7.5: Texture
Texture
What Happens to a Melt As It Cools?
As a melt cools, minerals crystallize and glasses can develop. In general, melts will cool slowly, over millennia to millions of years (beyond a human lifespan), or rapidly, in seconds to decades (within a human lifespan). There are a number of factors that dictate the cooling rate of a melt, but the major control is the cooling environment or realm of the melt. Does the melt cool intrusively, as magma? Or does the melt cool extrusively, as lava? A melt that cools in the intrusive Links to an external site., or plutonic, realm cools slowly over millennia to millions of years. A melt that cools in the extrusive Links to an external site., or volcanic, realm cools rapidly, in seconds to decades. The extrusive realm is typically associated with volcanics, and if humans are present they may witness the eruption. The cooling realm is typically the driving factor of the rock’s texture. Generally, rocks that cool intrusively have visible crystals (minerals), whereas rocks that cool extrusively have microscopically small crystals (minerals) or contain glass. In addition to the cooling realm, the size and shape of the melt will also dictate the overall cooling rate (Figure 7.7).
Figure 7.7: An illustration depicting the effect of size and shape on cooling rate. (CC-BY 4.0; Chloe Branciforte)
The end product of a cooling melt is an igneous rock. This rock may contain minerals, glasses, clasts, or some combination of those materials.
The classification of igneous rocks is based not just on composition, but also on texture. Recall a rocks’ texture Links to an external site. refers to the size and arrangement of minerals, glass, fragmented material, or vesicles (holes).
Igneous Rock Textures
Crystals form while the magma is cooling. Therefore, the size of the crystals is related to the cooling process. Each mineral derives its chemical composition directly from the magma and has a certain temperature interval during which that particular mineral can form. The chemical elements that become part of the mineral must migrate from the liquid magma to link or bond with other elements in a certain way to form the crystal structure that is unique for that mineral. Whether the magma’s temperature drops quickly or slowly affects the time necessary for the migration of the chemical elements to form a mineral crystal.
When magma cools slowly, there is plenty of time for the migration of the needed chemical elements to form a certain mineral; that particular mineral can become quite large in size, large enough for a person to see without the aid of a microscope. As a result, this igneous rock with its visible minerals is said to have a phaneritic texture Links to an external site. (phaneros = visible) (Figure 7.8). Granite Links to an external site., diorite Links to an external site., and gabbro Links to an external site. are all phaneritic igneous rocks with different compositions (felsic, intermediate, and mafic, respectively).
Figure 7.8: A phaneritic (coarse-grained) igneous rock. (CC-BY 4.0; Chloe Branciforte)
If the rock contains crystals that are larger than your thumbnail, throughout the entire rock, the textural term is pegmatitic that forms a rock called pegmatite Links to an external site. (Figure 7.9).
Figure 7.9: A pegmatitic igneous rock. (CC-BY 4.0; Chloe Branciforte)
Magma that cools relatively quickly has less time for the migration of the chemical elements to form a mineral, and as a result the minerals will not have time to form large crystals. Therefore, many small crystals of a particular mineral will form in the magma. Igneous rocks that are composed of crystals too small to see (unless you have a microscope) are called aphanitic Links to an external site. igneous rocks (Figure 7.10).
Figure 7.10: An aphanitic igneous rock. (CC-BY 4.0; Chloe Branciforte)
Basalt Links to an external site., andesite Links to an external site., and rhyolite Links to an external site. are all aphanitic igneous rocks with different compositions (mafic, intermediate, and felsic, respectively). It is important to note that basalt and gabbro both have the same composition – mafic – but one rock represents a lava that cooled fast (basalt), and the other represents a mafic magma that cooled slowly (gabbro). The same can be said for the other rock compositions: the felsic rocks rhyolite and granite have identical compositions, but one cooled fast (rhyolite) and the other cooled slowly (granite). The intermediate rocks diorite and andesite also represent melts that cooled slower or more rapidly, respectively.
Sometimes there are some visible crystals in an otherwise aphanitic rock, such as the andesite shown; this texture is referred to as porphyritic Links to an external site. (or more accurately porphyritic-aphanitic; Figure 7.11). Two different crystal sizes within the same igneous rock indicates that the cooling rate was complex, and there may be multiple levels of cooling represented. The larger crystals are called phenocrysts Links to an external site. and the smaller crystals are called the groundmass Links to an external site.. While the magma was cooling slowly, larger crystals can form. If the magma moves upward and erupts from a volcano, the remaining lava crystallizes quickly around the already formed phenocrysts, to form the small-size crystals in the groundmass. A phaneritic rock can also be referred to as a porphyritic-phaneritic if it contains some very large crystals (phenocrysts), in addition to the other smaller, but still visible crystals in the groundmass.
