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Geologic provinces of the world (USGS) Shield

Platform

Orogen

Basin

Large igneous province

Extended crust

Oceanic crust: 0–20 Ma

20–65 Ma

>65 Ma

Igneous rockFrom Wikipedia, the free encyclopedia

Igneous rock (derived from the Latinword ignis meaning fire) is one of thethree main rock types, the others beingsedimentary and metamorphic. Igneousrock is formed through the cooling andsolidification of magma or lava. Igneousrock may form with or withoutcrystallization, either below the surfaceas intrusive (plutonic) rocks or on thesurface as extrusive (volcanic) rocks.This magma can be derived from partialmelts of pre­existing rocks in either aplanet's mantle or crust. Typically, themelting is caused by one or more ofthree processes: an increase intemperature, a decrease in pressure, or achange in composition. Over 700 typesof igneous rocks have been described,most of them having formed beneath thesurface of Earth's crust.

Contents

1 Geological significance2 Morphology and setting

2.1 Intrusive2.2 Extrusive2.3 Hypabyssal

3 Classification3.1 Texture3.2 Chemicalclassification andPetrology3.3 History ofclassification

4 Mineralogical classification

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4.1 Example ofclassification

5 Magma origination5.1 Decompression5.2 Effects of water andcarbon dioxide5.3 Temperature increase5.4 Magma evolution

6 Etymology7 See also8 References9 Additional Reading10 External links

Geological significance

Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume.[1]

Igneous rocks are geologically important because:

their minerals and global chemistry give information about the composition of the mantle, fromwhich some igneous rocks are extracted, and the temperature and pressure conditions that allowedthis extraction, and/or of other pre­existing rock that melted;their absolute ages can be obtained from various forms of radiometric dating and thus can becompared to adjacent geological strata, allowing a time sequence of events;their features are usually characteristic of a specific tectonic environment, allowing tectonicreconstitutions (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 andplatinum are commonly associated with gabbros.

Morphology and setting

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

Intrusive

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Forming of igneous rock

Close­up of granite (an intrusiveigneous rock) exposed in Chennai,India.

Extrusive igneous rock is made fromlava released by volcanoes

Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet,surrounded by pre­existing rock (called country rock); the magma cools slowly and, as a result, these rocksare 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 tothe other formations into which it intrudes. Typical intrusive formations are batholiths, stocks, laccoliths,sills and dikes. When the magma solidifies within theearth's crust, it cools slowly forming coarse texturedrocks, such as granite, gabbro, or diorite.

The central cores of major mountain ranges consist ofintrusive igneous rocks, usually granite. When exposedby erosion, these cores (called batholiths) may occupyhuge areas of the Earth's surface.

Coarse grained intrusive igneous rocks that form atdepth within the crust are termed as abyssal; intrusiveigneous rocks that form near the surface are termedhypabyssal.

Extrusive

Extrusive igneous rocks, also known as volcanic rocks,are formed at the crust's surface as a result of the partial melting ofrocks within the mantle and crust. Extrusive igneous rocks cool andsolidify quicker than intrusive igneous rocks. They are formed bythe cooling of molten magma on the earth's surface. The magma,which is brought to the surface through fissures or volcaniceruptions, solidifies at a faster rate. Hence such rocks are smooth,crystalline and fine grained. Basalt is a common extrusive igneousrock and forms lava flows, lava sheets and lava plateaus. Somekinds of basalt solidify to form long polygonal columns. The Giant'sCauseway found in Antrim, Northern Ireland is an example.

The melted rock, with or without suspended crystals and gasbubbles, is called magma. It rises because it is less dense than therock from which it was created. When magma reaches the surfacefrom beneath water or air, it is called lava. Eruptions of volcanoesinto air are termed subaerial, whereas those occurring underneaththe ocean are termed submarine. Black smokers and mid­oceanridge basalt are examples of submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varieswith plate tectonic setting. Extrusive rock is produced in thefollowing proportions:[2]

divergent boundary: 73%convergent boundary (subduction zone): 15%

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Basalt (an extrusive igneous rock inthis case); light coloured tracks showthe direction of lava flow.

hotspot: 12%.

