EARTH SCIENCES

the fields of study concerned with the solid Earth, its waters, and the air that envelops it. Included in the Earth sciences are the geologic, hydrologic, and the atmospheric sciences. Each of the three major families of the Earth sciences is conventionally divided into disciplines and subdisciplines according to its particular subject matter. the fields of study concerned with the solid Earth, its waters, and the air that envelops it. Included are the geologic, hydrologic, and atmospheric sciences. The broad aim of the Earth sciences is to understand the present features and the past evolution of the Earth and to use this knowledge, where appropriate, for the benefit of humankind. Thus the basic concerns of the Earth scientist are to observe, describe, and classify all the features of the Earth, whether characteristic or not, to generate hypotheses with which to explain their presence and their development, and to devise means of checking opposing ideas for their relative validity. In this way the most plausible, acceptable, and long-lasting ideas are developed. The physical environment in which humans live includes not only the immediate surface of the solid Earth, but also the ground beneath it and the water and air above it. Early man was more involved with the practicalities of life than with theories, and thus his survival depended on his ability to obtain metals from the ground to produce, for example, alloys, such as bronze from copper and tin, for tools and armour, to find adequate water supplies for establishing dwelling sites, and to forecast the weather, which had a far greater bearing on human life in earlier times than it has today. Such situations represent the foundations of the three principal component disciplines of the modern Earth sciences. The rapid development of science as a whole over the past century and a half has given rise to an immense number of specializations and subdisciplines, with the result that the modern Earth scientist, perhaps unfortunately, tends to know a great deal about a very small area of study but only a little about most other aspects of the entire field. It is therefore very important for the lay person and the researcher alike to be aware of the complex interlinking network of disciplines that make up the Earth sciences today, and that is the purpose of this article. Only when one is aware of the marvelous complexity of the Earth sciences and yet can understand the breakdown of the component disciplines is one in a position to select those parts of the subject that are of greatest personal interest. It is worth emphasizing two important features that the three divisions of the Earth sciences have in common. First is the inaccessibility of many of the objects of study. Many rocks, as well as water and oil reservoirs, are at great depths in the Earth, while air masses circulate at vast heights above it. Thus the Earth scientist has to have a good three-dimensional perspective. Second, there is the fourth dimension: time. The Earth scientist is responsible for working out how the Earth evolved over millions of years. For example, what were the physical and chemical conditions operating on the Earth and the Moon 3,500,000,000 years ago? How did the oceans form, and how did their chemical composition change with time? How has the atmosphere developed? And finally, how did life on Earth begin, and from what did man evolve? Today the Earth sciences are divided into many disciplines, which are themselves divisible into six groups: Those subjects that deal with the water and air at or above the solid surface of the Earth. These include the study of the water on and within the ground (hydrology), the glaciers and ice caps (glaciology), the oceans (oceanography), the atmosphere and its phenomena (meteorology), and the world's climates (climatology). In this article such fields of study are grouped under the hydrologic and atmospheric sciences and are treated separately from the geologic sciences, which focus on the solid Earth. Disciplines concerned with the physical-chemical makeup of the solid Earth, which include the study of minerals (mineralogy), the three main groups of rocks (igneous, sedimentary, and metamorphic petrology), the chemistry of rocks (geochemistry), the structures in rocks (structural geology), and the physical properties of rocks at the Earth's surface and in its interior (geophysics). The study of landforms (geomorphology), which is concerned with the description of the features of the present terrestrial surface and an analysis of the processes that gave rise to them. Disciplines concerned with the geologic history of the Earth, including the study of fossils and the fossil record (paleontology), the development of sedimentary strata deposited typically over millions of years (stratigraphy), and the isotopic chemistry and age dating of rocks (geochronology). Applied Earth sciences dealing with current practical applications beneficial to society. These include the study of fossil fuels (oil, natural gas, and coal); oil reservoirs; mineral deposits; geothermal energy for electricity and heating; the structure and composition of bedrock for the location of bridges, nuclear reactors, roads, dams, and skyscrapers and other buildings; hazards involving rock and mud avalanches, volcanic eruptions, earthquakes, and the collapse