Igneous rock
Extended crust | Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma |
Igneous rock (
The magma can be derived from
Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.
Geological significance
Igneous and metamorphic rocks make up 90–95% of the top 16 kilometres (9.9 mi) of the Earth's crust by volume.[1] Igneous rocks form about 15% of the Earth's current land surface.[note 1] 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 lower crust or upper mantle from which their parent magma was extracted, and the temperature and pressure conditions that allowed this extraction;[3]
- their geological time scale;[4]
- their features are usually characteristic of a specific tectonic environment, allowing tectonic reconstructions (see plate tectonics);
- in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin,[5] and uranium[6] are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros.[7]
Geological setting
Igneous rocks can be either
Intrusive
Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within the crust of a planet. Bodies of intrusive rock are known as
The central cores of major mountain ranges consist of intrusive igneous rocks. 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 much finer-grained, often resembling volcanic rock.[8] Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths, or phacoliths.
Extrusive
Extrusive igneous rock, also known as volcanic rock, is formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or
The molten rock, which typically contains suspended crystals and dissolved gases, is called
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:[14]
- divergent boundary: 73%
- subduction zone): 15%
- hotspot: 12%.
The behaviour of lava depends upon its
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
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 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
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.[citation needed]
Texture
Texture is an important criterion for the naming of volcanic rocks. The
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
The
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.[20]
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.[20]
Where the mineralogy of an 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.[20]
Chemical classification and petrology
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:
The single most important component is silica, SiO2, 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.
- Intermediaterocks 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% SiO2) |
Intermediate (52% to 63% SiO2) |
Mafic (45% to 52% SiO2) |
Ultramafic (<45% SiO2) |
Intrusive | Granite | Diorite | Gabbro | Peridotite |
Extrusive | Rhyolite | Andesite | Basalt | Komatiite |
The percentage of
Other refinements to the basic TAS classification include:
- Ultrapotassic – rocks containing molar K2O/Na2O >3.
- Peralkaline – rocks containing molar (K2O + Na2O)/Al2O3 >1.[22]
- Peraluminous – rocks containing molar (K2O + Na2O + CaO)/Al2O3 <1.[22]
In older terminology, silica oversaturated rocks were called silicic or acidic where the SiO2 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.
Magmas are further divided into three series:
- The tholeiitic series – basaltic andesites and andesites.
- The calc-alkaline series – andesites.
- The 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.[23]
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.[24]
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
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.[27] The word granite goes back at least to the 1640s and is derived either from French granit or Italian granito, meaning simply "granulate rock".[28] The term rhyolite was introduced in 1860 by the German traveler and geologist Ferdinand von Richthofen[29][30][31] The naming of new rock types accelerated in the 19th century and peaked in the early 20th century.[32]
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.[33][34]
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.[33] 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.[35]
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.[36] His definition of the alkali series, and the term calc-alkali, continue in use as part of the widely used[37] Irvine-Barager classification,[38] along with W.Q. Kennedy's tholeiitic series.[39]
By 1958, there were some 12 separate classification schemes and at least 1637 rock type names in use. In that year, Albert Streckeisen wrote a review article on igneous rock classification that ultimately led to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989 a single system of classification had been agreed upon, which was further revised in 2005. The number of recommended rock names was reduced to 316. These included a number of new names promulgated by the Subcommission.[32]
Origin of magmas
The Earth's crust averages about 35 kilometres (22 mi) thick under the continents, but averages only some 7–10 kilometres (4.3–6.2 mi) beneath the oceans. The continental crust is composed primarily of sedimentary rocks resting on a crystalline basement formed of a great variety of metamorphic and igneous rocks, including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro. Both continental and oceanic crust rest on peridotite of the mantle.[citation needed]
Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition of water), to an increase in temperature, or to a combination of these processes.[citation needed]
Other mechanisms, such as melting from a meteorite impact, are less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometres of our early Earth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for the extensive basalt magmatism of several large igneous provinces.[citation needed]
Decompression
Decompression melting occurs because of a decrease in pressure.[40]
The
Decompression melting creates the ocean crust at
Effects of water and carbon dioxide
The change of rock composition most responsible for the creation of magma is the addition of water. Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of about 100 kilometres, peridotite begins to melt near 800 °C in the presence of excess water, but near or above about 1,500 °C in the absence of water.
The addition of carbon dioxide is relatively a much less important cause of magma formation than the addition of water, but genesis of some silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions. In the presence of carbon dioxide, experiments document that the peridotite solidus temperature decreases by about 200 °C in a narrow pressure interval at pressures 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 were determined to be 450 °C to 600 °C lower than for the same composition with no carbon dioxide.[43] Magmas of rock types such as nephelinite, carbonatite, and kimberlite are among those that may be generated following an influx of carbon dioxide into mantle at depths greater than about 70 km.[citation needed]
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 at a
Magma evolution
Most
As magma cools, minerals typically
Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions.[citation needed]
Etymology
- The word igneous rock means composed of fire. and is derived from the Latin root words of igni-,[45] meaning fire, and -eous[46] meaning composed of.
