Igneous rocks are formed by the solidification of magma, a silicate liquid generated by partial melting of the upper mantle or the lower crust. Different environments of formation, and the cooling rates associated with these, create very different textures and define the two major groupings within igneous rocks:
Volcanic rocks Volcanic rocks form when magma rises to the surface and erupts, either as lava or pyroclastic material. The rate of cooling of the magma is rapid, and crystal growth is inhibited. Volcanic rocks are characteristically fine-grained. Volcanic rocks often exhibit structures caused by their eruption, e.g. flow banding (formed by shearing of the lava as it flows), and vesicles (open cavities that represent escaped gasses).
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Plutonic rocks Plutonic rocks form when magma cools within the Earth’s crust. The rate of cooling of the magma is slow, allowing large crystals to grow. Plutonic rocks are characteristically coarse-grained.
Textures of igneous rocks
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The environment of formation produces characteristic textures in igneous rocks which aid in their identification. These textures are:
Phaneritic – This texture describes a rock with large, easily visible, interlocking crystals of several minerals. The crystals are randomly distributed and not aligned in any consistent direction. A phaneritic texture is developed by the slow cooling and crystallisation of magma trapped within the Earth’s crust and is characteristic of plutonic rocks.
Porphyritic – This texture describes a rock that has well-formed crystals visible to the naked eye, called phenocrysts, set in a very fine grained or glassy matrix, called the groundmass. A porphyritic texture is developed when magma that has been slowly cooling and crystallising within the Earth’s crust is suddenly erupted at the surface, causing the remaining uncrystallised magma to cool rapidly. This texture is characteristic of most volcanic rocks.
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Aphanitic – This texture describes very fine grained rock where individual crystals can be seen only with the aid of a microscope, i.e. the rock is mostly groundmass. An aphanitic texture is developed when magma is erupted at the Earth’s surface and cools too quickly for large crystals to grow. This texture is exhibited by some volcanic rocks.
Eutaxitic (applies only to welded ignimbrites) – This texture describes a rock with a planar fabric in which flattened pumice clasts are surrounded by a fine grained groundmass of sintered ash. The flattened pumice clasts are lenticular (lens-shaped) in cross-section and are called fiamme (Italian for flame). An eutaxitic texture is developed when hot, pumice-rich material is erupted explosively and is then compressed by overlying material while still in a hot, plastic state.
Other features
The chemical composition of the magma determines which minerals will form and in what proportions they will occur. Therefore, identification of the minerals present in the rock is an important step in being able to correctly identify the rock. Magmas that are relatively low in silica (SiO 2) crystallise olivine, pyroxene ( augite) and calcium-rich plagioclase, while magmas that are high in SiO 2 crystallise quartz, sodium-rich plagioclase, orthoclase, biotite and hornblende. As with minerals, igneous rocks can be broadly divided into mafic and felsic types. Mafic rocks are generally darker, and have higher abundances of mafic minerals. Felsic rocks are generally lighter in colour, having a higher concentration of felsic minerals.
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