C (2219–2300 °F);[13] it is strongly reduced in the presence of water, down to 650 °C at a few hundred megapascals of pressure.[14]

Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present.

Chemical composition

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QAPF diagram with the granite field in pink

A worldwide average of the chemical composition of granite, by mass percent, based on 2485 analyses:[15]

SiO2 72.04% (silica)
 
Al2O3 14.42% (alumina)
 
K2O 4.12%
 
Na2O 3.69%
 
CaO 1.82%
 
FeO 1.68%
 
Fe2O3 1.22%
 
MgO 0.71%
 
TiO2 0.30%
 
P2O5 0.12%
 
MnO 0.05%
 

The medium-grained equivalent of granite is microgranite.[16] The extrusive igneous rock equivalent of granite is rhyolite.[17]

Occurrence

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Granitic rock is widely distributed throughout the continental crust.[18] Much of it was intruded during the Precambrian age; it is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents. Outcrops of granite tend to form tors, domes or bornhardts, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. 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.[19]

Origin

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Mafic enclave in granite rock, at Yosemite National Park

Granite forms from silica-rich (felsic) magmas. Felsic magmas are thought to form by addition of heat or water vapor to rock of the lower crust, rather than by decompression of mantle rock, as is the case with basaltic magmas.[20] It has also been suggested that some granites found at convergent boundaries between tectonic plates, where oceanic crust subducts below continental crust, were formed from sediments subducted with the oceanic plate. The melted sediments would have produced magma intermediate in its silica content, which became further enriched in silica as it rose through the overlying crust.[21]

Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon.[22] Further fractionation reduces the content of iron, calcium, and titanium.[23] This is reflected in the high content of alkali feldspar and quartz in granite.

The presence of granitic rock in island arcs shows that fractional crystallization alone can convert a basaltic magma to a granitic magma, but the quantities produced are small.[24] For example, granitic rock makes up just 4% of the exposures in the South Sandwich Islands.[25] In continental arc settings, granitic rocks are the most common plutonic rocks, and batholiths composed of these rock types extend the entire length of the arc. There are no indication of magma chambers where basaltic magmas differentiate into granites, or of cumulates produced by mafic crystals settling out of the magma. Other processes must produce these great volumes of felsic magma. One such process is injection of basaltic magma into the lower crust, followed by differentiation, which leaves any cumulates in the mantle. Another is heating of the lower crust by underplating basaltic magma, which produces felsic magma directly from crustal rock. The two processes produce different kinds of granites, which may be reflected in the division between S-type (produced by underplating) and I-type (produced by injection and differentiation) granites, discussed below.[24]

Alphabet classification system

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Mineral assemblage of igneous rocks

The composition and origin of any magma that differentiates into granite leave certain petrological evidence as to what the granite's parental rock was. The final texture and composition of a granite are generally distinctive as to its parental rock. For instance, a granite that is derived from partial melting of metasedimentary rocks may have more alkali feldspar, whereas a granite derived from partial melting of metaigneous rocks may be richer in plagioclase. It is on this basis that the modern "alphabet" classification schemes are based.

The letter-based Chappell & White classification system was proposed initially to divide granites into I-type (igneous source) granite and S-type (sedimentary sources).[26] Both types are produced by partial melting of crustal rocks, either metaigneous rocks or metasedimentary rocks.

I-type granites are characterized by a high content of sodium and calcium, and by a strontium isotope ratio, 87Sr/86Sr, of less than 0.708. 87Sr is produced by radioactive decay of 87Rb, and since rubidium is concentrated in the crust relative to the mantle, a low ratio suggests origin in the mantle. The elevated sodium and calcium favor crystallization of hornblende rather than biotite. I-type granites are known for their porphyry copper deposits.[24] I-type granites are orogenic (associated with mountain building) and usually metaluminous.[27]

S-type granites are sodium-poor and aluminum-rich. As a result, they contain micas such as biotite and muscovite instead of hornblende. Their strontium isotope ratio is typically greater than 0.708, suggesting a crustal origin. They also commonly contain xenoliths of metamorphosed sedimentary rock, and host tin ores. Their magmas are water-rich, and they readily solidify as the water outgasses from the magma at lower pressure, so they less commonly make it to the surface than magmas of I-type granites, which are thus more common as volcanic rock (rhyolite).[24] They are also orogenic but range from metaluminous to strongly peraluminous.[27]

Although both I- and S-type granites are orogenic, I-type granites are more common close to the convergent boundary than S-type. This is attributed to thicker crust further from the boundary, which results in more crustal melting.[24]

A-type granites show a peculiar mineralogy and geochemistry, with particularly high silicon and potassium at the expense of calcium and magnesium[28] and a high content of high field strength cations (cations with a small radius and high electrical charge, such as zirconium, niobium, tantalum, and rare earth elements.)[29] They are not orogenic, forming instead over hot spots and continental rifting, and are metaluminous to mildly peralkaline and iron-rich.[27] These granites are produced by partial melting of refractory lithology such as granulites in the lower continental crust at high thermal gradients. This leads to significant extraction of hydrous felsic melts from granulite-facies resitites.[30][31] A-type granites occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica.[32] The rhyolites of the Yellowstone Caldera are examples of volcanic equivalents of A-type granite.[33]

M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle.[34] Although the fractional crystallisation of basaltic melts can yield small amounts of granites, which are sometimes found in island arcs,[35] such granites must occur together with large amounts of basaltic rocks.[24]

H-type granites were suggested for hybrid granites, which were hypothesized to form by mixing between mafic and felsic from different sources, such as M-type and S-type.[36] However, the big difference in rheology between mafic and felsic magmas makes this process problematic in nature.[37]

Granitization

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Migmatite featuring felsic minerals, at Morton Gneiss Complex

Granitization is an old, and largely discounted, hypothesis that granite is formed in place through extreme metasomatism. The idea behind granitization was that fluids would supposedly bring in elements such as potassium, and remove others, such as calcium, to transform a metamorphic rock into granite. This was supposed to occur across a migrating front. However, experimental work had established by the 1960s that granites were of igneous origin.[38] The mineralogical and chemical features of granite can be explained only by crystal-liquid phase relations, showing that there must have been at least enough melting to mobilize the magma.[39]

However, at sufficiently deep crustal levels, the distinction between metamorphism and crustal melting itself becomes vague. Conditions for crystallization of liquid magma are close enough to those of high-grade metamorphism that the rocks often bear a close resemblance.[40] Under these conditions, granitic melts can be produced in place through the partial melting of metamorphic rocks by extracting melt-mobile elements such as potassium and silicon into the melts but leaving others such as calcium and iron in granulite residues. This may be the origin of migmatites. A migmatite consists of dark, refractory rock (the melanosome) that is permeated by sheets and channels of light granitic rock (the leucosome). The leucosome is interpreted as partial melt of a parent rock that has begun to separate from the remaining solid residue (the melanosome).[41] If enough partial melt is produced, it will separate from the source rock, become more highly evolved through fractional crystallization during its ascent toward the surface, and become the magmatic parent of granitic rock. The residue of the source rock becomes a granulite.

