The Average Chemical Composition of Igneous Rocks

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At high temperatures only minerals that have structures stable under those conditions will be able to crystallize.

Typically those minerals are olivine, pyroxene, and plagioclase feldspar. Therefore igneous rock that forms at high temperature would be composed of these minerals and hence be dark or mafic in color. In this paper an attempt is made to use these data to construct a simple chemical classification of the volcanic rocks.

Silica content is used to divide the rocks into seven major classes; and the major classes are then subdivided using soda, potash, alumina and lime.

Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide. A simple classification of volcanic rocks. Authors Authors and affiliations E. This process is experimental and the keywords may be updated as the learning algorithm improves. This is a preview of subscription content, log in to check access. Ahrens, L. Acta, 28 , p.

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Igneous rock - Wikipedia

Chayes, F. In McBirney, A. State of Oregon, Dept. Carnegie Instn. Correns, C. Springer Verlag, New York, pp. Cross, W. Daly, R. McGraw Hill, New York, pp.

4 Igneous Processes and Volcanoes

Dawson, J. Nature, Lond. Such magmas, and those derived from them, build up island arcs such as those in the Pacific Ring of Fire. These magmas form rocks of the calc-alkaline series, an important part of the continental crust. The addition of carbon dioxide is relatively a much less important cause of magma formation than the addition of water, but genesis of some silica-undersaturated magmas has been attributed to the dominance of carbon dioxide over water in their mantle source regions.

Increase in temperature is the most typical mechanism for formation of magma within continental crust.

Chapter 7 - Igneous Rocks and Processes

Such temperature increases can occur because of the upward intrusion of magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in continental crust thickened by compression at a plate boundary. The plate boundary between the Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal continental crust.

Studies of electrical resistivity deduced from magnetotelluric data have detected a layer that appears to contain silicate melt and that stretches for at least 1, kilometers within the middle crust along the southern margin of the Tibetan Plateau. Temperature increases also may contribute to the melting of lithosphere dragged down in a subduction zone.

Most magmas only entirely melt for small parts of their histories.


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More typically, they are mixes of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have different densities, and so they can separate as magmas evolve. As magma cools, minerals typically crystallize from the melt at different temperatures fractional crystallization.

As minerals crystallize, the composition of the residual melt typically changes. If crystals separate from the melt, then the residual melt will differ in composition from the parent magma. For instance, a magma of gabbroic composition can produce a residual melt of granitic composition if early formed crystals are separated from the magma.

Incompatible elements are concentrated in the last residues of magma during fractional crystallization and in the first melts produced during partial melting: either process can form the magma that crystallizes to pegmatite , a rock type commonly enriched in incompatible elements. Bowen's reaction series is important for understanding the idealised sequence of fractional crystallisation of a magma.

Magma composition can be determined by processes other than partial melting and fractional crystallization. For instance, magmas commonly interact with rocks they intrude, both by melting those rocks and by reacting with them. Magmas of different compositions can mix with one another. In rare cases, melts can separate into two immiscible melts of contrasting compositions. There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon , oxygen , aluminium, sodium , potassium , calcium , iron, and magnesium.

These are the elements that combine to form the silicate minerals , which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks is expressed differently for major and minor elements and for trace elements.

Classification of Igneous Rocks

Contents of major and minor elements are conventionally expressed as weight percent oxides e. Abundances of trace elements are conventionally expressed as parts per million by weight e. The term "trace element" is typically used for elements present in most rocks at abundances less than ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1, ppm.

Felsic and mafic rocks

The diversity of rock compositions has been defined by a huge mass of analytical data—over , rock analyses can be accessed on the web through a site sponsored by the U. The word " igneous " is derived from the Latin ignis , meaning "of fire". Volcanic rocks are named after Vulcan , the Roman name for the god of fire.

Intrusive rocks are also called "plutonic" rocks, named after Pluto , the Roman god of the underworld. Kanaga volcano in the Aleutian Islands with a lava flow in the foreground. Devils Tower , an eroded laccolith in the Black Hills of Wyoming. A cascade of molten lava flowing into Aloi Crater during the Mauna Ulu eruption of Kilauea volcano.

Columnar jointing in the Alcantara Gorge, Sicily. A laccolith of granite light-colored that was intruded into older sedimentary rocks dark-colored at Cuernos del Paine, Torres del Paine National Park , Chile. An igneous intrusion cut by a pegmatite dike, which in turn is cut by a dolerite dike. Volcanic rocks : Subvolcanic rocks : Plutonic rocks :. Picrite basalt Peridotite. Basalt Diabase Dolerite Gabbro.