IGNEOUS ROCK (derived from the
* 1 Geological significance
* 2 Geological setting
* 2.1 Intrusive * 2.2 Extrusive
* 3 Classification
* 3.1 Texture
* 3.2 Chemical classification and
* 4 Mineralogical classification
* 4.1 Example of classification
* 6 Etymology * 7 See also * 8 Notes * 9 References * 10 Additional Reading * 11 External links
Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock.
Igneous rocks are also geologically important because:
* their minerals and global chemistry give information about the composition of the mantle, from which some igneous rocks are extracted, and the temperature and pressure conditions that allowed this extraction, and/or of other pre-existing rock that melted; * their absolute ages can be obtained from various forms of radiometric dating and thus can be compared to adjacent geological strata , allowing a time sequence of events; * their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstitutions (see plate tectonics ); * in some special circumstances they host important mineral deposits (ores ): for example, tungsten , tin , and uranium are commonly associated with granites and diorites , whereas ores of chromium and platinum are commonly associated with gabbros .
Forming of igneous rock
In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic ).
Close-up of granite (an intrusive igneous rock) exposed in
Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet, surrounded by pre-existing rock (called country 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. Intrusive rocks can also be 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 , stocks, laccoliths , sills and dikes . When the magma solidifies within the earth's crust, it cools slowly forming coarse textured rocks, such as granite, gabbro, or diorite.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by erosion, these cores (called batholiths ) may occupy huge areas of the Earth's surface.
Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal ) rocks and are usually coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually medium-grained. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths , or phacoliths .
Extrusive igneous rock is made from lava released by volcanoes
Sample of basalt (an extrusive igneous rock), found in
Extrusive igneous rocks, also known as volcanic 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. They are formed by the cooling
of molten magma on the earth's surface. The magma, which is brought to
the surface through fissures or volcanic eruptions, solidifies at a
faster rate. Hence such rocks are smooth, crystalline and
The molten rock, with or without suspended crystals and gas bubbles, is called magma . It rises because it is less dense than the rock from which it was created. When magma reaches the surface from beneath water or air, it is called lava . Eruptions of volcanoes into air are termed subaerial , whereas those occurring underneath the ocean are termed submarine . Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:
* divergent boundary : 73% * convergent boundary (subduction zone ): 15% * hotspot : 12%.
Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but also carbon dioxide . Explosively erupted pyroclastic material is called tephra and includes tuff , agglomerate and ignimbrite . Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas.
Because lava usually cools and crystallizes rapidly, it is usually fine-grained. If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rock obsidian ). If the cooling of the lava happened more slowly, the rock would be coarse-grained.
Because the minerals are mostly fine-grained, it is much more difficult to distinguish between the different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope , so only an approximate classification can usually be made in the field .
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.
The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock. Feldspars , quartz or feldspathoids , olivines , pyroxenes , amphiboles , and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates .
In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz , and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.
Igneous rocks that have crystals large enough to be seen by the naked eye are called phaneritic ; those with crystals too small to be seen are called aphanitic . Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one.
An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry . Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material.
Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of the mineral grains or crystals of which the rock is composed.
Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff , a pyroclastic lava or a simple lava .
However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash.
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 also tend 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 dikes (apophyses). Mineralogical classification is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".
Basic classification scheme for igneous rocks on their mineralogy . If the approximate volume fractions of minerals in the rock are known, the rock name and silica content can be read off the diagram. This is not an exact method, because the classification of igneous rocks also depends on other components than silica, yet in most cases it is a good first guess.
CHEMICAL CLASSIFICATION AND PETROLOGY
Total alkali versus silica classification scheme (TAS) as proposed in Le Maitre's 2002 Igneous Rocks - A classification and glossary of terms :237
Igneous rocks can be classified according to chemical or mineralogical parameters.
Chemical: total alkali-silica content (TAS diagram ) for volcanic rock classification used when modal or mineralogic data is unavailable:
* felsic 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 ) * mafic igneous rocks have low silica 45 – 52% and typically high iron – magnesium content (example gabbro and basalt ) * ultramafic rock igneous rocks with less than 45% silica. (examples picrite , komatiite and peridotite ) * 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 )
Chemical classification also extends to differentiating rocks that are chemically similar according to the TAS diagram, for instance;
* Ultrapotassic ; rocks containing molar K2O/Na2O >3 * Peralkaline ; rocks containing molar (K2O + Na2O)/ Al2O3 >1 * Peraluminous ; rocks containing molar (K2O + Na2O)/ Al2O3