Difference between revisions of "Granite" - New World Encyclopedia

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In [[geology]], granite is called an intrusive, igneous rock. An igneous rock is one that is formed when molten rock (magma) cools and solidifies. The term "intrusive" indicates that the rock solidified below the [[Earth]]'s surface and intrudes other rocks.
 
In [[geology]], granite is called an intrusive, igneous rock. An igneous rock is one that is formed when molten rock (magma) cools and solidifies. The term "intrusive" indicates that the rock solidified below the [[Earth]]'s surface and intrudes other rocks.
  
Granite is currently known only on Earth, where it forms a major part of the [[continental crust]]*.  It has been found as relatively small (less than 100 km<sup>2</sup>), stock-like masses and also as huge formations called "[[batholith]]*s" (igneous intrusive rock formed deep in the Earth's crust), often associated with [[mountain]] ranges formed by "orogeny" (the processes of plate tectonics). Most granite intrusions are emplaced at depths between 1.5&nbsp;kilometers (km) and 50&nbsp;km in the continental crust. Small [[dike (geology)|dikes]]* of granitic composition called "aplites" are associated with granite margins.
+
Granite is currently known only on Earth, where it forms a major part of the [[continental crust]]*.  It has been found as relatively small (less than 100 km<sup>2</sup>), stock-like masses and also as huge formations called "[[batholith]]*s", often associated with [[mountain]] ranges formed by "orogeny" (the mountain-forming processes of plate tectonics). Most granite intrusions are emplaced at depths between 1.5&nbsp;kilometers (km) and 50&nbsp;km in the continental crust. Small [[dike (geology)|dikes]]* of granitic composition called "aplites" are associated with granite margins.
  
 
Granite has been intruded into the [[Earth|Earth's crust]] during all [[Geologic ages|geologic periods]]; much of it is of [[Precambrian]] age. The origin of granite is contentious and has led to varied schemes of classification. There is a French scheme, a British scheme, and an American scheme. This confusion arises because the classification schemes define granite by different means. The "alphabet-soup" classification is often used because it classifies granite based on origin of the "parental" magma from which the granite was formed.
 
Granite has been intruded into the [[Earth|Earth's crust]] during all [[Geologic ages|geologic periods]]; much of it is of [[Precambrian]] age. The origin of granite is contentious and has led to varied schemes of classification. There is a French scheme, a British scheme, and an American scheme. This confusion arises because the classification schemes define granite by different means. The "alphabet-soup" classification is often used because it classifies granite based on origin of the "parental" magma from which the granite was formed.
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===Granitization===
 
===Granitization===
  
The granitization theory states that granite is formed in place by extreme [[metamorphism]]. 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 [[gneiss]]es. 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 volatiles which act as a [[catalyst]] by lowering the [[solidus]] temperature of the rock.
+
The granitization theory states that granite is formed in place by extreme [[metamorphism]]*&mdash;that is, the solid-state recrystallization of pre-existing rock. The production of granite by metamorphic heat is difficult, but is observed to occur in certain [[amphibolite]]* and [[granulite]]* terrains. 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 volatiles which act as a [[catalyst]] by lowering the [[solidus]] temperature of the rock.
  
 
==Emplacement mechanisms==
 
==Emplacement mechanisms==

Revision as of 16:06, 21 August 2006

File:Granite-Mt Bago-USGS-mfe00742.jpg
Jagged blades and pinnacles of granite on the south side of Mount Bago, Kings Canyon National Park, California.

Granite is a common and widely-occurring type of intrusive, felsic, igneous rock.

Granite is widely distributed throughout the continental crust of the Earth and is the most abundant "basement rock" that underlies the relatively thin veneer of sedimentary rock of the continents.

Granites are usually white, black, or buff colored, and are medium to coarse grained, 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, rounded massifs, and terrains of rounded boulders cropping out of flat, sandy soils. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive, hard and tough, and it is for this reason it has gained widespread use as a construction stone.

The average density of granite is 2.75 g·cm−3 with a range of 1.74 g·cm−3 to 2.80 g·cm−3.

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Mineralogy

Figure 1. QAPF diagram of granitoids and phaneritic foidolites (plutonic rocks).

Granite consists of a number of minerals, primarily orthoclase and plagioclase feldspars, quartz, hornblende, and muscovite or biotite micas. In addition, it contains minor amounts of other minerals, such as magnetite, garnet, zircon, and apatite. On rare occasions, a pyroxene is present.

The classification of granite is based on its mineralogic composition, using what is called the "QAPF diagram" for coarse-grained plutonic rocks (granitoids). QAPF is an acronym for four mineral groups: Quartz, Alkali feldspar (orthoclase), Plagioclase feldspar, and Feldspathoid (foid). Plutonic rocks are rocks formed when magma solidifies underground. (See the QAPF diagram on the right.)

