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Igneous rock

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Volcanic rock on North America
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   Volcanic rock on North America
   Plutonic rock on North America
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   Plutonic rock on North America

   Igneous rocks are formed when molten rock (magma) cools and solidifies,
   with or without crystallization, either below the surface as intrusive
   (plutonic) rocks or on the surface as extrusive ( volcanic) rocks. This
   magma can be derived from partial melts of pre-existing rocks in either
   the Earth's mantle or crust. Typically, the melting is caused by one or
   more of the following processes -- an increase in temperature, a
   decrease in pressure, or a change in composition. Over 700 types of
   igneous rocks have been described, most of them formed beneath the
   surface of the Earth's crust. The word "igneous" is derived from the
   Latin igneus, meaning "of fire".

Magma origination

   The Earth's crust averages about 35 kilometers thick under the
   continents, but averages only some 7-10 kilometers beneath the oceans.
   The continental crust is composed primarily of sedimentary rocks
   resting on 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.

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

Decompression

   Decompression melting occurs because of a decrease in pressure. The
   solidus temperatures of most rocks increase with increasing pressure in
   the absence of water (The solidus temperature of a rock at a given
   pressure is the maximum temperature below which that rock is completely
   crystalline.). Experimental studies of appropriate peridotite samples
   document that the solidus temperatures increase by 3°C to 4°C per
   kilometer. Peridotite at depth in the Earth's mantle may be hotter than
   its solidus temperature at some shallower level. If such rock rises
   during the convection of solid mantle, it will cool slightly as it
   expands in an adiabatic process, but the cooling is only about 0.3°C
   per kilometer. The rock may rise far enough so that its temperature is
   at the solidus at that shallower depth. If the rock rises higher, it
   will begin to melt. Melt droplets can coalesce into larger volumes and
   be intruded upwards. This process of melting from upward movement of
   solid mantle is critical in the evolution of the earth.

   Decompression melting creates the ocean crust at mid-ocean ridges.
   Decompression melting caused by the rise of mantle plumes is
   responsible for creating ocean islands like those of the Hawaiian
   islands. Plume-related decompression melting also is the most common
   explanation for flood basalts and oceanic plateaus (two types of large
   igneous provinces), although other causes such as melting related to
   meteorite impact have been proposed for some of these huge volumes of
   igneous rock.

Addition of water and carbon dioxide

   The change of rock composition most responsible for 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 kilometers,
   peridotite begins to melt near 800°C in the presence of excess water,
   but near or above about 1500°C in the absence of water (Grove and
   others, 2006). Water is driven out of the ocean lithosphere in
   subduction zones, and it causes melting in the overlying mantle.
   Hydrous magmas of basalt and andesite composition are produced directly
   and indirectly as results of dehydration during the subduction process.
   Such magmas and those derived from them build up island arcs such as
   those in the Pacific ring of fire. These magmas have contributed much
   of the material to form continental crust.

   The addition of carbon dioxide is relatively a much less important
   cause of magma formation than 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. Magmas
   of rock types such as nephelinite, carbonatite, and kimberlite are
   among those that may be generated following an influx of carbon dioxide
   into a mantle volume at depths greater than about 70 km.

Temperature increase

   Increase of 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 plate boundary. The plate boundary
   between the Indian and Asian continental masses provides a well-studied
   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 from
   magnetotelluric data have detected a layer that appears to contain
   silicate melt and that stretches for at least 1000 kilometers within
   the middle crust along the southern margin of the Tibetan Plateau
   (Unsworth and others, 2005). Granite and rhyolite are types of igneous
   rock commonly interpreted as products of melting of continental crust
   because of increases of temperature. Temperature increases also may
   contribute to the melting of lithosphere dragged down in a subduction
   zone.

Magma evolution

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

   As magma cools, minerals typically crystallize from the melt at
   different temperatures ( fractional crystallization). As minerals
   crystallize, the composition of the residual melt typically changes. If
   crystals separate from melt, then the residual melt will differ in
   composition from the parent magma. For instance, a magma of gabbro
   composition can produce a residual melt of granite composition if early
   formed crystals are separated from the magma. Gabbro may have a
   liquidus temperature near 1200°C, and derivative granite-composition
   melt may have a liquidus temperature as low as about 700°C.
   Incompatible elements are concentrated in the last residues of magma
   during fractional crystallization and in the first melts produced
   during partial melting: either process can form the magma that
   crystallizes to pegmatite, a rock type commonly enriched in
   incompatible elements. Bowen's reaction series is important for
   understanding the idealised sequence of fractional crystallisation of a
   magma.

   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.

   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 which 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" typically is 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 1000 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).

Geologic significance

   Igneous rocks make up approximately ninety five percent of the upper
   part of the Earth's crust, but their great abundance is hidden on the
   Earth's surface by a relatively thin but widespread layer of
   sedimentary and metamorphic rocks.

   Igneous rocks are geologically important because:
     * their minerals and global chemistry gives information about the
       composition of the mantle, from where 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, whereas ores of chromium and platinum are
       commonly associated with gabbros.

Morphology and setting

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

Intrusive igneous rocks

   Intrusive igneous rocks are formed from magma that cools and solidifies
   within the earth. 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. The extrusive types
   usually are called lavas.

   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.

