Granite

Granite is a coarse-grained, intrusive igneous rock composed chiefly of quartz, alkali feldspar, and plagioclase. It forms when silica-rich, felsic magma cools slowly beneath the Earth’s surface, allowing large, visible crystals to develop. This slow crystallisation typically occurs within continental crust, where granite forms a major component of batholiths, stocks, and other intrusive bodies that constitute the deep foundations of many mountain belts. Granite belongs to a broader family of granitic rocks known as granitoids, all characterised by visibly interlocking grains composed predominantly of feldspar and quartz.

Mineralogy, Texture, and Classification

Granite derives its name from the Latin granum, referring to its grainy, holocrystalline texture. It typically appears white, pink, or grey, depending on its mineral proportions. The rock consists of an interlocking mosaic of feldspar, quartz, and darker minerals such as biotite or hornblende. Textures range from equigranular to porphyritic, the latter marked by larger feldspar phenocrysts embedded within a finer-grained matrix. Although most granites contain significant dark minerals, leucogranites are notable exceptions, being composed almost entirely of light-coloured constituents.
Petrographic classification relies on the QAPF diagram, which distinguishes different granitoid types based on the relative proportions of quartz (Q), alkali feldspar (A), and plagioclase (P). True granite contains between roughly 20 and 60 per cent quartz and is dominated by alkali feldspar. Varieties richer in plagioclase are designated granodiorite or tonalite, whereas those with particularly high alkali feldspar content are classed as alkali-feldspar granites. When quartz exceeds approximately 60 per cent, the rock may be described as quartz-rich granitoid or quartzolite. Further subdivisions, such as syenogranite or monzogranite, reflect the proportion of alkali feldspar relative to total feldspar.
Granites may also be grouped according to mineral content and geochemical features. Two-mica granites, containing both biotite and muscovite, tend to be enriched in potassium and are commonly associated with crustal melting processes. Metaluminous granites contain feldspar-forming quantities of aluminium, sodium, and potassium, while peralkaline and peraluminous granites exhibit chemical excesses that lead to formation of distinctive accessory minerals such as riebeckite or muscovite.

Physical and Chemical Properties

Granite is renowned for its durability. It is massive, meaning it lacks internal layering, and ranks between 6 and 7 on the Mohs hardness scale. Its compressive strength commonly exceeds 200 megapascals, making it suitable for structural and decorative uses. The density typically ranges between about 2.6 and 2.7 grams per cubic centimetre. Although granite has low primary permeability, secondary permeability can occur where fractures and joints are present.
Chemically, granite is dominated by silica, alumina, sodium, potassium, and calcium oxides. It represents one end of the igneous compositional spectrum, being far more silica-rich than mafic rocks such as basalt. The presence of water significantly reduces its melting temperature, which otherwise lies above 1100°C at low pressures. The extrusive equivalent of granite is rhyolite, while medium-grained intrusive equivalents are termed microgranites.

Distribution and Geological Occurrence

Granite is widely distributed in the continental crust and forms the basement rocks beneath much of the Earth’s sedimentary cover. The most extensive exposures occur within ancient shields, where Precambrian granites constitute large portions of the crystalline crust. Granite also appears in orogenic belts as large batholiths emplaced during mountain-building processes. Smaller intrusions occur as stocks, dykes, and sills. In some landscapes, granite outcrops form dome-like masses, tors, or inselbergs due to weathering processes that exploit joint patterns in the rock. Coarse-grained pegmatites, often associated with the margins of granite intrusions, may contain exceptionally large crystals and host economically valuable minerals.

Origins and Magmatic Processes

Granite forms from felsic magmas generated primarily within continental crust. Unlike basaltic magma, which often originates in the mantle via decompression melting, granitic magma typically results from heating or hydration of lower crustal rocks. Processes include:

• Addition of water or heat: facilitating partial melting of crustal material.
• Fractional crystallisation: progressive removal of early-formed mafic minerals enriches remaining melt in silica and alkali elements, promoting felsic character.
• Crustal assimilation: incorporation and melting of surrounding rock during magma ascent.

At convergent plate boundaries, particularly continental arcs, large batholiths of granitic composition form due to prolonged magmatic activity and repeated magma injections into the crust. Although fractional crystallisation alone can generate small amounts of granite from basaltic melts, the immense volumes seen in major batholiths require additional processes such as basaltic underplating and extensive crustal melting.
In some tectonic settings, such as island arcs, granitic rocks represent a relatively small proportion of exposed material, whereas in continental arcs they dominate the intrusive suites. This variation reflects differences in crustal thickness, heat flow, and the availability of crustal material for melting.

Alphabet Classification and Petrogenesis

Granite classification schemes based on source material help interpret the geological history of granitic bodies. The widely applied alphabet system defines several types:

• I-type (igneous source): derived from meta-igneous rocks, generally richer in plagioclase and exhibiting higher sodium and calcium contents.
• S-type (sedimentary source): produced by melting of metasedimentary rocks, typically richer in potassium and aluminium, and often containing muscovite or other aluminium-rich minerals.

These distinctions correlate with isotopic signatures, accessory mineral assemblages, and field relationships. The origins inferred from such classifications provide insights into crustal evolution, tectonic processes, and thermal regimes at the time of granite formation.

Uses and Cultural Significance

Granite’s hardness, resistance to weathering, and attractive appearance have made it a favoured material in construction throughout history. It has been used for monuments, building facades, paving, and architectural details. Polished granite remains popular for decorative applications, including countertops and interior design. The stone’s durability also ensures its continued use in engineering projects, such as bridge supports and kerbstones.
Because large granite masses form prominent landscape features, they have influenced settlement patterns, fortification sites, and cultural landmarks. The study of granite has further contributed to understanding crustal processes, continental growth, and the thermal evolution of the Earth’s lithosphere.