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.
Like lava flow rocks, these are also extrusive igneous rocks. In a rock with a porphyritic texture, we use the above table to define the grain size of the groundmass or matrix, and this table to describe the phenocrysts:. Another aspect of texture, particularly in medium to coarse grained rocks is referred to as fabric. Fabric refers to the mutual relationship between the grains. Three types of fabric are commonly referred to: If most of the grains are euhedral - that is they are bounded by well-formed crystal faces.
The fabric is said to be idomorphic granular. If most of the grains are subhedral - that is they bounded by only a few well-formed crystal faces, the fabric is said to be hypidiomorphic granular. If most of the grains are anhedral - that is they are generally not bounded by crystal faces, the fabric is said to be allotriomorphic granular. If the grains have particularly descriptive shapes, then it is essential to describe the individual grains. Some common grain shapes are: Tabular - a term used to describe grains with rectangular tablet shapes.
Equant - a term used to describe grains that have all of their boundaries of approximately equal length. Fibrous - a term used to describe grains that occur as long fibers. Acicular - a term used to describe grains that occur as long, slender crystals.
Prismatic - a term used to describe grains that show an abundance of prism faces. Other terms may apply to certain situations and should be noted if found in a rock. Vesicular - if the rock contains numerous holes that were once occupied by a gas phase, then this term is added to the textural description of the rock.
Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic. Amygdular - if vesicles have been filled with material usually calcite, chalcedonay, or quartz, then the term amygdular should be added to the textural description of the rock. An amygdule is defined as a refilled vesicle.
Graphic - a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites. Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.
Obicular - a texture usually restricted to coarser grained rocks that consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals. Other textures that may be evident on microscopic examination of igneous rocks are as follows: Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites.
Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros. Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains.
Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral. Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides. Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.
There are nine main types of igneous rock textures: Phaneritic, vesicular, aphanitic, porphyritic, poikilitic, glassy, pyroclastic, equigranular, and spinifex.
Each kind of texture has a variety of different characteristics that make them unique. What determines igneous rock texture? It all comes down to the rate at which the rock cools.
Other factors include the diffusion rate, which is how atoms and molecules move through the liquid. The rate of crystal growth is another factor, and that's how quickly new constituents come to the surface of the growing crystal. New crystal nucleation rates, which is how enough chemical components can come together without dissolving, is another factor affecting the texture. Texture is comprised of grains, and there are a few main types of igneous rock grains: Equant grains are those with boundaries of equal lengths; rectangular tablet shapes are known as tabular grains; acicular grains are slender crystals; long fibers are known as fibrous grains, and a grain that is prismatic is one that has different types of prisms.
Aphanitic "AY-fa-NIT-ic" rocks have mineral grains that are mostly too small to be seen with the naked eye or a hand lens, like this rhyolite. Basalt is another igneous rock with the aphanitic texture. This example is a granite. Glassy or hyaline or vitreous rocks have no or almost no grains at all, as in this quickly chilled pahoehoe basalt or in obsidian.
Phaneritic "FAN-a-RIT-ic" rocks have mineral grains that are large enough to be seen with the naked eye or a hand lens, like this granite.
Poikilitic "POIK-i-LIT-ic" texture is one in which large crystals, like this feldspar grain, contain small grains of other minerals scattered inside them. In other words, they display two distinctive sizes of grains that are visible to the naked eye. Rocks with pyroclastic "PY-ro-CLAS-tic" texture are made of pieces of volcanic material that are created in an explosive eruption, like this welded tuff. Spinifex texture, found only in komatiite, consists of large crisscrossing platy crystals of olivine.
Spinifex is a spiny Australian grass. Rocks with vesicular "ve-SIC-ular" texture are full of bubbles. It always indicates a volcanic rock, like this scoria.
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