Granite

• 1. Introduction

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  Granite takes its name from the Latin word granum, meaning a grain, a nod to the coarse crystals that interlock across its surface. It is a hard rock, sitting around 6.5 on the Mohs hardness scale, and it forms underground when silica-rich magma cools slowly and solidifies. It is the most abundant basement rock beneath the continents, the foundation hidden under a thin veneer of sediment. Yet this single rock raises a tangle of questions. How does a mass of molten material shove aside cold, brittle crust to make room for itself? Why does it harden with age, foiling Medieval stoneworkers? And why might the slab on a kitchen counter carry a trace of radiation?

• 2. A Family Of Crystals

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  Quartz, alkali feldspar, mica, and plagioclase make up the bulk of granite, locking together into a somewhat equigranular matrix. The lighter feldspar and quartz are peppered with darker biotite mica and amphibole, often hornblende. Granite can be white, pink, or gray, depending on which minerals dominate. When some crystals, called phenocrysts, grow larger than the surrounding groundmass, the texture is described as porphyritic, and the rock becomes a granite porphyry.

  True granite is only one member of a wider clan called granitoids, sorted by the QAPF classification. By modern petrologic convention, true granite contains between 20% and 60% quartz by volume, with alkali feldspar making up 35% to 90% of the total feldspar. Rocks poorer in quartz become syenites or monzonites; those dominated by plagioclase become granodiorites or tonalites. A few granitoids, the leucogranites, contain almost no dark minerals at all.

  The classification grows finer still. A granite whose feldspar is 65% to 90% alkali feldspar is a syenogranite, while monzogranite sits between 35% and 65%. A granite carrying both muscovite and biotite is called a binary or two-mica granite. Granites also split by their metal ratios into metaluminous, peralkaline, and peraluminous, the peralkaline kind holding unusual sodium amphiboles such as riebeckite.

• 3. Born From Deep Magma

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  Felsic magma, rich in silica, is the parent of granite, and it forms by adding heat or water vapor to rock of the lower crust rather than by decompressing the mantle. At convergent boundaries, where oceanic crust subducts beneath continental crust, subducted sediments may melt into a magma that grows richer in silica as it rises. Early fractional crystallisation strips the melt of magnesium and chromium while enriching it in iron, sodium, potassium, aluminum, and silicon.

  Fractional crystallization alone struggles to explain granite's sheer volume. In the South Sandwich Islands, granitic rock makes up just 4% of the exposures. In continental arcs, by contrast, granitic rocks are the most common plutonic rocks, with batholiths running the entire length of the arc. Geologists point to other processes, such as injecting basaltic magma into the lower crust or heating that crust by underplating, to generate felsic magma directly.

  The parent rock leaves fingerprints, and these underpin the letter-based Chappell and White classification. I-type granites, with a strontium isotope ratio below 0.708, are high in sodium and calcium and known for porphyry copper deposits. S-type granites are sodium-poor and aluminum-rich, carry biotite and muscovite, host tin ores, and often hold xenoliths of metamorphosed sedimentary rock. A-type granites, high in silicon and potassium, form over hot spots and continental rifting; examples occur in the Koettlitz Glacier Alkaline Province in the Royal Society Range, Antarctica.

• 4. The Room Problem

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  Granite magma has a density of 2.4 Mg/m3, well below the 2.8 Mg/m3 of high-grade metamorphic rock, giving it tremendous buoyancy once enough accumulates. Ascent then becomes inevitable. But how such a vast mass pushes aside surrounding country rock, the so-called room problem, remains a matter of research. Two main mechanisms compete: Stokes diapirism and fracture propagation.

  Stokes diapirism imagines magma rising as a single buoyant mass, heating wall rocks so they flow around it like a power-law fluid. This works in the warm, ductile lower crust but stalls in the cold, brittle upper crust, where the magma would cool and solidify before climbing higher. Fracture propagation, favored by many geologists, sends magma up through small self-propagating dykes along faults and shear zones, the first magma solidifying to insulate what follows.

  The mechanisms can also work together. A diapir may rise through stoping, cracking its roof so blocks of crust sink while magma takes their place. This appears as piecemeal stoping, as cauldron subsidence, or as roof foundering with a caldera eruption. There is evidence for cauldron subsidence at the Mt. Ascutney intrusion in eastern Vermont. Assimilation offers another path, the magma melting upward and contaminating itself with crustal material, a process detectable in isotope ratios.

• 5. Wearing Down

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  Exfoliation joints crack granite apart on a large scale, the rock expanding and fracturing as overlying material erodes away and pressure is relieved. Chemical weathering works more quietly: dilute carbonic acid and other acids in rain and soil water alter feldspar through hydrolysis, turning potassium feldspar into kaolinite and leaving potassium, bicarbonate, and silica in solution. The coarse crumbled remains of disintegrated granite are called grus.