The partial melting of solid rocks requires high temperatures and the addition of water or other volatiles which lower the solidus temperature (temperature at which partial melting commences) of these rocks. It was long debated whether crustal thickening in orogens (mountain belts along convergent boundaries) was sufficient to produce granite melts by radiogenic heating, but recent work suggests that this is not a viable mechanism.[42] In-situ granitization requires heating by the asthenospheric mantle or by underplating with mantle-derived magmas.[43]

Ascent and emplacement

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Granite magmas have a density of 2.4 Mg/m3, much less than the 2.8 Mg/m3 of high-grade metamorphic rock. This gives them tremendous buoyancy, so that ascent of the magma is inevitable once enough magma has accumulated. However, the question of precisely how such large quantities of magma are able to shove aside country rock to make room for themselves (the room problem) is still a matter of research.[44]

Two main mechanisms are thought to be important:

Schematic diagram illustrating the ascent and emplacement of magmas

Of these two mechanisms, Stokes diapirism has been 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 intrusion allowing it to pass without major heat loss.[45] 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 diapir it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust.

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 fracture or fault systems and networks of active shear zones.[46] As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.

These mechanisms can operate in tandem. For example, diapirs may continue to rise through the brittle upper crust through stoping, where the granite cracks the roof rocks, removing blocks of the overlying crust which then sink to the bottom of the diapir while the magma rises to take their place. This can occur as piecemeal stopping (stoping of small blocks of chamber roof), as cauldron subsidence (collapse of large blocks of chamber roof), or as roof foundering (complete collapse of the roof of a shallow magma chamber accompanied by a caldera eruption.) There is evidence for cauldron subsidence at the Mt. Ascutney intrusion in eastern Vermont.[47] Evidence for piecemeal stoping is found in intrusions that are rimmed with igneous breccia containing fragments of country rock.[44]

Assimilation is another mechanism of ascent, where the granite melts its way up into the crust and removes overlying material in this way. This is limited by the amount of thermal energy available, which must be replenished by crystallization of higher-melting minerals in the magma. Thus, the magma is melting crustal rock at its roof while simultaneously crystallizing at its base. This results in steady contamination with crustal material as the magma rises. This may not be evident in the major and minor element chemistry, since the minerals most likely to crystallize at the base of the chamber are the same ones that would crystallize anyway, but crustal assimilation is detectable in isotope ratios.[48] Heat loss to the country rock means that ascent by assimilation is limited to distance similar to the height of the magma chamber.[49]

Weathering

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Further information: Weathering
Grus sand and granitoid from which it derived

Physical weathering occurs on a large scale in the form of exfoliation joints, which are the result of granite's expanding and fracturing as pressure is relieved when overlying material is removed by erosion or other processes.

Chemical weathering of granite occurs when dilute carbonic acid, and other acids present in rain and soil waters, alter feldspar in a process called hydrolysis.[50][51] As demonstrated in the following reaction, this causes potassium feldspar to form kaolinite, with potassium ions, bicarbonate, and silica in solution as byproducts. An end product of granite weathering is grus, which is often made up of coarse-grained fragments of disintegrated granite.

2 KAlSi3O8 + 2 H2CO3 + 9 H2O → Al2Si2O5(OH)4 + 4 H4SiO4 + 2 K+ + 2 HCO3−

Climatic variations also influence the weathering rate of granites. For about two thousand years, the relief engravings on Cleopatra's Needle obelisk had survived the arid conditions of its origin before its transfer to London. Within two hundred years, the red granite has drastically deteriorated in the damp and polluted air there.[52]

Soil development on granite reflects the rock's high quartz content and dearth of available bases, with the base-poor status predisposing the soil to acidification and podzolization in cool humid climates as the weather-resistant quartz yields much sand.[53] Feldspars also weather slowly in cool climes, allowing sand to dominate the fine-earth fraction. In warm humid regions, the weathering of feldspar as described above is accelerated so as to allow a much higher proportion of clay with the Cecil soil series a prime example of the consequent Ultisol great soil group.[54]

Fires can also contribute to the weathering of granite. The high temperatures reached during a fire—often exceeding 1000 °C—can cause significant physical and chemical processes that alter the rock. Among the physical processes, the differential thermal expansion of individual mineral grains, the anisotropic expansion of certain minerals, and polymorphic transformations, such as the alpha-beta quartz transition, induce substantial volume changes and generate internal stresses that damage the granite.[55][56] Additionally, the decomposition of certain granite constituents, such as phyllosilicates, at specific temperatures further contributes to granite degradation. As a result, granite becomes micro-fractured, its total porosity increases, and its mechanical strength is significantly reduced.[55][56][57]

Natural radiation

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Pegmatitic granite composed of orthoclase and quartz

Granite is a natural source of radiation, like most natural stones. Potassium-40 is a radioactive isotope of weak emission, and a constituent of alkali feldspar, which in turn is a common component of granitic rocks, more abundant in alkali feldspar granite and syenites. Some granites contain around 10 to 20 parts per million (ppm) of uranium. By contrast, more mafic rocks, such as tonalite, gabbro and diorite, have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts.

Many large granite plutons are 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.

Cellars and basements built into soils over granite can become a trap for radon gas,[58] which is formed by the decay of uranium.[59] Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking.[60]

Thorium occurs in all granites.[61] Conway granite has been noted for its relatively high thorium concentration of 56±6 ppm.[62]

There is some concern that some granite sold as countertops or building material may be hazardous to health.[63] Dan Steck of St. Johns University has stated[64] that approximately 5% of all granite is of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slab types have been tested. Resources from national geological survey organizations 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.

A study of granite countertops was done (initiated and paid for by the Marble Institute of America) in November 2008 by National Health and Engineering Inc. of USA. In this test, all of the 39 full-size granite slabs that were measured for the study showed radiation levels well below the European Union safety standards (section 4.1.1.1 of the National Health and Engineering study) and radon emission levels well below the average outdoor radon concentrations in the US.[65]

Industry and uses

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Granite and related marble industries are considered one of the oldest industries in the world, existing as far back as Ancient Egypt.[66] Major modern exporters of granite include China, India, Italy, Brazil, Canada, Germany, Sweden, Spain and the United States.[67][68]

Antiquity

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Cleopatra's Needle, London

The Red Pyramid of Egypt (c. 2590 BC), named for the light crimson hue of its exposed limestone surfaces, is the third largest of Egyptian pyramids. Pyramid of Menkaure, likely dating 2510 BC, was constructed of limestone and granite blocks. The Great Pyramid of Giza (c. 2580 BC) contains a 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, which is 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.[69] How the Egyptians worked the solid granite is still a matter of debate. Tool marks described by the Egyptologist Anna Serotta indicate the use of flint tools on finer work with harder stones, e.g. when producing the hieroglyphic inscriptions.[70] Patrick Hunt[71] has postulated that the Egyptians used emery, which has greater hardness.

