The word igneous comes from the Latin word ignis meaning fire. Whilst not literally on fire, Igneous rocks do form from magma, molten rock beneath the Earth’s surface. When magma reaches the surface it is called lava. The reservoir of hot magma below ground is known as a Batholith. When it forces itself horizontally between layers of overlying rocks it is called a Sill. Vertical, columnar structures are called Dikes or Pipes, whilst any lens-like bodies are known as Laccoliths. Strictly speaking, these names are only given to these structures after the magma has cooled and solidified into solid rock.

A major characteristic of igneous rocks is the lack of layers or bands. Instead they have interlocked crystalline structures and a uniform texture irrespective of the direction of the rock. They are usually hard and over 99% of the chemical composition consists of just nine elements; oxygen, silicon, aluminium, iron, calcium, sodium, potassium, magnesium and titanium. These combine in various proportions to form eight primary minerals which make up the majority of igneous rock types. These are subdivided into two groups of four minerals each; the Felsic and Mafic rocks. In addition accessory minerals such as Apatite, Garnet, Sphene, Spinel and Zircon may also be present, but in much smaller quantities, usually as small crystals embedded in the primary minerals of the igneous rock.

Felsic Igneous Rocks

The Felsic rocks, also called acidic igneous rocks, are rich in minerals that are both lighter in colour and less dense than Mafic rocks. They are rich in silicates and the specific minerals are Quartz, Feldspars, Feldspathiods (similar to feldspars but containing less silica) and Muscovite. The name Felsic refers to the Feldspar and silicate content of these rocks. They are collectively known as Essential Minerals, which means that if a rock lacks any of them it cannot be classified as a Felsic Igneous Rock. However it is entirely possible for one or more to dominate which conversely means that the others might be present in proportionately smaller quantities. Examples of Felsic Igneous Rocks include Granite and Diorite.

Mafic Igneous Rocks

The Mafic rocks are also called basic or "dark-coloured" igneous rocks. Their chemical composition is dominated by the heavy elements iron and magnesium which give these rocks their high density and much greater weight. The composition gives the rock its name taking the Ma from Magnesium and Fi from Ferric, pertaining to the iron. Unlike Felsic rocks, these are low in silica, the main constituent minerals being Olivine, Pyroxene, Amphibole and Biotite. Basalt is the most common Mafic rock. It typically forms the ocean floor and the heavy layer of the Earth’s crust underneath the granitic continents.

Crystallinity of Igneous Rocks

In addition the classification based on the chemical composition, igneous rocks can also be categorized into four categories that describe their crystalline form.

• Crystalline (also called holo­crystalline) – rocks composed entirely of well-defined crystals, such as Granite
• Hemicrystalline (also called hypo­crystalline) – rocks that contain mostly crystals but contain some glassy amorphous mass
• Hemihyaline (also called hypo­hyaline) – rocks that are mostly igneous glass, but they have a small degree of crystal structure
• Hyaline (also called holo-hyaline) ­ rocks that are completely glass, such as Obsidian.

Granularity of Igneous Rocks

Another way of classifying igneous rocks is based on their granularity, which describes the relative size of the most common crystals (‘grains’) in the rock.

• Phaneritic (‘coarse­grained’) - individual mineral grains can be seen without magnification; grains are more than 5mm in diameter
• Aphanitic (‘medium­grained’) – individual grains can only be seen with a hand magnifier; grains are between 1 and 5mm in diameter
• Microcrystalline (‘fine­grained’) – individual grains can only be seen under a microscope; grains are between 0.01 and 1 mm in diameter
• Cryptocrystalline (‘very fine-grained’) – individual grains cannot easily be seen with a normal microscope; grains less than 0.01 mm in diameter.
• Hyaline (Subaphanitic) ­ no individual grains at all; usually amorphous or glassy.

Colour Classification of Igneous Rocks

The next common classification of igneous rocks is based on their colour, which reflects their Mafic content or the percentage of basic or dark-coloured minerals in their composition.

• Leucocratic - less than one-third Mafic minerals. The rock is therefore light coloured. Examples are Granite, Monzonite and Syenite
• Mesocratic – ranging from one third to two thirds Mafic minerals. Therefore medium coloured rock. Examples are Andesite and Diorite.
• Melanocratic – more than two thirds mafic minerals. The rock is therefore dark. Examples are Basalt and Gabbro

Classification of Igneous Rocks based on formation

Igneous rocks are also classified into two large groups based on where they solidified.

• Intrusive Igneous Rocks - solidified below the Earth's surface either as the original magma reservoir or in the body of other preceding rock. Examples are Gabbro, Granite, and Syenite. ?
• Extrusive Igneous rocks - solidified above the Earth's surface following their eruption from below as lavas. Examples are Basalt and Obsidian.

