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Granite () is a coarse-grained (
phaneritic A phanerite is an igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rom ...
)
intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magma leaving behind igneous rock * Saltwater intrusion, the movement of saline water into freshwa ...
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock i ...
composed mostly of
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
,
alkali feldspar Feldspars () are a group of rock-forming tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. ...
, and
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
. It forms from
magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others ...

magma
with a high content of
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

silica
and
alkali metal oxide The alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origi ...
s that slowly cools and solidifies underground. It is common in the
continental crust Continental crust is the layer of igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...
of Earth, where it is found in
igneous intrusion , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, ...
s. These range in size from
dike Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), the Greek goddess of moral justice * Dikes, diagonal pliers, diag ...
s only a few centimeters across to
batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magm ...
s exposed over hundreds of square kilometers. Granite is typical of a larger family of ''granitic rocks'', or ''
granitoid A granitoid is a generic term for a diverse collection of coarse-grained igneous rocks that consist predominately of quartz Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. The atoms are linked in a continuous framew ...
s'', 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 A QAPF diagram is a double ternary diagram A diagram is a symbolic representation Representation may refer to: Law and politics *Representation (politics) Political representation is the activity of making citizens "present" in public policy ...
), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain
mica Micas ( ) are a group of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs natural ...

mica
or
amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their s ...

amphibole
minerals, though a few (known as
leucogranite Leucogranite is a light-colored, granitic A granitoid is a generic term for a diverse collection of coarse-grained Igneous rock, igneous rocks that consist predominately of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase ...
s) contain almost no dark minerals. Granite is nearly always massive (lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history.


Description

The word "granite" comes from the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
''granum'', a grain, in reference to the coarse-grained structure of such a completely crystalline rock. Granitic rocks mainly consist of
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity ...
,
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
,
mica Micas ( ) are a group of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs natural ...

mica
, and
amphibole Amphibole () is a group of Silicate minerals, inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedron, tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their s ...

amphibole
mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

mineral
s, which form an interlocking, somewhat
equigranular An equigranular material is composed chiefly of crystals of similar orders of magnitude to one another. Basalt Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and ...
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryoti ...
of feldspar and quartz with scattered darker
biotite Biotite is a common group of phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science conce ...

biotite
mica and amphibole (often
hornblende Hornblende is a complex inosilicate series of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock ...

hornblende
) peppering the lighter color minerals. Occasionally some individual crystals (
phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss Swiss may refer to: * the adjectival form of Switzerland *Swiss people Places *Swiss, Missouri *Swiss, North Carolina *Swiss, West Virginia *Swiss, Wisconsin Other uses * ...
s) are larger than the
groundmass The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ord ...
, in which case the texture is known as
porphyritic porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada (U.S.), Sierra Nevada, Rock Creek Canyon, California. Porphyritic is an adjective used in geology, specifical ...
. A granitic rock with a porphyritic texture is known as a granite porphyry.
Granitoid A granitoid is a generic term for a diverse collection of coarse-grained igneous rocks that consist predominately of quartz Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. The atoms are linked in a continuous framew ...
is a general, descriptive
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grassl ...

field
term for lighter-colored, coarse-grained igneous rocks.
Petrographic Petrography is a branch of petrology Image:LvMS-Lvm.jpg, A volcanic lithic fragment (geology), sand grain seen under the microscope, with plane-polarized light in the upper picture, and cross polarized light in the lower picture. Scale box is 0. ...
examination is required for identification of specific types of granitoids. Granites can be predominantly white, pink, or gray in color, depending on their
mineralogy Mineralogy is a subject of geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is com ...

mineralogy
. The alkali feldspar in granites is typically
orthoclase Orthoclase, or orthoclase feldspar (endmember An endmember (also end-member or end member) in mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical ...

orthoclase
or
microcline Microcline (KAlSi3O8) is an important igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in ...

microcline
and is often perthitic. The plagioclase is typically sodium-rich
oligoclase Oligoclase is a rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks o ...
. Phenocrysts are usually alkali feldspar. Granitic rocks are classified according to the
QAPF diagram A QAPF diagram is a double ternary diagram A diagram is a symbolic Depiction, representation of information using Visualization (graphics), visualization techniques. Diagrams have been used since prehistoric times on Cave painting, walls of cave ...
for coarse grained
plutonic rocks , an igneous ''intrusion'' exposed when the surrounding softer rock eroded away Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form ''Igneous intrusion, intrusions'', such as batholiths, ...

plutonic rocks
and are named according to the percentage of
quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra, ...

quartz
, alkali
feldspar Feldspars are a group of rock-forming aluminium Aluminium (aluminum in and ) is a with the  Al and  13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity ...
(
orthoclase Orthoclase, or orthoclase feldspar (endmember An endmember (also end-member or end member) in mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical ...

