Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive ( plutonic ) rocks or on the surface as extrusive ( volcanic ) rocks. This magma can be melting is caused by one or more three processes: an increase in temperature, a decrease in pressure, or a change in composition.
When igneous rocks are formed, they solidify either below the surface or on the surface of the earth; in which case they are called intrusive rocks (or plutonic rocks) and extrusive rocks (or volcanic rocks) respectively. Igneous rocks may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses.
The difference between magma and lava is that magma is molten rock beneath the surface of the earth while lava is a molten rock on the surface of the earth. Igneous rocks are very hard rocks that have had vast usefulness in building and construction for thousands of years.
Examples of Igneous rocks include granite, andesite, diorite, rhyolite, gabbro, obsidian, pumice, basalt, etc. These have diverse properties, depending on their composition and how they were formed.
Volcanic rocks (that is extrusive rocks) are fine-grained (or glass) due to their rapid solidification; intrusive rocks (plutonic) are generally coarse-grained, which indicates that the magma crystallized slowly underground. Experiments show that the slower cooling of liquids results in larger crystals.
The preexisting solid rock into which the magma intrudes during the formation of plutonic rocks is called country rocks. Country rocks appear to have been forcibly broken by an intruding liquid, with the magma flowing into the fractures that developed. Country rock, incidentally, is an accepted term for any older rock into which an igneous body intruded.
There are various ways of classifying igneous rocks. The most significant are mineralogical and chemical composition and rock texture (geological environment). Igneous rock is either formed Intrusive and Extrusive Rocks.
Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. Surrounded by pre-existing rock (calledcountry rock), the magma cools slowly, and as a result, these rocks are coarse-grained. The mineral grains in such rocks can generally be identified with the naked eye.
Extrusive igneous rocks are formed at the crust’s surface as a result of the partial melting of rocks within the mantle and crust. Extrusive Igneous rocks cool and solidify quicker than intrusive igneous rocks. Since the rocks cool very quickly they are fine-grained.
Criteria for distinguishing igneous rock
In describing any rock, one should proceed from the general to the particular, nothing firstly it’s colour, behaviour on weathering and any other striking features and then deciding whether it is igneous or other types of rock. The outstanding characteristics of the igneous rocks are given below but must be emphasized that one characteristic by itself proof positive that the rock belongs to a certain class.
Composition of Igneous Rocks
Since both crust and upper mantle are composed largely of silicate minerals, that is, minerals that contain both silicon and oxygen, igneous rocks also consist mostly of silicate minerals. Minerals found in rocks include quartz, olivine, orthoclase feldspar, plagioclase feldspar, amphibole, mica, etc.
The classification of igneous rocks based on their mineralogy is described below:
Silicic rocks: These are also called or felsic rocks. They are generally light-coloured (white, tan, pink, light grey). They are high in silica, potassium and aluminium; and low in iron and magnesium. The dominant minerals in these rocks are quartz and orthoclase feldspar. Biotite, amphibole, and plagioclase feldspar may also be present in small quantities. Olivine should not be present in silicic rocks.
Intermediate rocks: They are usually medium to dark brown or grey (sometimes blackish) and are intermediate in silica, iron, and magnesium content. The dominant minerals are pyroxene and sodic plagioclase, but biotite and amphibole are also common.
Mafic rocks: They are called basic rocks. They are usually dark-colored (black or dark green). They are low in silica; and high in iron, magnesium, and calcium. Mafic rocks are composed chiefly of pyroxene and calcic plagioclase; ultramafic rocks consist of pyroxene and olivine. Quartz should not be present in mafic rocks.
Ultramafic rocks: They are rocks which are composed entirely or almost entirely of ferromagnesian minerals. No feldspars are present and, of course, no quartz. Peridotite, a coarse-grained rock composed of pyroxene and olivine, is the most abundant ultramafic rock. Chemically, these rocks contain less than 45% silica.
Textures of Igneous Rocks
Thе tеxturе оf rocks іѕ a vital property fоr thеіr іdеntіfісаtіоn аnd сlаѕѕіfісаtіоn. Texture rеfеrѕ tо a rосk’ѕ арреаrаnсе with rеѕресt to thе size, shape, аnd аrrаngеmеnt оf its grains оr other соnѕtіtuеntѕ. Mоѕt (but not аll) іgnеоuѕ rосkѕ аrе сrуѕtаllіnе; thаt is, they are made of іntеrlосkіng сrуѕtаlѕ (оf, fоr instance, quartz аnd feldspar). Thе texture оf аn іgnеоuѕ rock rеflесtѕ іtѕ cooling hіѕtоrу, but mау аlѕо be influenced bу its chemical composition. The mоѕt significant aspect оf tеxturе in igneous rосkѕ іѕ the grаіn or the сrуѕtаl ѕіzе.
