EARTH

terrestrial body whose solid surface, abundant waters, and oxygen-rich atmosphere have combined to create conditions suitable for life. This article discusses the structure, composition, and properties of the solid Earth. For detailed treatment of its surface features, atmosphere, and waters, see the articles atmosphere, hydrosphere, ocean, river, lake, and continental landform. For a discussion of the Earth as a member of the Solar System, see the article on planet Earth. If the Earth were reduced to a tabletop globe 50 centimetres (20 inches) in diameter, the portion accessible to direct observation through even the deepest mines and boreholes would be the equivalent of a very thin skin less than 1 millimetre (0.04 inch) thick. It is therefore not surprising that scientific investigators did not develop a picture of the Earth's interior until well into the 20th century, and only since the 1960s have they come to understand the dynamic processes that shape the terrestrial surface. The Earth is a nearly spherical body with an equatorial radius of slightly more than 6,378 kilometres (3,963 miles). Compared with the other planets of the solar system, it is only of intermediate size and is substantially smaller than the giant planets Jupiter, Saturn, Uranus, and Neptune. The Earth's Moon, which has a radius of 1,738 kilometres, is relatively large in comparison to the Earth itself. This fact has been of great importance in determining the history of the Earth's rotation, for the Earth and the Moon raise tides in the bodies of one another, resulting in the dissipation of energy into heat, which in turn leads to the slowing of the Earth's spin velocity on its axis and the recession of the Moon. In fact, if the present rates of slowing and recession are linearly extrapolated backward in time, the Moon is found to have been impossibly close to the Earth at a time within the geologic recorda seemingly unexplainable paradox. Only very recently has it been shown that the present distribution of continents and oceans produces an anomalously high rate of slowing, and so the Moon need never have been extremely close to the Earth. The outstanding feature of the Earth as a planet is the presence of liquid water. Water is vital not only for the biosphere but also for the geologic processes of erosion, transport, and deposition that shape the Earth's surface. Yet, if the Earth were closer to the Sun, the water would be vaporized; if farther, it would turn to ice. Two-thirds of the terrestrial surface is covered by oceans. It was long thought that the continents, constituting the remaining one-third of the surface, had been fixed in position throughout the Earth's history. Gradually some Earth scientists dared to suggest that there had been major continental displacements, and finally, during the 1960s, investigators developed the full picture of seafloor spreading and plate tectonics. The continents, though constantly in motion, are in fact the oldest portions of the Earth's surface, for the seafloor is created at ridges and consumed at trenches on a geologically short time scale. Other planets, notably Mars and Venus, have surface features that suggest some elements of plate tectonics, but none is known to be undergoing the constant rejuvenation of the surface as is the Earth. The fundamental laws of geologic succession, of the differences between igneous rocks (those crystallized from a melt) and sedimentary rocks (those formed by diagenesis of sediments deposited by surface processes), began to be understood toward the end of the 18th century. At about the same time the measurement of the constant in Newton's law of gravitation showed that the specific gravity of the Earth was about 5.5, whereas that of a typical crustal rock was only about 2.7. Obviously, the interior must be much denser, and it became apparent that pressure alone could not explain the difference. Instead, there have to be differences in chemical composition, involving a decrease with depth of the light elements abundant in the crust (oxygen, silicon, and aluminum) and an increase of heavier elements such as iron. With reference again to the other planets of the solar system, it is seen that their densities tend to fall into two groups: the inner planets with densities close to that of the Earth, and the giant planets with appreciably lower densities. The members of the first group probably have iron cores (proposed for the Earth in the early years of the 20th century), while those of the second must contain large amounts of very light elements such as hydrogen. If the Earth were completely static, it would be virtually impossible to obtain information about the interior apart from the mean density. Dynamism is, however, the mark of the Earth: tectonic plates move, probably driven by slow convection currents within the Earth's mantle; earthquakes occur; and, from the study of earthquake waves, the broad outlines of the interior can be established. The resulting picture of the Earth includes a solid inner core, a fluid outer