Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and

petrology Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...

in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

, physics, or other earth sciences. Petrography,

crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The wor ...

, and isotopic studies are common methods used in igneous petrology.

Methods

Determination of chemical composition

The composition of igneous rocks and minerals can be determined via a variety of methods of varying ease, cost, and complexity. The simplest method is observation of

hand sample A hand is a prehensile, multi- fingered appendage located at the end of the forearm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs. A few other vertebrates such as the koala (which has two opposable thumbs on eac ...

s with the naked eye and/or with a hand lens. This can be used to gauge the general mineralogical composition of the rock, which gives an insight into the composition. A more precise but still relatively inexpensive way to identify minerals (and thereby the bulk chemical composition of the rock) with a petrographic microscope. These microscopes have polarizing plates, filters, and a conoscopic lens that allow the user to measure a variety of crystallographic properties. Another method for determining mineralogy is to use

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

, in which a powdered sample is bombarded by X-rays, and the resultant spectrum of crystallographic orientations is compared to a set of standards. One of the most precise ways of determining chemical composition is by the use of an electron microprobe, in which tiny spots of materials are sampled. Electron microprobe analyses can detect both bulk composition and trace element composition.

Dating methods

The dating of igneous rocks determines when magma solidified into rock.

Radiogenic isotopes A radiogenic nuclide is a nuclide that is produced by a process of radioactive decay. It may itself be radioactive (a radionuclide) or stable (a stable nuclide). Radiogenic nuclides (more commonly referred to as radiogenic isotopes) form some ...

are frequently used to determine the age of igneous rocks.

Potassium–argon dating

In this dating method the amount of ⁴⁰Ar trapped in a rock is compared to the amount of ⁴⁰K in the rock to calculate the amount of time ⁴⁰K must have been decaying in the solid rock to produce all ⁴⁰Ar that would have otherwise not have been present there.

Rubidium–strontium dating

The rubidium–strontium dating is based on the natural

decay Decay may refer to: Science and technology * Bit decay, in computing * Software decay, in computing * Distance decay, in geography * Decay time (fall time), in electronics Biology * Decomposition of organic matter * Tooth decay (dental caries) ...

of ⁸⁷Rb to ⁸⁷Sr and the different behaviour of these elements during

fractional crystallization Fractional crystallization may refer to: * Fractional crystallization (chemistry), a process to separate different solutes from a solution * Fractional crystallization (geology) Fractional crystallization, or crystal fractionation, is one of the ...

of magma. Both Sr and Rb are found in most magmas; however, as fractional crystallization occurs, Sr will tend to be concentrated in plagioclase crystals while Rb will remain in the melt for a longer time. ⁸⁷Rb decays in magma and elsewhere so that every 1.42×10¹¹ years half of the amount has been converted into ⁸⁷Sr. Knowing the decay constant and the amount of ⁸⁷Rb and ⁸⁷Sr in a rock it is possible to calculate the time that the ⁸⁷Rb must have needed before the rock reached closure temperature to produce all ⁸⁷Sr, yet considering that there was an initial ⁸⁷Sr amount not produced by ⁸⁷Rb in the magmatic body. Initial values of ⁸⁷Sr, when the magma started fractional crystallization, might be estimated by knowing the amounts of ⁸⁷Rb and ⁸⁷Sr of two igneous rocks produced at different times by the same magmatic body.

Other methods

Stratigraphic principles may be useful to determine the

relative age Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age (i.e., estimated age). In geology, rock (geology), rock or s ...

of volcanic rocks. Tephrochronology is the most common application of stratigraphic dating on volcanic rocks.

Thermobarometry methods

the mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things make this method especially useful: first, clinopyroxene is a common phenocryst in igneous rocks easy to identify; and secondly, the

crystallization Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...

of the

jadeite Jadeite is a pyroxene mineral with composition sodium, Naaluminium, Alsilicon, Si2oxygen, O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, bu ...

component of clinopyroxene implies a growth in molar volume being thus a good indicator of pressure.

Thermochronometry

Publications

Most contemporary ground breaking in igneous petrology has been published in prestigious American and British

scientific journal In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. Content Articles in scientific journals are mostly written by active scientists such as s ...

s of worldwide circulation such as Science and Nature. Study material, overviews of certain topics and older works are often found as books. Many works before the plate tectonics

paradigm shift A paradigm shift, a concept brought into the common lexicon by the American physicist and philosopher Thomas Kuhn, is a fundamental change in the basic concepts and experimental practices of a scientific discipline. Even though Kuhn restricted t ...

in the 1960s and 1970s contains inaccurate information regarding the origin of magmas.

Notable igneous petrologists

Norman L. Bowen Norman Levi Bowen FRS (June 21, 1887 – September 11, 1956) was a Canadian geologist. Bowen "revolutionized experimental petrology and our understanding of mineral crystallization". Beginning geology students are familiar with Bowen's reaction s ...

Nicolas Desmarest Nicolas Desmarest (16 September 1725 – 20 September 1815) was a French geologist and contributor to the ''Encyclopédie ou Dictionnaire raisonné des sciences, des arts et des métiers'', in particular, the multi-volume ''Géographie-physi ...

* Louis Cordier *

Harry von Eckermann Harry von Eckermann (1886–1969) was a Swedish industrialist, mineralogist and geologist. His studies were centered around anorogenic alkaline igneous rocks occurring in the Baltic Shield. Following this line he studied the Alnö Complex, Norra ...

Antoine Lacroix Antoine François Alfred Lacroix (4 February 186312 March 1948) was a French mineralogist and geologist. He was born in Mâcon, Saône-et-Loire. Education Lacroix completed a D. s Sc. in Paris in 1889, as student of Ferdinand André Fouqué. Fo ...

Akiho Miyashiro was a Japanese geologist. Career Miyashiro was known for his contributions to metamorphic and igneous petrology. He also made contributions to the study of tectonics and meteorites. In the 1960s he introduced the concept of paired metamorphic b ...

Paul Niggli Paul Niggli (26 June 1888 – 13 January 1953) was a Swiss crystallographer, mineralogist, and petrologist who was a leader in the field of X-ray crystallography. Education and career Niggli was born in Zofingen and studied at the Swiss Feder ...

* Hans Ramberg *

Jakob Sederholm Jakob Johannes Sederholm (20 July 1863 – 26 June 1934) was a Finnish petrologist most associated with his studies of migmatites. Troubled by illness throughout his life, Sederholm originally chose to study geology to allow him to work outdoors ...

* Albert Streckeisen *

Marjorie Wilson Marjorie Wilson (also known as B. Marjorie Wilson) (born 1951) is an English geologist and petrology, petrologist known for her formative work on the origin of igneous rocks. Her most significant book is ''Igneous Petrogenesis: A Global Tectonic ...

Peter John Wyllie Peter John Wyllie (born 8 February 1930, in London, England) is a British petrologist and academic. He was Professor of Geology at the California Institute of Technology from 1983 until his retirement in 1999. Prior to this, he held positions at ...

* Lawrence Wager

References

{{DEFAULTSORT:Igneous Petrology Igneous rocks