Figure 7.11: A porphyritic igneous rock. Phenocrysts, the white minerals, are labeled with the yellow arrow. Goundmass, fine-grained gray regions, are labeled with the orange arrow. (CC-BY 4.0; Chloe Branciforte)
Sometimes the melt cools so quickly that there isn’t time to form any minerals, as the chemical elements in the magma do not have time to migrate into any crystal structure. When this happens, the magma becomes a dense glass called obsidian Links to an external site.. By definition, glass is a chaotic arrangement of the chemical elements, and therefore is not considered to be a mineral; igneous rocks composed primarily of glass are said to have a glassy texture Links to an external site. (Figure 7.12). The identification of a glassy rock such as obsidian is easy once you recall the properties of glass: any thick glass pane or a glass bottle that is broken will have a smooth, curve-shaped pattern on the broken edge called conchoidal fracture Links to an external site.. Even though obsidian is naturally occurring, and not synthetic (human-made), it still breaks in the same way as glass you are likely familiar with. Obsidian appears quite dark in color because light has difficulty passing through the dense glass; however, chemically it is mostly silicon and oxygen and therefore more felsic in composition.
Figure 7.12: A glassy igneous rock. (CC-BY 4.0; Chloe Branciforte)
Another igneous rock that is also composed primarily of glass is called pumice, which will actually float on water due to its low density. The glass in this rock has been aerated during a volcanic eruption. This aeration produces a texture referred to as vesicular Links to an external site. or porous, and the rock typically has many holes (imagine freezing the foamy head on root beer) (Figure 7.13). Pumice Links to an external site. can display a wide range of colors but typically has a composition between felsic to intermediate. Scoria Links to an external site., the other vesicular rock, will not float on water. This is because scoria contains an abundance of iron and magnesium, resulting in a dark gray, black or red color, representing a more mafic composition.
Figure 7.13: A vesicular igneous rock. (CC-BY 4.0; Chloe Branciforte)
The final texture is unique in that the rock contains clasts, fragments, or pieces of pre-existing rock that were violently produced during an explosive, gas-rich, volcanic eruption. This texture is termed pyroclastic Links to an external site. or fragmental (Figure 7.14) and has a distinct gritty or grainy feel.
Figure 7.14: A fragmental igneous rock. (CC-BY 4.0; Chloe Branciforte)
The different crystal sizes and presence or absence of glass in an igneous rock is primarily controlled by the rate of magma cooling. Generally, rocks that cool intrusively have visible crystals (minerals), whereas rocks that cool extrusively have small crystals (minerals) or contain no crystals, instead exhibiting glasses, holes, or clasts. Recall, magmas that cool below the surface in the intrusive or plutonic realm Links to an external site. cool slowly over millennia to millions of years, because the rock surrounding the magma acts as an insulator (similar to a coffee thermos). Lavas that cool at the surface, in the extrusive or volcanic realm Links to an external site., cool rapidly, in seconds to decades.
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Texture |
Composition |
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|---|---|---|---|---|
|
Felsic High SiO2 Low Fe & Mg |
Intermediate |
Mafic |
Ultramafic Low SiO2 High Fe & Mg |
|
|
Minerals present (crystalline rocks only) |
Quartz, Potassium feldspar, Na-rich plagioclase feldspar |
Na-rich plagioclase feldspar, Amphibole |
Ca-Plagioclase feldspar, Pyroxene |
Olivine, Pyroxene, Ca-Plagioclase feldspar |
|
Pegmatite* |
Granite pegmatite |
-- |
Gabbro pegmatite |
-- |
|
Phaneritic* |
Granite Syenite Monzonite |
Diorite |
Gabbro |
Peridotite |
|
Porphyritic*^ |
Granite porphyry Rhyolite porphyry |
Andesite porphyry |
Basalt porphyry |
-- |
|
Aphanitic^ |
Rhyolite Trachyte |
Andesite Dacite |
Basalt |
Komatiite |
|
Glassy^ |
Obsidian |
-- |
-- |
|
|
Vesicular^ |
Pumice |
Scoria |
-- |
|
|
The rocks below have no set igneous composition. Instead, the size of the fragments (clasts) are used to identify. |
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|
Pyroclastic^ (Fragmental) |
Fine (<4 mm) |
Volcanic Tuff |
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|
Coarse (>4 mm) |
Volcanic Breccia |
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*intrusive ^extrusive *^complex cooling history |
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