The Main Classification of Igneous Rocks

The main classification of Igneous rocks is based on a combination of Mineral composition and granularity. The rocks are then subdivided into extrusive vs extrusive rocks. Below are descriptions of the most commonly encountered igneous rocks.

Intrusive Igneous Rocks

Intrusive igneous rocks (also called plutonic rocks, named after Pluto the Roman god of the underworld) formed from magma that cooled and solidified below the Earth's surface. Because these rocks cool slowly, being insulated by surrounding rocks, their crystals have time to grow. The greater the depth of cooling and the longer the time of cooling, the larger their crystals and therefore the coarser the texture. Consequently intrusive igneous rocks usually have coarse to medium sized mineral grains.

DIORITE - composed of medium to fine crystalline grains. The mineral content is mostly Amphibole, Biotite, Plagioclase Feldspar and Pyroxene (commonly Augite), a mixture of Felsic and Mafic species. Grey to dark green in colour, often dark because of additional Hornblende. Relatively hard and moderately dense. Quartz is sometime present between 5% and 20%, but if quartz exceeds 20% the rock is called a "grano-diorite." This is an intermediate rock in a series which eventually ends in Granite. Diorite is the result of fractional melting of mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in areas associated with cordilleran mountain building. It emplaces in large batholiths and sends magma to the surface to produce composite volcanoes with andesite lavas.

GABBRO – composed mostly of Plagioclase Feldspars and Pyroxenes, with traces of Hornblende, Ilmenite, and Olivine. Dark coloured ranging from dark grey to greenish-black. Medium to fine texture. Relatively hard and moderately dense. Dolerite (also called Diabase) is a fine-grained variety of Gabbro. The rock is typically found on continents in dikes, sills, and laccoliths from slow cooling of mafic magma, mostly derived from hot spots. Gabbro is also part of the oceanic lithosphere (ophiolite suite) forming as a thick layer below the uppermost pillow lavas across all the oceanic floors.

GRANITE - the most common intrusive igneous rock. Typically composed of about 30% Quartz, 60% K-feldspars and 10% Ferromagnesian minerals such as Hornblende. The K-feldspar usually gives a particular granite its distinctive colour (grey, pink or red) whilst the dark ferromagnesian content gives rise to the dark contrast. The overall colour is typically light but can vary enormously depending on the relative proportions of its constituent minerals. The rock is usually hard and weather resistant, forming many landscape features where exposed by the erosion of the surrounding rock. Mineral grains are most often medium to fine. Overall, amongst most igneous rocks, granite has the highest Quartz content, resulting in the relative hardness.

GRANODIORITE – similar to granite, but the feldspar is mostly Plagioclase (whilst granitic content is mostly K-feldspar). Granite and Granodiorite are often difficult to distinguish and they commonly grade into each other which increases diagnostic difficulty. Granodiorite colour is similar to Granite but it is usually darker grey and less pink. MONZONITE - similar to granite, but with a quartz content of 10 or less, and similar to Syenite, but with more Plagioclase Feldspar. Colour is mostly light to medium grey. Medium hardness.

PEGMATITE – a coarse grained, highly crystalline rock, often forming dikes or veins which penetrate other intrusive igneous or metamorphic rock. Individual crystals can be several centimeters in length. Many accessory minerals crystallise in pegmatitic cavities from igneous vapours and solutions. Their clarity hardness and size means that many are used as valuable gemstones. Examples are Beryl, Topaz, Tourmaline, Zircon and Corundum var. Ruby and Emerald. Pegmatite rock is usually characterised by its coarse grains compared to the bedrock that it invades.

PERIDOTITE - a dark coloured rock, generally dark green to black. Mostly made of ferromagnesian Felsic minerals including Pyroxenes and Olivine with occasional Hornblende. Dunite is a variety of Peridotite that is yellowish-green, due to its almost total Olivine content. Peridotite has a generally dull luster. It easily alters into Serpentine under metamorphic conditions. Peridotite sometimes bears economically valuable ores of chromium, nickel, and native platinum. Kimberlite, a type of Peridotite, is the famous diamond­bearing rock of South Africa.

PORPHYRY - either an intrusive or extrusive igneous rock, and classified according to its origin – Granitic Porphyry (intrusive) or Basaltic Porphyry (extrusive). Porphyry has Phenocrysts (large crystals) mixed with a fine grains resembling the sedimentary rock conglomerate. This lack of uniform texture means that Porphyry is difficult to classify using any of the common igneous rock classification schemes.

SYENITE - similar to granite, but with little or no quartz. Mostly made of K-Feldspars with some Biotite or Hornblende. A rarer mineral, Nepheline, may be present in small amounts. Syenite is usually pale grey but it may have pink or yellow tints. The crystal grains are usually small and the texture uniform. Medium hardness and low density compared to other igneous rocks.

