Granite ( /ˈɡrænɪt/) is a common type of felsic intrusive igneous
rock that is granular and phaneritic in texture. Granites can be
predominantly white, pink, or gray in color, depending on their
mineralogy. The word "granite" comes from the
Latin granum, a grain,
in reference to the coarse-grained structure of such a holocrystalline
rock. Strictly speaking, granite is an igneous rock with between 20%
and 60% quartz by volume, and at least 35% of the total feldspar
consisting of alkali feldspar, although commonly the term "granite" is
used to refer to a wider range of coarse grained igneous rocks
containing quartz and feldspar.
The term "granitic" means granite-like and is applied to granite and a
group of intrusive igneous rocks with similar textures and slight
variations in composition and origin. These 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. The extrusive igneous rock
equivalent of granite is rhyolite.
Granite is nearly always massive (lacking any internal structures),
hard and tough, and therefore it has gained widespread use throughout
human history as a construction stone. The average density of granite
is between 2.65 and 2.75 g/cm3 (165.4 - 171.7 lb/ft3), its
compressive strength usually lies above 200 MPa, and its viscosity
near STP is 3–6 • 1019 Pa·s.
The melting temperature of dry granite at ambient pressure is
1215–1260 °C (2219–2300 °F); it is strongly reduced in the
presence of water, down to 650 °C at a few kBar pressure.
Granite has poor primary permeability overall, but strong secondary
permeability through cracks and fractures if present.
1.1 Chemical composition
3.1 Geochemical origins
3.2 Chappell & White classification system
4 Ascent and emplacement
6 Natural radiation
8.2.1 Sculpture and memorials
8.2.4 Other uses
9 Rock climbing
10 See also
12 Further reading
13 External links
QAPF diagram for classification of plutonic rocks
Mineral assemblage of igneous rocks
Granite is classified according to the
QAPF diagram for coarse grained
plutonic rocks and is named according to the percentage of quartz,
alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase
feldspar on the A-Q-P half of the diagram. True granite (according to
modern petrologic convention) contains both plagioclase and alkali
feldspars. When a granitoid is devoid or nearly devoid of plagioclase,
the rock is referred to as alkali feldspar granite. When a granitoid
contains less than 10% orthoclase, it is called tonalite; pyroxene and
amphibole are common in tonalite. A granite containing both muscovite
and biotite micas is called a binary or two-mica granite. Two-mica
granites are typically high in potassium and low in plagioclase, and
are usually S-type granites or A-type granites.
A worldwide average of the chemical composition of granite, by weight
percent, based on 2485 analyses:
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The Cheesewring, a granite tor
A granite peak at Huangshan, China
Granite rock in the cliff of Gros la Tête – Aride Island,
Seychelles. The thin (1-3 cm wide) brighter layers are quartz
veins, formed during the late stages of crystallization of granitic
magmas. They are also sometimes called “hydrothermal veins”
Granite containing rock is widely distributed throughout the
continental crust. 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 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.
Granite has a felsic composition and is more common in recent geologic
time in contrast to Earth's ultramafic ancient igneous history. Felsic
rocks are less dense than mafic and ultramafic rocks, and thus they
tend to escape subduction, whereas basaltic or gabbroic rocks tend to
sink into the mantle beneath the granitic rocks of the continental
cratons. Therefore, granitic rocks form the basement of all land
Granitoids have crystallized from magmas that have compositions at or
near a eutectic point (or a temperature minimum on a cotectic curve).
Magmas will evolve to the eutectic because of igneous differentiation,
or because they represent low degrees of partial melting. Fractional
crystallisation serves to reduce a melt in iron, magnesium, titanium,
calcium and sodium, and enrich the melt in potassium and silicon –
alkali feldspar (rich in potassium) and quartz (SiO2), are two of the
defining constituents of granite.
This process operates regardless of the origin of the parental magma
to the granite, and regardless of its chemistry. However, the
composition and origin of the magma that differentiates into granite
leaves certain geochemical and mineral evidence as to what the
granite's parental rock was. The final mineralogy, texture and
chemical composition of a granite is often distinctive as to its
origin. For instance, a granite that is formed from melted sediments
may have more alkali feldspar, whereas a granite derived from melted
basalt may be richer in plagioclase feldspar. It is on this basis that
the modern "alphabet" classification schemes are based.
Granite has a
slow cooling process which forms larger crystals.
Chappell & White classification system
The letter-based Chappell & White classification system was
proposed initially to divide granites into I-type granite (or igneous
protolith) granite and S-type or sedimentary protolith granite.
Both of these types of granite are formed by the melting of high grade
metamorphic rocks, either other granite or intrusive mafic rocks, or
buried sediment, respectively.
M-type or mantle derived granite was later proposed to cover those
granites that were clearly sourced from crystallized mafic magmas,
generally sourced from the mantle. These are rare, because it is
difficult to turn basalt into granite via fractional crystallisation.
A-type or anorogenic granites are formed above volcanic "hot spot"
activity and have a peculiar mineralogy and geochemistry. These
granites are formed by the melting of the lower crust under conditions
that are usually extremely dry. 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.
H-type or hybrid granites are formed following a mixing of two
granitic magmas from different sources, e.g. M-type and S-type.
An old, and largely discounted theory, granitization states that
granite is formed in place by extreme metasomatism by fluids bringing
in elements, e.g. potassium, and removing others, e.g. calcium, to
transform the metamorphic rock into a granite. This was supposed to
occur across a migrating front. The production of granite by
metamorphic heat is difficult, but is observed to occur in certain
amphibolite and granulite terrains. In-situ granitisation or melting
by metamorphism is difficult to recognise except where leucosome and
melanosome textures are present in migmatites. Once a metamorphic rock
is melted it is no longer a metamorphic rock and is a magma, so these
rocks are seen as a transitional between the two, but are not
technically granite as they do not actually intrude into other rocks.