orthoclase
,
sanidine Sanidine is the high temperature form of potassium Potassium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the propert ...

sanidine
, or
microcline Microcline (KAlSi3O8) is an important igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in ...

microcline
) and
plagioclase Plagioclase is a series of tectosilicate Silicate minerals are rock-forming mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition ...
feldspar on the A-Q-P half of the diagram. True granite (according to modern
petrologic Image:LvMS-Lvm.jpg, A volcanic lithic fragment (geology), sand grain seen under the microscope, with plane-polarized light in the upper picture, and cross polarized light in the lower picture. Scale box is 0.25 mm. Petrology (from the grc, wikt: ...
convention) contains between 20% and 60% quartz by volume, with 35% to 90% of the total feldspar consisting of
alkali feldspar Feldspars () are a group of rock-forming tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. ...
. Granitic rocks poorer in quartz are classified as
syenite Image:Nepheline-syenite-2005.jpg, Leucocratic variety of nepheline syenite from Sweden (särnaite) Syenite is a coarse-grained Intrusion, intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, ...

syenite
s or
monzonite Monzonite is an igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, know ...

monzonite
s, while granitic rocks dominated by plagioclase are classified as
granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from grani ...
s or
tonalite Tonalite is an igneous rock, igneous, plutonic (Intrusive rock, intrusive) rock (geology), rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar ...

tonalite
s. 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 Quartzolite or silexite is an intrusive igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken ...
. 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{{Infobox rock , name=Monzogranite , type=Igneous , image=File:Monzogranite col Croix.JPG , caption=Monzogranite (Col des Croix, Haute-Saône, France) Monzogranites are biotite granite rocks that are considered to be the final Fractional crystalliz ...
is 35% to 65% alkali feldspar. A granite containing both muscovite and biotite
mica Micas ( ) are a group of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs natural ...

mica
s is called a binary or ''two-mica'' granite. Two-mica granites are typically high in
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
and low in plagioclase, and are usually S-type granites or A-type granites, as described
below Below may refer to: *Earth *Ground (disambiguation) *Soil *Floor *Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Fred Below (1926–1988), American blues drummer *Fritz von Below (1853 ...
. 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 Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter ...

CaO
> > 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 Riebeckite is a sodium Sodium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms th ...

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 Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate Silicate minerals are rock-forming mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'' ...

muscovite
.


Physical properties

The average
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its per unit . The symbol most often used for density is ''ρ'' (the lower case Greek letter ), although the Latin letter ''D'' can also ...

density
of granite is between , its
compressive strength In mechanics Mechanics (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populat ...
usually lies above 200 MPa, and its
viscosity The viscosity of a fluid In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, ...

viscosity
near STP is 3–6·1020 Pa·s. The melting temperature of dry granite at ambient pressure is ; it is strongly reduced in the presence of water, down to 650 °C at a few hundred megapascals of pressure. Granite has poor primary permeability overall, but strong secondary permeability through cracks and fractures if they are present.


Chemical composition

A worldwide average of the chemical composition of granite, by weight percent, based on 2485 analyses:Blatt and Tracy 1996, p.66 The medium-grained equivalent of granite is microgranite. The
extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extrusive rock refers to the mode of igneous volcanic ...
igneous rock equivalent of granite is
rhyolite Rhyolite ( ) is the most silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and ...

rhyolite
.


Occurrence

Granitic rock is widely distributed throughout the
continental crust Continental crust is the layer of igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...
. Much of it was intruded during the
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale The geologi ...

Precambrian
age; it is the most abundant
basement rock upright=1.25, Gneiss outcrop, basement rock, Scotland">outcrop.html" ;"title="Gneiss outcrop">Gneiss outcrop, basement rock, Scotland In geology, basement and crystalline basement are the rocks below a sedimentation, sedimentary platform (geology ...
that underlies the relatively thin
sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentatio ...

sedimentary
veneer of the continents.
Outcrop An outcrop or rocky outcrop is a visible exposure of bedrock Bedrock in geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , ...

Outcrop
s of granite tend to form
tors Tors may refer to: * Tor (rock formation) A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing rock outcrop An outcrop or rocky outcrop is a visible exposure of bedrock Bedrock in ...
,
domes A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
or
bornhardt A bornhardt () is a dome-shaped, steep-sided, bald rock outcropping at least in height and several hundred metres in width. They are named after Friedrich Wilhelm Conrad Eduard Bornhardt, Wilhelm Bornhardt (1864–1946), a German geologist and ...
s, and rounded
massif In geology, a massif ( or ) is a section of a planet's Crust (geology), crust that is demarcated by geologic fault, faults or Lithospheric flexure, flexures. In the Plate tectonics, movement of the crust, a massif tends to retain its internal s ...
s. Granites sometimes occur in circular
depression Depression may refer to: Mental health * Depression (mood), a state of low mood and aversion to activity * Mood disorders characterized by depression are commonly referred to as simply ''depression'', including: ** Dysthymia ** Major depressive ...
s surrounded by a range of hills, formed by the metamorphic aureole or
hornfels Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusi ...