The Seokguram Grotto in Korea is a Buddhist shrine and part of the Bulguksa temple complex. Completed in 774 AD, it is an artificial grotto constructed entirely of granite. The main Buddha of the grotto is a highly regarded piece of Buddhist art,[72] and along with the temple complex to which it belongs, Seokguram was added to the UNESCO World Heritage List in 1995.[73]

Rajaraja Chola I of the Chola Dynasty in South India built the world's first temple entirely of granite in the 11th century AD in Tanjore, India. The Brihadeeswarar Temple dedicated to Lord Shiva was built in 1010. The massive Gopuram (ornate, upper section of shrine) is believed to have a mass of around 81 tonnes. It was the tallest temple in south India.[74]

Imperial Roman granite was quarried mainly in Egypt, and also in Turkey, and on the islands of Elba and Giglio. Granite became "an integral part of the Roman language of monumental architecture".[75] The quarrying ceased around the third century AD. Beginning in Late Antiquity the granite was reused, which since at least the early 16th century became known as spolia. Through the process of case-hardening, granite becomes harder with age. The technology required to make tempered metal chisels was largely forgotten during the Middle Ages. As a result, Medieval stoneworkers were forced to use saws or emery to shorten ancient columns or hack them into discs. Giorgio Vasari noted in the 16th century that granite in quarries was "far softer and easier to work than after it has lain exposed" while ancient columns, because of their "hardness and solidity have nothing to fear from fire or sword, and time itself, that drives everything to ruin, not only has not destroyed them but has not even altered their colour."[75]

Modern

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Sculpture and memorials

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The graves of Emperor Pedro I of Brazil (also King of Portugal as Pedro IV) and his two wives Maria Leopoldina (not pictured, facing his grave) and Amélie (left), in the Monument to the Independence of Brazil, are made of green granite. The walls as well as the floor are clad with the same material.[76]

In some areas, granite is used for gravestones and memorials. Granite is a hard stone and requires skill to carve by hand. Until the early 18th century, in the Western world, granite could be carved only by hand tools with generally poor results.

A key breakthrough was the invention of steam-powered cutting and dressing tools by Alexander MacDonald of Aberdeen, inspired by seeing ancient Egyptian granite carvings. In 1832, the first polished tombstone of Aberdeen granite to be erected in an English cemetery was installed at Kensal Green Cemetery. It caused a sensation in the London monumental trade and for some years all polished granite ordered came from MacDonald's.[77] As a result of the work of sculptor William Leslie, and later Sidney Field, granite memorials became a major status symbol in Victorian Britain. The royal sarcophagus at Frogmore was probably the pinnacle of its work, and at 30 tons one of the largest. It was not until the 1880s that rival machinery and works could compete with the MacDonald works.

Modern methods of carving include using computer-controlled rotary bits and sandblasting over a rubber stencil. Leaving the letters, numbers, and emblems exposed and the remainder of the stone covered with rubber, the blaster can create virtually any kind of artwork or epitaph.

The stone known as "black granite" is usually gabbro, which has a completely different chemical composition.[78]

Buildings

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The granite castle of Aulanko in Hämeenlinna, Finland

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. Aberdeen in Scotland, which is constructed principally from local granite, is known as "The Granite City". Because of its abundance in New England, granite was commonly used to build foundations for homes there. 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.[79]

Engineering

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Engineers have traditionally used polished granite surface plates to establish a plane of reference, since they are relatively impervious, inflexible, and maintain good dimensional stability. 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. Granite tables are used extensively as bases or even as the entire structural body of optical instruments, CMMs, and very high precision CNC machines because of granite's rigidity, high dimensional stability, and excellent vibration characteristics. A most unusual use of granite was as the material of the tracks of the Haytor Granite Tramway, Devon, England, in 1820.[80] Granite block is usually processed into slabs, which can be cut and shaped by a cutting center.[81] In military engineering, Finland planted granite boulders along its Mannerheim Line to block invasion by Russian tanks in the Winter War of 1939–40.[82]

Paving

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Granite is used as a pavement material. This is because it is extremely durable, permeable and requires little maintenance. For example, in Sydney, Australia black granite stone is used for the paving and kerbs throughout the Central Business District.[83]Granite can be crushed using industrial equipment such as VSI or cone crusher.[84][85]

Curling stones

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Curling stones

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 rarity of this granite, the best stones can cost as much as US$1,500. Between 60 and 70 percent of the stones used today are made from Ailsa Craig granite. Although the island is now a wildlife reserve, it is still quarried under license for Ailsa granite by Kays of Scotland for curling stones.[86]

Countertops

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In the United States, granite is a popular choice for countertops due to its affordability, aesthetic appeal, and convenience.

Rock climbing

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Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction.[87] Well-known venues for granite climbing include the Yosemite Valley, the Bugaboos, the Mont Blanc massif (and peaks such as the Aiguille du Dru, the Mourne Mountains, the Adamello-Presanella Alps, the Aiguille du Midi and the Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram (especially the Trango Towers), the Fitzroy Massif and the Paine Massif in Patagonia, Baffin Island, Ogawayama, the Cornish coast, the Cairngorms, Sugarloaf Mountain in Rio de Janeiro, Brazil, and the Stawamus Chief, British Columbia, Canada.

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See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Granite

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Granite is a coarse-grained, intrusive igneous rock formed by the slow crystallization of silica-rich magma deep within the Earth's crust, primarily composed of quartz, alkali feldspar, plagioclase feldspar, and minor amounts of biotite or muscovite mica. It typically exhibits a light-colored, speckled appearance due to its visible interlocking crystals and is a major component of the continental crust, often occurring in large plutons and batholiths. With a high silica content exceeding 66 weight percent, granite is classified as a felsic rock, distinguishing it from more mafic igneous varieties like basalt. The formation of granite involves partial melting of crustal rocks, magma mixing, and fractional crystallization under pressures of 1-10 kbar (typically 2-6 kbar) and temperature (700-950°C), commonly linked to tectonic settings such as subduction zones, continental collisions, or extensional rifts. Mineralogically, it features 20-60% quartz, 60-80% total feldspars (including 35-65% alkali feldspar such as orthoclase, microcline, and perthitic varieties, and 10-40% plagioclase), and 5-15% micas, with accessory minerals like hornblende, amphibole, or zircon depending on the granite type (e.g., I-type from igneous sources or S-type from sedimentary). This slow cooling process underground allows for the development of its phaneritic texture, where crystals are large enough to identify without magnification. Granite's durability, low water absorption, and resistance to weathering and acidic solutions make it a preferred material for construction, including building facades, paving, curbstones, and monuments, as seen in sites like Mount Rushmore. Its color ranges from pink and red (due to alkali feldspars) to gray or white, influenced by mineral variations and iron content, while potential issues like spalling or efflorescence can arise from moisture and freeze-thaw cycles in exposed settings. Geologically, granite provides key insights into crustal evolution and mineralization, often hosting economic deposits of copper, tin, and tungsten.

Definition and Composition

Mineralogical Composition

Granite is primarily composed of felsic minerals, with quartz and feldspars forming the essential components that define its light-colored, coarse-grained appearance. Quartz typically constitutes 20-60% of the rock by volume, appearing as anhedral to subhedral crystals that contribute to the rock's hardness and resistance to weathering. Alkali feldspar, including orthoclase, microcline, and perthitic intergrowths, makes up 35-90% of the feldspar content, often dominating as large, euhedral crystals that impart a pinkish or reddish hue to many granites. Plagioclase feldspar, usually oligoclase or andesine, is present in proportions up to 50%, comprising 10-65% of the total feldspar content, and occurs as blocky crystals that add subtle variations in color and cleavage. Accessory minerals are present in minor amounts, typically less than 5-15% combined, and include mafic silicates such as biotite and muscovite micas, which provide darker flecks and schistose tendencies in some varieties, as well as hornblende in amphibole-bearing types. Other common accessories are opaque oxides like magnetite, and trace silicates including apatite, zircon, and occasionally garnet, which occur as small, disseminated grains and influence the rock's trace element geochemistry without altering its overall felsic character. Variations in mineral proportions lead to distinct granite subtypes. Alkali granites are enriched in quartz (often >40%) and alkali feldspar (>60% of the feldspar), with minimal plagioclase, resulting in a highly siliceous, peralkaline composition. Syenogranites feature lower quartz content (20-30%) and predominant alkali feldspar, sometimes with muscovite as a key accessory, emphasizing peraluminous traits. Monzogranites exhibit a more balanced mix, with quartz around 30%, roughly equal alkali and plagioclase feldspars, and biotite or hornblende as common mafics, representing the most typical granite variety. Certain textural intergrowths highlight mineral interactions in granite. Graphic granite displays skeletal quartz crystals intergrown with alkali feldspar in a eutectic-like pattern, often on a millimeter scale, reflecting late-stage magmatic crystallization. Myrmekite consists of vermicular quartz rods embedded in sodic plagioclase, typically forming at boundaries with alkali feldspar, and indicates metasomatic or subsolidus replacement processes. These features underscore the felsic mineral dominance and phaneritic texture of granite, which collectively determine its durability in geological and engineering contexts.