Magma that erupts from a volcano behaves according to itsviscosity, determined by temperature, composition, and crystalcontent. High­temperature magma, most of which is basaltic incomposition, behaves in a manner similar to thick oil and, as itcools, treacle. Long, thin basalt flows with pahoehoe surfaces arecommon. Intermediate composition magma, such as andesite, tendsto form cinder cones of intermingled ash, tuff and lava, and mayhave a viscosity similar to thick, cold molasses or even rubber whenerupted. Felsic magma, such as rhyolite, is usually erupted at lowtemperature and is up to 10,000 times as viscous as basalt.Volcanoes with rhyolitic magma commonly erupt explosively, andrhyolitic lava flows are typically of limited extent and have steepmargins, because the magma is so viscous.

Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the release ofdissolved gases—typically water vapour, but also carbon dioxide. Explosively erupted pyroclastic materialis called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and formsash tuff deposits, which can often cover vast areas.

Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as to preventthe formation of even small crystals after extrusion, the resulting rock may be mostly glass (such as the rockobsidian). If the cooling of the lava happened more slowly, the rocks would be coarse­grained.

Because the minerals are mostly fine­grained, it is much more difficult to distinguish between the differenttypes of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, themineral constituents of fine­grained extrusive igneous rocks can only be determined by examination of thinsections of the rock under a microscope, so only an approximate classification can usually be made in thefield.

Hypabyssal

Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks. These areformed due to cooling and resultant solidification of rising magma just beneath the earth surface.Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths,lopoliths, or phacoliths.

Classification

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 informationabout the conditions under which they formed. Two important variables used for the classification ofigneous rocks are particle size, which largely depends on the cooling history, and the mineral compositionof the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are all importantminerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks.

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Gabbro specimen showing phaneritictexture; Rock Creek Canyon, easternSierra Nevada, California; scale bar is2.0 cm.

All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessoryminerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks includethose 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 magnesiumminerals present. Rocks containing quartz (silica in composition) are silica­oversaturated. Rocks withfeldspathoids are silica­undersaturated, because feldspathoids cannot coexist in a stable association withquartz.

Igneous rocks that have crystals large enough to be seen by the naked eye are called phaneritic; those withcrystals 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 finer­grained matrix is termedporphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the mainmass of the magma crystallizes as finer­grained, uniform material.

Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, andarrangement of the mineral grains or crystals of which the rock is composed.

Texture

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 lavaor a simple lava.

However, the texture is only a subordinate part of classifyingvolcanic rocks, as most often there needs to be chemical informationgleaned from rocks with extremely fine­grained groundmass or fromairfall tuffs, which may be formed from volcanic ash.

Textural criteria are less critical in classifying intrusive rocks wherethe majority of minerals will be visible to the naked eye or at leastusing a hand lens, magnifying glass or microscope. Plutonic rocksalso tend to be less texturally varied and less prone to gainingstructural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, forinstance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes (apophyses).Mineralogical classification is most often used to classify plutonic rocks. Chemical classifications arepreferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine­bearing picrite"or "orthoclase­phyric rhyolite".

see also List of rock textures and Igneous textures

Chemical classification and Petrology

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Basic classification scheme for igneous rocks ontheir mineralogy. If the approximate volumefractions of minerals in the rock are known, therock name and silica content can be read off thediagram. This is not an exact method, because theclassification of igneous rocks also depends on othercomponents than silica, yet in most cases it is agood first guess.

Total alkali versus silica classification scheme (TAS) as proposed inLe Maitre's 2002 Igneous Rocks ­ A classification and glossary ofterms[3]:237

Igneous rocks can be classified according to chemical or mineralogical parameters.