hornfels
. Granite often occurs as relatively small, less than 100 km2 stock masses (
stocks Stocks are feet restraining devices that were used as a form of and . Form and application The stocks, , and pranger each consist of large wooden boards with hinges; however, the stocks are distinguished by their restraint of the feet. The s ...
) and in
batholith A batholith (from Greek ''bathos'', depth + ''lithos'', rock) is a large mass of intrusive Intrusive may refer to: * Intrusiveness, a typically unwelcome behavior, interrupting and disturbing to others * Intrusive rock; intrusion of molten magm ...
s that are often associated with
orogenic An orogeny is an event that leads to both structural Deformation (physics), deformation and compositional differentiation of the Earth's lithosphere (Crust (geology), crust and uppermost Mantle (geology), mantle) at Convergent boundary, converge ...
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

mountain
ranges. Small
dikes Dyke or dike may refer to: General uses * Dyke (slang) The term ''dyke'' is a slang Slang is language (words, phrases, and usages) of an informal register. It also sometimes refer to the language generally exclusive to the members of ...
of granitic composition called
aplite Aplite () is an in which the mineral composition is the same as , but in which the grains are much finer, under 1 mm across. and are the dominant s. The term 'aplite' or 'aplitic' is often used as a textural term to describe veins of ...
s are often associated with the margins of granitic
intrusion In geology Geology (from the γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is a branch of concerned with both the liquid and , the of which it is composed, and the processes by which they change over ...
s. In some locations, very coarse-grained
pegmatite A pegmatite is an igneous rock showing a very coarse Texture (geology), texure, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a si ...
masses occur with granite.


Origin

Granite forms from silica-rich (
felsic In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the processes ...
) 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
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extr ...

basalt
ic magmas. It has also been suggested that some granites found at convergent boundaries between
tectonic plate This is a list of tectonic plates on Earth's surface Earth is the third planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibri ...
s, where
oceanic crust The oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates This is a list of tectonic plates on Earth's surface. Tectonic plates are pieces of Earth's crust 350px, Plates in the crust of Earth Earth's crust i ...
subducts below continental crust, were formed from
sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
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. Early fractional crystallisation serves to reduce a melt in magnesium and chromium, and enrich the melt in iron, sodium, potassium, aluminum, and silicon. Further fractionation reduces the content of iron, calcium, and titanium. This is reflected in the high content of alkali feldspar and quartz in granite. The presence of granitic rock in
island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries (such as the Ring of Fire). Most island arcs originate on oceanic crust and have resulted from the descent of the litho ...

island arc
s shows that fractional crystallization alone can convert a basaltic magma to a granitic magma, but the quantities produced are small. For example, granitic rock makes up just 4% of the exposures in the
South Sandwich Islands ) , anthem = "God Save the Queen "God Save the Queen", alternatively "God Save the King" (dependent on the gender of the reigning monarch), is the or in most s, their territories, and the British . The author of the tune is unknown, and ...

South Sandwich Islands
. 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 Magmatic underplating occurs when basaltic Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (''mafic '' lava) exposed at or very near the surface of a te ...
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.


Alphabet classification system

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). 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. I-type granites are orogenic (associated with mountain building) and usually metaluminous. S-type granites are sodium-poor and aluminum-rich. As a result, they contain
mica Micas ( ) are a group of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs natural ...

mica
s 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). They are also orogenic but range from metaluminous to strongly peraluminous. 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. A-type granites show a peculiar mineralogy and geochemistry, with particularly high silicon and potassium at the expense of calcium and magnesium 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.) They are not orogenic, forming instead over hot spots and continental rifting, and are metaluminous to mildly peralkaline and iron-rich. 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. A-type granites occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica. The rhyolites of the Yellowstone Caldera are examples of volcanic equivalents of A-type granite. M-type granite was later proposed to cover those granites that were clearly sourced from crystallized mafic magmas, generally sourced from the mantle. Although the fractional crystallisation of basaltic melts can yield small amounts of granites, which are sometimes found in island arcs, such granites must occur together with large amounts of basaltic rocks. 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. However, the big difference in rheology between mafic and felsic magmas makes this process problematic in nature.


Granitization

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. 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. 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. 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). 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. In-situ granitization requires heating by the asthenospheric mantle or by underplating with mantle-derived magmas.