Extrusive Igneous Rocks

Extrusive igneous rocks (also called volcanic rocks, named after Vulcan the Roman god of fire) formed from magma that escaped from volcanic vents or crustal fissures and cooled and solidified on the Earth’s surface. Due to the very rapid cooling crystals are either very small or absent. There usually isn’t enough time for these to form. Consequently their textures are usually very smooth or glassy, if there are no crystals present at all. Extrusive rocks are generally subdivided into four classes depending their formation and consistency:

• Compact – dense rocks which usually formed from cooling lava flows. These typically contain fine to very fine mineral grains due to rapid cooling. Examples are Andesite and Basalt.
• Porous – rocks containing vesicles caused by escaping volcanic gasses. These make the rocks very light and some may even float on water. Examples are Pumice and Scoria.
• Glassy – dense rocks with glass-like consistency. No visible crystal grains at all. Formed during very rapid cooling before any crystals could form. Examples are Obsidian and Volcanic Glass.
• Pyroclastic - irregular fragments of rock, debris and ash forcefully ejected from a volcano and subsequently cemented together on the ground. The texture therefore tends to be highly variable depending on individual composition.

However any simple classification is complicated by the fact that many very structurally different rocks have an identical chemical composition. Therefore it is impossible to reconcile both structure and chemistry in one scheme. For example Pumice, Obsidian and Rhyolite are chemically identical but are completely different from each other in formation and appearance. Consequently most geologists use one scheme or the other interchangeably depending on whether they are thinking about rocks in chemical or structural terms.

ANDESITE - the extrusive equivalent of Diorite. Contains Biotite, Hornblende, Plagioclase Feldspar and Pyroxene, with little or no Quartz. It is intermediate between Basalt and Rhyolite in colour and composition. Usually brown, grey or green. Medium hardness and medium grained. Often contains phenocrysts of Feldspar or dark ferromagnesian minerals.

BASALT - the extrusive equivalent of Gabbro. Basalt is a dark grey to black lava rock, occasionally with dark green or dark red inclusions. Composed mostly of Plagioclase Feldspar and Pyroxene, usually Augite. Often contains phenocrysts containing Olivine. Mineral grains very small and the close packing means that the density is higher than granite. Basalt is the most common extrusive rock forming the ocean floor and the mass of rock underlying the continents. Some basalts, including the Deccan Flats in India are some of the largest lava flows ever erupted in the Earth’s history. Basalt often forms spectacular pentagonal or hexagonal columns when erupted on land. Cavities often contain silica-poor minerals such as zeolites. Scoria is a variety of porous basalt containing volcanic gas bubbles.

DACITE - the extrusive equivalent of Quartz Diorite. Usually light to mid-grey and brown. Contains Biotite, Plagioclase Feldspar, Hornblende, Pyroxenes and at least 5% Quartz. Porphyritic with a range of light and dark phenocrysts. EJECTA - rocks explosively thrown out of volcanoes. The same composition as local lava. The fragments often cool and solidify whilst flying through the air. Large ejecta over 10cm in diameter are called Volcanic Bombs. Smaller fragments between 5 and 10cms are called Lapilli, whilst even finer material is known as cinders and ash.

OBSIDIAN – a volcanic glass. The same chemical composition as Granite, Rhyolite, and Pumice. High in the silicate minerals Quartz and Feldspar. Forms when lava rapidly cools, hence crystals have no time to form or grow. Glassy texture and dark-coloured (usually black, but some­times reddish-brown). Glassy (vitreous) luster and usually transparent in thin sections. Conchoidal fracture and relatively hard. Consequently freshly-made chips are razor-sharp and have been used as tools by Stone Age people.

PUMICE - a porous, spongy, extrusive rock, whose air pockets were formed by hot gases bubbling up through lava. Pumice has the same composition as Obsidian, Granite and Rhyolite. Due to their low density and lightness many pumice specimens will float on water. The rock is usually light-coloured.

RHYOLITE – a pale, fine-grained extrusive equivalent of Granite, having an identical chemical composition. Rhyolite is often banded indicating the succession and direction of lava flow. Usually pale shades of yellow, red, grey or brown. Many fresh exposures darken when weathered. Often porphyritic with very small phenocrysts of Quartz and Feldspar. Relatively hard. Pitchstone is a brown coloured Rhyolite.

TRACHYTE - an extrusive rock equivalent to Syenite. Light coloured, sometimes containing light or dark phenocrysts. Medium grained of medium hardness. Composed mainly of Biotite, Hornblende and K-Feldspar, often without Quartz.

TUFF - rock made from compacted volcanic ejecta. Usually whitish, grey yellow or brown. It is generally layered corresponding to successive episodes of volcanic activity. Tuff is relatively lightweight and may form the ‘cement’ holding together breccia made from large rock fragments including non-volcanic ones. However the preferred use of the name is for volcanic rock compacted from smaller particles of pyroclastic material.