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 that 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 that, for the most part, are observable on the surface of the Earth.