Igneous rocks can be classified into four categories based on their chemical composition: ultramafic, mafic, intermediate, and felsic. This classification is based on the amount of silica (SiO2) in the rock.
Ultramafic Igneous rocks
Peridotite is the name given to intrusive ultramafic rocks, whereas komatiite is the name given to extrusive ultramafic rocks. Peridotite and komatiite are compositionally identical. Their textures, however, are different reflecting their mode of formation. Peridotite appears to be the dominant rock type of the upper mantle.
It is very rarely exposed at the surface. Chunks of peridotite, however, are often brought up by other magmas (of the mafic family- see below). These included chunks are called inclusions or xenoliths (meaning "foreign rock"). Komatiites, the extrusive equivalents of peridotite are relatively rare. They are significant, though, in that most of them are older than about 2 billion years (2 Ga where "Ga" stands for Giga-annum). Because these magmas require the most heat to form, it is inferred that the Earth has cooled down since 2 Ga so that these magmas can no longer form.
Mafic Igneous rocks
Mafic igneous rocks are a group of rocks characterized by their dark color, high magnesium and iron content, and dominance of specific silicate minerals.Mafic igneous Rocks composed mostly of pyroxene, calcium-rich plagioclase, and minor amounts of olivine make up the mafic family of igneous rocks. The mafic magmas are somewhat more viscous than the ultramafic magmas, but they are still fairly fluid.
Additionally, they contain somewhat more gas than do the ultramafic magmas, but not nearly so much as the felsic magmas. Gabbro is the name given to intrusive mafic rocks, whereas basalt is the name given to extrusive mafic rocks.
Mafic rocks are produced in a variety of plate tectonic settings (divergent plate boundaries such as mid-ocean ridges, continental rifts such as our own Basin and Range Province in Arizona, and mantle hot spots like Hawaii).
Mafic rocks, regardless of tectonic setting, have one thing in common. All are produced by partial melting of the uppermost asthenosphere. Because of their relatively low viscosity, mafic lavas may travel great distances and "flood" the landscapes over which they flow.
Mafic lava flows are thin and sometimes called plateau or flood lavas. Volcanoes made up of mafic flows are typically shield volcanoes and/or cinder cones. Mafic flows erupting beneath water form bulbous masses called pillow lavas.
Intermediate Igneous rocks
Intermediate igneous rocks represent a middle ground between felsic and mafic rocks in terms of their mineral composition and silica content. They typically contain 5% to 20% quartz and a mixture of felsic and mafic minerals.Intermediate igneous Rocks composed mostly of hornblende and intermediate plagioclase feldspars make up the intermediate family of igneous rocks. Intermediate magmas are somewhat more viscous than the mafic magmas. Additionally, they contain somewhat more gas than do the mafic magmas, but not quite as much as the felsic magmas (see below).
Diorite is the name given to intrusive intermediate rocks, whereas andesite is the name given to extrusive intermediate rocks. Intermediate rocks are produced primarily in convergent plate boundaries in which an oceanic plate is subducting beneath either another oceanic plate (such as in Japan) or a continental plate (such as along the Andes Mountains of South America).
Intermediate magmas tend to form thicker lava flows and higher, steeper volcanoes (called composite or stratovolcanoes) than do mafic magmas. Typical composite cones are Mt. Rainier near Seattle, Washington, Mt. Fuji in Japan, and our own San Francisco Peaks near Flagstaff, Arizona.
Felsic Igneous rocks
Felsic igneous rocks are distinguished by their high silicon (Si) content, typically ranging from 65% to 75% silica (SiO₂). This high silica content results in a light color and a predominance of light-colored minerals like feldspar and quartz.Felsic igneous Rocks composed mostly of biotite, muscovite, sodium-rich plagioclase feldspars, potassium feldspars, and quartz make up the felsic family of igneous rocks.
Felsic magmas are much more viscous than the intermediate magmas. Additionally, felsic magmas may have very high gas contents. Granite is the name given to intrusive felsic rocks, whereas rhyolite is the name given to extrusive felsic rocks.
Felsic rocks are produced primarily in convergent plate boundaries in which an oceanic plate is subducting beneath either another oceanic plate (such as in Japan) or a continental plate (such as along the Andes Mountains of South America). Because of their high viscosity, felsic magmas do not reach the surface as often as do intermediate or mafic magmas. However, because of its high gas content, when felsic magma does erupt, the eruption is the most violent.
The explosive eruption often results in the emplacement of ashflow tuffs and volcanic breccias. Rhyolites tend to form domes or plugs rather than extensive lava flows. In addition, the viscosity of the magma hinders crystal growth. Consequently, obsidian or volcanic glass is most common in the cooling of felsic lavas.
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