Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. The way that magma turns into a solid rock gives it a distinctive igneous texture.
For example, magma that becomes a pluton by slowly crystallizing growing minerals within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock.
Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.
Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals.
Examples include obsidian and pumice. Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive the size ranges from a few centimetres to several metres. This is typical of pegmatites.
As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye.
Examples of phaneritic igneous rocks are gabbro, diorite and granite. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.
The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained aphanitic matrix. The result is an aphanitic rock with some larger crystals phenocrysts imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
Figure 1. Different cooling rate and gas content resulted in these different textures. Let us start with textures associated with rocks formed by lava flows.
Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture.
Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.
This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice.
Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. National Geographic Society. For information on user permissions, please read our Terms of Service.
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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Metamorphic rocks start as one type of rock and—with pressure, heat, and time—gradually change into a new type of rock. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom.
Encyclopedic Entry Vocabulary. Pele's hair. Download digital geologic maps for entire states Where can I find information about the geology and natural history of National Parks? Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. The website has listings for regions of the country What is the difference between a rock and a mineral? A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties.
Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite. A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral What are metamorphic rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are Filter Total Items: 2.
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