According to modern petrology, true granite contains both plagioclase and orthoclase feldspars. When a granitoid is extremely rich in orthoclase, the rock is referred to as alkali granite. If quartz is either absent or present in relatively small amounts (under 5%), the rock is called syenite. 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.

Chemical Composition

The average proportions of the different chemical components in granites, on a worldwide basis, is approximately as follows (in descending order)[1]:

  • Silicon dioxide (SiO2) — 70.18%
  • Aluminum oxide (Al2O3) — 14.47%
  • Potassium oxide (K2O) — 4.11%
  • Sodium oxide (Na2O) — 3.48%
  • Calcium oxide (CaO) — 1.99%
  • Iron(II) oxide (FeO) — 1.78%
  • Iron(III) oxide (Fe2O3) — 1.57%
  • Magnesium oxide (MgO) — 0.88%
  • Water (H2O) — 0.84%
  • Titanium dioxide (TiO2) — 0.39%
  • Diphosphorus pentoxide (P2O5) — 0.19%
  • Manganese Oxide (MnO) — 0.12%


Origin and occurrence

In geology, granite is called an intrusive, igneous rock. An igneous rock is one that is formed when molten rock (magma) cools and solidifies. The term "intrusive" indicates that the rock solidified below the Earth's surface and intrudes other rocks.

Granite is currently known only on Earth, where it forms a major part of the continental crust. It has been found as relatively small (less than 100 km2), stock-like masses and also as huge formations called "batholiths", often associated with mountain ranges formed by "orogeny" (the mountain-forming processes of plate tectonics). Most granite intrusions are emplaced at depths between 1.5 kilometers (km) and 50 km in the continental crust. Small dikes of granitic composition called "aplites" are associated with granite margins.

Granite has been intruded into the Earth's crust during all geologic periods; much of it is of Precambrian age. The origin of granite is contentious and has led to varied schemes of classification. There is a French scheme, a British scheme, and an American scheme. This confusion arises because the classification schemes define granite by different means. The "alphabet-soup" classification is often used because it classifies granite based on origin of the "parental" magma from which the granite was formed.

Geochemical origins

As magma in the Earth's crust cools, it undergoes the process of "fractional crystallization"—a process by which some minerals crystallize before others. This process enriches the melt in silicon, aluminum, and potassium (the major constituents of granite), and lowers the content of iron, magnesium, titanium, calcium, and sodium.

This process operates regardless of the chemistry and origin of the magma, but it leaves geochemical and mineralogical evidence of the composition and origin of the parental magma from which granite was formed. The final mineralogy, texture, and chemical composition of a granite is often distinctive, based on its origin. For instance, a granite formed from melted sediments will contain more alkali feldspar (oligoclase), whereas a granite derived from melted basalt will be richer in plagioclase feldspar. Modern classification schemes are built on this basis.

Alphabet Soup Classification

The "alphabet soup" scheme of Chappell and White was proposed initially to divide granites into "I-type" (igneous protolith) and "S-type" (sedimentary protolith). Both of these types of granite were formed by the melting of precursor rock called the protolith.

Later, "M-type" (mantle-derived) granite was proposed, to cover granites that clearly arose from magmas sourced from the mantle. This type of granite is rare.

"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. An example of an A-type granite is a caldera at Yellowstone National Park.

Granitization

The granitization theory states that granite is formed in place by extreme metamorphism—that is, the solid-state recrystallization of pre-existing rock. The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains. 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 volatiles which act as a catalyst by lowering the solidus temperature of the rock.

Emplacement mechanisms

The problem of emplacing large volumes of molten rock within the solid Earth has faced geologists for over a century, and is not entirely resolved. Granite 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
  • Diapirism where the density of the lighter granite causes relative buoyancy and the granite pushes upwards, warping and folding the rock above it
  • 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 phenomenon can be used to explain granite intrusions, and that not all granites can be explained by one or another mechanism.

Uses

Antiquity

The Red Pyramid of Ancient Egypt (c.26th century B.C.E.), 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 B.C.E.) 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, [1] 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 [2] 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. In fact, the amount of granite in them is comparable to the Great Pyramid of Giza. [3]

Modern

Granite has been extensively used as a dimension stone and as flooring tiles in public and commercial buildings and monuments. 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 has been a popular choice for kitchen countertops due to its high durability and aesthetic qualities. The Black Galaxy granites from the Cheemakurthy area of Andhra Pradesh in India are world known for their elegance.

Engineers have traditionally used polished granite surfaces to establish a plane of reference, since they are relatively impervious and inflexible.

In the world of sports, curling rocks are traditionally fashioned of granite.

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.

See also

Footnotes

  1. E. H. Shackleton (1971). Lakeland Geology, 3rd edition, Clapham (via Lancaster): Dalesman, 49. 

External links

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