Igneous rock

February 4, 2013, 3:09 pm
Content Cover Image

Pictured above is the igneous rock known as diorite. (Image Source: Wikimedia Commons, Photographer Siim Sepp. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.)




Types of Igneous Rocks


Geologists have classified the chemistry of igneous rocks into four basic types: felsic, intermediate, mafic, and ultramafic (Figure 1). Igneous rocks derived from felsic magma contain relatively high quantities of sodium, aluminum, and potassium and are composed of more than 65% silica. Rocks formed from felsic magma include granite, granodiorite, dacite, and rhyolite (Figure 1 and Table 1). All of these rock types are light in color because of the dominance of quartz, potassium and sodium feldspars, and plagioclase feldspar minerals. Dacite and granodiorite contain slightly more biotite and amphibole minerals than granite and rhyolite. Rhyolite and dacite are produced from continental lava flows that solidify quickly. The quick solidification causes the mineral crystals in these rocks to be fine grained. Granite and granodiorite are common intrusive igneous rocks that are restricted to the Earth's continents. Large expanses of these rocks were formed during episodes of mountain building on the Earth. Because granite and granodiorite form beneath the Earth's surface, their solidification is a relatively slow process. This slow solidification produces a rock with a coarse mineral grain.

Mafic magma produces igneous rocks rich in calcium, iron, and magnesium and are relatively poor in silica (silica amounts from 45 to 52%). Some common mafic igneous rocks include fine-grained basalt and coarse-grained gabbro. Mafic igneous rocks tend to be dark in color because they contain a large proportion of minerals rich in iron and magnesium (pyroxene, amphiboles, and olivine). Basalt is much more common than gabbro (Table 1). It is found in the upper portion of the oceanic crust and also in vast continental lava flows that cover parts of Washington, Oregon, Idaho, and California. Gabbro is normally found in the lower parts of oceanic crust and sometimes in relatively small intrusive features in continental crust.

Andesite and diorite are intermediate igneous rocks that have a chemistry between mafic and felsic (silica amounts between 53 to 65%). These rocks are composed predominantly of the minerals plagioclase feldspar, amphibole, and pyroxene. Andesite is a common fine-grained extrusive igneous rock that forms from lavas erupted by volcanoes located along continental margins (Table 1). Coarse-grained diorite is found in intrusive igneous bodies associated with continental crust.


Figure 1: The  classification of igneous rocks. This graphic model describes the  difference between nine common igneous rocks based on texture of mineral  grains, temperature of crystallization, relative  amounts of typical rock-forming elements, and relative proportions of silica and some common minerals. (Source:  PhysicalGeography.net)

Table 1: Common igneous rock types.

caption Rhyolite (Image Source: Wikimedia Commons, Photographer Benhoff. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.) caption Granite (Image Source: Wikimedia Commons, Photographer Friman. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.)
caption Dacite (Image Source: US Geological Survey)   caption Granodiorite (Image Source: Wikimedia Commons, Photographer Rudolf Pohl. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.)
  caption Andesite (Image Source: Wikimedia Commons, Photographer Beatrice Murch. This image is licensed under the Creative Commons Attribution 2.0 Generic license.)   caption Diorite (Image Source: Wikimedia Commons, Photographer Siim Sepp. This image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.)
  caption Basalt (Image Source: US Geological Survey)   caption Gabbro (Image Source: US Geological Survey)

Igneous Rocks and the Bowen Reaction Series

 In the 1920s, N. L. Bowen created the following model to explain the origin of the various types of igneous rocks (Figure 2). This model, known as the Bowen reaction series, suggests that the type of igneous rocks that form from magma solidification depends on the temperature of crystallization and the chemical composition of the originating magma.



Bowen theorized that the formation of minerals, which make up igneous rocks, begins with two different chemical sequences at high temperatures that eventually merge into a single series at cooler temperatures. One sequence, the discontinuous series, involves the formation of chemically unique minerals at discrete temperature intervals from iron- and magnesium-rich mafic magma. In the other sequence, known as the continuous series, temperature reduction causes a gradual change in the chemistry of the minerals that form calcium- and sodium-rich felsic magma. The discontinuous series starts with the formation of rocks that are primarily composed of the mineral olivine. Continued temperature decreases change the minerals dominating the composition of the rock from pyroxene, to amphibole, and then biotite. The continuous series produces light-colored rocks rich in plagioclase feldspar minerals. At high temperatures, the plagioclase feldspar minerals are dominated with the element calcium. With continued cooling, the calcium in these minerals is gradually replaced with sodium. The convergence of both series occurs with a continued drop in magma temperature. In the merged series, the minerals within the crystallizing rock become richer in potassium and silica and we get the formation of first potassium feldspars and then the mineral muscovite. The last mineral to crystallize in the Bowen reaction series is quartz. Quartz (SiO2) which is a silicate mineral composed of just silicon and oxygen.

Further Reading

  • Grotzinger, J., T.H. Jordan, F. Press, and R. Siever. 2007. Understanding Earth. 5th Edition. W.H. Freeman and Company, New York.
  • McConnell, D., D. Steer, C. Knight, K. Owens, and L. Park. 2010. The Good Earth. 2nd Edition. McGraw-Hill, Dubuque, Iowa.
  • Plummer, C., D. Carlson, and L. Hammersle. 2010. Physical Geology. 13th Edition. McGraw-Hill, Dubuque, Iowa.
  • Tarbuck, E.J., F.K. Lutgens, and D. Tasa. 2009. Earth Science. 12th Edition. Prentice Hall, Upper Saddle River, New Jersey.


Pidwirny, M. (2013). Igneous rock. Retrieved from http://www.eoearth.org/view/article/153764


To add a comment, please Log In.