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Igneous rocks are formed when molten rock cools and solidifies, with or without
crystallization, either below the surface as
intrusive rocks or on the surface as extrusive 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 ignis, meaning "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
Earth's mantle.
The
Earth's mantle immediately below the crust typically is relatively rigid, and this relatively rigid mantle and the overlying crust comprise the
lithosphere. Because of the typical increase of temperature with depth, the mantle becomes less rigid and convects even though it is completely or mostly solid; the convecting mantle immediately below the lithosphere makes up the asthenospheric mantle. The
mantle, which extends to a depth of nearly 3,000 kilometers, is the source of much magma. Most magmas are generated at temperatures between 600 to 1600 °C. Most of the higher-temperature magmas, those with temperatures between about 900° and 1600°C, are formed in the upper two hundred kilometers or so 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: the solidus temperature of a rock at a given pressure is the maximum temperature below which that rock is completely crystalline. Rock at depth in the earth 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 0.5°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 , 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
The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the solidus temperatures of rocks. 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.
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 the lower part of crust thickened by compression at a plate boundary.
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 . 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.
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.
Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.
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 rock 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 ;
- in some special circumstances they host important mineral deposits : 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 or extrusive .
Intrusive igneous rocks
Intrusive igneous rocks are formed from magma that cools and solidifies within the earth. Surrounded by pre-existing 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 may occupy huge areas of the 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.
Extrusive igneous rocks
Extrusive igneous rocks are formed at the Earth's surface as a result of the melting of rocks within the
mantle.The melted rock, called magma rises because of contrasting density with the surrounding mantle. 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 temperature and composition, which cause a highly different range of
viscosity. High temperature magma, which is usually basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. This forms
pahoehoe type lava. 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. These volcanoes rarely form lava flows, and usually erupt explosively.
Felsic and intermediate rocks which erupt at surface often do so violently, with explosions driven by release of gases such as
carbon dioxide trapped in the magma. Such volcanic deposits are called pyroclastic deposits, and 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 the resulting rock may be a glass .
Because of this fine grained texture 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 .
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 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.
The crystals embedded in fine grained igneous rocks are termed porphyritic. The porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma consolidates into the finer grained uniform material.
Texture
- 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 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 for
volcanic rock classification used when modal or mineralogic data is unavailable:
- acid igneous rocks containing a high silica content, greater than 63% SiO2
- intermediate igneous rocks containing between 52 - 63% SiO2
- basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content
- ultrabasic igneous rocks with less than 45% silica.
- alkalic igneous rocks with 5 - 15% alkali content or with a molar ratio of alkali to silica greater than 1:6.
- 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 K2O/Na2O >3
- Peralkaline; rocks containing molar / Al2O3 >1
- Peraluminous; rocks containing molar / Al2O3 <1
An idealized 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 are usually light coloured, and have low density.
- mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic plagioclase; these rocks are usually dark coloured, and have higher density than felsic rocks.
- ultramafic rock, with more than 90% of mafic minerals
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.
For a more detailed classification see
QAPF diagram.
Example of classification
Granite is an igneous intrusive rock , with felsic composition and phaneritic, subeuhedral texture . 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.
Reference
- Le Maitre, L.E., ed., Igneous Rocks: A Classification and Glossary of Terms 2nd edition, Cambridge.
See also
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External links
