Igneous rock




 

Igneous rocks (etymology from Latin ignis, fire) are 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.

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 metamorphic rocks.

Igneous rocks are geologically important because:

  • their minerals and global chemistry give information about the composition of the mantle, from which 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, 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. lava flows.

The central cores of major mountain ranges consist of intrusive igneous rocks, usually 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.  

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 basalt are examples of submarine volcanic activity.

The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock is produced in the following proportions:[1]

  • divergent boundary: 73%
  • convergent boundary (subduction zone): 15%
  • hotspot: 12%

Magma which erupts from a basalt. Volcanoes with rhyolitic magma 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 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). If the cooling of the lava happened slowly, the rocks would be coarse-grained.

Because the minerals are mostly 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 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 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. carbonates.

In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of feldspathoids are silica-undersaturated, because feldspathoids cannot coexist in a stable association 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 discernible 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

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 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.

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 porphyry stocks and subvolcanic dikes (apophyses). Mineralogical classification is used most often 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".

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% SiO2 (examples rhyolite)
  • intermediate igneous rocks containing between 52 - 63% SiO2 (example dacite)
  • basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium content (example basalt)
  • komatiite)
  • alkalic igneous rocks with 5 - 15% trachyte)
Note: the acid-basic terminology is used more broadly in older (generally British) geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.

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 (K2O + Na2O)/ Al2O3 >1
  • Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1

An idealized mineralogy (the nephelinite.

History of classification

In 1902 a group of American petrographers 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 was much of the existing terminology and argued that as the chemical composition of an igneous rock was its most fundamental characteristic it should be elevated to prime position.

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.[2] [3]

Mineralogical classification

For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important criterion is the aphanitic, chemical classification must be used to properly identify a volcanic rock.

Mineralogic contents - felsic versus mafic

  • feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light coloured, and have low density.
  • plagioclase; these rocks (example, basalt, gabbro) are usually dark coloured, and have a higher density than felsic rocks.
  • 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 Felsic Intermediate Mafic Ultramafic
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite
Essential rock forming silicates
Felsic Intermediate Mafic Ultramafic
Coarse Grained Granite Diorite Gabbro Peridotite
Medium Grained Diabase
Fine Grained Rhyolite Andesite Basalt Komatiite

For a more detailed classification see QAPF diagram.

Example of classification

subeuhedral texture (minerals are visible to the unaided eye and commonly some of them retain original crystallographic shapes).

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 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 peridotite samples document that the solidus temperatures increase by 3°C to 4°C per kilometer. If the rock rises far enough, 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 ridgess. Decompression melting caused by the rise of mantle plumes is responsible for creating ocean islands like 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.

Effects of water and carbon dioxide

The change of rock composition most responsible for creation of magma is the addition of water. Water lowers the calc-alkaline series, an important part of continental crust.

The addition of kimberlite are among those that may be generated following an influx of carbon dioxide into mantle 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 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

Main article: Igneous differentiation

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, 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: 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% SiO2, and 1.50% TiO2). 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).

Etymology

The word "igneous" is derived from the Latin igneus, meaning "of fire". 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.

See also

References

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