- The word volcanic rock is derived from the Latin root words of Vulcan,[47] the Roman the god of fire, and -ic,[48] meaning having some characteristics of.
- The word meaning having some characteristics of.
Gallery
-
Kanaga volcano in the Aleutian Islandswith a 1906 lava flow in the foreground
-
A "skylight" hole, about 6 m (20 ft) across, in a solidified lava crust reveals molten lava below (flowing towards the top right) in an eruption of Kīlauea in Hawaii
-
Devils Tower, an eroded laccolith in the Black Hills of Wyoming
-
A cascade of molten lava flowing into Aloi Crater during the 1969–1971 Mauna Ulu eruption of Kilauea volcano
-
Columnar jointing in the Alcantara Gorge, Sicily
-
A laccolith of granite (light-coloured) that was intruded into older sedimentary rocks (dark-coloured) at Cuernos del Paine, Torres del Paine National Park, Chile
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An igneous intrusion cut by adoleritedike
See also
- List of rock types – List of rock types recognized by geologists
- Metamorphic rock – Rock that was subjected to heat and pressure
- Migmatite – Mixture of metamorphic rock and igneous rock
- Petrology – Branch of geology that studies the formation, composition, distribution and structure of rocks
- Sedimentary rock – Rock formed by the deposition and cementation of particles
Notes
References
- ISBN 978-0-7167-3905-0.
- doi:10.1130/B26457.1.
- ISBN 978-0-521-88006-0.
- ^ Philpotts & Ague 2009, p. 295.
- .
- ISBN 978-1-5015-0919-3. Retrieved 13 February 2021.
- ^ Philpotts & Ague 2009, p. 96, 387–388.
- ^ Philpotts & Ague 2009, p. 139.
- ^ Philpotts & Ague 2009, pp. 52–59.
- ^ Philpotts & Ague 2009, pp. 19–26.
- ^ Philpotts & Ague 2009, pp. 28–35.
- S2CID 220886233.
- ^ Philpotts & Ague 2009, pp. 365–374.
- ISBN 3-540-12756-9.
- ^ Philpotts & Ague 2009, pp. 23–26, 59–73.
- ^ Philpotts & Ague 2009, pp. 73–77.
- ^ a b Philpotts & Ague 2009, pp. 139–143.
- ^ ISBN 978-0-13-240342-9.
- S2CID 28548230.
- ^ a b c Le Bas & Streckeisen 1991.
- .
- ^ ISBN 0-7167-2438-3.
- ^ Philpotts & Ague 2009, pp. 143–146.
- ^ "Mafic magma types" (PDF). University of Washington. Archived from the original (PDF) on 1 August 2020. Retrieved 2 December 2020.
- ISSN 0016-7037.
- .
- . Retrieved 19 August 2020.
- ^ Biek. "Granite". Online Etymology Dictionary. Douglas Harper. Retrieved 2 December 2020.
- ^ Richthofen, Ferdinand Freiherrn von (1860). "Studien aus den ungarisch-siebenbürgischen Trachytgebirgen" [Studies of the trachyte mountains of Hungarian Transylvania]. Jahrbuch der Kaiserlich-Königlichen Geologischen Reichsanstalt (Wein) [Annals of the Imperial-Royal Geological Institute of Vienna] (in German). 11: 153–273.
- ^ Simpson, John A.; Weiner, Edmund S. C., eds. (1989). Oxford English Dictionary. Vol. 13 (2nd ed.). Oxford: Oxford University Press. p. 873.
- ISBN 0-691-10279-1.
- ^ ISBN 978-0-521-66215-4.
- ^ a b public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. p. 330. One or more of the preceding sentences incorporates text from a publication now in the
- ^ Cross, C.W.; Iddings, J.P.; Pirsson, L.V.; Washington, H.S. (1903). Quantitative Classification of Igneous Rocks. Chicago: University of Chicago Press.
- .
- S2CID 140563237.
- ^ Philpotts & Ague 2009, p. 143.
- doi:10.1139/e71-055.
- .
- ISBN 0-521-42740-1.
- ISBN 978-1-4051-6148-0.
- .
- S2CID 95932394.
- S2CID 4359642.
- ^ "igneous". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
- ^ "-eous". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
- ^ "Volvano". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
- ^ "ic". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
- ^ "Pluto". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
- ^ "ic". Dictionary.com. Archived from the original on 15 October 2022. Retrieved 15 October 2022.
External links
- USGS Igneous Rocks Archived 21 February 2013 at the Wayback Machine
- Igneous rock classification flowchart
- Igneous Rocks Tour, an introduction to Igneous Rocks
- The IUGS systematics of igneous rocks