igneous rock classification

igneous rock classification Igneous rocks are those that form from molten rock (magma), itself a product of the melting of parts of the Earth's interior. Igneous rocks display a variety of compositions, colours, grain sizes, and other attributes. Much scientific effort has focused on how igneous rocks have formed but, because it is difficult to observe and measure them at the relevant temperature and pressures, the search for the explanation of igneous rock formation has centred upon the rocks themselves. The study of the way in which rocks are formed is known as petrogenesis. Research on igneous rocks has included the identification of the minerals and the determination of the concentrations of the chemical elements in them. Each type of study has produced its own system of nomenclature and, hence, classification. These classifications place the rocks into pigeonholes which, although useful, tend to conceal the fact that there is a continuum of rock compositions.

The proliferation of igneous rock names that were based upon the names of the localities at which type specimens of the rocks were found introduced unnecessary confusion. The Subcommission on the Systematics of Igneous Rocks of the International Union of Geological Sciences (IUGS) has a glossary of more than 1500 rock terms, from ‘A-type granite’ and ‘abessedite’ to ‘zobtenite’ and ‘zutterite’ (an erroneous spelling of ‘rutterite’). The subcommision has made a strong recommendation that the naming of rocks should be simplified and standardized in accordance with the guidelines laid out in their book (see Further reading).

Acid–basic

One simple classification is based upon the silica (SiO₂) content of the rock. Rocks with less than 45 per cent by weight SiO₂ are termed ‘ultrabasic’; 45–52 per cent SiO₂, ‘basic’; 52–66 per cent ‘intermediate’; and more than 66 per cent ‘acidic’. The terms ‘acidic’ and ‘basic’ are not intended to express the idea that the rocks are acidic or alkaline in the customary sense; they are employed merely to describe the silica content of the rock. Rocks that are rich in silica (‘acid’ or silicic) are commonly (with the notable exception of obsidian, a black volcanic glass) leucocratic (pale in colour), while ‘basic’ rocks are darker, that is melanocratic. The terms leucocratic and melanocratic may be replaced by felsic and mafic, respectively. The simple categories ‘ultrabasic’, ‘basic’, ‘intermediate’, and ‘acidic’ can be subdivided in a number of ways by consideration of grain size, concentrations of other elements, or the mineral content of the rock.

Grain-size classification

Rocks of identical composition can be distinguished from each other on the basis of their grain size. Figure 1 shows how rocks in each compositional group can be additionally subdivided in this way. Rocks of smaller grain size are those that have cooled quickly; they tend to be associated with volcanoes and near-surface intrusions, coarser-grained rocks are most likely to be plutonic (formed within the crust) in origin. This method of classification by composition and grain size can be enhanced by the addition of qualifiers; for example, a gabbro in which olivine is prominent is called an olivine gabbro, or a rhyolite that is strongly banded may be called a banded rhyolite.

The methods of classification detailed above are extremely useful for the naming of rocks in the field. Although a field geologist cannot necessarily identify all the minerals in the rock, and almost certainly cannot calculate the silica content, the clues yielded by colour, grain size, and readily identifiable minerals allow a field name based upon the system to be assigned. Laboratory analysis of the concentrations of numerous elements in igneous rocks makes it possible to define smaller and more specific categories that can, for example, hinge upon the relative concentrations of potassium and sodium. Some other methods of rock analysis and classification are summarized below.

Although the terms ‘acidic’, ‘intermediate’, ‘basic’, and ‘ultrabasic’ are potentially misleading from the viewpoint of the chemical composition of a rock, their use persists because they are well-established words and because all geologists understand their true meaning.

CIPW-norm

An igneous rock of given chemical composition (of elements) usually contains crystals of the same minerals (crystalline molecules) as other rocks of that composition. Figure 2 shows the minerals that are associated with the various silica contents of igneous rocks. This led to the use of the CIPW-norm in the classification of rocks. The CIPW-norm was developed in 1902 by four eminent petrologists who also left the initial letters of their surnames (Cross, Iddings, Pirrson, and Washington) to posterity. Using their knowledge of coarse grained rocks and the results of chemical analysis, they were able to reconstitute their samples theoretically as if they were coarse-grained. This operation allowed rocks of different grain size to be compared in terms of their bulk chemistry with recourse to only a few simple rules and some elementary, but tedious, arithmetic. Comparison of the normative composition of basalt, which is very fine grained, with gabbro, which is coarse grained, shows that the two are identical in CIPW-norm terms. This implies that, apart from their cooling history, they have a similar petrogenesis. Modem techniques for the determination of bulk chemistry have made analyses widely available for comparison with one another, and the CIPW-norm as an intermediary has become largely redundant. However, as many rock analyses in the literature have been classified in terms of the CIPW-norm, it is still a standard method of classification.