of tunnels; and coastal, cliff, and soil erosion. The study of the rock record on the Moon and the planets and their satellites (astrogeology). This field includes the investigation of relevant terrestrial featuresnamely, tektites (glassy objects resulting from meteorite impacts) and astroblemes (meteorite craters). With such intergradational boundaries between the divisions of the Earth sciences (which, on a broader scale, also intergrade with physics, chemistry, biology, mathematics, and certain branches of engineering), researchers today must be versatile in their approach to problems. Hence, an important aspect of training within the Earth sciences is an appreciation of their multidisciplinary nature. Brian Frederick Windley Additional reading The history of the Earth sciences is recounted in Frank Dawson Adams, The Birth and Development of the Geological Sciences (1938, reprinted 1954), the best general account for the years prior to 1830; Asit K. Biswas, History of Hydrology (1970), a factual chronicle of developments since the earliest times; Henry Faul and Carol Faul, It Began with a Stone: A History of Geology from the Stone Age to the Age of Plate Tectonics (1983); A. Hallam, A Revolution in the Earth Sciences (1973), a summary of the historical development of ideas from seafloor spreading to plate tectonics, and Great Geological Controversies (1983), an evaluation of celebrated controversies from Neptunism to continental drift; Robert Muir Wood, The Dark Side of the Earth: The Battle for the Earth Sciences, 18001980 (1985), a history of important controversies; Richard J. Chorley, Antony J. Dunn, and Robert P. Beckinsale, The History of the Study of Landforms; or, The Development of Geomorphology, vol. 1, Geomorphology Before Davis (1964), an expansive account covering developments to the end of the 19th century; Charles C. Gillispie, Genesis and Geology: A Study in the Relations of Scientific Thought, Natural Theology, and Social Opinion in Great Britain, 17901850 (1951, reprinted 1969), an analysis of the impact of developments in geology upon Christian beliefs in the decades before Darwin (extensive bibliography); C.P. Idyll (ed.), Exploring the Ocean World: A History of Oceanography, rev. ed. (1972), a symposium treating each of the several branches of oceanography in historical format; Joseph Needham, Science and Civilisation in China, vol. 3, Mathematics and the Sciences of the Heavens and the Earth (1959), containing a comprehensive and elaborately illustrated account of the history of Earth science in China to around AD 1500; Cecil J. Schneer, The Rise of Historical Geology in the 17th Century, Isis, vol. 45, part 3, no. 141, pp. 256268 (September 1954), an analysis of the points at issue in the fossil controversy; Cecil J. Schneer (ed.), Toward a History of Geology (1969), 25 essays on the history of geologic thought, mainly of the 18th and 19th centuries; Napier Shaw, Manual of Meteorology, vol. 1, Meteorology in History (1926, reprinted 1932), a rambling but literate and entertaining history of meteorology from the earliest to modern times; Evelyn Stokes, Fifteenth Century Earth Science, Earth Sciences Journal, 1(2):130148 (1967), an analysis of classical and medieval views of nature, especially those reflected in Caxton's Mirrour of the World; Philip D. Thompson et al., Weather, rev. ed. (1980), an introduction to meteorology with much historical material, well illustrated; Stephen Toulmin and June Goodfield, The Discovery of Time (1965, reprinted 1983), which traces the history of the idea of geologic time; William Whewell, History of the Inductive Sciences from the Earliest to the Present Time, 3rd ed., 3 vol. (1857, reissued 1976)vol. 2 containing an analysis of uniformitarian and catastrophist views of Earth history; and Karl Alfred Von Zittel, History of Geology and Palaeontology to the End of the Nineteenth Century (1901, reissued 1962; originally published in German, 1899, reprinted 1965), best for its history of paleontology. Brian Frederick Windley The gravitational field of the Earth The nature of gravity It is a familiar phenomenon that an object released above the Earth's surface accelerates toward the Earth. This phenomenon is a special case of universal gravitationall mass within the universe attracts all other mass. The acceleration in this special case is known as the acceleration due to gravity, denoted g. Reference has already been made above to the fact that g varies over the Earth's surface and that this variation is intimately related to the shape of the sea-level surface, or geoid. In this section the nature of gravity, its measurement, and the relationship of gravity variations to the internal structure of the Earth are discussed. Newton put forth the law of gravitation for particles of mass m1 and m2 separated by a distance r in the form where F12 is the force of attraction of either particle on the other and G is a constant whose numerical value depends on the system of units employed. The application of Newton's law to bodies rather than to particles involves, in general, integration of the effects between differential elements; however, Newton also established the very convenient result that a uniform sphere attracts as though its mass were concentrated at the centre. This led to the possibility of measuring G in the laboratory, first exploited