  Climate dictates the pace. For about two thousand years, the relief engravings on Cleopatra's Needle survived the arid conditions of their origin, only to deteriorate drastically within two hundred years in the damp, polluted air of London. Soils over granite tilt acidic and sandy in cool humid climates through podzolization, while warm humid regions accelerate feldspar weathering to yield far more clay, with the Cecil soil series a prime example of the resulting Ultisol.

  Fire adds its own assault. Temperatures often exceeding 1000 °C drive differential thermal expansion of mineral grains and polymorphic shifts such as the alpha-beta quartz transition. Phyllosilicates decompose at specific temperatures, and the rock ends up micro-fractured, more porous, and significantly weaker.

• 6. A Quiet Source Of Radiation

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  Potassium-40, a weakly emitting radioactive isotope, rides inside the alkali feldspar that fills granitic rocks, most abundantly in alkali feldspar granite and syenites. Some granites carry around 10 to 20 parts per million of uranium, far above the 1 to 5 ppm found in mafic rocks like tonalite, gabbro, and diorite. Conway granite has been noted for a relatively high thorium concentration of 56 plus or minus 6 ppm. Many large granite plutons feed roll-front uranium ore deposits as uranium washes into sediments from the granite uplands.

  The decay of that uranium produces radon gas, which can collect in cellars and basements built into soils over granite. Radon poses real health concerns and ranks as the number two cause of lung cancer in the US behind smoking. Concern has extended to granite sold as countertops. Dan Steck of St. Johns University has stated that roughly 5% of all granite is of concern, while cautioning that only a tiny fraction of the tens of thousands of slab types have been tested.

  Testing has tempered the alarm. In November 2008, a study initiated and paid for by the Marble Institute of America and carried out by National Health and Engineering Inc. measured 39 full-size granite slabs. All showed radiation levels well below European Union safety standards and radon emission well below average outdoor radon concentrations in the US.

• 7. Stone Of The Ancients

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  The Great Pyramid of Giza, dating to around 2580 BC, holds a granite sarcophagus fashioned of Red Aswan Granite. The Pyramid of Menkaure, likely from about 2510 BC, mixed limestone and granite blocks. The ruined Black Pyramid of Amenemhat III once wore a polished granite pyramidion, now displayed in the main hall of the Egyptian Museum in Cairo. How the Egyptians worked such hard stone is still debated; the Egyptologist Anna Serotta found tool marks pointing to flint on finer work, while Patrick Hunt has postulated the use of emery.

  Granite shrines rose far beyond Egypt. The Seokguram Grotto in Korea, completed in 774 AD as part of the Bulguksa temple complex, is an artificial grotto built entirely of granite and was added to the UNESCO World Heritage List in 1995. In South India, Rajaraja Chola I built the world's first temple entirely of granite, the Brihadeeswarar Temple dedicated to Lord Shiva, finished in 1010 in Tanjore. Its massive Gopuram is believed to weigh around 81 tonnes.

  Imperial Roman granite came mainly from Egypt, along with Turkey and the islands of Elba and Giglio, becoming an integral part of the Roman language of monumental architecture before quarrying ceased around the third century AD. Through case-hardening, granite grows harder with age. Medieval stoneworkers, lacking tempered metal chisels, were forced to use saws or emery on ancient columns. Giorgio Vasari noted in the 16th century that quarried granite was far softer and easier to work than after it had lain exposed.

• 8. From Tombstones To Curling Rinks

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  Alexander MacDonald of Aberdeen, inspired by ancient Egyptian granite carvings, invented steam-powered cutting and dressing tools that transformed memorial work. In 1832, the first polished tombstone of Aberdeen granite in an English cemetery was installed at Kensal Green Cemetery, causing a sensation in the London monumental trade. With sculptors William Leslie and later Sidney Field, granite memorials became a major status symbol in Victorian Britain. The royal sarcophagus at Frogmore, at 30 tons, was probably the pinnacle of MacDonald's work.

  Granite shaped American building and transport too. Aberdeen in Scotland, built principally from local granite, is known as The Granite City. The Granite Railway, America's first railroad, was built in the 1820s to haul granite from quarries in Quincy, Massachusetts, to the Neponset River. Finland planted granite boulders along its Mannerheim Line to block Russian tanks in the Winter War of 1939-40. Engineers prize polished granite surface plates for their dimensional stability, using granite as the body of CMMs and high-precision CNC machines.

  Curling carries one of granite's most particular legacies. Curling stones are traditionally fashioned of Ailsa Craig granite, first made in the 1750s from the island of Ailsa Craig in Scotland. Because the granite is rare, the best stones can cost as much as US$1,500, and between 60 and 70 percent of stones used today come from this source. Though Ailsa Craig is now a wildlife reserve, it is still quarried under license by Kays of Scotland, keeping a centuries-old supply alive for the rinks of today.