Extrusive rocks are typically fine-grained rocks, in which most of the grains are smaller than 1 millimetre. The grains of these rocks, if they are crystals, are small because as described above, the magma cools rapidly at the Earth’s surface, and so they have less time to form. Some plutonic rocks are also fine-grained; this occurs when these rocks solidify near the surface upon intrusion into relatively cold country rock (probably within a couple of kilometres of the Earth’s surface). Types of fine-grained igneous rocks include: Basalt, andesite, and rhyolite.
As discussed above, because of their great depth of solidification, plutonic rocks are mostly coarse-grained. This is because at these depths, they solidify slowly. According to Carlson et al (2011), the crystals or grains of most fine-grained rocks are considerably smaller than 1 millimetre and cannot be distinguished by the unaided eye. So, for practical purposes, if you can discern the individual grains, regard the rock as coarse-grained; if not, consider it fine-grained.
Different textures of rocks are described below:
Aphanitic Texture: A rock is aphanitic when the individual grains are too small to be recognized or identified. As explained, aphanitic igneous rocks are of volcanic (extrusive) origin.
Phaneritic Texture: A rock is phaneritic when the individual grains are visible, more or less the same general size, and interlock like elements of a jigsaw puzzle.
Porphyritic Texture: A porphyritic rock refers to large, more-or-less well-formed crystals (called phenocrysts) surrounded by a fine-grained or aphanitic material (the groundmass). Thus, in porphyritic rocks, large crystals are enclosed in a groundmass of finer-grained crystals or glass.
Vesicular Texture: These are light rocks with bubble holes (vesicles) formed by escaping gas. They indicate a volcanic origin.
Explain the igneous rock classification according to the texture and chemical and mineral composition.
Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance, porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dykes. Mineralogical classification is used most often to classify plutonic rocks.
Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, example “olivine-bearing picrite” or “orthoclase-phyric rhyolite”.
Igneous rocks can be classified according to chemical or mineralogical parameters.
Chemical with total alkali-silica content for volcanic rock classification used when modal or mineralogic data is unavailable:
- acid igneous rocks containing a high silica content, greater than 63% SiO2 (examples granite and rhyolite)
- intermediate igneous rocks containing between 52 – 63% SiO2 (example andesite and dacite)
- basic igneous rocks have low silica 45 – 52% and typically high iron – magnesium content (example gabbro and basalt)
- ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite)
- alkalic igneous rocks with 5 – 15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6. (examples phonolite and trachyte)
Identification of Igneous Rocks
Plutonic rocks are easier to identify than extrusive rocks because of their larger mineral grains. The physical properties of each mineral in a plutonic rock can be determined more readily. And, of course, knowing what minerals are present makes rock identification a simpler task. As explained above, the texture and mineralogy knowledge of the rock helps in the identification of the rocks. The properties of some common igneous rocks are described below:
Gabbro: Gabbro is an intrusive igneous rock that formed by magma that cools very slowly into the hard rock below or within the Earth’s crust. It is formed of coarse-grained ferromagnesian minerals and grey, plagioclase feldspar. Ferromagnesian minerals are silicates that contain iron and magnesium —amphibole, pyroxene, olivine, and biotite. It is dark grey-black, has shiny surfaces of visible feldspar.
Basalt: This is the extrusive counterpart of gabbro. It is composed of feldspar, olivine, pyroxene, and amphibole. Basalt is solidified lava, like rhyolite. However, it flows much quicker because it is less viscous. Its characteristics include dark-grey to black colour, frothy with small visible holes where gas escaped while the lava cooled.
Granite: Granite is an intrusive igneous rock that formed by magma that cools very slowly into the hard rock below or within the Earth’s crust. It is composed of feldspar, quartz, mica, and hornblende. Its characteristics include visible crystals of pink feldspar, white or grey quartz, and black mica. There is no horizontal banding in granite. Granite is the most abundant intrusive rock found in the continents.
Rhyolite: This is an extrusive counterpart of granite. Rhyolite is formed by magma that has reached the Earth’s surface (lava) and therefore cools very quickly. It is composed predominantly of feldspars (usually white or pink) and quartz.
Diorite: Diorite is an intrusive igneous rock composed of feldspars and significant amounts of ferromagnesian minerals (30–50%). The minerals can be identified and their percentages estimated to indicate diorite. It has a medium-grey or medium-green colour.
Andesite: This is the extrusive counterpart of diorite.
From our observation of the igneous rock in the laboratory, it is also having significance in the construction industry. The geologist and the engineers working on projects have to determine the origin of the igneous rock and the mineralogy of the rocks.
A rock which originated as molten magma from beneath the earth’s surface and subsequently came to the surface as extrusion, or remained below ground as an intrusion. The nature of the rock depends in part on the rate at which it cooled; as intrusions of magma slowly solidify, enough time elapses for large crystals to form whereas extrusions cool quickly, leaving little time for crystal growth.
Thus, a coarse-grained, intrusive igneous rock has a fine-grained, extrusive counterpart; granite is coarse rhyolite and gabbro is coarse basalt. Igneous rocks are also classified as an acid or basic, according to whether their silica content is high (e.g. granite), or low (e.g. basalt).