Chemical Composition

Granite exhibits a felsic bulk chemical composition dominated by silica, with SiO₂ comprising 65-75 wt.% of the rock, reflecting its derivation from silica-rich magmas. Aluminum oxide (Al₂O₃) typically ranges from 14-18 wt.%, while alkali oxides are prominent, including 4-7 wt.% K₂O and 3-5 wt.% Na₂O, which contribute to the peraluminous to metaluminous nature of most granites. Lesser components include 1-4 wt.% CaO, 2-5 wt.% total iron as FeO and Fe₂O₃, <2 wt.% MgO, and <1 wt.% TiO₂, with these variations influencing the rock's geochemical affinity. Trace element patterns in granite are distinctive, featuring elevated concentrations of large-ion lithophile elements such as Rb (often >100 ppm), Ba (300-1000 ppm), and Sr (100-500 ppm), which are incompatible during fractional crystallization. In contrast, granite is depleted in heavy metals and compatible elements like Ni (<20 ppm) and Cr (<20 ppm), due to the exclusion of mafic phases from the source or melt evolution. The International Union of Geological Sciences (IUGS) classifies granite within the QAP diagram for plutonic rocks, where naming relies on modal percentages of quartz (Q: 20-60%), alkali feldspar (A: 20-60%), and plagioclase (P: 5-60%), often estimated from chemical analyses via normative calculations. Geochemically, granites fall into series such as calc-alkaline, characterized by moderate FeO/(FeO+Fe₂O₃) ratios and K₂O enrichment with increasing SiO₂, or alkaline, with higher total alkalis (Na₂O + K₂O >8 wt.%) and steeper rare earth element patterns. Further distinction includes I-type granites, derived from igneous (mafic to intermediate) crustal sources and exhibiting metaluminous compositions with hornblende, versus S-type granites from sedimentary protoliths, which are peraluminous with higher P₂O₅ and lower Na₂O. Strontium isotopic ratios provide evidence of source materials, with initial ⁸⁷Sr/⁸⁶Sr values exceeding 0.706 in most granites, indicating significant involvement of evolved continental crust rather than primitive mantle (which has ratios ~0.702-0.704). I-type granites typically show ratios of 0.704-0.706, while S-type often exceed 0.710, reflecting radiogenic enrichment from older crustal Rb/Sr ratios.

Physical and Optical Properties

Texture and Structure

Granite exhibits a phaneritic texture, defined by its coarse-grained nature where individual mineral crystals are visible to the naked eye, typically ranging from 1 to 5 mm in size for medium-grained varieties and up to 5-15 mm in coarser examples. This texture arises from slow cooling at depth, allowing sufficient time for crystal growth. Granites may display equigranular fabrics, with uniformly sized grains, or porphyritic variants featuring larger phenocrysts (up to several millimeters) set in a finer matrix of the same minerals. The overall fabric of granite is predominantly massive, lacking aligned mineral orientations, though deformed granites can develop foliated or gneissic structures where minerals align into banded patterns due to tectonic stress. At the microscopic scale, key microstructures include perthitic exsolution in alkali feldspars, manifesting as irregular lamellae of sodic feldspar (albite) within potassic feldspar (orthoclase or microcline), formed through subsolidus unmixing during cooling below the solvus temperature. Myrmekitic intergrowths appear as vermicular, worm-like quartz rods embedded in plagioclase, often along grain boundaries with K-feldspar, resulting from metasomatic reactions involving sodium and calcium exchange. Color variations in granite stem from the relative proportions of its minerals; pink tones predominate in varieties rich in K-feldspar, while gray shades arise from higher plagioclase content, with quartz contributing neutral white to gray hues. Diagnostic features include the absence of vesicles, amygdules, or flow banding, which distinguishes granite from volcanic equivalents like rhyolite, as its plutonic origin precludes rapid surface cooling and gas entrapment.

Mechanical and Thermal Properties

Granite exhibits a density typically ranging from 2.65 to 2.75 g/cm³, though values can vary slightly based on mineral composition and porosity, with averages around 2.7 g/cm³ reported for many continental granites. This density contributes to its role as a stable crustal material, influencing gravitational loading in geological settings. The mineral components, particularly quartz and feldspars, impart a Mohs hardness of 6 to 7, making granite resistant to scratching and abrasion under normal conditions. In terms of strength, granite demonstrates high compressive strength, generally between 100 and 250 MPa, which reflects its ability to withstand significant overburden pressures in plutonic environments. Conversely, its tensile strength is much lower, ranging from 7 to 25 MPa, highlighting a common anisotropy in rock behavior where failure often initiates under tension. Elastic properties include a Young's modulus of approximately 50 to 70 GPa and a Poisson's ratio of 0.25 to 0.3, indicating moderate stiffness and lateral strain response under uniaxial stress. Thermally, granite has a low coefficient of linear thermal expansion, around 8 to 10 × 10⁻⁶ /°C, which minimizes dimensional changes during temperature fluctuations in shallow crustal depths. Its thermal conductivity is moderate, typically 1.5 to 3 W/m·K, facilitating heat dissipation in geothermal contexts without excessive gradients. Porosity is generally low, less than 1%, contributing to its impermeability to fluids and enhancing structural integrity against fluid-induced weakening.
PropertyTypical Range/ValueKey Influence
Density2.65–2.75 g/cm³Affects gravitational stability
Mohs Hardness6–7Resistance to mechanical wear
Compressive Strength100–250 MPaWithstands lithostatic pressure
Tensile Strength7–25 MPaLimits under tensile loading
Young's Modulus50–70 GPaMeasures elastic deformation
Poisson's Ratio0.25–0.3Indicates lateral strain response
Thermal Expansion Coefficient8–10 × 10⁻⁶ /°CControls thermal stress buildup
Thermal Conductivity1.5–3 W/m·KGoverns heat flow in rock masses
Porosity<1%Enhances fluid impermeability

Optical Properties

Granite is opaque to visible light due to its crystalline structure and mineral composition. Its luster ranges from dull to sub-vitreous, often appearing grainy with pearly or vitreous sheen in polished surfaces or mica-rich areas. The streak of granite is white.

