Principles of Igneous and Metamorphic Petrology

PETROLOGY AND ITS SCOPE

Petrology is the science dealing with the description, classification. modes of occurrence, and theories of the origins of rocks. Its emphasis is commonly chemical and mineralogical, but it draws heavily on many disciplines, including the basic physical sciences, mathematics, geophysics, structural geology, and geochemistry. Its tools range from the simple hammer and hand lens, to sophisticated devices such as the electron microprobe or the laboratory equipment capable of reproducing conditions deep within the Earth. Its goal is to provide an understanding of the great diversity of rocks found on the surface of the Earth (and other planets), and to provide insight into the nature of those materials within the Earth which are not accessible to direct observation but play such important roles in the Earth’s history.

Rocks can be divided into three main groups: igneous, sedimentary, and metamorphic. Those formed from the solidification of molten material are termed igneous, whereas those that originate from the deposition of material from water or air are termed sedimentary, and those formed from a previously existing rock by some process of change are termed metamorphic.

The study of igneous and metamorphic rocks, the subject of this book, is commonly treated separately from the study of sedimentary rocks, mainly because of the different approaches used. Sedimentary rocks are formed by processes which, for the most part, are observable on the surface of the Earth. Careful examination of present-day environments of deposition can, therefore, provide information on the origins of most sedimentary rocks. Igneous and metamorphic rocks, on the other hand, are formed largely by processes operating within the earth and therefore not directly accessible to observation; their origins must, consequently, be deduced through physical-chemical arguments. Also, at the higher temperatures existing within the Earth, reactions proceed more rapidly than on the surface, and thus principles of chemical equilibrium are more applicable to the study of igneous and metamorphic rocks than they are to most sedimentary ones.

Petrologic studies fall into two general categories: the dentification and classification of rocks, and the interpretaion of these data and the generation of theories on the origin f rocks. The early emphasis in petrology, as in other natural ciences, was on description and classification. There were, onetheless, many ltvely discussions concerning the origins of ocks, such as that in the early nineteenth century between Werner and his student von Buch on whether basalt was a sedinentary or volcanic rock. But most of the early work involved he cataloging of the constituents of the Earth's crust. During he latter half of the nineteenth and early part of the twentieth enturies vast amounts of petrologic data were collected, from which came an enormous number of rock names and many lifferent classifications. Despite the surfeit of names, generalzations concerning rock associations and mineral assemblages did emerge, which, in turn, allowed for simplificattons in the classifications. Many different rocks that had previously been given separate names could be considered varieties of a single type, and this naturally led to theories explaining the associations. Fortunately, the era of rock naming is over, and modern petrology employs only a small number of rock names.

With recent investigations of the ocean floors, the inventory of rock types available for study in the Earth’s crust is almost complete, and today most petrologists are concerned mainly with the genesis of rocks. This change in emphasis has been stimulated by development of experimental techniques that allow us to imitate, in a limited way, rock-forming condittons and processes within the Earth's crust and upper mantle. In fact, with such techniques it is possible to investigate the petrology of parts of the Earth that, while being of great importance as source regions for many igneous rocks, were virtually unavailable for examination and classification by the early petrologists.

Most processes involved in the formation of igneous and metamorphic rocks occur within the Earth and hence are not subject to direct observation (volcanic eruptions are obvious exceptions). Ideas on the origin of these rocks are, therefore, based on interpretations of field observations in the light of experimental and theoretical studies. Nature provides fragmentary evidence of events and processes that have formed .. .