Chapter 1
An introduction to magmas and magmatic rocks

WHY STUDY MAGMATIC ROCKS?

The purpose of this book is to stimulate the reader's interest in magmatic rocks and processes, to develop key skills of describing, classifying and naming such rocks, and to show how much we can learn about igneous processes from careful, informed interpretation of rock textures, mineralogy and geochemistry. The book is aimed primarily at the intermediate-level student of geology who already has a basic knowledge of igneous rocks, but anyone starting from scratch should find that the opening chapter and relevant boxes - together with the Glossary - provide the minimum introduction they require. The emphasis throughout the book will be on practical investigation, mainly by means of the polarizing microscope; basic mineral-identification data have therefore been included to provide - between one set of covers - all that the student needs during a typical igneous practical class.

The logical place to begin any 'ig. pet.' course is to ask what purpose the petrologist, geologist or volcanologist hopes to accomplish in studying igneous rocks. Why do we do it? What kinds of things do we hope to learn? What answers are we trying to find? Such questions should always engage the mind of a petrologist who embarks on a petrographic or geochemical study; petrological science has moved on a long way from the early days when merely describing an igneous rock was an end in itself. In real life, a petrologist may study a suite of igneous rocks with one or more objectives in mind, including:

• understanding eruptive processes;
• assessing from previously erupted products the hazard presented by a volcano to surrounding communities;
• investigating magma evolution in a sub-volcanic magma chamber;
• documenting the structure and formation of oceanic or continental crust;
• inferring past tectonic environments (e.g. mid-ocean ridge, island arc) from the compositions of ancient igneous rocks;
• understanding the formation of economic mineral deposits associated with igneous rocks.
• establishing the absolute age of a succession of sedimentary and volcanic rocks (igneous rocks being easier to date isotopically than sedimentary rocks);
• identifying the source from which a magma has originated, and under what conditions melting occurred (i.e. investigating 'magma genesis');
• identifying from erupted magmatic rocks the character and distribution of geochemical domains in the underlying mantle, and their evolution in time.

In every such investigation, there is likely to be a role for carefully describing the igneous rocks involved, but the ultimate goal is usually to learn about magmatic processes, or the conditions under which those processes operate. That goal - of studying igneous rocks to learn about process - will come up again and again in this book, because understanding what goes on in magmatic systems is the modern petrologist's principal aim in life. Igneous rocks can tell us not only about processes taking place on the Earth's surface at the present time, but also:

• about processes that have taken place earlier in Earth history; and
• about processes that operate in parts of the Earth that are not directly accessible to us, for example in a magma chamber that originally lay 5 km below an active volcano (but whose contents - or erupted products - are now exposed at the surface).

Today, anyone working with igneous rocks has to apply a range of skills, including the analysis of field relationships, hand specimen identification in the field, the description and interpretation of thin sections, the allocation of informative rock names, the quantitative interpretation of rock and mineral analyses (often including trace elements and isotope ratios), and the interpretation of experimental equilibria and phase diagrams. This book provides a basic introduction to all but the first of these practical and interpretive skills. The book is not intended to take the place of advanced texts dealing with theories of igneous petrogenesis.

The remainder of this chapter is devoted to introducing the basic vocabulary that will be needed for a clear explanation of igneous rocks.

WHAT IS 'MAGMA'?

Igneous rocks are those that form from molten products of the Earth's interior. Petrologists use two words for molten rock. Magma1 is the more general term that embraces mixtures of melt and any crystals that may be suspended in it: a good example would be flowing lava which contains crystals suspended in the melt (Fig. 1.1): the term magma refers to the entire assemblage, embracing both solid and liquid states of matter present in the lava. Melt, on the other hand, refers to the molten state on its own, excluding any solid material which might be suspended in or associated with it. The difference becomes clearer if one considers how one would chemically analyse the distinct chemical compositions of the magma and melt, once the lava flow had solidified (Fig. 1.1). The magma composition could be estimated by crushing up a sample of the solidified lava, including both phenocrysts and groundmass (ensuring they are present in representative proportions). Analysing the melt composition, however, would require the groundmass or glassy matrix - the solidified equivalent of the melt between the phenocrysts - to be physically separated out and analysed on its own.

Fig. 1.1 Terminology used to designate the different constituents of (a) a molten lava and (b) the same lava in the solid state.

In fact, 'magma' may be used in a still broader sense. An ascending magma body, as it approaches the surface, commonly contains gas bubbles as well as phenocrysts, bubbles formed by gas that has escaped from the melt due to the fall in pressure that accompanies ascent (see Box 1.4). The term 'magma' is generally understood to embrace melt, crystals and any gas bubbles present (Fig. 1.1). Once erupted on the surface, on the other hand, and having lost some of its gas content to the atmosphere, the molten material is more appropriately called 'lava'. Determining a representative chemical analysis of the original magma composition, including the gaseous component, would, however, be difficult: as the melt solidified and contracted on cooling, the gaseous contents of the vesicles would escape to the atmosphere (and they would in any case be lost during crushing of the rock prior to analysis). Determining the concentrations of these volatile magma constituents - from the solid rock that the magma eventually becomes - therefore requires a different analytical approach that will be discussed later.

Magmas are originally formed by melting deep within the Earth (Chapter 2). The initial melting event most commonly takes place in the mantle, though passage of hot magma into or through the continental crust may cause additional melting to occur there as well, adding to the chemical and petrological complexity of continental magmatic rocks. In oceanic and continental areas, mantle-derived magmas are liable to undergo cooling and partial crystallization in storage reservoirs (magma chambers) within the crust (Chapter 3), and such processes widen considerably the diversity of magma compositions that eventually erupt at the surface.

THE DIVERSITY OF NATURAL MAGMA COMPOSITIONS

What do we mean by magma (or rock) composition?

The overall composition of an igneous rock can be expressed in two alternative ways:

• as a quantitative geochemical analysis, giving the percentage by mass of each of the main chemical constituents (Box 1.1);
• as a list of the minerals present in the rock as seen under a microscope, perhaps including an estimate - qualitative or quantitative - of their relative proportions.

Though correlated, these two forms of analysis are not entirely equivalent in the information they convey. As a quantitative statement of chemical composition that can be plotted on graphs (e.g. Fig. 1.2) and used in calculations, a geochemical analysis provides the more exact information. The bulk analysis (also known as a 'whole-rock' analysis) of a volcanic rock approximates closely - except for volatile components - to the composition of the magma from which it formed, considered at a stage before it had begun to crystallize. Careful analysis of geochemical data can reveal a lot about the source of the melt and the conditions (pressure, depth, extent of melting) under which the melt originally formed.

In some circumstances, however, other forms of rock analysis are of more practical use. Geochemical analyses, requiring elaborate laboratory facilities, are not usually available at the field stage of an investigation, when a geologist will normally find mineralogical and textural observations on hand specimens a more practical way of characterizing, and discriminating between, the different rock types present in the area. Moreover, the occurrence in thin section of certain key indicator minerals - such as quartz, olivine, nepheline,...