Igneous rocks may be simply classified according to their chemical/mineral composition as felsic, intermediate, mafic, and ultramafic, and by texture or grain size: intrusive rocks are course grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained (microscopic crystals) or glass . Intrusive Igneous Rocks - usually medium to coarse grained texture due to slow cooling. Typically light coloured. Extusive Igneous Rocks - glassy or fine-grained. Igneous rocks are formed when magma cools and solidifies. They are classified by using grain size, silica content, and/or silica saturation.


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Because lava usually cools and crystallizes rapidly, it is usually fine-grained.

If the cooling has been so rapid as to prevent the formation of even small crystals after extrusion, the resulting rock igneous rock classification be mostly glass such as the rock obsidian. If the cooling of the lava happened more slowly, the rock 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 microscopeso only igneous rock classification approximate classification can usually igneous rock classification 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.

Igneous Rock Classificiation

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 on the cooling history, and the mineral composition of the igneous rock classification.

Feldsparsquartz or feldspathoidsolivinespyroxenesamphibolesand micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification igneous rock classification these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory igneous rock classification.

Australian Museum

Types of igneous rocks igneous rock classification other essential minerals are very rare, and these rare rocks include those with essential carbonates. In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartzigneous rock classification in rocks with no feldspar or quartz, the type of iron or magnesium minerals present.

  • Classification of igneous rocks - Australian Museum
  • Intrusive or Extrusive

Rocks containing quartz silica in composition are silica-oversaturated. Rocks with igneous rock classification are silica-undersaturated, because feldspathoids cannot coexist in a stable association with quartz.

Igneous rocks that 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. Silica under saturation is when SiO2 is low enough there is not only not enough to form quartz, there is insufficient igneous rock classification form other minerals such as feldspars.

Igneous Rock Classification

The result is silica poor feldspathoid minerals, such as nephaline and sodalite. Igneous rock classification saturation is when enough SiO2 exists for quartz to crystallize out. If SiO2 is high enough it is possible to have a basalt with quartz, an association not commonly thought to exist.

Go to table link. Alkali indexes greater than 1 indicate high Ca content typical of the top of.

Igneous rock

Iron enrichment declines steadily with fractionation. This is a measure of the decrease in the importance of ferromagnesium minerals down the reaction series.


Iron is low in the Komatiite suite because the ultramafic components Mg, Ni, and Cr are so high. Igneous rock evolution can igneous rock classification both within and among the suites. Within suite evolution occurs when, for example, a calcalkaline suite evolves from a diorite to a granite, or a komatiite suite evolves from a peridotite to a basalt to an andesite.

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