The classification of igneous rocks

Igneous rocks are solid forms of magma that has been extruded by volcanic processes. The word “igneous” derives from “ignis”, the Latin for “fire”. Magma is molten rock that rises from deep within the Earth’s crust and which cools and solidifies as it approaches the surface. Igneous rocks can therefore be classified according to the manner in which the cooling took place, and also according to the chemical composition of the magma from which they were formed.

Chemical Composition

Nine elements make up about 99% of all igneous rocks, with the most common compound being silica (silicon dioxide, SiO₂). The rocks can therefore be classified according to the proportion of silica they contain. If this is greater than 65% the rock is said to be “acid”, and if lower than 55% it is “basic”. Rocks between these points are “intermediate”, whereas those with a silica percentage lower than 45% are “ultrabasic”. Where the silica percentage is low, that of other (basic) oxides is high, and vice versa.  Acid rocks are generally lighter in colour and weight than basic rocks.

Examples of rocks of the various types are:

Acid: granite, obsidian

Intermediate: diorite, andesite

Basic: gabbro, basalt

Ultrabasic: peridotite

Cooling of the Magma

Where the magma cools has much to do with the rate at which it cools. Not all magma reaches the surface, and it may therefore cool slowly at some point below the surface. The magma from a single event can cool at different rates depending on how close it gets to the surface, and may therefore produce a wide range of igneous rocks.

Rocks formed from magma that has reached the surface are termed “extrusive” whereas those formed below the surface, and exposed by later erosion or earth movements, are termed “intrusive”.

Cooling magma will produce crystals of nine silicate minerals, each being produced at different temperatures, from olivine to quartz.

The rate of cooling will determine the size of the crystals, such that the longer the process takes, the larger will be the crystals, with some having been found at 40 feet of length in the Black Hills of South Dakota.

Rocks formed from very slow cooling of intrusive magma are termed “plutonic” and are compact, coarse-textured and large-crystalled, examples being granite, diorite, gabbro and peridotite.

Rocks formed from rapid cooling at the surface are termed “volcanic”. These contain very small crystals or are glassy in appearance (e.g. obsidian). Non-glassy volcanic rocks include rhyolite, andesite and basalt.

Sometimes magma will penetrate weaknesses in the original rock and cool at a rate that is intermediate between plutonic and volcanic rocks. The magma may cool at different rates as it progresses, thus producing crystals of varying sizes. These are termed “hypabyssal”, of which porphyry (in its various forms) is an example.

The Igneous Rocks Matrix

The two classifications mentioned above, namely according to chemical composition and rate of cooling, cross each other and thus produce a matrix.

We can therefore distinguish the following groups of igneous rocks (with examples; but note that not every logically possible combination is apparent in terms of actual rocks):

Acid plutonic (granite)

Intermediate plutonic (diorite)

Basic plutonic (gabbro)

Ultrabasic plutonic (peridotite)

Acid hypabyssal (granophyre)

Intermediate hypabyssal (porphyries)

Basic hypabyssal (dolerite)

Acid volcanic (rhyolite, obsidian)

Intermediate volcanic (andesite)

Basic volcanic (basalt)

Mention should also be made of “pyroclasts”, which are rocks formed during volcanic eruptions from rough balls of material that are spat out and comprise a mixture of lava, cinders, ash and dust.

© John Welford