Igneous Rock

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  • Topic: Igneous rock, Basalt, Petrology
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Igneous rock
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Geologic provinces of the world (USGS)
Shield
Platform
Orogen
Basin
Large igneous province
Extended crust
Oceanic crust:
0–20 Ma
20–65 Ma
>65 Ma
Igneous rock (derived from the Latin word ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form 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 a planet's mantle or crust. Typically, the melting is caused by one or more of three 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 having formed beneath the surface of Earth's crust. Contents [hide]

1 Geological significance
2 Morphology and setting
2.1 Intrusive
2.2 Extrusive
2.3 Hypabyssal
3 Classification
3.1 Texture
3.2 Chemical classification
3.3 History of classification
4 Mineralogical classification
4.1 Example of classification
5 Magma origination
5.1 Decompression
5.2 Effects of water and carbon dioxide
5.3 Temperature increase
5.4 Magma evolution
6 Etymology
7 See also
8 Notes
9 References
10 External links
[edit]Geological significance

Igneous and metamorphic rocks make up 90-95% of the top 16 km of the Earth's crust by volume.[1] 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, tungsten, tin, and uranium are commonly associated with granites and diorites, whereas ores of chromium and platinum are commonly associated with gabbros. [edit]Morphology and setting

In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive (volcanic) or hypabyssal. [edit]Intrusive

Close-up of granite (an intrusive igneous rock) exposed in Chennai, India. Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a planet. 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 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 crust are termed as abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal. [edit]Extrusive

Extrusive igneous rock is made from lava released by volcanoes

Basalt (an extrusive igneous rock in this case); light coloured tracks show the direction of...
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