Formation Processes

Magmatic Crystallization

Granite primarily forms through the slow crystallization of felsic magma within the Earth's crust, a process dominated by igneous differentiation mechanisms that evolve mantle-derived melts toward silica-rich compositions. This magmatic crystallization occurs in continental settings, often associated with subduction zones where hydrous fluids from the subducting slab trigger partial melting of the overlying mantle wedge, producing initial basaltic magmas that undergo further modification. In hotspot environments, such as beneath oceanic islands or continental rifts, upwelling mantle plumes can similarly generate basaltic melts that interact with the crust to yield granitic magmas. A key mechanism in this evolution is fractional crystallization, where early-formed mafic minerals like olivine and pyroxene settle out or are removed from the cooling magma, progressively enriching the residual melt in silica, alkali metals, and incompatible elements to achieve the felsic composition typical of granite (over 70% SiO₂). This process is particularly effective in subduction-related arc magmas, where repeated cycles of crystallization and melt extraction concentrate quartz and feldspar components. Complementary to this, magma mixing plays a crucial role, as hotter basaltic melts from the mantle intrude and hybridize with cooler, silica-rich crustal partial melts, resulting in intermediate to felsic hybrids that crystallize into granite. Such interactions are evidenced by mafic enclaves and disequilibrium textures in granitic rocks, indicating rapid blending under convective conditions in magma chambers. These hybrid compositions often exhibit trace element patterns intermediate between mantle and crustal sources. The crystallization of granite occurs under plutonic conditions at depths of 5-10 km, where slow cooling rates—spanning 10⁵ to 10⁶ years—facilitate the growth of large, interlocking crystals (phaneritic texture) due to prolonged diffusion of ions in the viscous melt. Phase equilibria dictate that granite minimum melts, saturated with water, reach eutectic points at temperatures of 650-700°C under pressures of 200-300 MPa, allowing quartz, plagioclase, and alkali feldspar to co-precipitate as the primary mineral assemblage. These conditions align with the Qz-Ab-Or ternary phase diagram, where the cotectic curve defines the low-temperature boundary for felsic melt stability. Geological evidence for these processes includes compositional zoning within plutons, where inner zones are more evolved (higher silica) due to prolonged fractional crystallization, contrasting with marginal zones influenced by wall-rock assimilation. Additionally, contact metamorphism aureoles surrounding granite intrusions reveal thermal gradients from 500-700°C near the contacts, confirming the heat budget and slow cooling of the crystallizing magma body.

Granitization and Metamorphism

Granitization refers to the transformation of pre-existing crustal rocks into granite-like compositions through solid-state processes, including partial melting and metasomatism, often occurring in situ without significant magmatic intrusion from deeper sources. This process typically involves the influx of fluids that facilitate dehydration and melting of metamorphic rocks, leading to the formation of migmatites—hybrid rocks exhibiting both metamorphic and igneous textures. Migmatites serve as transitional stages, where leucosomes (melt-rich layers) segregate from paleosomes (residual metamorphic components), ultimately contributing to granite protoliths. These transformations occur under high metamorphic grades, primarily in the amphibolite to granulite facies, where temperatures exceed 650°C to initiate anatexis (partial melting). In amphibolite facies conditions (approximately 500–700°C), fluid-present melting promotes initial melt generation, while granulite facies (>700°C) involves fluid-absent dehydration melting, yielding higher melt fractions up to 70 vol%. Fluid influx lowers the melting point, enabling potassium enrichment through metasomatic alteration, which replaces sodium-rich minerals with K-feldspar and mica. Debated models distinguish anatectic granites, formed dominantly by melting of crustal sources, from metasomatic variants, where element mobility via fluids drives compositional changes toward granitic norms. Evidence for granitization includes the preservation of relic metamorphic fabrics, such as foliation and schlieren in otherwise granitic textures, indicating incomplete homogenization during melting. Isotopic disequilibrium, observed in zircon and monazite populations with varying U-Pb ages and oxygen isotope ratios, further supports rapid, non-equilibrium processes rather than prolonged magmatic equilibration. Historically, H.H. Read's 1940s theory emphasized granitization as a pervasive replacement mechanism to resolve the "room problem" of granite emplacement, challenging earlier igneous models. However, modern plate tectonic frameworks largely favor magmatic origins for most granites, relegating granitization to hybrid roles in continental collision zones, particularly for S-type granites exhibiting peraluminous signatures from sedimentary precursors.

Geological Occurrence and Distribution

Global Settings

Granite dominates the composition of the continental crust, with felsic, granitic rocks accounting for approximately 85% of the upper crust by volume. This prevalence reflects the role of granite as the primary lithology in forming and stabilizing continental masses over geological time. In orogenic belts associated with convergent plate margins, granite manifests as extensive batholiths, particularly in settings like the Cordilleran orogeny, exemplified by the Sierra Nevada batholith in California, which spans over 30,000 km². Similar large-scale intrusions occur in the Variscan orogeny across Europe, where granitic plutons form elongated belts along ancient collision zones, and in the Himalayan orogeny, where syn-tectonic granites intrude the thickened crust. These structures highlight granite's association with subduction-related magmatism and continental collision. Anorogenic granites, by contrast, develop in intraplate environments within stable cratons, away from active plate boundaries. Notable examples include rapakivi granites in Precambrian shields, such as those in the Fennoscandian Shield of Finland and Russia or the Amazonian Craton in South America, characterized by their massive, subhorizontal plutons emplaced during periods of tectonic quiescence. The global distribution of granite exhibits a broad age spectrum, ranging from Archean greenstone belts in regions like the Pilbara Craton in Australia, where ancient tonalitic-granodioritic gneisses date back over 3.5 billion years, to Cenozoic arcs in active subduction zones such as the Andes. This temporal range underscores granite's persistent formation across Earth's history. Significant estimates for individual plutonic complexes illustrate the scale of granitic magmatism; for instance, the Idaho Batholith in the northern Rocky Mountains is primarily composed of Cretaceous granodiorites and granites.

Notable Deposits and Formations

One of the classic granite localities is Dartmoor in southwest England, where late Carboniferous (approximately 280-310 million years old) granitic intrusions form prominent tors—isolated, castellated outcrops shaped by subaerial weathering and periglacial processes during the Pleistocene. These formations are closely associated with historical tin mineralization, as cassiterite veins occur within the granite and surrounding killas (metasedimentary rocks), supporting extensive mining from prehistoric times through the 20th century. In the United States, the granitic rocks of Yosemite National Park in California exemplify Cretaceous-age (roughly 80-120 million years old) intrusions from the Sierra Nevada batholith, featuring dramatic glacial polish that creates smooth, reflective surfaces on domes and cliffs like those at Half Dome and El Capitan. This polish results from abrasion by Pleistocene glaciers, highlighting the interplay between magmatic emplacement and subsequent ice-age erosion in shaping iconic landforms. Economically significant deposits include the Barre granite in Vermont, USA, a fine-grained, light gray variety quarried since the 1830s from the Barre pluton (late Devonian age, approximately 330-380 million years old), renowned for its uniformity and use in monumental dimension stone. The Rock of Ages E.L. Smith Quarry there is the world's largest deep-hole dimension granite operation, reaching depths of over 570 feet and producing hundreds of thousands of cubic feet annually. Another major economic feature is the Peninsular Ranges batholith, spanning southern California and Baja California, Mexico, a vast Mesozoic (primarily 130-80 million years old) composite of tonalite, granodiorite, and granite that forms the backbone of the region's mountains and supports aggregate and dimension stone extraction. Unique granite types include the A-type rapakivi granites of Finland, such as those in the Wiborg batholith (Proterozoic, dated to 1640-1615 million years old), characterized by rounded orthoclase phenocrysts mantled by plagioclase, formed in anorogenic settings with associated anorthosite and metaluminous compositions indicative of mantle-derived magmas. In Nigeria, alkaline granites of the Younger Granite Province (Mesozoic, 215-140 million years old) occur in ring complexes like those around Jos, featuring peralkaline to metaluminous varieties with riebeckite and arfvedsonite, linked to extensional tectonics and enriched in rare metals such as tin and niobium. Granite-associated pegmatites yield valuable gems, notably in western Maine, USA, where Devonian-age (around 400 million years old) granitic pegmatites at sites like Mount Mica and the Dunton Quarry host exceptional tourmaline crystals, including gem-quality elbaite varieties up to several carats, formed through late-stage magmatic differentiation. For conservation, granite formations like the tor landscapes in Belitong UNESCO Global Geopark, Indonesia, showcase weathered granite outcrops from Cretaceous intrusions (approximately 100 million years old), protected for their geodiversity and contrast with basaltic sites such as Giant's Causeway by emphasizing subaerial erosion over volcanic columnar jointing.