Chemical: total alkali­silica content (TAS diagram) forvolcanic rock classification used when modal ormineralogic data is unavailable:

felsic igneous rocks containing a high silicacontent, greater than 63% SiO2 (examples granite

and rhyolite)intermediate igneous rocks containing between52 – 63% SiO2 (example andesite and dacite)

mafic igneous rocks have low silica 45 – 52% andtypically 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 andtrachyte)

Chemical classification also extends todifferentiating rocks that are chemicallysimilar according to the TAS diagram,for instance;

Ultrapotassic; rocks containingmolar K2O/Na2O >3

Peralkaline; rocks containingmolar (K2O + Na2O)/ Al2O3 >1

Peraluminous; rocks containingmolar (K2O + Na2O)/ Al2O3 <1

An idealized mineralogy (the normativemineralogy) can be calculated from thechemical composition, and thecalculation is useful for rocks too fine­grained or too altered for identificationof minerals that crystallized from the melt. For instance, normative quartz classifies a rock as silica­

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oversaturated; an example is rhyolite. In an older terminology, silica oversaturated rocks were called silicicor acidic where the SiO2 was greater than 66% and the family term quartzolite was applied to the mostsilicic. A normative feldspathoid classifies a rock as silica­undersaturated; an example is nephelinite.

History of classification

In 1902, a group of American petrographers proposed that all existing classifications of igneous rocksshould be discarded and replaced by a "quantitative" classification based on chemical analysis. Theyshowed how vague, and often unscientific, much of the existing terminology was and argued that as thechemical composition of an igneous rock was its most fundamental characteristic, it should be elevated toprime position.

Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria for thediscrimination of rock species—were relegated to the background. The completed rock analysis is first tobe interpreted in terms of the rock­forming minerals which might be expected to be formed when themagma crystallizes, e.g., quartz feldspars, olivine, akermannite, Feldspathoids, magnetite, corundum, andso on, and the rocks are divided into groups strictly according to the relative proportion of these minerals toone another.[4][5]

Mineralogical classification

For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criterionis the phenocryst species, followed by the groundmass mineralogy. Often, where the groundmass isaphanitic, chemical classification must be used to properly identify a volcanic rock.

Mineralogic contents – felsic versus mafic

felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/orfeldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, andhave low density.mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic mineralspyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro) are usually darkcoloured, and have a higher density than felsic rocks.ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g., dunite).

For intrusive, plutonic and usually phaneritic igneous rocks (where all minerals are visible at least viamicroscope), the mineralogy is used to classify the rock. This usually occurs on ternary diagrams, where therelative proportions of three minerals are used to classify the rock.

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

CompositionMode of occurrence Felsic Intermediate Mafic UltramaficIntrusive Granite Diorite Gabbro Peridotite

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Extrusive Rhyolite Andesite Basalt KomatiiteEssential rock forming silicates

Felsic Intermediate Mafic UltramaficCoarse Grained Granite Diorite Gabbro PeridotiteMedium Grained DiabaseFine Grained Rhyolite Andesite Basalt Komatiite

For a more detailed classification see QAPF diagram.

Example of classification

Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica andpredominately quartz plus potassium­rich feldspar plus sodium­rich plagioclase) and phaneritic,subeuhedral texture (minerals are visible to the unaided eye and commonly some of them retain originalcrystallographic shapes).

Magma origination

The Earth's crust averages about 35 kilometers thick under the continents, but averages only some 7–10kilometers beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting ona crystalline basement formed of a great variety of metamorphic and igneous rocks, including granulite andgranite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust reston peridotite of the mantle.

Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition ofwater), to an increase in temperature, or to a combination of these processes.

Other mechanisms, such as melting from a meteorite impact, are less important today, but impacts duringthe accretion of the Earth led to extensive melting, and the outer several hundred kilometers of our earlyEarth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million yearshave been proposed as one mechanism responsible for the extensive basalt magmatism of several largeigneous provinces.

Decompression

Decompression melting occurs because of a decrease in pressure.[6]

The solidus temperatures of most rocks (the temperatures below which they are completely solid) increasewith increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter thanits solidus temperature at some shallower level. If such rock rises during the convection of solid mantle, itwill cool slightly as it expands in an adiabatic process, but the cooling is only about 0.3 °C per kilometer.Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by3 °C to 4 °C per kilometer. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce intolarger volumes and be intruded upwards. This process of melting from the upward movement of solidmantle is critical in the evolution of the Earth.

