Granite
Granite is a coarse-grained intrusive igneous rock composed mostly of quartz, alkali feldspar, mica and plagioclase. The word granite comes from the Latin granum, meaning grain, in reference to its completely crystalline structure. True granite contains between 20% and 60% quartz by volume. It also requires that 35% to 90% of the total feldspar consists of alkali feldspar. These specific percentages define true granite within the QAPF classification system used by petrologists. Granitic rocks poorer in quartz are classified as syenites or monzonites instead. Rocks dominated by plagioclase become granodiorites or tonalites. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Such stones are typically high in potassium and low in plagioclase. They often fall into S-type or A-type categories based on their chemical makeup. Some granites contain almost no dark minerals and are known as leucogranites. Others feature phenocrysts larger than the groundmass, creating a porphyritic texture. This specific texture defines a granite porphyry.
Granite forms from silica-rich magmas that slowly cool and solidify underground. Felsic magmas form by adding heat or water vapor to rock in the lower crust. This process differs from basaltic magma formation which involves decompression of mantle rock. Early fractional crystallisation reduces magnesium and chromium while enriching iron, sodium, potassium, aluminum, and silicon. Further fractionation lowers iron, calcium, and titanium content. Granite makes up just 4% of exposures in the South Sandwich Islands but dominates continental arc settings. Batholiths composed of these rocks extend the entire length of such arcs. One mechanism involves injecting basaltic magma into the lower crust followed by differentiation. Another method heats the lower crust via underplating basaltic magma. These processes produce different kinds of granites reflected in I-type and S-type divisions. I-type granites show high sodium and calcium with strontium isotope ratios below 0.708. S-type granites are sodium-poor and aluminum-rich with ratios typically greater than 0.708. A-type granites form over hot spots and continental rifting rather than at convergent boundaries. They contain particularly high silicon and potassium at the expense of calcium and magnesium.
The average density of granite lies between 2.65 and 2.75 grams per cubic centimeter. Its compressive strength usually exceeds 200 megapascals or 29,000 psi. Granite has poor primary permeability overall but strong secondary permeability through cracks if present. Physical weathering occurs on a large scale as exfoliation joints result from pressure relief during erosion. Chemical weathering happens when dilute carbonic acid alters feldspar through hydrolysis. This process causes potassium feldspar to form kaolinite while releasing ions into solution. An end product of this breakdown is grus made up of coarse-grained fragments. Climatic variations influence how fast these rocks degrade. Cleopatra's Needle obelisk survived arid conditions for about two thousand years before transfer to London. Within two hundred years in damp polluted air, the red granite drastically deteriorated. Fires exceeding 1000 degrees Celsius cause differential thermal expansion that damages the rock. Such heat induces substantial volume changes generating internal stresses. The decomposition of phyllosilicates at specific temperatures further contributes to degradation. Granite becomes micro-fractured with increased porosity and significantly reduced mechanical strength.
Granite industries exist as far back as Ancient Egypt where it was used for monumental architecture. The Red Pyramid of Egypt dates to the third largest of Egyptian pyramids. The Great Pyramid of Giza around 2580 BC contains a sarcophagus fashioned of Red Aswan Granite. The mostly ruined Black Pyramid once had a polished granite pyramidion now displayed in Cairo. Tool marks described by Egyptologist Anna Serotta indicate flint tools were used on finer work. Patrick Hunt postulated that Egyptians used emery which has greater hardness than stone. The Seokguram Grotto in Korea completed in 774 AD is an artificial grotto constructed entirely of granite. Rajaraja Chola I built the world's first temple entirely of granite in Tanjore India during the 11th century. The Brihadeeswarar Temple dedicated to Lord Shiva was finished in 1010. Its massive Gopuram is believed to have a mass of around 81 tonnes. Imperial Roman granite was quarried mainly in Egypt, Turkey, Elba and Giglio. Quarrying ceased around the third century AD before reuse began in Late Antiquity.
A key breakthrough occurred when Alexander MacDonald invented steam-powered cutting and dressing tools inspired by ancient Egyptian carvings. In 1832 the first polished tombstone of Aberdeen granite appeared at Kensal Green Cemetery. It caused a sensation in the London monumental trade for some years. All polished granite ordered came from MacDonald's workshop until rival machinery emerged in the 1880s. Granite became known as The Granite City due to its abundance in Aberdeen Scotland. The Granite Railway America's first railroad hauled granite from Quincy Massachusetts to Neponset River in the 1820s. Engineers use polished granite surface plates to establish reference planes since they maintain dimensional stability. Finland planted granite boulders along its Mannerheim Line to block Russian tanks in the Winter War of 1939-40. Curling stones are traditionally fashioned of Ailsa Craig granite with best examples costing up to US$1,500. Between 60% and 70% of stones used today come from this Scottish island source. Granite countertops remain popular choices in the United States due to affordability and aesthetic appeal.
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Common questions
What is granite composed of by volume?
True granite contains between 20% and 60% quartz by volume. It also requires that 35% to 90% of the total feldspar consists of alkali feldspar.
How does granite form underground?
Granite forms from silica-rich magmas that slowly cool and solidify underground. Felsic magmas form by adding heat or water vapor to rock in the lower crust.
When was the Great Pyramid of Giza built with Red Aswan Granite?
The Great Pyramid of Giza around 2580 BC contains a sarcophagus fashioned of Red Aswan Granite. The mostly ruined Black Pyramid once had a polished granite pyramidion now displayed in Cairo.
Where did Alexander MacDonald invent steam-powered cutting tools for granite?
Alexander MacDonald invented steam-powered cutting and dressing tools inspired by ancient Egyptian carvings. In 1832 the first polished tombstone of Aberdeen granite appeared at Kensal Green Cemetery.
Why do some granites become micro-fractured during fires?
Fires exceeding 1000 degrees Celsius cause differential thermal expansion that damages the rock. Such heat induces substantial volume changes generating internal stresses.