Igneous Rocks Composition (Ultramafic, Mafic, Intermediate and Felsic)

Igneous rocks can be classified based on their chemical and mineral composition into four primary categories: ultramafic, mafic, intermediate, and felsic. Here’s a detailed overview of each category including their composition, characteristics, examples, and formation processes.

Ultramafic Rocks

Classify Igneous Rocks Into (Ultramafic, Mafic, Intermediate and Felsic)

Ultramafic rocks are often associated with tectonic settings like:

• Mantle Plumes: Rising magma from deep within the Earth.
• Mid-Ocean Ridges: Areas where tectonic plates are diverging.
• Ophiolites: Fragments of the oceanic crust and upper mantle that have been emplaced onto continental crust.

Mafic rocks

Mafic rocks contain approximately 45% to 52% silica and are characterized by a 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.

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. 

 Mafic Igneous rocks Examples: 

• Basalt: A fine-grained, dark volcanic rock that is the most common type of mafic rock, found primarily in oceanic crust.
• Gabbro: A coarse-grained intrusive rock that corresponds to basalt, containing similar minerals but crystallized at a slower rate.

Intermediate Rocks

Intermediate rocks have silica content between 52% and 63%, representing a mix of mafic and felsic characteristics.

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. 

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.

 Intermediate Igneous rocks Examples: 

• Andesite: A fine-grained volcanic rock, often found in volcanic arcs associated with subduction zones.
• Diorite: A coarse-grained intrusive rock with a speckled appearance due to the intermingling of light and dark minerals.

Classification diagram for igneous rocks. Igneous rocks are classified according to the relative abundances of minerals they contain. A given rock is represented by a vertical line in the diagram. In the mafic field, the arrows represent a rock containing 48% pyroxene and 52% plagioclase feldspar. The name an igneous rock gets depends not only on composition, but on whether it is intrusive or extrusive.

Felsic 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.

Felsic Igneous rocks Examples: Granite, rhyolite, pegmatite

• Granite: A coarse-grained rock often used in construction, characterized by its visible crystals of quartz and feldspar.
• Rhyolite: A fine-grained volcanic rock that corresponds to granite; often contains glassy textures.
• Pumice: A light, porous volcanic rock formed from explosive eruptions; can float on water due to its vesicular structure.

The classification of igneous rocks into ultramafic, mafic, intermediate, and felsic categories is essential for geologists and earth scientists. It provides insight into the formation processes, geological settings, and the evolution of the Earth's crust.