Igneous petrology

Key takeaways

Igneous petrology belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Igneous petrology to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Igneous petrology from memory before moving on to harder problems.

Reference excerpt

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 in general. The modern study of igneous rocks uses a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, 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. 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, 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 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 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 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 of volcanic rocks. Tephrochronology is the most common application of stratigraphic dating on volcanic rocks.

Thermobarometry methods

In petrology 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 of the jadeite 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 journals 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 in the 1960s and 1970s contains inaccurate information regarding the origin of magmas.

Notable igneous petrologists Norman L. Bowen Nicolas Desmarest Louis Cordier Harry von Eckermann Antoine Lacroix Akiho Miyashiro Paul Niggli Hans Ramberg Jakob Sederholm Albert Streckeisen Marjorie Wilson Peter John Wyllie Lawrence Wager

References

Worked examples

Example 1 — a first encounter with Igneous petrology

Start with the simplest possible case. Write down what Igneous petrology claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Igneous petrology before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Igneous petrology ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Igneous petrology

In research: Igneous petrology appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Igneous petrology in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Igneous petrology is common in secondary-school and first-year university syllabi. It links to neighbouring topics Igneous petrology, Igneous rocks, so understanding it makes those chapters shorter.
In everyday life: Look for Igneous petrology outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Igneous petrology in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Igneous petrology means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Igneous petrology out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Igneous petrology in simple terms?

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 in general.

Why does Igneous petrology matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Igneous petrology?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Igneous petrology.