Granite () is a common and widely occurring type of
intrusive,
felsic,
igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the
groundmass forming a rock known as
porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy.
Outcrops of granite tend to form
tors, and rounded
massifs. Granites sometimes occur in circular
depressions surrounded by a range of hills, formed by the
metamorphic aureole or
hornfels.
Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average
density of granite is 2.75 g/cm
3 and its viscosity at standard temperature and pressure is ~4.5 • 10
19 Pa·s.
The word granite comes from the
Latin granum, a grain, in reference to the coarse-grained structure of such a
crystalline rock.
Mineralogy
Granite is classified according to the
QAPF diagram for coarse grained
plutonic rocks (granitoids) and is named according to the percentage of
quartz, alkali
feldspar (
orthoclase,
sanidine, or
microcline) and
plagioclase feldspar on the A-Q-P half of the diagram.
True granite according to modern
petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called
tonalite;
pyroxene and
amphibole are common in tonalite. A granite containing both muscovite and biotite
micas is called a binary or
two-mica granite. Two-mica granites are typically high in
potassium and low in plagioclase, and are usually S-type granites or A-type granites. The
volcanic equivalent of
plutonic granite is
rhyolite.
Granite has poor primary
permeability but strong secondary permeability.
Chemical composition
A worldwide average of the average proportion of the different chemical components in granites, in descending order by weight percent, is:
Occurrence
Granite is currently known only on Earth where it forms a major part of
continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (
stocks) and in
batholiths that are often associated with
orogenic mountain ranges. Small
dikes of granitic composition called
aplites are often associated with the margins of granitic
intrusions. In some locations very coarse-grained
pegmatite masses occur with granite.
Granite has been intruded into the
crust of the
Earth during all
geologic periods, although much of it is of
Precambrian age. Granitic rock is widely distributed throughout the
continental crust of the Earth and is the most abundant
basement rock that underlies the relatively thin
sedimentary veneer of the continents.
Origin
Granite is an
igneous rock and is formed from
magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional and include French, British, and American systems.
Geochemical origins
Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a
eutectic point (or a temperature minimum on a
cotectic curve). Magmas will evolve to the eutectic because of
igneous differentiation, or because they represent low degrees of partial melting.
Fractional crystallisation serves to reduce a melt in
iron,
magnesium,
titanium,
calcium and
sodium, and enrich the melt in
potassium and
silicon - alkali feldspar (rich in potassium) and
quartz (SiO
2), are two of the defining constituents of granite.
This process operates regardless of the origin of the parental magma to the granite, and regardless of its chemistry. However, the composition and origin of the magma which differentiates into granite, leaves certain geochemical and 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 which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted
basalt may be richer in
plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are based.
Chappell & White classification system
The letter-based Chappell & White classificiation 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 melting of high grade
metamorphic rocks, either other granite or intrusive mafic rocks, or buried sediment, respectively.
M-type or
mantle derived granite was proposed later, to cover those granites which were clearly sourced from 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 peculiar mineralogy and
geochemistry. These granites are formed by melting of the lower
crust under conditions that are usually extremely dry. The rhyolites of the
Yellowstone caldera are examples of volcanic equivalents of A-type granite.
[Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M., and Larson, P., 2005, Large-volume, low-δ18O rhyolites of the central Snake River Plain, Idaho, USA: Geology 33: 821–824.][C.D. Frost, M. McCurry, R. Christiansen, K. Putirka and M. Kuntz, Extrusive A-type magmatism of the Yellowstone hot spot track 15th Goldschmidt Conference Field Trip AC-4. Field Trip Guide, University of Wyoming (2005) 76 pp., plus an appended map.]Granitization
An old, and largely discounted theory,
granitization states that granite is formed in place by extreme
metasomatism by fluids bringing in elements e.g. potassium and removing others e.g. calcium to transform the metamorphic rock into a granite. This was supposed to occur across a migrating front. The production of granite by metamorphic heat is difficult, but is observed to occur in certain
amphibolite and
granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where
leucosome and
melanosome textures are present in
gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires high temperature, and also
water or other
volatiles which act as a
catalyst by lowering the
solidus temperature of the rock.
Ascent and emplacement
thumb|Roche Rock, CornwallThe 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 (Weinberg, 1994). This is entirely feasible in the warm,
ductile lower crust where rocks are easily deformed, but runs into problems in the upper crust which is far colder and more brittle. Rocks there do not deform so easily: for magma to rise as a pluton it would expend far too much energy in heating wall rocks, thus cooling and solidifying before reaching higher levels within the crust.
Nowadays
fracture propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust. Magma rises instead in small channels along self-propagating
dykes which form along new or pre-existing
fault systems and networks of active shear zones (Clemens, 1998). As these narrow conduits open, the first magma to enter solidifies and provides a form of insulation for later magma.
Granitic magma must make room for itself or be intruded into other rocks in order to form an intrusion, and several mechanisms have been proposed to explain how large
batholiths have been emplaced:
- Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust
- Assimilation, where the granite melts its way up into the crust and removes overlying material in this way
- Inflation, where the granite body inflates under pressure and is injected into position
Most geologists today accept that a combination of these phenomena can be used to explain granite intrusions, and that not all granites can be explained entirely by one or another mechanism.
