Introduction to Igneous and Metamorphic Petrology

Preface This text is designed for use in advanced undergraduate or early graduate courses in igneous and metamorphic petrology. The book is extensive enough to be used in separate igneous and metamorphic courses, but I use it for a one-semester combined course by selecting from the available chapters. The nature of geological investigations has largely shaped the approach that I follow. Geology is often plagued by the problem ofinaccessibility.Geological observers really see only a tiny fraction of the rocks that compose the Earth. Uplift and erosion exposes some deep-seated rocks, whereas others are delivered as xenoliths in magma, but their exact place of origin is vague at best. As a result, a large proportion of our information about the Earth is indirect, coming from melts of subsurface material, geophysical studies, or experiments conducted at elevated temperatures and pressures. The problem of inaccessibility has a temporal aspect as well. Most Earth processes are exceedingly slow. As a result, we seldom are blessed with the opportunity of observing even surface processes at rates that lend themselves to ready interpretation (volcanism is a rare exception for petrologists). In most other sciences, theories can be tested by experiment. In geology, as a rule, flat experiment has run to its present state and is impossible to reproduce. Our common technique is to observe the results and infer what the experiment was. Most of our work is thus inferential and deductive. Rather than being repulsed by this aspect of our work, I believe most geologists are attracted by it. The nature of how geology is practiced has changed dramatically in recent years. Early geologists worked strictly in the observational and deductive fashion described above. The body of knowledge resulting from the painstaking accumulation of data observable with the naked eye or under a light microscope is impressive, and most of the theories concerning how the Earth works that were developed by the mid-20th century are still considered valid today, at least in broad terms. Modern post-war technology, however, has provided geologists with the means to study the Earth using techniques borrowed from our colleagues in the fields of physics and chemistry. We have mapped and sampled much of the ocean basins; we have probed the mantle using variations in gravity and seismic waves; we can perform chemical analyses of rocks and minerals quickly and with high precision; we can also study natural and synthetic specimens at elevated temperatures and pressures in the laboratory to approximate the conditions at which many rocks formed within the Earth. These and other techniques, combined with theoretical models and computing power, have opened new areas of research and have permitted us to learn more about the materials and processes of the Earth's interior. These modern techniques have been instrumental in the development of plate tectonic theory, the encompassing paradigm that guides much present geologic thought. Given the limitations of inaccessibility mentioned above, it is impressive how much we have learned about our planet. Modern petrology, because it, addresses processes that occur hidden from view deep within the Earth, must rely heavily on data other than simple observation. In the pages that follow I shall attempt to explain the techniques employed, and the resulting insights they provide into the creation of the igneous and metamorphic rocks now found at the surface of the Earth. The reader should be aware, however, that the results of our investigations, however impressive and consistent they may appear, are still based in large part on indirect evidence and inferential reasoning. I'm sure that the many researchers whose painstaking work we shall review would join me in urging a healthy skepticism lest we become too dogmatic in our perspective. Ideas and theories are always in a state of flux. Many of today'sWinter, John D. is the author of 'Introduction to Igneous and Metamorphic Petrology', published 2001 under ISBN 9780132403429 and ISBN 0132403420.