   Coarse grained intrusive igneous rocks which form at depth within the
   earth are termed as abyssal; intrusive igneous rocks which form near
   the surface are termed hypabyssal.
   Igneous rock - light coloured tracks show the direction of lava flow
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   Igneous rock - light coloured tracks show the direction of lava flow

Extrusive igneous rocks

   Extrusive igneous rocks are formed at the Earth's surface as a result
   of the partial melting of rocks within the mantle and crust.

   The melt, with or without suspended crystals and gas bubbles, is called
   magma. Magma rises because it is less dense than the rock from which it
   was created. When it reaches the surface, magma extruded onto the
   surface either beneath water or air, is called lava. Eruptions of
   volcanoes under the 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.

   Magma which erupts from a volcano behaves according to its viscosity,
   determined by temperature, composition, and crystal content.
   High-temperature magma, most of which is basaltic in composition,
   behaves in a manner similar to thick oil and, as it cools, treacle.
   Long, thin basalt flows with pahoehoe surfaces are common. Intermediate
   composition magma such as andesite tends to form cinder cones of
   intermingled ash, tuff and lava, and may have viscosity similar to
   thick, cold molasses or even rubber when erupted. Felsic magma such as
   rhyolite is usually erupted at low temperature and is up to 10,000
   times as viscous as basalt. Volcanoes with rhyolitic magama commonly
   erupt explosively, and rhyolitic lava flows typically are of limited
   extent and have steep margins, because the magma is so viscous.

   Felsic and intermediate magmas that erupt often do so violently, with
   explosions driven by release of dissolved gases -- typically water but
   also carbon dioxide. Explosively erupted material is called tephra, and
   volcanic deposits are called pyroclastic, and they include tuff,
   agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms
   ash 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 prevent the formation of even small
   crystals after extrusion, the resulting rock may be mostly glass (such
   as the rock obsidian).

   Because the minerals are 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.

Classification

   Igneous rock 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 upon the cooling
   history, and the mineral composition of the rock. Feldspars, quartz,
   olivines, pyroxenes, amphiboles, and micas are all important minerals
   in the formation of igneous rocks, and they are basic to the
   classification of these rocks. All other minerals present are regarded
   as nonessential (called accessory minerals).

   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 with
   quartz.

   Igneous rocks which 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 discernable 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.

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 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 which are 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
   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
   apophyses. Mineralogical classification is used most often to classify
   plutonic rocks and chemical classifications are preferred to classify
   volcanic rocks, with phenocryst species used as a prefix, eg;
   "olivine-bearing picrite" or "orthoclase-phyric rhyolite".
     * see also List of rock textures

Chemical classification

   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:
     * acid igneous rocks containing a high silica content, greater than
       63% SiO[2] (examples rhyolite and dacite)
     * intermediate igneous rocks containing between 52 - 63% SiO[2]
       (example andesite)
     * basic igneous rocks have low silica 45 - 52% and typically high
       iron - magnesium content (example basalt)
     * ultrabasic igneous rocks with less than 45% silica. (examples
       picrite and komatiite)
     * alkalic igneous rocks with 5 - 15% alkali (K[2]O + Na[2]O) content
       or with a molar ratio of alkali to silica greater than 1:6.
       (examples phonolite and trachyte)

          Note: the acid-basic terminology is used more broadly in older
          geological literature.

   Chemical classification also extends to differentiating rocks which are
   chemically similar according to the TAS diagram, for instance;
     * 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
     * Peraluminous; rocks containing molar (K[2]O + Na[2]O)/ Al[2]O[3] <1

   An idealized mineralogy (the normative mineralogy) can be calculated
   from the chemical composition, and the calculation is useful for rocks
   too fine-grained or too altered for identification of minerals that
   crystallized from the melt. For instance, normative quartz classifies a
   rock as silica-oversaturated; an example is rhyolite. A normative
   feldspathoid classifies a rock as silica-undersaturated; an example is
   nephelinite.

Mineralogical classification

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

   Mineralogic contents - felsic versus mafic
     * felsic rock, with predominance of quartz, alkali feldspar and/or
       feldspathoids: the felsic minerals; these rocks (e.g., granite) are
       usually light coloured, and have low density.
     * mafic rock, with predominance of mafic minerals pyroxenes, olivines
       and calcic plagioclase; these rocks (example, basalt) are usually
       dark coloured, and have higher density than felsic rocks.
     * ultramafic rock, 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 via microscope, the mineralogy is used to
   classify the rock. This usually occurs on ternary diagrams, where the
   relative proportions of three minerals are used to classify the rock.

   The following table is a simple subdivision of igneous rocks according
   both to their composition and mode of occurrence.
                                   Composition
   Mode of occurrence Acid     Intermediate Basic  Ultrabasic
   Intrusive          Granite  Diorite      Gabbro Peridotite
   Extrusive          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 and with more than 10% of felsic
   minerals) and phaneritic, subeuhedral texture (minerals are visible for
   the unaided eye and some of them retain original crystallographic
   shapes). Granite is the most abundant intrusive rock that can be found
   in the continents.

Etymology

   Volcanic rocks are named after Vulcan, the Roman name for the god of
   fire.
   Intrusive rocks are also called plutonic rocks, named after Pluto, the
   Roman god of the underworld.
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