GEOLOGIC SCIENCES

the fields of study concerned with the solid Earth. Included are sciences such as mineralogy, geodesy, and stratigraphy. An introduction to the geochemical and geophysical sciences logically begins with mineralogy because the Earth's rocks are composed of mineralsinorganic elements or compounds that have a fixed chemical composition and that are made up of regularly aligned rows of atoms. Today, one of the principal concerns of mineralogy is the chemical analysis of the some 3,000 known minerals that are the chief constituents of the three different rock types: sedimentary (formed by diagenesis of sediments deposited by surface processes); igneous (crystallized from magmas either at depth or at the surface as lavas); and metamorphic (formed by a recrystallization process at temperatures and pressures in the Earth's crust high enough to destabilize the parent sedimentary or igneous material). Geochemistry is the study of the composition of these different types of rocks. During mountain building, rocks became highly deformed, and the primary objective of structural geology is to elucidate the mechanism of formation of the many types of structures (e.g., folds and faults) that arise from such deformation. The allied field of geophysics has several subdisciplines, which make use of different instrumental techniques. Seismology, for example, involves the exploration of the Earth's deep structure through the detailed analysis of recordings of elastic waves generated by earthquakes and man-made explosions. Earthquake seismology has largely been responsible for defining the location of major plate boundaries and of the dip of subduction zones down to depths of about 700 kilometres at those boundaries. In other subdisciplines of geophysics, gravimetric techniques are used to determine the shape and size of underground structures; electrical methods help to locate a variety of mineral deposits that tend to be good conductors of electricity; and paleomagnetism has played the principal role in tracking the drift of continents. Geomorphology is concerned with the surface processes that create the landscapes of the worldnamely, weathering and erosion. Weathering is the alteration and breakdown of rocks at the Earth's surface caused by local atmospheric conditions, while erosion is the process by which the weathering products are removed by water, ice, and wind. The combination of weathering and erosion leads to the wearing down or denudation of mountains and continents, with the erosion products being deposited in rivers, internal drainage basins, and the oceans. Erosion is thus the complement of deposition. The unconsolidated accumulated sediments are transformed by the process of diagenesis and lithification into sedimentary rocks, thereby completing a full cycle of the transfer of matter from an old continent to a young ocean and ultimately to the formation of new sedimentary rocks. Knowledge of the processes of interaction of the atmosphere and the hydrosphere with the surface rocks and soils of the Earth's crust is important for an understanding not only of the development of landscapes but also (and perhaps more importantly) of the ways in which sediments are created. This in turn helps in interpreting the mode of formation and the depositional environment of sedimentary rocks. Thus the discipline of geomorphology is fundamental to the uniformitarian approach to the Earth sciences according to which the present is the key to the past. Geologic history provides a conceptual framework and overview of the evolution of the Earth. An early development of the subject was stratigraphy, the study of order and sequence in bedded sedimentary rocks. Stratigraphers still use the two main principles established by the late 18th-century English engineer and surveyor William Smith, regarded as the father of stratigraphy: (1) that younger beds rest upon older ones and (2) different sedimentary beds contain different and distinctive fossils, enabling beds with similar fossils to be correlated over large distances. Today, biostratigraphy uses fossils to characterize successive intervals of geologic time, but as relatively precise time markers only to the beginning of the Cambrian Period, about 540,000,000 years ago. The geologic time scale, back to the oldest rocks, some 3,900,000,000 years ago, can be quantified by isotopic dating techniques. This is the science of geochronology, which in recent years has revolutionized scientific perception of Earth history and which relies heavily on the measured parent-to-daughter ratio of radiogenic isotopes (see below). Paleontology is the study of fossils and is concerned not only with their description and classification but also with an analysis of the evolution of the organisms involved. Simple fossil forms can be found in early Precambrian rocks as old as 3,500,000,000 years, and it is widely considered that life on Earth must have begun before the appearance of the oldest rocks. Paleontological research of the fossil record since the Cambrian Period has contributed much