The origin of granite has for long fascinated geologists though serious debate on the topic may be said to date from a famous meeting of the Geological Society of France in Read, I entered his study as an amateur fossil collector and left it as a committed granite petrologist - after just ten minutes!
I can hardly aspire to convert my reader in so dramatic a way, yet this book is an attempt, however inadequate, to pass on the enthusiasm that I inherited, and which has been reinforced by innumerable discussions on the outcrop with granitologists of many nationalities and of many shades of opinion. Since the s, interest in granites has been greatly stimulated by the thesis that granites image their source rocks in the inaccessible deep crust, and that their diversity is the result of varying global tectonic context.
So great a body of new data and new ideas has accumulated that my attempt to review the whole field of granite studies must carry with it a possible charge of arrogance, especially as I have adopted the teaching device of presenting the material from a personal point of view with its thinly disguised prejudices. The sources of granitic magmas in their various global.
The Nature and Origin of Granite
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. 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 analyses: Granite containing rock is widely distributed throughout the continental crust. 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.
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 continents. 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 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 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.
Granite - Wikipedia
It is on this basis that the modern "alphabet" classification schemes are based. Granite has a slow cooling process which forms larger crystals. 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.
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. An old, and largely discounted theory, granitization states that granite is formed in place by extreme metasomatism by fluids bringing in elements, e.
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.
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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.
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. Rocks there do not deform so easily: 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.
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:. 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.
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, alter feldspar in a process called hydrolysis.
An end product of granite weathering is 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 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. Potassium 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. Thorium occurs in all granites as well. There is some concern that some granite sold as countertops or building material may be hazardous to health.
Dan Steck of St.
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 by National Health and Engineering Inc. 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.
Granite and related marble industries are considered one of the oldest industries in the world; existing as far back as Ancient Egypt. The Red Pyramid of Egypt c.
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Menkaure's Pyramid , likely dating to the same era, was constructed of limestone and granite blocks. The Great Pyramid of Giza c. 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 , sills , jambs , and wall and floor veneer.
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Patrick Hunt  has postulated that the Egyptians used emery , which has greater hardness on the Mohs scale. The Brihadeeswarar Temple dedicated to Lord Shiva was built in 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". 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 colour.
In some areas, granite is used for gravestones and memorials.