Natural radiation
Granite is a natural source of
radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.
Some granites contain around 10 to 20 parts per million of
uranium. By contrast, more mafic rocks such as tonalite,
gabbro or
diorite have 1 to 5
ppm uranium, and
limestones and
sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for
palaeochannel-hosted or roll front
uranium ore deposits, where the uranium washes into the
sediments from the granite uplands and associated, often highly radioactive, pegmatites. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils over granite can become a trap for
radon gas, which is formed by the decay of uranium. Radon can also be introduced into houses by wells drilled into granite
. Radon gas poses significant health concerns, and is the #2 cause of
lung cancer in the US behind smoking
.
There is some concern that materials sold as granite countertops or as building material may be hazardous to health. One expert, Dr. Dan Steck of St. Johns University, has stated that approximately 5% of all granites will be of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slabs have been actually 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 , and found that 18 of the 39 full size granite slabs that were measured for the study failed to meet the European Union safety standards (section 4.1.1.1 of the National Health and Engineering study).
Furthermore, all but one of the 39 full size slabs tested in the E,H,& E study had Activity Concentration Indexes above that which the EU regulations require dose assements (Section 4.3.1 of the E,H,&E study). The Marble Institute dealt with this issue by stating that the European Union granite countertop regulations were flawed. The stones tested include types of granite that comprise approximately 80 percent of the annual U.S. market share for granite countertops, based on the most recent market data available."
Other researchers and organizations do not agree with the Marble Institute's stated position on granite safety, including AARST (American Association of Radon Scientists and Technicians) and the CRCPD (Conference of Radiation Control Program Directors, an organization of state radiation protection officials). Both organizations have committees currently setting maximum allowed levels of radiation/radon as well as protocols for measuring radiation/radon from granite countertops. The European Union regulations will likely serve as the basis for new EPA based regulations for granite building materials in the U.S.
Uses
Antiquity
Life-size elephant and other creatures carved in granite;
Mahabalipuram, India.
The
Red Pyramid of
Egypt (c.26th century BC), named for the light crimson hue of its exposed granite surfaces, is the third largest of
Egyptian pyramids.
Menkaure's Pyramid, likely dating to the same era, was constructed of
limestone and granite blocks. The
Great Pyramid of Giza (c.
2580 BC) contains a huge granite
sarcophagus fashioned of "Red
Aswan Granite." The mostly ruined
Black Pyramid dating from the reign of
Amenemhat III once had a polished granite
pyramidion or capstone, now on display in the main hall of the
Egyptian Museum in
Cairo (see
Dahshur). Other uses in
Ancient Egypt include
columns, door
lintels,
sills,
jambs, and wall and floor
veneer. How the
Egyptians worked the solid granite is still a matter of debate.
Dr. Patrick Hunt has postulated that the Egyptians used
emery shown to have higher
hardness on the
Mohs scale.
Many large Hindu temples in southern India, particularly those built by the 11th century king
Rajaraja Chola I, were made of granite. There is a large amount of granite in these structures. They are comparable to the Great Pyramid of Giza.
Modern
Building
Polished red granite tombstone
Granite has been extensively used as a
dimension stone and as flooring tiles in public and commercial buildings and monuments. Because of its abundance, granite was commonly used to build foundations for homes in
New England. The
Granite Railway, America's first railroad, was built to haul granite from the quarries in
Quincy, Massachusetts, to the
Neponset River in the 1820s. With increasing amounts of
acid rain in parts of the world, granite has begun to supplant
marble as a monument material, since it is much more durable. Polished granite is also a popular choice for
kitchen countertops due to its high durability and aesthetic qualities. In building and for countertops, the term "granite" is often applied to all igneous rocks with large crystals, and not specifically to those with a granitic composition.
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 particular rarity of the granite, the best stones can cost as much as US$1,500. Between 60–70 percent of the stones used today are made from Ailsa Craig granite, although the island is now a wildlife reserve and is no longer used for quarrying.
In some areas granite is used for gravestones and memorials. Granite is a hard stone and requires skill to carve by hand. 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.
Engineering
Engineers have traditionally used polished granite surfaces to establish a
plane of reference, since they are relatively impervious and inflexible. Sandblasted
concrete with a heavy
aggregate content has an appearance similar to rough granite, and is often used as a substitute when use of real granite is impractical. A most unusual use of granite was in the construction of the rails for the
Haytor Granite Tramway, Devon, England, in 1820.
Rock climbing
Granite is one of the rocks most prized by climbers, for its steepness, soundness, crack systems, and friction. Well-known venues for granite climbing include
Yosemite, the
Bugaboos, the
Mont Blanc massif (and peaks such as the
Aiguille du Dru, the
Mountains of Mourne, the
Aiguille du Midi and the
Grandes Jorasses), the
Bregaglia,
Corsica, parts of the
Karakoram, the Fitzroy Massif,
Patagonia,
Baffin Island, the
Cornish coast and the
Cairngorms.
Granite
rock climbing is so popular that many of the artificial rock
climbing walls found in gyms and theme parks are made to look and feel like granite.
See also
- Aberdeen, Scotland's third largest city nicknamed "The Granite City"