Geological Evolution

Ascent and Emplacement Mechanisms

Granite magma ascent is primarily driven by buoyancy, where less dense felsic melts rise through denser surrounding crustal material. In the ductile lower crust, diapirism allows for the upward migration of partially molten granite bodies, facilitated by the power-law rheology of the surrounding rocks that permits flow under differential stress. However, as magma approaches shallower crustal levels, diapirism becomes less viable due to increasing viscosity contrasts and brittle behavior, transitioning to fracture propagation mechanisms such as dyke ascent, where magma exploits tensile fractures to propagate vertically. This dyke propagation is particularly effective for crystal-poor granitic melts, enabling rapid transport over tens of kilometers. Emplacement of granite magma occurs through diverse styles adapted to local crustal conditions, including the formation of laccoliths, where buoyant magma intrudes as concordant sheets that dome overlying strata, as observed in the Gangotri granite of the Himalayas. Sheeted complexes represent another style, involving multiple subhorizontal intrusions that stack to form tabular plutons, exemplified by the Dinkey Creek pluton in the Sierra Nevada. Stoping, or the assimilation of discrete blocks of country rock, also plays a key role, particularly in the upper crust, where thermal weakening allows magma to engulf and partially dissolve wall-rock fragments, creating space for further intrusion. Structural features exert significant control on granite emplacement, with magma often exploiting pre-existing weaknesses such as fault zones and shear bands. In transtensional settings, strike-slip faults and dilational jogs provide pathways for intrusion, as seen in the Los Pedroches batholith where shear zones channeled magma flow. Shear bands within ductile crust facilitate focused ascent by localizing strain and dilation, enabling magma to migrate along planes of weakness during regional deformation. The timescales of granite magma ascent and emplacement are relatively rapid, typically spanning 10³ to 10⁵ years, allowing for efficient transport before significant cooling. Viscous flow models, incorporating melt viscosities of 10⁴ to 10⁶ Pa·s, demonstrate that buoyancy-driven ascent in dykes can achieve velocities of meters per year under moderate strain rates of ~10⁻¹⁴ s⁻¹. These short durations are consistent with the incremental assembly of plutons from multiple magma pulses. Evidence for these processes includes mafic microgranular enclaves (MMEs) that provide evidence of magma mixing, where mafic magma injections interact mechanically with the felsic host magma, preserving textural disequilibrium indicative of mechanical interaction. Thermal metamorphic halos, or contact aureoles, surrounding plutons further attest to the heat budget of emplacement, with narrow zones of recrystallization in wall rocks signaling rapid intrusion and limited conductive cooling. Such features underscore the dynamic interplay between magma ascent and host rock response.

Weathering and Alteration

Granite, exposed at the Earth's surface after uplift and erosion, undergoes progressive deterioration through weathering processes that include physical, chemical, and biological mechanisms. These processes collectively break down the rock's mineral components and structure, transforming it from a coherent mass into loose sediments over geological timescales. Physical weathering involves mechanical forces that fragment the rock without altering its chemical composition, while chemical weathering decomposes minerals through reactions with water, oxygen, and acids. Biological weathering enhances both by introducing organic agents that accelerate breakdown. The interplay of these processes is influenced by climate, with rates varying significantly between temperate and tropical regions. Physical weathering of granite primarily occurs via exfoliation and frost action. Exfoliation, also known as sheeting, results from the release of confining pressure as overlying rock is eroded, causing the granite to expand and fracture parallel to the surface into curved slabs or sheets. This process is evident in the formation of granite domes, such as those in Yosemite National Park, where repeated unloading produces large, rounded exfoliation sheets. Frost action, prevalent in colder climates, exploits joints and cracks by repeated freezing and thawing of water, widening fractures and leading to the development of tors—isolated, rounded granite boulders or outcrops, as seen in regions like Dartmoor in England and parts of Alaska. These mechanical processes are facilitated by granite's inherent resistance to deformation, which allows it to maintain structural integrity until surface exposure. Chemical weathering targets granite's primary minerals, particularly feldspars and biotite, leading to their transformation into secondary products. Hydrolysis of plagioclase and orthoclase feldspars reacts with water and carbonic acid to form kaolinite clay, releasing soluble ions like sodium, potassium, and silica into solution. Biotite, a ferromagnesian mineral, undergoes oxidation, where iron is oxidized to form iron oxides and hydroxides, alongside hydrolysis that converts it to vermiculite or other clays, weakening the rock matrix. These reactions are enhanced in humid environments, producing a friable residue. Biological contributions, such as lichen colonization, further accelerate feldspar breakdown; lichen thalli secrete organic acids like oxalic and citric acid, which chelate metal ions and promote mineral dissolution at rates up to several times higher than abiotic processes alone. The products of granite weathering include grus, a coarse, granular residue consisting of separated quartz, feldspar, and mica grains, and corestones, which are unweathered spherical boulders surrounded by decomposed material. Grus forms through the disintegration of intergranular bonds, often in arid to semi-arid settings, while corestones result from spheroidal weathering where chemical attack rounds the rock from the exterior inward. Weathering rates for granite typically range from 0.1 to 1 mm per 1000 years in temperate climates, reflecting slower physical and chemical processes under moderate temperature and moisture conditions; in tropical regions, rates can exceed 10 mm per 1000 years due to intensified hydrolysis and biological activity in warm, humid environments.

Special Properties

Natural Radioactivity

Granite exhibits natural radioactivity primarily due to the presence of primordial radionuclides such as uranium-238 (U-238), thorium-232 (Th-232), and potassium-40 (K-40), which are incorporated into its mineral structure during formation. Typical concentrations in granitic rocks range from 0.4 to 20 parts per million (ppm) for uranium, 2.5 to 42 ppm for thorium, and 1.6% to 4.9% total potassium (with K-40 comprising about 0.0117% of natural potassium). These elements are enriched in felsic igneous rocks like granite compared to the Earth's crustal average, often concentrated in accessory minerals such as zircon, apatite, and monazite, where radioactivity can be significantly higher—monazite, for instance, may contain up to several percent thorium oxide. The primary environmental concern arises from the decay chains of U-238 and Th-232, which produce radon-222 (Rn-222) through alpha decay processes; Rn-222 is a noble gas that can emanate from granite surfaces and diffuse into enclosed spaces. This emanation occurs as alpha particles displace radon atoms from mineral lattices, with rates varying based on grain size, porosity, and moisture content in the rock. Gamma radiation from the decay series contributes to external exposure, while beta and alpha emissions from K-40 and other daughters are largely absorbed within the material. Accessory minerals like monazite amplify local radiation due to their elevated thorium and uranium content. Radiation levels from granite are generally low but measurable, with typical gamma dose rates at the surface ranging from 0.09 to 0.40 microsieverts per hour (µSv/h), depending on composition and proximity; annual effective doses from prolonged exposure in granite-rich environments are estimated at 0.005 to 0.18 millisieverts (mSv) for scenarios like kitchen countertops. Higher levels, up to several µSv/h, can occur near thorium-rich accessories like monazite. These are assessed using gamma-ray spectrometry, which detects emissions from U-238 (at 1.76 MeV from Bi-214), Th-232 (at 2.62 MeV from Tl-208), and K-40 (at 1.46 MeV) to quantify activity concentrations. Regulatory limits focus on radon, with the U.S. Environmental Protection Agency recommending mitigation if indoor levels exceed 4 picocuries per liter (pCi/L or 148 Bq/m³). Health implications center on elevated indoor radon concentrations from granite in basements, flooring, or countertops, which can increase lung cancer risk as a second leading cause of the disease after smoking; however, contributions from typical granite installations are minimal compared to soil or water sources, often below action levels unless in high-uranium variants. Direct gamma exposure poses negligible risk, equivalent to natural background in many regions.

