Igneous petrology is the study of

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

s—those that are formed from

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

. As a branch of

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ea ...

, igneous petrology is closely related to

volcanology Volcanology (also spelled vulcanology) is the study of volcanoes, lava, magma and related geological, geophysical and geochemical phenomena (volcanism). The term ''volcanology'' is derived from the Latin word '' vulcan''. Vulcan was the an ...

tectonophysics Tectonophysics, a branch of geophysics, is the study of the physical processes that underlie tectonic deformation. The field encompasses the spatial patterns of stress, strain, and differing rheologies in the lithosphere and asthenosphere of th ...

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

physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

, or other

earth sciences Earth science or geoscience includes all fields of natural science related to the planet Earth. This is a branch of science dealing with the physical, chemical, and biological complex constitutions and synergistic linkages of Earth's four spheres ...

Petrography Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The class ...

, crystallography, 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 samples with the naked eye and/or with a

hand lens A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to crea ...

. 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 A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology whi ...

. 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, 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 An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It ...

, in which tiny spots of materials are sampled. Electron microprobe analyses can detect both bulk composition and

trace element __NOTOC__ A trace element, also called minor element, is a chemical element whose concentration (or other measure of amount) is very low (a "trace amount"). They are classified into two groups: essential and non-essential. Essential trace elements ...

composition.

Dating methods

The dating of igneous rocks determines when magma solidified into rock. Radiogenic isotopes 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 The rubidium-strontium dating method is a radiometric dating technique, used by scientists to determine the age of rocks and minerals from their content of specific isotopes of rubidium (87Rb) and strontium (87Sr, 86Sr). One of the two naturally ...

is based on the natural decay of ⁸⁷Rb to ⁸⁷Sr and the different behaviour of these elements during fractional crystallization of magma. Both Sr and Rb are found in most magmas; however, as fractional crystallization occurs, Sr will tend to be concentrated in

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

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 In radiometric dating, closure temperature or blocking temperature refers to the temperature of a system, such as a mineral, at the time given by its radiometric date. In physical terms, the closure temperature is the temperature at which a syste ...

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 Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostra ...

principles may be useful to determine the relative age of volcanic rocks.

Tephrochronology 250px, Tephra horizons in south-central Iceland. The thick and light coloured layer at the height of the volcanologist's hands is rhyolitic tephra from Hekla. Tephrochronology is a Geochronology, geochronological technique that uses discrete l ...

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

Thermobarometry methods

In petrology the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...

is used for

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

calculations of the

that produced

containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things make this method especially useful: first, clinopyroxene is a common

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

in igneous rocks easy to identify; and secondly, the crystallization of the jadeite component of clinopyroxene implies a growth in

molar volume In chemistry and related fields, the molar volume, symbol ''V''m, or \tilde V of a substance is the ratio of the volume occupied by a substance to the amount of substance, usually given at a given temperature and pressure. It is equal to the molar ...

being thus a good indicator of

Thermochronometry

Publications

Most contemporary ground breaking in igneous petrology has been published in prestigious American and British scientific journals of worldwide circulation such as

Science Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earli ...

and

Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...

. Study material, overviews of certain topics and older works are often found as books. Many works before the

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

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

Notable igneous petrologists

* Norman L. Bowen * Nicolas Desmarest *

Louis Cordier Pierre Louis Antoine Cordier (31 March 1777 – 30 March 1861)
Annales.org, accessed 20 September 2009 was a French

* Harry von Eckermann * Antoine Lacroix * Akiho Miyashiro * Paul Niggli *

Hans Ramberg Hans Ramberg (15 March 1917 – 7 June 1998) was a Norwegian- Swedish geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes t ...

* Jakob Sederholm *

Albert Streckeisen Albert Streckeisen (8 November 1901 – 29 September 1998) was a Swiss petrographer and petrologist, the son of Basel forensic scientist Adolf Streckeisen. Biography He studied geology, mineralogy and petrology in Basel, Zürich and Berne. He su ...

* Marjorie Wilson * Peter John Wyllie *

Lawrence Wager Lawrence Rickard Wager, commonly known as Bill Wager, (5 February 1904 – 20 November 1965) was a British geologist, explorer and mountaineer, described as "one of the finest geological thinkers of his generation"Vincent and best remembered for ...

References

{{DEFAULTSORT:Igneous Petrology Igneous rocks