to the theory of evolution of life on Earth. Several disciplines of the geologic sciences have practical benefits for society. The geologist is responsible for the discovery of minerals, oil, gas, and coal, which are the main economic resources of the Earth; for the application of knowledge of subsurface structures and geologic conditions to the building industry; and for the prevention of natural hazards or at least providing early warning of their occurrence. (For further examples, see below Practical applications.) Astrogeology is important in that it contributes to understanding the development of the Earth within the solar system. The U.S. Apollo program of manned missions to the Moon, for example, provided scientists with firsthand information on lunar geology, including observations on such features as meteorite craters that are relatively rare on Earth. Unmanned space probes have yielded significant data on the surface features of many of the planets and their satellites. Since the 1970s even such distant planetary systems as those of Jupiter, Saturn, and Uranus have been explored by probes. Additional reading Popular introductions include Peter Cattermole and Patrick Moore, The Story of the Earth (1985); and Frank Press and Raymond Siever, Earth, 4th ed. (1986). Other readable general accounts are J. Dercourt and J. Paquet, Geology: Principles and Methods, trans. from French (1985), an introduction to the modern Earth; and Richard Fifield (ed.), The Making of the Earth (1985), which reviews recent major breakthroughs in the Earth sciences. (Study of the composition, structure, and surface features of the Earth): Martin H.P. Bott, Interior of the Earth: Its Structure, Constitution, and Evolution, 2nd ed. (1982), a comprehensive synthesis of the geophysics of the solid Earth; G.C. Brown and A.E. Mussett, The Inaccessible Earth (1981), a readable treatment of the geophysics of the Earth's core, mantle, and crust; Kent C. Condie, Plate Tectonics and Crustal Evolution, 2nd ed. (1982), an advanced but accessible account of the origin and development of the Earth's crust; John Suppe, Principles of Structural Geology (1985), a modern introduction; W. Kenneth Hamblin, The Earth's Dynamic Systems: A Textbook in Physical Geology, 4th ed. (1985), an introduction to physical geology and tectonics; Arthur Holmes, Holmes Principles of Physical Geology, 3rd ed. rev. by Doris L. Holmes (1978), a thorough and stimulating coverage of physical geology; Akiho Miyashiro, Keiiti Aki, and A.M. Cell Sengr, Orogeny (1982; originally published in Japanese, 1979), which describes orogenic theory based on plate tectonics; Cliff Ollier, Tectonics and Landforms (1981), an explanation of the interrelationships between these subjects; Edward J. Tarbuck and Frederick K. Lutgens, The Earth: An Introduction to Physical Geology (1984); Saiya Uyeda, The New View of the Earth: Moving Continents and Moving Oceans (1978), a good introduction to global plate tectonics; P.J. Wyllie, The Way the Earth Works (1976), a readable introduction and overview of the global geology of plate tectonics; Peter Francis, Volcanoes (1976), a popular account; Eric A.K. Middlemost, Magmas and Magmatic Rocks: An Introduction to Igneous Petrology (1985); Haroun Tazieff and Jean-Christophe Sabroux (eds.), Forecasting Volcanic Events (1983); and Howel Williams and Alexander R. McBirney, Volcanology (1979), an authoritative and comprehensive account of volcanoes. (Historical geology, stratigraphy, and paleontology): P.J. Brenchley (ed.), Fossils and Climate (1984), symposium proceedings explaining the influence of past climates on the evolution of life and paleobiogeography; W.S. McKerrow (ed.), The Ecology of Fossils (1978), a comprehensive and illustrated guide; Carl K. Seyfert and Leslie A. Sirkin, Earth History and Plate Tectonics: An Introduction to Historical Geology, 2nd ed. (1979), with emphasis on the Phanerozoic period; Steven M. Stanley, Earth and Life Through Time (1986), which relates the physical history of the Earth and the history of life; and Brian F. Windley, The Evolving Continents, 2nd ed. (1984), a detailed, comprehensive synthesis of evidence related to continental evolution throughout geologic time. (Astrogeology): Michael H. Carr (ed.), The Geology of the Terrestrial Planets (1984), an authoritative account; Billy P. Glass, Introduction to Planetary Geology (1982), a well-illustrated comprehensive volume on the Earth, the other planets, and the asteroids; and Ronald Greely, Planetary Landscapes (1985), abundant photos of the planets and their satellites and text explaining their geomorphology and geology. (Applied Earth sciences): Donald R. Coates, Geology and Society (1985), basic concepts of environmental geology; Charles S. Hutchison, Economic Deposits and Their Tectonic Setting (1983), a comprehensive account of economic geology; and David W. Simpson and Paul G. Richards (eds.), Earthquake Prediction: An International Review (1981), which provides a broad spectrum of case histories and an overview of the subject. Brian Frederick Windley