Granite Petrology & Igneous Rock Information

Granite is a common and widely-occurring group of intrusive felsic igneous rocks that forms at great depths and pressures under continents. Granite consists of orthoclase, plagioclase quartz, hornblende, biotite, muscovite and minor accessories such as magnetite, garnet, zircon and apatite minerals. Rarely a pyroxene is present. Ordinary granite always carries a small amount of plagioclase, but when this is absent the rock is referred to as alkali granite. An increasing proportion of plagioclase feldspar causes granite to pass into granodiorite. A rock consisting of equal proportions of orthoclase and plagioclase plus quartz may be considered a quartz monozonite. A granite containing both muscovite and biotite micas is called a binary granite.

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

Granite occurs as stock-like masses and as batholiths often associated with mountain ranges and frequently of great extent. Granite has been intruded into the crust of the Earth during all geologic periods, except perhaps the most recent; much of it is of Precambrian age. Granite is widely distributed throughout the Earth.

Because of its hardness and comparative cheapness in relation to marble, granite is often used to make kitchen countertops. A granite countertop can be cut in any shape, and it is virtually unscratchable. Very hot pots must not be placed onto it though, because the temperature differential could possibly crack the granite.

Average density: 2750 kg/m3 (range 1741 to 2800)


Igneous rocks are crystalline or glassy rocks formed by the cooling and solidification of molten magma. Igneous rocks comprise one of the three principal classes of rocks, the others being metamorphic and sedimentary.

Igneous rocks are formed from the solidification of magma, which is a hot (600 deg.C - 1300 deg.C, or 1100 deg. - 2400 deg. F) molten or partially molten rock material. The Earth is composed predominantly of a large mass of igneous rock with a very thin covering of sedimentary rock. Whereas sedimentary rocks are produced by processes operating mainly at the Earth's surface such as weathering and erosion, igneous--and metamorphic--rocks are formed by internal processes that cannot be directly observed.

Magma is thought to be generated within the asthenosphere (the layer of partially molten rock underlying the Earth's crust) at a depth below about 60-100 kilometers (40-60 miles). Because magma is less dense than the surrounding solid rocks, it rises toward the surface. It may settle within the crust or erupt at the surface from a volcano as a lava flow. Rocks formed from the cooling and solidification of magma deep within the crust are distinct from those erupted at the surface mainly owing to the differences in conditions in the two environments. Within the Earth crust the temperatures and pressures are much higher than at its surface; consequently, the hot magma cools slowly and crystallizes completely. The slow cooling promotes the growth of minerals large enough to be identified visually without the aid of a microscope (called phaneritic, from the Greek phaneros, meaning "visible"). On the other hand, magma erupted at the surface is chilled so quickly that the individual minerals have little or no chance to grow. As a result, the rock is either composed of minerals that can be seen only with the aid of a microscope (called aphanitic, from the Greek aphanes, meaning "invisible") or contains no minerals at all (in the latter case, the rock is composed of glass, which is really a viscous, non-crystalline liquid). This results in two groups of igneous rocks: (1) plutonic or intrusive igneous rocks that solidified deep within the earth and (2) volcanic, or extrusive, igneous rocks formed at the Earth's surface.

The deep-seated plutonic rocks can be exposed at the surface for study only after a long period of weathering or by some tectonic forces that push the crust upward or by a combination of the two. The exposed intrusive rocks are found in a variety of sizes, from small dikes to massive dome-shaped batholiths, which cover hundreds of square miles and make up the cores of many mountain ranges.

Extrusive rocks occur in two forms: (1) as lava flows that flood the land surface much like a river and (2) as fragmented pieces of magma of various sizes (pyroclastic materials), which often are blown through the atmosphere and blanket the Earth's surface upon settling. The coarser pyroclastic materials accumulate around the erupting volcano, but the finest pyroclasts can be found as thin layers located hundreds of miles from the opening. Most lava flows do not travel far from the volcano, but some low-viscosity flows that erupted from long fissures have accumulated in thick sequences. Both intrusive and extrusive magmas have played a vital role in the spreading of the ocean basin, in the formation of the oceanic crust, and in the formation of the continental margins. Igneous processes have been active since the formation of the Earth some 4.6 billion years ago.


Igneous rocks are formed when molten rock (magma derived from the mantle, or, pre-existing rocks molten by extreme temperature) cools and solidifies, with or without crystallization. Over 700 types of igneous rocks have been described, most of them intrusive.

Igneous rock are geologically important because:

their minerals and global chemistry gives information about the composition of the mantle where they were extracted from, and the temperature and pressure conditions that allowed this extraction, or (below) their minerals and global chemistry gives information about the composition of the country pre-existing rock that melted

their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent strata, allowing a time sequence of events

their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions

in some special circumstances they host important mineral deposits, of, for example, tungsten, tin or uranium, commonly associated with granites they can be explored as ornamental stone

Igneous rocks are classified according to mode of occurrence, texture, chemical composition, and the geometry of the igneous body.

Modes Of Occurrence

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic) or extrusive (effusive).

Intrusive rocks crystallize within the crust interior.

Extrusive rocks are the result of volcanic eruptions and, therefore, solidify in atmospheric conditions.


The most important distinction in igneous rocks textures is related to grain size.

Phaneritic rocks contain minerals with grains (crystals) visible to the unaided eye and are commonly intrusive (as the slower cooling rates allow the formation of large crystals). In the extreme, such rocks may contain extremely large crystals, in which case they are termed pegmatitic.

In extrusive rocks, where cooling is much more rapid, the individual mineral crystals are usually not visible and these rocks are termed aphanitic.

Porphyritic textures are an intermediate situation between the previous two: the groundmass of the rock has an aphanitic texture, but crystals (termed in this particular occurrence as phenocrystals) are visible to unaided eye.

If a molten magma cools at extremely high rates, allowing no crystallization, the result is a vulcanic glass called obsidian.

Crystal Shapes

Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euedral, subeuedral or anedral:

Euedral, if the crystallographic shape is preserved

Subeuedral, if only part is preserved

Anedral, if the crystal presents no recognizable crystallographic direction

Chemical Composition

Igneous rocks can be subdivided according to two main chemical parameters:

contents of silica:

acid igneous rocks present a high silica content (ex: granite)

basic igneous rocks have low silica content (ex: basalt)

intermediate igneous rocks

contents of quartz, alkali feldspar and feldspatoids: the felsic minerals

felsic rock, with predominance of felsic minerals; these rocks (ex: granite) are usually light coloured

mafic rock, with predominance of mafic minerals (all other minerals, excluding the felsic); these rocks (ex: basalt) are usually dark coloured

ultramaphic rock, with more that 90% of mafic minerals (ex: dunite)

Note that light coloured rocks, such as limestone or sandstone cannot be classified as felsic, because their origin is not igneous, it is sedimentary.

The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.

Mode of

Geometry Of The Igneous Body

Igneous rocks can also classified according to the shape and size of the intrusive body and its relation to the other formations into which it intrudes. Typical intrusive formations are batholiths, laccoliths, sills and dikes. The extrusive types usually are called lavas.

Example Of Classification

Granite is an igneous, intrusive rock (crystallised at depth), with felsic acid composition (rich in silica and with more than 10% of felsic minerals) and phaneritic, subeuedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes).


Rocks Classification Image

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