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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 either the Earth's mantle or pre-existing rocks made molten by extreme temperature and pressure changes. 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".
The Earth's crust is about 35 kilometers (22 miles) thick under the continents, but averages only some 7 kilometers (4.3 miles) beneath the oceans. It is made up of rocks which have a relatively low density, and beneath the crust there is the denser rock of the mantle, which extends to a depth of nearly 3,000 kilometers (1,860 miles). Most of the magma which forms igneous rocks is generated within the upper parts of the mantle at temperatures estimated between 600 to 1600 °C.
As magma cools, minerals crystallize from the melt at different temperatures (fractional crystallization). There are relatively few minerals which are important in the formation of igneous rocks. This is 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.
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 (see plate tectonics);
- in some special circumstances they host important mineral deposits (ores): for example, tungsten, tin, and uranium, are commonly associated with granites.
Igneous rocks are classified according to mode of occurrence, texture, chemical composition, and the geometry of the igneous body.
Mode of occurrence
Intrusive igneous rocks crystallize within the crust interior.
Intrusive igneous rocks (also called plutonic rocks, named after Pluto, the Roman god of the underworld) are formed from magma that cools and hardens within the earth. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are course grained. The mineral grains in such rocks can generally be identified with the naked eye.
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 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 are the result of volcanic eruptions and, therefore, solidify in atmospheric conditions.
Extrusive igneous rocks (also called volcanic rocks, named after Vulcan, the Roman name for the god of fire) are formed at the Earth's surface as a result of volcanic activity. The temperatures only a few kilometers beneath the surface of the earth are higher than the temperatures at which most rocks would melt at the surface. These rocks remain solid, however, due to the pressure exerted by the overlying rocks. If the rocks fracture the pressure may be released and a sizeable volume of rock will melt. The resulting magma will be forced up through the fractures to the surface, forming a volcano.
Molten or partly molten rock (called lava) will flow from the volcano and spread out. 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 (such as the rock obsidian).
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.
Material may be violently forced out of the volcano during igneous activity as blocks of rock, pellets, and ash. This material is called pyroclastic rock (also called fragmented igneous rock) and may fall nearby, forming part of the volcano itself, of may be spread over great distances by the wind.
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.
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.
The most important distinction in igneous rocks is texture, which is related to the size and shape of the constituent crystallite grains.
According to the size of the grains, igneous rocks may be classified as pegmatic (very large grains), phaneritic (only large grains), porphyritic (some large grains and some small grains), apahnitic (only small grains) or glassy (no grains).
- Phaneritic rocks contain minerals with grains (crystals) visible to the unaided eye and are commonly intrusive (as the slower cooling rates allow the formation of large crystals). In the extreme, such rocks may contain extremely large crystals, in which case they are termed pegmatitic.
- In extrusive rocks, where cooling is much more rapid, the individual mineral crystals are usually not visible and these rocks are termed aphanitic.
- Porphyritic textures are an intermediate situation between the previous two: the groundmass of the rock has an aphanitic texture, but crystals (termed in this particular occurrence as phenocrysts) are visible to unaided eye.
- If a molten magma cools at extremely high rates, allowing no crystallization, the result is a volcanic glass called obsidian.
Crystal shape is also an important factor in the texture of an igneous rock. Crystals may be euhedral, subeuhedral or anhedral:
- Euhedral, if the crystallographic shape is preserved.
- Subeuhedral , if only part is preserved.
- Anhedral , if the crystal presents no recognizable crystallographic direction.
Chemical - mineralogical composition
Igneous rocks can be subdivided according to two main chemical parameters:
- Chemical - silica versus iron - magnesium content:
- 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 that 90% of mafic minerals (e.g., dunite)
The following table is a simple subdivision of igneous rocks according both to their composition and mode of occurrence.
|Mode of occurrence||Acid||Intermediate||Basic||Ultrabasic|
Geometry of the igneous body
Igneous rocks can also 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.
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, subeuedral texture (minerals are visible for the unaided eye and some of them retain original crystallographic shapes).
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