Durability and Engineering Relevance

Granite exhibits exceptional resistance to erosion, making it a key material for long-term geological stability and engineering applications. In natural settings, granite's abrasion rate is notably low, typically ranging from 0.01 to 0.1 mm per year under glacial or subaerial conditions, as determined through cosmogenic nuclide analysis of exposed surfaces. This slow erosion rate allows granite outcrops to preserve surface features for millions of years, enabling the use of cosmogenic nuclides like ^{10}Be and ^{26}Al for dating landscape evolution and glacial histories. In engineering contexts, this durability supports its selection for structures exposed to abrasive environments, such as coastal defenses or road bases, where minimal material loss over decades ensures structural integrity. Seismic properties further underscore granite's engineering relevance, particularly in regions prone to earthquakes. Intact granite propagates P-waves at velocities of 5-6 km/s, reflecting its high rigidity and low porosity, which contribute to efficient energy transmission in the crust. Its fracture toughness, approximately 1-2 MPa·m^{0.5}, indicates resistance to crack propagation under dynamic loading, making it suitable for foundation materials in seismic zones. These attributes are critical for assessing site stability in civil engineering projects, where granite bedrock provides a reliable anchor against vibrational stresses. Hydrothermal alteration can compromise granite's durability in mineralized environments, with sericitization representing a primary process that reduces mechanical strength. This alteration involves the replacement of feldspars by fine-grained sericite through fluid-rock interactions in granite-hosted ore deposits, leading to increased porosity and susceptibility to fracturing. In engineering evaluations of such altered granites near mining operations, sericitization zones require careful mapping to avoid zones of weakened rock that could fail under load. Over geological timescales, granite plays a pivotal role in stabilizing continental cratons, enduring for billions of years as a refractory component of the lithosphere. Granitic intrusions contribute to crustal thickening and cooling, forming stable nuclei that resist tectonic deformation since the Archean era, approximately 3-2.5 billion years ago. This long-term persistence is evident in ancient cratons like the Kaapvaal, where granite-dominated compositions have maintained integrity against mantle convection and plate collisions. In modern engineering practice, granite's compressive strength is rigorously tested using ASTM standards to ensure suitability for foundational applications. ASTM C170 outlines methods for measuring compressive strength, typically exceeding 131 MPa for dimension granite, which verifies its capacity to bear heavy loads in buildings and bridges without deformation. These standards guide geotechnical assessments, confirming granite's reliability in load-bearing scenarios while accounting for variability due to mineralogy and alteration.

Historical and Cultural Uses

Ancient Applications

In the Neolithic period, around 4000 BCE, communities in regions like England utilized granite for crafting durable stone tools such as axes and maceheads, valued for their hardness in agricultural and woodworking tasks. At sites like those near Langdale, though primarily known for volcanic tuffs, nearby granite sources contributed to tool production, with polished axes distributed widely across Britain as evidenced by archaeological finds. These implements, often hafted to wooden handles, represented early mastery of hard stone working, enabling forest clearance and monument construction during the transition to settled farming societies. Ancient Egyptians extensively quarried granite from Aswan starting in the Old Kingdom, employing it for monumental obelisks that symbolized divine power and solar rays. By the 18th Dynasty, around 1400 BCE, pharaohs like Hatshepsut commissioned obelisks from Aswan's rose granite for sites such as Luxor Temple, where one surviving pair stands over 25 meters tall. Quarrying techniques involved pounding the granite with dolerite balls—harder stone tools weighing up to 10 kilograms each—to fracture and shape blocks, a labor-intensive process that could remove granite at rates of about 450 cubic centimeters per hour per worker. This method, combined with wooden levers and wedges soaked in water to exploit natural fissures, allowed extraction of massive pieces despite granite's resistance to carving. In classical Greece and Rome, granite's use contrasted with the prevalent marble, highlighting shifts in material preferences for architecture. While the Parthenon (447–432 BCE) featured Pentelic marble columns for their fine carving qualities and luminous white finish, ideal for sculptural detail in Doric order temples, Romans imported Egyptian granite for structural endurance in grander projects. The Pantheon's portico, completed around 126 CE under Hadrian, incorporates 16 monolithic granite columns—each over 11 meters tall and weighing about 60 tons—sourced from quarries like Mons Claudianus, underscoring Rome's imperial reach and granite's role in load-bearing elements over decorative marble. Transporting such heavy granite pieces relied on ingenious low-tech methods, including wooden rollers, sledges, and lubricated surfaces to move colossi weighing up to 1000 tons over land. Egyptians pulled statues on sledges across desert paths, with workers pouring water onto sand to form a stiff crust that reduced friction by half, as demonstrated by wall paintings from the tomb of Djehutihotep (circa 1900 BCE) showing 172 men hauling a 60-ton statue. For longer distances, like from Aswan to the Nile (over 800 kilometers), blocks floated on barges during flood season, combining human labor with seasonal hydrology. Granite's enduring quality imbued it with profound cultural symbolism in ancient Egypt, representing permanence and the pharaoh's eternal journey to the afterlife, particularly in pyramid interiors. In structures like the Great Pyramid of Giza (circa 2580 BCE), red Aswan granite lined the King's Chamber, its unyielding nature mirroring beliefs in immortality and divine resurrection, as the stone's resistance to decay echoed the soul's (ka) transcendence beyond death. Obelisks and sarcophagi of granite further reinforced this, serving as conduits for solar worship and royal apotheosis, ensuring the deceased's integration into the cosmic order.