core whose radius is more than half the planet's radius, a predominantly solid mantle, and a chemically distinct thin crust that contains most of the familiar geologic features. The rigid plates that are driven over the surface consist of the crust and some uppermost mantle, which together make up a mechanically distinct regime called the lithosphere. In addition to producing earthquakes, the motions of the Earth's interior bring to the surface rocks typical of the deep interior as well as heat. Indeed, scientists now realize that it is the heat escaping from the outer part of the Earth that drives the motions that shape the surface. Smaller bodies of the solar system (e.g., Mars and the Moon) have probably cooled to such an extent that convectively driven tectonics no longer operate. The heat now escaping from the Earth's surface probably comes in the main from radioactivity throughout the mantle. Some heat may still come from that generated when the dense core fell into the Earth's centre, and some may be original heat. The question of whether the Earth began hot or cold is not definitely settled, although majority opinion favours a cold origin with intense early heating through radioactivity and the separation of the metallic core. Ages determined by the analysis of radioactive isotopes and their daughter products provide a clue to early history. The Earth as a distinct body is known to have an age of 4.6 109 years, and samples of lunar material show a similar age. The oldest continental rocks currently found, however, in Canada, have ages (since the time they solidified) of approximately 3.9 109 years. Presumably the record at the Earth's surface of the first 700 million years was erased by the elevated temperatures that prevailed in those times. That the Earth has a magnetic field has been known at least since the 11th century when the directional properties of suspended magnetic rock (magnetite; also called lodestone) were first used for navigation. Over the centuries the characteristics of this changing field have become better understood, until it now appears that the only plausible cause is some system of motions in the Earth's liquid outer core. These motions, which may be thermally driven like the slow convection currents in the mantle, constitute an electromagnetic dynamo whose electric currents sustain the field. Many problems remain: Why, for example, should the field have reversed polarity at irregular intervals through geologic time? But again, the existence of the field is in all likelihood simply further evidence of the Earth's dynamic structure. Some other planets, notably Jupiter and Mercury, are known to have magnetic fields. It is interesting that Jupiter and Mercury differ appreciably in density and therefore composition, but both planets must have the internal motions necessary to constitute a dynamo. The Moon probably once had a magnetic field, but its internal dynamo apparently ceased long ago as the fluid interior froze. The Earth's magnetic field shields the planet from the most direct effects of the ionized gas that constitutes the solar wind, carving out a cavity known as the magnetosphere. The existence of the magnetosphere has in all likelihood played a fundamental role in determining the nature of the Earth's atmosphere and its climate and therefore in the development of life. Yet, the verification of the magnetosphere's existence is a fairly recent accomplishment, dating only from the International Geophysical Year of 195758. George D. Garland designated in astronomy, third planet outward from the Sun. Its single most outstanding feature is that its near-surface environments are the only places in the universe known to harbour life. Scientists have applied the full battery of modern instrumentation to studying the Earth in ways that have not yet been possible for the other planets; thus, much more is known about its structure and composition. It is convenient to consider separate parts of the planet in terms of roughly spherical regions extending from the interior outward: the core and mantle, the lithosphere (the rocky, near-surface crust of land), the hydrosphere (dominantly the oceans, which fill in low places in the crust), the atmosphere (itself divided into spherical zones such as the troposphere, where weather occurs), and the magnetosphere (which includes the interface with the upper atmospheric ionosphere, the radiation belts, and the bow shock). These parts of the planet are treated briefly, in turn, in this article, while they are treated in detail elsewhere. The interior, surface, and physical and chemical properties of the planet are discussed in detail in the article Earth. The geologic and biological history of the Earth, including its surface features and the processes by which they are created and modified, are discussed in geochronology, continental landform, and plate tectonics. The behaviour of the atmosphere and of its tenuous, ionized outer reaches is treated in atmosphere, while the water cycle and major hydrologic features are