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Decompression melting creates the ocean crust at mid­ocean ridges. It also causes volcanism in intraplateregions, such as Europe, Africa and the Pacific sea floor. There, it is variously attributed either to the rise ofmantle plumes (the "Plume hypothesis") or to intraplate extension (the "Plate hypothesis").[7]

Effects of water and carbon dioxide

The change of rock composition most responsible for the creation of magma is the addition of water. Waterlowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100kilometers, peridotite begins to melt near 800 °C in the presence of excess water, but near or above about1,500 °C in the absence of water.[8] Water is driven out of the oceanic lithosphere in subduction zones, andit causes melting in the overlying mantle. Hydrous magmas composed of basalt and andesite are produceddirectly and indirectly as results of dehydration during the subduction process. Such magmas, and thosederived from them, build up island arcs such as those in the Pacific Ring of Fire. These magmas form rocksof the calc­alkaline series, an important part of the continental crust.

The addition of carbon dioxide is relatively a much less important cause of magma formation than theaddition of water, but genesis of some silica­undersaturated magmas has been attributed to the dominanceof carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experimentsdocument that the peridotite solidus temperature decreases by about 200 °C in a narrow pressure interval atpressures corresponding to a depth of about 70 km. At greater depths, carbon dioxide can have more effect:at depths to about 200 km, the temperatures of initial melting of a carbonated peridotite composition weredetermined to be 450 °C to 600 °C lower than for the same composition with no carbon dioxide.[9] Magmasof rock types such as nephelinite, carbonatite, and kimberlite are among those that may be generatedfollowing an influx of carbon dioxide into mantle at depths greater than about 70 km.

Temperature increase

Increase in temperature is the most typical mechanism for formation of magma within continental crust.Such temperature increases can occur because of the upward intrusion of magma from the mantle.Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression ata plate boundary. The plate boundary between the Indian and Asian continental masses provides a well­studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick,roughly twice the thickness of normal continental crust. Studies of electrical resistivity deduced frommagnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least1,000 kilometers within the middle crust along the southern margin of the Tibetan Plateau.[10] Granite andrhyolite are types of igneous rock commonly interpreted as products of the melting of continental crustbecause of increases in temperature. Temperature increases also may contribute to the melting oflithosphere dragged down in a subduction zone.

Magma evolution

Most magmas only entirely melt for small parts of their histories. More typically, they are mixes of meltand crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have differentdensities, and so they can separate as magmas evolve.

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Schematic diagrams showing the principles behind fractionalcrystallisation in a magma. While cooling, the magma evolves incomposition because different minerals crystallize from the melt. 1:olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxeneand plagioclase crystallize; 4: plagioclase crystallizes. At the bottomof the magma reservoir, a cumulate rock forms.

As magma cools, minerals typically crystallize from the melt at different temperatures (fractionalcrystallization). As minerals crystallize, the composition of the residual melt typically changes. If crystalsseparate from the melt, then the residual melt will differ in composition from the parent magma. Forinstance, a magma of gabbroic composition can produce a residual melt of granitic composition if earlyformed crystals are separated from the magma. Gabbro may have a liquidus temperature near 1,200 °C, andthe derivative granite­composition meltmay have a liquidus temperature as lowas about 700 °C. Incompatible elementsare concentrated in the last residues ofmagma during fractional crystallizationand in the first melts produced duringpartial melting: either process can formthe magma that crystallizes topegmatite, a rock type commonlyenriched in incompatible elements.Bowen's reaction series is important forunderstanding the idealised sequence offractional crystallisation of a magma.

Magma composition can be determinedby processes other than partial meltingand fractional crystallization. Forinstance, magmas commonly interactwith rocks they intrude, both by melting those rocks and by reacting with them. Magmas of differentcompositions can mix with one another. In rare cases, melts can separate into two immiscible melts ofcontrasting compositions.

