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the (326), and (154), rocks (121), rock (102), igneous (92), are (82), from (51), magma (48), classification (46), for (36), volcanic (33), with (32), composition (28), #intrusive (25), which (25), can (25), crust (23), minerals (23), series (21), that (21), was (18), magmas (18), lava (18), temperature (18), basalt (17), about (16), melting (16), earth (16), extrusive (16), have (16), most (16), such (16), may (15), types (15), doi (15), geological (15), than (15), other (14), granite (14), alkali (14), 2009 (14), some (14), melt (14), silica (14), these (14), citation (13), mantle (13), philpotts (13), ague (13), into (13), alkaline (13), wikipedia (12), mineral (12), formed (12), also (12), needed (12), form (12), important (12), water (12), but (12), october (11), 2022 (11), retrieved (11), peridotite (11), sio (11), felsic (11), content (11), bibcode (11), isbn (11), chemical (11), more (11), composed (11), called (11), grained (11), this (10), all (10), gabbro (10), classified 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ous 15 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 16 because volcanic rocks are mostly fine grained or glassy 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 although classification by mineral makeup is preferred by the iugs this is often impractical and chemical classification is done instead using the tas classification 17 classification further information list of rock types close up of granite an intrusive igneous rock 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 igneous rocks can provide 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 but include carbonatites which contain essential carbonates 17 in a simplified compositional classification igneous rock types are categorized into felsic or mafic based on the abundance of silicate minerals in the bowen s series rocks dominated by quartz plagioclase alkali feldspar and muscovite are felsic mafic rocks are primarily composed of biotite hornblende pyroxene and olivine generally felsic rocks are light colored and mafic rocks are darker colored 18 for textural classification 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 or plutonic indicating slow cooling aphanitic are extrusive or volcanic indicating rapid cooling 18 an igneous rock with larger clearly discernible crystals embedded in a finer grained matrix is termed porphyry porphyritic texture develops when the larger crystals called phenocrysts grow to considerable size before the main mass of the magma crystallizes as finer grained uniform material called groundmass grain size in igneous rocks results from cooling time so porphyritic rocks are created when the magma has two distinct phases of cooling 18 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 19 texture gabbro specimen showing phaneritic texture from rock creek canyon eastern sierra nevada california main article rock microstructure 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 citation needed 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 showing distinctive 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 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 citation needed mineralogical classification basic classification scheme for igneous rocks based on their mineral composition 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 yet in most cases it is a good first guess qapf diagram the iugs recommends classifying igneous rocks by their mineral composition whenever possible this is straightforward for coarse grained intrusive igneous rock but may require examination of thin sections under a microscope for fine grained volcanic rock and may be impossible for glassy volcanic rock the rock must then be classified chemically 20 mineralogical classification of an intrusive rock begins by determining if the rock is ultramafic a carbonatite or a lamprophyre an ultramafic rock contains more than 90 of iron and magnesium rich minerals such as hornblende pyroxene or olivine and such rocks have their own classification scheme likewise rocks containing more than 50 carbonate minerals are classified as carbonatites while lamprophyres are rare ultrapotassic rocks both are further classified based on detailed mineralogy 21 in the great majority of cases the rock has a more typical mineral composition with significant quartz feldspars or feldspathoids classification is based on the percentages of quartz alkali feldspar plagioclase and feldspathoid out of the total fraction of the rock composed of these minerals ignoring all other minerals present these percentages place the rock somewhere on the qapf diagram which often immediately determines the rock type in a few cases such as the diorite gabbro anorthite field additional mineralogical criteria must be applied to determine the final classification 21 where the mineralogy of a volcanic rock can be determined it is classified using the same procedure but with a modified qapf diagram whose fields correspond to volcanic rock types 21 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 22 blue area is roughly where alkaline rocks plot yellow area is where subalkaline rocks plot when it is impractical to classify a volcanic rock by mineralogy the rock must be classified chemically there are relatively few minerals that are important in the formation of common igneous rocks because the magma 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 silicate minerals which account for over ninety percent of all igneous rocks the chemistry of igneous rocks is expressed differently for major and minor elements and for trace elements contents of major and minor elements are conventionally expressed as weight percent oxides e g 51 sio 2 and 1 50 tio 2 abundances of trace elements are conventionally expressed as parts per million by weight e g 420 ppm ni and 5 1 ppm sm the term trace element is typically used for elements present in most rocks at abundances less than 100 ppm or so but some trace elements may be present in some rocks at abundances exceeding 1 000 ppm the diversity of rock compositions has been defined by a huge mass of analytical data 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 citation needed the single most important component is silica sio 2 whether occurring as quartz or combined with other oxides as feldspars or other