described in hydrosphere, ocean, and river. The present discussion complements these more detailed articles by providing an overview of the Earth, chiefly in comparison with the other planets in the solar system. Since the Copernican revolution of the 16th century, at which time the Polish astronomer Nicolaus Copernicus proposed a Sun-centred model of the universe, enlightened thinkers have regarded the Earth as a planet like the others of the solar system. Concurrent sea voyages provided practical proof that the Earth is a globe, just as Galileo's use of his newly invented telescope in the early 17th century soon showed various other planets to be globes as well. It was only after the dawn of the space age, however, when photographs from rockets and orbiting spacecraft first captured the dramatic curvature of the Earth's horizon that the conception of the Earth as a roughly spherical planet rather than as a flat entity was verified by direct human observation. Figure 1: The Earth, as photographed from the Galileo spacecraft during its December 1990 flyby of Humans for the first time saw the Earth as a complete globe in December 1968 when Apollo 8 carried astronauts around the Moon. In December 1990 the Galileo spacecraft, outfitted with an array of remote-sensing instruments, studied the Earth during the first of its two gravity-assisted flybys en route to the planet Jupiter. The information about the Earth gathered from Galileo was meagre compared with that obtained by the swarm of artificial satellites that have orbited the globe throughout the space age, but it provided some unique portraits of the Earth as a planet. (See Figure 1.) Viewed from another planet, the Earth would appear bright and bluish in colour. Most readily apparent would be its atmospheric features, chiefly the swirling white cloud patterns of mid-latitude and tropical storms, ranged in roughly latitudinal belts around the planet. The polar regions also would appear a brilliant white owing to the clouds above and the snow and ice below. Beneath the changing patterns of clouds are the much darker, blue-black oceans, interrupted by occasional tawny patches of desert lands. The green landscapes that harbour most human life would not be easily seen from space; not only do they constitute a modest fraction of the land area, which itself is a small fraction of the Earth's surface, but they are often obscured by clouds. Over the course of the seasons, some seasonal changes in the storm patterns and cloud belts on Earth would be observed. Also prominent would be the growth and recession of the winter snowcap across land areas of the Northern Hemisphere. the third planet in distance outward from the Sun. It is the only planetary body in the solar system that has conditions suitable for life, at least as known to modern science. Additional reading The works by William K. Hartmann and Ron Miller, The History of Earth: An Illustrated Chronicle of an Evolving Planet (1991), lavishly illustrated; Preston Cloud, Oasis in Space: Earth History from the Beginning (1988); and Jonathan Weiner, Planet Earth (1986), the companion volume to a PBS television series, are good introductions to the planet for the general reader. An older but excellent summary of the historical development of plate tectonics is the book by Ursula B. Marvin, Continental Drift: The Evolution of a Concept (1973). Virgil L. Sharpton and Peter D. Ward (eds.), Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality (1990), is a technical but definitive collection of papers on the role of giant impacts in the Earth's history. Earth (bimonthly), published in Waukesha, Wis., is for the general reader. Clark R. Chapman Additional reading The figure of the Earth Recent standard works include G. Bomford, Geodesy, 4th ed. (1980, reprinted 1983); Weikko A. Heiskanen and Helmut Moritz, Physical Geodesy (1967); Wolfgang Torge, Geodesy, an Introduction (1980; originally published in German, 1975), a classical treatment; and Petr Vancek and Edward J. Krakiwsky, Geodesy, the Concepts, rev. ed. (1986). See also International Association Of Geodesy, Systeme geodesique de reference 1967: Geodetic Reference System 1967 (1971). The Earth's gravitational field Texts include Michele Caputo, The Gravity Field of the Earth, from Classical and Modern Methods (1967); W.A. Heiskanen and F.A. Vening Meinesz, The Earth and Its Gravity Field (1958); and W.M. Telford et al., Applied Geophysics (1976). The Journal of Geophysical Research contains much recent work, including the following articles: LeRoy M. Dorman and Brian T.R. Lewis, Experimental Isostasy, 1: Theory of the Determination of the Earth's Isostatic Response to a Concentrated Load, 75(17):335765 (June 10, 1970), Experimental Isostasy, 2: An Isostatic Model for the U.S.A. Derived from Gravity and Topographic Data, 75(17):336786 (June 10, 1970), and Experimental Isostasy, 3: Inversion of the Isostatic Green Function and Lateral Density Changes, 77(17):306877 (June 10, 1972); G.D. Karner and A.B. Watts, Gravity Anomalies and Flexure of