Architectural and Artistic Roles

Granite's enduring durability and distinctive aesthetic qualities have made it a preferred material for architectural and artistic applications from medieval times onward, particularly in monumental structures and sculptures where permanence and visual impact are essential. In medieval and Renaissance-era architecture across the UK, granite was extensively used in regions with abundant local quarries, such as Scotland and Devon, for constructing cathedrals, churches, and palaces that emphasized grandeur and longevity. Scottish baronial castles, a style blending medieval fortifications with Renaissance ornamentation, frequently incorporated granite for their robust walls, towers, and turrets; Balmoral Castle, rebuilt in the 1850s, exemplifies this with its granite sourced from the nearby Invergelder quarry, creating a pink-tinted facade that highlights the stone's warm hues and textural variety. In England, medieval churches like those in Drewsteignton utilized granite for structural elements, showcasing its role in enduring ecclesiastical architecture despite the prevalence of softer limestones elsewhere. During the Renaissance, granite offered a stark contrast to the marble favored by Italian masters like Michelangelo for sculptures such as David (1504), which prized marble's carvability for fine anatomical details; in contrast, northern traditions, particularly in Aberdeen, Scotland, harnessed local granite for bold, figurative works that leveraged the stone's hardness for monumental scale. Aberdeen's granite sculptors developed expertise in crafting life-sized statues, as evidenced by the statue of George Gordon, 5th Duke of Gordon (unveiled 1844), Scotland's oldest surviving large-scale granite sculpture, depicting the figure in military attire and demonstrating the material's suitability for polished, heroic forms despite its challenging workability. This tradition built on earlier regional uses, producing public monuments that endured harsh climates better than marble. Artistic techniques for granite emphasize its transformation from rough block to refined artwork through processes like polishing, which creates a glossy sheen that accentuates the stone's crystalline structure and depth, and selective highlighting of natural veining to exploit color variations—often grays, pinks, and blacks—for dynamic patterns and visual interest. These methods, involving progressive grinding with abrasives from coarse diamonds to fine pastes, allow sculptors to balance the material's opacity with luminous effects, enhancing both architectural facades and standalone pieces; in Aberdeen's monumental trade, polishing became key to elevating granite memorials with a fashionable, reflective finish. Granite's symbolic strength has cemented its role in memorials, where it conveys permanence and reverence. The Lincoln Memorial (dedicated 1922) incorporates Milford Pink granite from Massachusetts quarries for its terrace walls and lower steps, providing a stable, reddish base that contrasts with the white marble upper facade and supports the structure's neoclassical dignity. Mount Rushmore National Memorial (carved 1927–1941) exploits the site's Harney Peak granite, a fine-grained variety from South Dakota's Black Hills, selected for its uniformity and erosion resistance—eroding at about 1 inch per 10,000 years—to sustain the 60-foot presidential carvings against environmental wear. In 20th-century icons, granite featured prominently in international and artistic contexts, underscoring themes of unity and abstraction. The United Nations Headquarters plaza in New York, developed during the 1940s–1950s construction, employs granite paving stones and structural elements to form its expansive, formal walkways and benches, symbolizing global solidarity through the material's unyielding presence amid the complex's modernist design. Modern sculptors like Henry Moore integrated granite into abstract works, often as pedestals or carved components to ground organic forms; for instance, Reclining Figure: Hand (1979–1981, edition 8/9) places a bronze figure on a granite base, merging the stone's solidity with Moore's biomorphic contours to evoke timeless human-nature connections in public spaces.

Modern Industrial Applications

Construction and Infrastructure

Granite serves as a premier dimension stone in modern construction, valued for its compressive strength and aesthetic appeal in structural elements. In building facades, it provides a robust, weather-resistant cladding; a notable example is the Empire State Building, completed in 1931, where granite forms the base and lower exterior, contributing to the structure's longevity amid urban environmental stresses. For interior applications, granite flooring endures high-traffic areas such as commercial lobbies and public spaces, where its Mohs hardness of 6-7 resists abrasion and staining over decades. In civil engineering projects, granite supports critical infrastructure like bridges and dams through its use in abutments and foundations. Urban sidewalks often incorporate granite curbing for its resistance to vehicular impact and freeze-thaw cycles, as exemplified in Charleston's historic districts, where uniform granite blocks line streets to maintain functionality and visual coherence. The quarrying of granite has evolved significantly since the 1800s, when controlled blasting with black powder and later dynamite enabled the extraction of large blocks while minimizing fractures in the stone. By the mid-20th century, wire saws improved precision, but the introduction of diamond wire saws in the 1980s transformed operations, allowing efficient, low-waste cutting of hard granite with speeds up to 3-5 m²/hour and reduced dust compared to traditional methods. As of 2025, advancements in waterless cutting technologies further reduce environmental impact in granite quarrying. These advancements, combined with modern blasting controls like electronic detonators, optimize block yield and safety in dimension stone production. Industry standards govern granite's application in construction to ensure performance and consistency. The American National Standards Institute (ANSI) endorses ASTM C615 for granite dimension stone, specifying minimum compressive strengths of 19,000 psi (131 MPa) for load-bearing uses and absorption rates under 0.4% to prevent moisture-related degradation. Finishes are standardized accordingly: polished surfaces achieve a high gloss via abrasive buffing for reflective interiors, while flamed finishes, created by thermal texturing, provide slip resistance (coefficient of friction >0.6) for exterior paving and steps. These specifications facilitate reliable integration in structural designs. Sustainability practices in granite use include recycling crushed material from quarry byproducts or deconstructed buildings as aggregate in concrete and road bases, substituting up to 30% of virgin materials and thereby reducing mining demands by conserving natural resources and lowering carbon emissions associated with extraction. This approach aligns with circular economy principles, as recycled granite aggregate maintains comparable strength to natural counterparts in non-structural applications. Leveraging its inherent durability, granite minimizes long-term maintenance in infrastructure, further enhancing environmental efficiency.

Specialized Uses

Granite finds niche applications in recreational, household, and specialized industrial contexts due to its unique physical properties, such as durability, low friction in certain varieties, and resistance to wear. In the sport of curling, high-quality granite from Ailsa Craig, Scotland, is primarily used to craft stones, with blue hone granite forming the critical running band for its exceptionally low water absorption—typically under 0.02%—which minimizes ice adhesion and ensures consistent low-friction gliding across the rink. This variety's fine-grained microgranite structure provides the necessary density and smoothness for predictable performance, while common green granite forms the stone's body for added stability. Standard curling stones weigh between 17.24 and 19.96 kg, including the handle, and have been produced from Ailsa Craig material since the mid-19th century, when Kays of Scotland began commercial manufacturing in 1851, though the sport's origins trace to earlier informal play. The quarry's restricted access due to environmental protections limits annual yields, making these stones a premium resource for Olympic and professional competitions. Granite countertops have become a staple in kitchens and laboratories for their aesthetic appeal and functional resilience, particularly when sealed to enhance stain resistance. In residential settings, sealing with resin or impregnators reduces porosity to less than 0.5% moisture absorption, effectively repelling common spills like oils, acids, and wines while maintaining the stone's natural veining. Laboratory applications leverage granite's hardness (6-7 on the Mohs scale) and thermal stability, making it suitable for work surfaces exposed to chemicals and heat, with sealed variants offering moderate resistance to corrosive substances. The popularity of granite countertops surged in the 1990s following advancements in quarrying, cutting, and global importation, transforming it from a luxury to an accessible option and contributing to the countertops market's growth from niche to a multi-billion-dollar industry by the early 2000s. Rock climbers prize granite formations for their frictional qualities, especially in traditional (trad) climbing routes where the stone's crystalline texture provides reliable hand and foot holds. The rough, interlocking quartz and feldspar crystals create a high-friction surface ideal for slab and face climbing, allowing climbers to "smear" shoes or edge fingers without traditional cracks or features. Iconic sites like Yosemite National Park feature massive granite walls, such as El Capitan, where this crystal roughness enables challenging friction-dependent ascents, demanding precise body positioning to exploit the stone's grippy patina formed by weathering. Beyond these, granite's machinability supports detailed engraving for tombstones, where its uniform density allows deep, erosion-resistant inscriptions using sandblasting or laser techniques, preserving memorials for centuries. Emerging applications include radiation shielding in medical facilities, where granite's high density (around 2.65-2.75 g/cm³) and low porosity attenuate gamma rays and neutrons effectively, as demonstrated by linear attenuation coefficients up to 0.29 cm⁻¹ for common isotopes; composites incorporating granite waste further enhance performance in X-ray rooms and radiotherapy suites without compromising structural integrity.

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