Granite ( ) is a common and widely occurring type
of
intrusive,
felsic,
igneous rock. Granites usually have a medium to
coarse grained texture. 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 sometimes known as a
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
depression 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:
Based on 2485 analyses
Occurrence
Granite is currently known only on Earth where it forms a major
part of
continental crust. Granite
often occurs as relatively small, less than 100 km² stock
masses (
stocks) and in
batholiths that are often associated with
orogenic mountain ranges.
Small
dikes of granitic composition
called
aplites are often associated with the
margins of granitic
intrusion.
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.
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.
File:Granite azul noce.jpg|
Azul Noche (Spain
)
File:Granite giallo.jpg|
Santa Cecilia (Brazil
)
File:Granite gran violet.jpg|
Gran Violet (Brazil)
File:Granite lavanda blue.jpg|
Lavanda Blue (Brazil)
Ascent and emplacement
Roche Rock, Cornwall
The ascent and emplacement of large volumes of granite within the
upper continental crust is a source of much debate amongst
geologists. There is a lack of field evidence for any proposed
mechanisms, so hypotheses are predominantly based upon experimental
data.There are two major hypotheses for the ascent of magma through
the crust:
- Stokes Diapir
- Fracture Propagation
Of these two mechanisms, Stokes
diapir was
favoured for many years in the absence of a reasonable alternative.
The basic idea is that magma will rise through the crust as a
single mass through
buoyancy. As it rises
it heats the
wall rocks,
causing them to behave as a
power-law
fluid and thus flow around the
pluton
allowing it to pass rapidly and without major heat loss (Weinberg,
1994). This is entirely feasible in the warm,
ductile lower crust where rocks are easily
deformed, but runs into problems in the upper crust which is far
colder and more brittle. Rocks there do not deform so easily: for
magma to rise as a pluton it would expend far too much energy in
heating wall rocks, thus cooling and solidifying before reaching
higher levels within the crust.
Nowadays
fracture propagation is the
mechanism preferred by many geologists as it largely eliminates the
major problems of moving a huge mass of magma through cold brittle
crust. Magma rises instead in small channels along self-propagating
dykes which form along new or
pre-existing
fault systems and
networks of active shear zones (Clemens, 1998). As these narrow
conduits open, the first magma to enter solidifies and provides a
form of insulation for later magma.
Granitic magma must make room for itself or be intruded into other
rocks in order to form an intrusion, and several mechanisms have
been proposed to explain how large
batholiths have been emplaced:
- Stoping, where the granite cracks the
wall rocks and pushes upwards as it removes blocks of the overlying
crust
- Assimilation, where the granite melts its way up into the crust
and removes overlying material in this way
- Inflation, where the granite body inflates under pressure and
is injected into position
Most geologists today accept that a combination of these phenomena
can be used to explain granite intrusions, and that not all
granites can be explained entirely by one or another
mechanism.
Natural radiation
Granite is a natural source of
radiation, 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
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
References
- Chappell, B.W. and White, A.J.R., 2001. Two contrasting granite
types: 25 years later. Australian Journal of Earth Sciences v.48,
p.489-499.
- 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.
- Radiation and Life
-
http://www.marble-institute.com/industryresources/assessingexposureexecutivesummary.pdf
External links
, Scotland's third largest city is nicknamed "The
Granite City"
"Granite Capital of the World", home of the
Rock of Ages
Corporation
, the "Granite Capital of the World"
, the "Granite State"
, Georgia
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