the Lithosphere at Mountain Ranges, 88(B12):10,44910,477 (Dec. 10, 1983); and Marcia McNutt, Implications of Regional Gravity for State of Stress in the Earth's Crust and Upper Mantle, 85(B11):637796 (Nov. 10, 1980). George D. Garland The Earth's magnetic field Brief surveys are Sydney Chapman, The Earth's Magnetism, 2nd ed. rev. (1951, reprinted 1961); and J.A. Jacobs, The Earth's Core and Geomagnetism (1963), and Reversals of the Earth's Magnetic Field (1984). For more in-depth coverage, consult Sydney Chapman and Julius Bartels, Geomagnetism, vol. 1, Geomagnetic and Related Phenomena (1940, reprinted 1962); George D. Garland, Introduction to Geophysics: Mantle, Core and Crust, 2nd ed. (1979), an informative overview; S. Matsushita, Solar Quiet and Lunar Daily Variation Fields, ch. III-1 in S. Matsushita and Wallace H. Campbell, Physics of Geomagnetic Phenomena, vol. 1 (1967), pp. 301424, an extensive treatment; Ronald T. Merrill and Michael W. McElhinny, The Earth's Magnetic Field: Its History, Origin, and Planetary Perspective (1983), an attempt to bridge the gap between dynamo theory and paleomagnetology; W.D. Parkinson, Introduction to Geomagnetism (1983), a logical, systematic, and highly readable treatment; and S.K. Runcorn, K.M. Creer, and J.A. Jacobs (eds.), The Earth's Core: Its Structure, Evolution, and Magnetic Field (1982), an excellent introduction to the modern literature on the topic. See also S.J. Peale, Consequences of Tidal Evolution, ch. 12 in Margaret G. Kivelson (ed.), The Solar System: Observations and Interpretations (1986), pp. 275288; and K.A. Wienert, Notes on Geomagnetic Observatory and Survey Practice (1970), an explanation of the basic principles of geomagnetic survey techniques. Robert L. McPherron Structure and composition of the solid Earth David G. Smith (ed.), The Cambridge Encyclopedia of Earth Sciences (1981), is a highly readable, illustrated collection of articles on the Earth's surface and interior. Two collections of articles from Scientific American are The Dynamic Earth (1983), on the geologic processes occurring inside and around the planet; and Robert Decker and Barbara Decker (eds.), Volcanoes and the Earth's Interior (1982), on volcanoes and the rocks occurring in the mantle. Minoru Ozima, The Earth: Its Birth and Growth (1981; originally published in Japanese, 1979), gives a general account of the evolution of the Earth based on geochemical research. Martin H.P. Bott, Interior of the Earth: Its Structure, Constitution, and Evolution, 2nd ed. (1982), provides a thorough summary of the nature of the Earth's interior. Two works with a more technical slant are Jean-Claude De Bremaecker, Geophysics, the Earth's Interior (1985), an introductory textbook; and Donald L. Turcotte and Gerald Schubert, Geodynamics: Applications of Continuum Physics to Geological Problems (1982), a mathematical text on processes occurring within the Earth. See also G.C. Brown and A.E. Mussett, The Inaccessible Earth (1981), on both geophysics and geochemistry. Raymond Jeanloz Major geologic features of the Earth's exterior David Alt, Physical Geology: A Process Approach (1982), is an up-to-date introduction to the principles of physical geology. Daniel S. Barker, Igneous Rocks (1983), discusses the characteristics of igneous rocks and their relationship to plate tectonics. Marland P. Billings, Structural Geology, 3rd ed. (1972), is a classic introduction to structural geology, which describes the deformation of the Earth's crust. A. Hallam, A Revolution in the Earth Sciences: From Continental Drift to Plate Tectonics (1973), is an excellent historical review of the development of ideas on continental drift, polar wandering, seafloor spreading, and plate tectonics. Arthur Holmes, Principles of Physical Geology, 2nd rev. ed. (1965), is a beautifully written, though somewhat dated, introduction to physical geology. Robley K. Matthews, Dynamic Stratigraphy: An Introduction to Sedimentation and Stratigraphy, 2nd ed. (1984), relates the study of the sequence of layered rocks to plate tectonics. William D. Thornbury, Principles of Geomorphology, 2nd ed. (1969), is a classic account of the shaping of the Earth's surface by natural forces, including a description of the relationship between structural geology and landforms. Tjeerd H. Van Andel, New Views on an Old Planet: Continental Drift and the History of Earth (1985), is an excellent introduction to the ideas of continental drift, polar wandering, seafloor spreading, and plate tectonics from the viewpoint of an oceanographer. Brian F. Windley, The Evolving Continents, 2nd ed. (1984), relates the geology of various parts of the Earth to plate tectonics and the Wilson cycle of ocean-basin formation. Studies of historical geology include Don L. Eicher, A. Lee McAlester, and Marcia L. Rottman, The History of the Earth's Crust (1984); and Carl K. Seyfert and Leslie A. Sirkin, Earth History and Plate Tectonics (1979), which relates the history of the Earth to plate tectonics, especially to the Wilson cycle. Carl Keenan Seyfert