There are relatively few minerals that are important in the formation of common igneous rocks, because themagma from which the minerals crystallize is rich in only certain elements: silicon, oxygen, aluminium,sodium, potassium, calcium, iron, and magnesium. These are the elements that combine to form the silicateminerals, which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks isexpressed differently for major and minor elements and for trace elements. Contents of major and minorelements are conventionally expressed as weight percent oxides (e.g., 51% SiO2, and 1.50% TiO2).Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppmNi, and 5.1 ppm Sm). The term "trace element" is typically used for elements present in most rocks atabundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundancesexceeding 1,000 ppm. The diversity of rock compositions has been defined by a huge mass of analyticaldata—over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S.National Science Foundation (see the External Link to EarthChem).

Etymology

The word "igneous" is derived from the Latin ignis, meaning "of fire". Volcanic rocks are named afterVulcan, the Roman name for the god of fire. Intrusive rocks are also called "plutonic" rocks, named afterPluto, the Roman god of the underworld.

See also

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List of mineralsList of rock typesLarge igneous provincePetrologyMetamorphic rocksSedimentary rocks

References

1. Prothero, Donald R.; Schwab, Fred (2004). Sedimentary geology : an introduction to sedimentary rocks andstratigraphy (2nd ed.). New York: Freeman. p. 12. ISBN 978­0­7167­3905­0.

2. Fisher, R. V. & Schmincke H.­U., (1984) Pyroclastic Rocks, Berlin, Springer­Verlag3. Ridley, W.I., 2012, Petrology of Igneous Rocks, Volcanogenic Massive Sulfide Occurrence Model, USGS

Scientific Report 2010­5070­C, Chapter 154. Cross, W. et al. (1903) Quantitative Classification of Igneous Rocks, Chicago, University of Chicago Press5. One or more of the preceding sentences incorporates text from a publication now in the public

domain: Chisholm, Hugh, ed. (1911). "Petrology"(https://archive.org/stream/encyclopaediabri21chisrich#page/323/mode/1up). Encyclopædia Britannica 21 (11thed.). Cambridge University Press. pp. 323–333.

6. Geoff C. Brown, C. J. Hawkesworth, R. C. L. Wilson (1992). Understanding the Earth(http://books.google.com/books?id=Kgk4AAAAIAAJ&pg=PA93) (2nd ed.). Cambridge University Press. p. 93.ISBN 0­521­42740­1.

7. Foulger, G.R. (2010). Plates vs. Plumes: A Geological Controversy(http://www.wiley.com/WileyCDA/WileyTitle/productCd­1405161485.html). Wiley­Blackwell. ISBN 978­1­4051­6148­0.

8. T. L. Grove, N. Chatterjee, S. W. Parman, and E. Medard, (2006)The influence of H2O on mantle wedge melting.

Earth and Planetary Science Letters, v. 249, p. 74­899. R. Dasgupta and M. M. Hirschmann (2007) Effect of variable carbonate concentration on the solidus of mantleperidotite. American Mineralogist, v. 92, p. 370­379

10. M. J. Unsworth et al. (2005) Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluricdata. Nature, v. 438, p. 78­81

Additional Reading

R. W. Le Maitre (editor) (2002) Igneous Rocks: A Classification and Glossary of Terms,Recommendations of the International Union of Geological Sciences, Subcommission of theSystematics of Igneous Rocks., Cambridge, Cambridge University Press ISBN 0­521­66215­X

External links

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Wikimedia Commons hasmedia related to Igneousrock.

Look up igneous inWiktionary, the freedictionary.

USGS Igneous Rocks

(http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/igneous_rocks.html)Igneous rock classification flowchart(http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/IUGS­IgneousClassFlowChart.htm)Igneous Rocks Tour, an introduction to Igneous Rocks(http://geology.cnsm.ad.csulb.edu/people/bperry/IgneousRocksTour/IntroToIgneousRocks.html)The IUGS systematics of igneous rocks(http://www.utdallas.edu/~aiken/SHAKEBAKE/rockclassification.pdf)

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Categories: Igneous petrology Igneous rocks Volcanology

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