In all cases, melting of solid rock requires high temperature, and
also water or other volatiles which act as a catalyst by lowering the
solidus temperature of the rock.
Ascent and emplacement
The ascent and emplacement of large volumes of granite within the
upper continental crust is a source of much debate amongst geologists.
There is a lack of field evidence for any proposed mechanisms, so
hypotheses are predominantly based upon experimental data. There are
two major hypotheses for the ascent of magma through the crust:
Of these two mechanisms, Stokes diapir was favoured for many years in
the absence of a reasonable alternative. The basic idea is that magma
will rise through the crust as a single mass through buoyancy. As it
rises, it heats the wall rocks, causing them to behave as a power-law
fluid and thus flow around the pluton allowing it to pass rapidly and
without major heat loss. This is entirely feasible in the warm,
ductile lower crust where rocks are easily deformed, but runs into
problems in the upper crust which is far colder and more brittle.
Rocks there do not deform so easily: for magma to rise as a pluton it
would expend far too much energy in heating wall rocks, thus cooling
and solidifying before reaching higher levels within the crust.
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. As these narrow conduits open, the first magma to enter
solidifies and provides a form of insulation for later magma.
Granitic magma must make room for itself or be intruded into other
rocks in order to form an intrusion, and several mechanisms have been
proposed to explain how large batholiths have been emplaced:
Stoping, where the granite cracks the wall rocks and pushes upwards as
it removes blocks of the overlying crust
Assimilation, where the granite melts its way up into the crust and
removes overlying material in this way
Inflation, where the granite body inflates under pressure and is
injected into position
Most geologists today accept that a combination of these phenomena can
be used to explain granite intrusions, and that not all granites can
be explained entirely by one or another mechanism.
Further information: Weathering
Grus sand and granitoid it derived from
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
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. 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
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.
Granite is a natural source of radiation, like most natural stones.
However, some granites have been reported to have higher
radioactivity, thereby raising some concerns about their
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.
Naturally, a geiger counter should register this low effect.
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 as well.
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. Various 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 126.96.36.199 of the
National Health and Engineering study) and radon emission levels well
below the average outdoor radon concentrations in the US.
Granite dimension stone quarry in Taivassalo, Finland.
Granite and related 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 .
Indian granite quarries have been mired in controversy over child
labor and slavery.
Cleopatra's Needle, London
Red Pyramid of Egypt (c. 26th century BC), named for the light
crimson hue of its exposed limestone surfaces, is the third largest of
Egyptian pyramids. Menkaure's Pyramid, likely dating to the same era,
was constructed of limestone and granite blocks. The Great Pyramid of
Giza (c. 2580 BC) contains a huge granite sarcophagus fashioned of
Aswan Granite". The mostly ruined
Black Pyramid dating from the
Amenemhat III once had a polished granite pyramidion or
capstone, which is now on display in the main hall of the Egyptian
Cairo (see Dahshur). Other uses in
Ancient Egypt include
columns, door lintels, sills, jambs, and wall and floor veneer.
Egyptians worked the solid granite is still a matter of
debate. Patrick Hunt has postulated that the
Egyptians used emery,
which has greater hardness on the Mohs scale.
Rajaraja Chola I
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,
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 Giglio.
Granite became "an
integral part of the Roman language of monumental architecture".
The quarrying ceased around the third century CE. Beginning in Late
Antiquity the granite was reused, which since at least the early 16th
century became known as spoliation. Through the process of
case-hardening, granite becomes harder with age. The technology
required to make tempered steel 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
Sculpture and memorials
Various granites (cut and polished surfaces)
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
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 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 on the stone, 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.
Granite has been extensively used as a dimension stone and as flooring
tiles in public and commercial buildings and monuments.
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
Engineers have traditionally used polished granite surface plates to
establish a plane of reference, since they are relatively impervious
and inflexible. Sandblasted concrete with a heavy aggregate content
has an appearance similar to rough granite, and is often used as a
substitute when use of real granite is impractical. A most unusual use
of granite was 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. Granite
tables are used extensively as bases for optical instruments because
of granite's rigidity, high dimensional stability, and excellent
vibration characteristics. In military engineering, Finland planted
granite boulders along its
Mannerheim Line to block invasion by
Russian tanks in the winter war of 1940.
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 Mauchline for curling stones.
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 Bugaboos, the
Mont Blanc massif (and peaks such as the Aiguille du Dru, the Mourne
Mountains, the Adamello-Presanella Alps, the
Aiguille du Midi
Aiguille du Midi and the
Grandes Jorasses), the Bregaglia, Corsica, parts of the Karakoram
(especially the Trango Towers), the Fitzroy Massif, Patagonia, Baffin
Island, Ogawayama, the Cornish coast, the Cairngorms, Sugarloaf
Mountain in Rio de Janeiro, Brazil, and the Stawamus Chief, British
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
Granite was used for setts on the
St. Louis riverfront and for the
piers of the
Eads Bridge (background)
The granite peaks of the
Cordillera Paine in the Chilean Patagonia
Half Dome, Yosemite National Park, a classic granite dome and popular
rock climbing destination
Rixö red granite quarry in Lysekil, Sweden
Falkenfelsen, or Falcon Rock
Fall River granite
List of rock types
Pikes Peak granite, Colorado
Stone Mountain, Georgia
Wicklow Mountains, Ireland
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