minerals both intrusive and volcanic rocks are grouped chemically by total silica content into broad categories felsic rocks have the highest content of silica and are predominantly composed of the felsic minerals quartz and feldspar these rocks granite rhyolite are usually light coloured and have a relatively low density intermediate rocks have a moderate content of silica and are predominantly composed of feldspars these rocks diorite andesite are typically darker in colour than felsic rocks and somewhat more dense mafic rocks have a relatively low silica content and are composed mostly of pyroxenes olivines and calcic plagioclase these rocks basalt gabbro are usually dark coloured and have a higher density than felsic rocks ultramafic rock is very low in silica with more than 90 of mafic minerals komatiite dunite this classification is summarized in the following table composition mode of occurrence felsic 63 sio 2 intermediate 52 to 63 sio 2 mafic 45 to 52 sio 2 ultramafic 45 sio 2 intrusive granite diorite gabbro peridotite extrusive rhyolite andesite basalt komatiite the percentage of alkali metal oxides na 2 o plus k 2 o is second only to silica in its importance for chemically classifying volcanic rock the silica and alkali metal oxide percentages are used to place volcanic rock on the tas diagram which is sufficient to immediately classify most volcanic rocks rocks in some fields such as the trachyandesite field are further classified by the ratio of potassium to sodium so that potassic trachyandesites are latites and sodic trachyandesites are benmoreites some of the more mafic fields are further subdivided or defined by normative mineralogy in which an idealized mineral composition is calculated for the rock based on its chemical composition for example basanite is distinguished from tephrite by having a high normative olivine content other refinements to the basic tas classification include ultrapotassic rocks containing molar k 2 o na 2 o 3 peralkaline rocks containing molar k 2 o na 2 o al 2 o 3 1 23 peraluminous rocks containing molar k 2 o na 2 o cao al 2 o 3 1 23 in older terminology silica oversaturated rocks were called silicic or acidic where the sio 2 was greater than 66 and the family term quartzolite was applied to the most silicic a normative feldspathoid classifies a rock as silica undersaturated an example is nephelinite afm ternary diagram showing the relative proportions of na 2 o k 2 o a for alkali earth metals feo fe 2 o 3 f and mgo m with arrows showing the path of chemical variation in tholeiitic and calc alkaline series magmas magmas are further divided into three series the tholeiitic series basaltic andesites and andesites the calc alkaline series andesites the alkaline series subgroups of alkaline basalts and the rare very high potassium bearing i e shoshonitic lavas the alkaline series is distinguishable from the other two on the tas diagram being higher in total alkali oxides for a given silica content but the tholeiitic and calc alkaline series occupy approximately the same part of the tas diagram they are distinguished by comparing total alkali with iron and magnesium content 24 these three magma series occur in a range of plate tectonic settings tholeiitic magma series rocks are found for example at mid ocean ridges back arc basins oceanic islands formed by hotspots island arcs and continental large igneous provinces 25 all three series are found in relatively close proximity to each other at subduction zones where their distribution is related to depth and the age of the subduction zone the tholeiitic magma series is well represented above young subduction zones formed by magma from relatively shallow depth the calc alkaline and alkaline series are seen in mature subduction zones and are related to magma of greater depths andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of the calc alkaline magmas some island arcs have distributed volcanic series as can be seen in the japanese island arc system where the volcanic rocks change from tholeiite calc alkaline alkaline with increasing distance from the trench 26 27 history of classification some igneous rock names date to before the modern era of geology for example basalt as a description of a particular composition of lava derived rock dates to georgius agricola in 1546 in his work de natura fossilium 28 the word granite goes back at least to the 1640s and is derived either from french granit or italian granito meaning simply granulate rock 29 the term rhyolite was introduced in 1860 by the german traveler and geologist ferdinand von richthofen 30 31 32 the naming of new rock types accelerated in the 19th century and peaked in the early 20th century 33 much of the early classification of igneous rocks was based on the geological age and occurrence of the rocks however in 1902 the american petrologists charles whitman cross joseph p iddings louis v pirsson and henry stephens washington proposed that all existing classifications of igneous rocks should be discarded and replaced by a quantitative classification based on chemical analysis they showed how vague and often unscientific much of the existing terminology was and argued that as the chemical composition of an igneous rock was its most fundamental characteristic it should be elevated to prime position 34 35 geological occurrence structure mineralogical constitution the hitherto accepted criteria for the discrimination of rock species were relegated to the background the completed rock analysis is first to be interpreted in terms of the rock forming minerals which might be expected to be formed when the magma crystallizes e g quartz feldspars olivine akermannite feldspathoids magnetite corundum and so on and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another 34 this new classification scheme created a sensation but was criticized for its lack of utility in fieldwork and the classification scheme was abandoned by the 1960s however the concept of normative mineralogy has endured and the work of cross and his coinvestigators inspired a flurry of new classification schemes 36 among these was the classification scheme of m a peacock which divided igneous rocks into four series the alkalic the alkali calcic the calc alkali and the calcic series 37 his definition of the alkali series and the term calc alkali continue in use as part of the...