Rock analysis

In the past, the concentrations of major elements (those that make up more than 1 per cent by weight) of the rock were determined by wet chemistry, a long and tedious process which had to be performed by specialists. Modern analytical tools include X-ray fluorescence, a technique by which the quantities of major and trace elements (those that make up less than 1 per cent by weight) of a rock can be determined. Analysis for major and trace elements is thus now relatively fast and can be performed by less expert chemists, provided that the equipment is available. This change in analytical procedure has led to a change of emphasis in rock nomenclature, now that there are enough analyses to consider the mean and variance of the rock compositions. A sample may include rocks that fall into a number of different named categories, even though analyses cluster as though they were the result of the same geological process or set of processes.

Harker plots

As a means of addressing these new data sets, the use of element–element plots (Harker diagrams), a method first used in 1909 by Alfred Harker, has become widespread (Fig.3). After plotting, the rocks can be divided and assigned names according to where they plot on the graph. An enormous variety of different indices has also been used to split groups of rocks more finely and thus assign names to the groups. The user should ascertain, before applying such an index plucked from the literature, what the index represents and what he or she expects from it.

Spider diagrams

The spider diagram (Fig. 4) has become a popular tool in the analysis of trace-element compositions. Spider diagrams overcome the difficulty of plotting elements whose concentrations vary over several orders of magnitude by normalizing the elemental concentrations to a carefully chosed standard known as a norm. Common normalization standards include the concentrations of trace elements in chondrites (meteorites that are thought to have the same composition as the Earth) and MORB, the mean composition of normal (as opposed to enriched) mid-ocean ridge basalts. The compositions relative to the norm are plotted against the element names and a comparison of rocks or groups of rocks is made. It should be borne in mind that normalized values express the relative concentrations of elements to the norm and that the actual values of the elemental concentrations have been obscured.

Rare-earth elements (the elements lanthanum (La) to lutetium (Lu) are also normalized and plotted in this way (Fig.5). Rare-earth elements (REE) are particularly useful in petrogenetic studies because they are chemically similar to one another and they are incompatible; when a small amount of melting occurs they move from the solid phase into the melt. Incompatible elements are hence very mobile and can move with extremely small melt fractions. This fact accounts for the extremely variable concentrations of these elements in rocks and the plethora of rock names that they have spawned. Incompatible elements are much invoked in the study of melting, particularly where small degrees of melting have occurred.

The naming of rocks began with field names that were assigned so that a rock, or group of rocks, could be labelled easily in the field. As methods of analysis proliferated, and hence the criteria that could be used to discriminate between rocks became more numerous, so the range of names has snowballed. Now that modern analytical techniques can determine the concentrations of almost every element in the periodic table, there is a movement away from intensive naming of rocks towards a statistical treatment of multi-element data. Erstwhile names such as kulaite (a particular type of volcanic rock from the village of Kula in western Turkey) have been subsumed by much broader categories like ‘alkali-basalt’. In time, more and more use will be made of statistical analysis of data to describe rocks that fall into a few named categories. Indeed, it is essential to replace a proliferation of rock names with a sound knowledge of the petrogenetic origin of the rocks but, at the same time, to avoid generating, in their place, too many complicated ‘black-box’ diagrams of one index against another. Modern petrogenetic studies are deeply involved in the study of the different environments in which igneous rocks occur, and in mo-delling the processes that lead to the range of rock types and compositions observed.

Judith M. Bunbury

Bibliography

le Maitre, R. W. (ed.) (1989) A classification of igneous rocks and glossary of terms, recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications, Oxford.
Kennet, P. and and Ross, C. A. (1983) Igneous rocks. Longman Group Resources Unit, York.
Wilson, M. (1989) Igneous petrogenesis: a global tectonic approach. Unwin Hyman, London.