Ascent and emplacement

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 (geology), country rock to make room for themselves (the ''room problem'') is still a matter of research. Two main mechanisms are thought to be important: *Stokes diapir *Fracture (geology), Fracture propagation 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 Country rock (geology), 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. This is entirely feasible in the warm, ductility, 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 (geology), 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 Dike (geology), dykes which form along new or pre-existing fracture or fault (geology), fault systems and networks of active shear zones. 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 (geology), 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. Evidence for piecemeal stoping is found in intrusions that are rimmed with ''igneous breccia'' containing fragments of country rock. 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. Heat loss to the country rock means that ascent by assimilation is limited to distance similar to the height of the magma chamber.


Weathering

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, mineral alteration, alter feldspar in a process called weathering, hydrolysis. 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 (geology), grus, which is often made up of coarse-grained fragments of disintegrated granite. Climatic variations also influence the weathering rate of granites. For about two thousand years, the relief engravings on Cleopatra's Needle (London), 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. 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 soil acidification, acidification and podzolization in cool humid climates as the weather-resistant quartz yields much sand. 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), Cecil soil series a prime example of the consequent Ultisol great soil group.


Natural radiation

Granite is a natural source of Particle radiation, radiation, like most natural stones. Potassium-40 is a radioactive isotope of weak emission, and a constituent of
alkali feldspar Feldspars () are a group of rock-forming tectosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. ...
, which in turn is a common component of granitic rocks, more abundant in alkali feldspar granite and
syenite Image:Nepheline-syenite-2005.jpg, Leucocratic variety of nepheline syenite from Sweden (särnaite) Syenite is a coarse-grained Intrusion, intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, ...

syenite
s. 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, which is formed by the decay of uranium. Radon gas poses significant health concerns and is the number two cause of lung cancer in the US behind smoking. Thorium occurs in all granites. Conway granite has been noted for its relatively high thorium concentration of 56±6 ppm. There is some concern that some granite sold as countertops or building material may be hazardous to health. Dan Steck of St. Johns University has stated 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.


Industry

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


Uses


Antiquity

The Red Pyramid of Ancient Egypt, Egypt (circa 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. 26th century BC, 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, 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, sill plate, sills, jambs, and wall and floor veneer. How the Egyptians worked the solid granite is still a matter of debate. Patrick Hunt (archaeologist), Patrick Hunt has postulated that the Egyptians used emery (rock), emery, which has greater Hardenability, hardness on the Mohs scale of mineral hardness, Mohs scale. 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. Imperial Roman granite was quarried mainly in Egypt, and also in Turkey, and on the islands of Elba and Isola del Giglio, Giglio. Granite became "an integral part of the Roman language of monumental architecture". 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 of rocks, case-hardening, granite becomes harder with age. The technology required to make tempered steel, 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."


Modern


Sculpture and memorials

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. 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#Royal Mausoleum, 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.


Buildings

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.


Engineering

Engineers have traditionally used polished granite surface plates to establish a plane (mathematics), plane of reference, since they are relatively impervious, inflexible, and maintain good dimensional stability. Sandblasted concrete with a heavy Aggregate (geology), 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, Coordinate-measuring machine, 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. Granite block is usually processed into slabs, which can be cut and shaped by a cutting center. In military engineering, Finland planted granite boulders along its Mannerheim Line to block invasion by Russian tanks in the Winter War of 1939–40.


Other uses

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 and is still used for quarrying under license for Ailsa granite by Kays of Scotland for curling stones.


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 the Yosemite Valley, the The Bugaboos, 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 Range, Bregaglia, Corsica, parts of the Karakoram (especially the Trango Towers), the Fitzroy Massif, Patagonia, Baffin Island, Ogawayama, the Geography of Cornwall, Cornish coast, the Cairngorms, Sugarloaf Mountain in Rio de Janeiro, Brazil, and the Stawamus Chief, British Columbia, Canada. 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. File:St. Louis wharf cobbles 20090121 1.jpg, Granite was used for setts on the St. Louis Mississippi River, riverfront and for the piers of the Eads Bridge (background) File:Torres del Paine, Patagonia (2004).jpg, The granite peaks of the Cordillera Paine in the Chilean Patagonia File:Yosemite 20 bg 090404.jpg, alt=Half Dome, Yosemeite National Park, Half Dome, Yosemite National Park, is actually a granite arête and is a popular rock climbing destination File:Rixö granitbrott 4.jpg, Rixö red granite quarry in Lysekil, Sweden File:Liberty Bell Mountain seen from Highway 20.jpg, Liberty Bell Mountain in the North Cascades File:Early Winters Spires.jpg, Early Winters Spires in the North Cascades


See also

* * * * * * * * * * * * * * , Colorado * * * , Georgia (U.S. state), Georgia * * , Ireland


References

; Citations


Further reading

* * *


External links

{{Authority control Granite, Felsic rocks National symbols of Finland Plutonic rocks Sculpture materials Symbols of Wisconsin Industrial minerals