by the English physicist and chemist Henry Cavendish, by observing the force between massive spheres. In the International System of Units (SI), the modern (as revised in 1986) value for the gravitational constant is G = 6.67259 10-11 m3s-2 kg-1. While the numerical value depends on the system of units, the apparently small magnitude of G is real. Gravitational forces between bodies of less than terrestrial size are indeed small, as compared, for example, to electrostatic forces or the magnetic forces between electric currents. The acceleration due to gravity, g, is the force on a unit mass. In equation (11 ), if m1 = 1 and m2 = M, the mass of the Earth, the value of g on a spherical, uniform, nonrotating Earth is found to be where a is the Earth's radius. On the real Earth, departures from the spherical shape and uniformity and the effect of rotation all cause g to vary over the planet's surface. For example, the average value of g is close to 980 centimetres per second per second, but values at sea level range from about 978 near the Equator to 983 at the poles. Superimposed on this variation are the effects of internal structure, usually a small part of one centimetre per second per second. To describe these variations, a smaller unit has been introduced. In geophysics, one centimetre per second per second is known as the gal (after Galileo); 1 10-3 gals equal one milligal (mgal), which is the usual unit in which internal effects are measured. Since g itself is very nearly 1,000 gals, the milligal is approximately one part in 1,000,000 of g itself. Modern methods of measuring gravity approach a precision of one microgal, or 1 10-3 mgal. Gravity at any point on the Earth is not constant in time but varies periodically with the tide-producing attractions of the Sun and Moon. The tidal variation has been measured for some years, and its analysis provides information on the yielding of the Earth under tidal forces. While the emphasis in this section is on the uses of gravity measurements to study the Earth itself, it should be noted that a knowledge of the value of g is required, particularly in standards laboratories, for the measurement or calibration of other physical quantities. Such is the case whenever the weight of a known mass m is used as a standard of force, as, for example, in the absolute pressure exerted by a column of mercury in a barometer. The geophysicist is usually called upon to provide the best value of g for these locations. Basic characteristics of the terrestrial field Variation with latitude Even if the Earth were of uniform density or uniformly stratified in layers of constant density, gravity at sea level would increase from the Equator to the poles because of the combined effects of the planet's rotation and spheroidal shape. The effect of rotation arises from the fact that any body on the Earth experiences a centripetal acceleration given by rw2, where r is the perpendicular distance to the axis of rotation and w is the angular velocity of rotation of the Earth on its axis. Part of the inward gravitational attraction of the Earth must provide centripetal acceleration simply to hold the body on the planet's surface and thus does not appear in the weight of the body or in measured g. Gravity therefore decreases as r increases from the poles to the Equator. Rotation, however, also distorts the sea-level shape into the ellipsoidal form, so that points near the poles are closer to the Earth's centre of attraction. The effects of rotation and shape are thus cumulative. As mentioned earlier, the variation of gravity on this ideal ellipsoidal Earth was investigated by Clairaut, who showed that the expected relationship was where go(f) is the sea-level value of gravity at latitude f and B is a constant incorporating the effects of shape and rotation. If the potential of this field is expanded as a series of spherical harmonics (see above), it is found to contain a single latitude-dependent term involving P2 (cos q) where q is colatitude. The numerical coefficient of this term is known as J2, o. The three quantities B, J2,o, and the flattening f of the ellipsoid are obviously interrelated, the relationships having been given above in equations (3 ) and (8 ). Clairaut's equation as given is accurate only to the order of f; analysis to higher order of small quantities shows that additional terms in (sin f) are involved. The quantity go is a fundamental reference against which measured values of g may be compared to study all effects other than those of latitude. In the pre-satellite era the constants in the expression were obtained by fitting the measured values of g, distributed over the Earth, to the theoretical form; adoption of the constants by the International Union of Geodesy and Geophysics led to the International Gravity Formula. With the international adoption of MG, a, and J2, o, it is more precise to compute go. The adopted values of the latter lead to The magnetic field of the Earth The Earth's steady magnetic field is produced by many sources, both above and below the planet's surface. From the core outward, these include the geomagnetic dynamo, crustal magnetization, ionospheric dynamo, ring current, magnetopause current, tail current, field-aligned currents, and auroral electrojets. The geomagnetic dynamo is the most important source because, without the field it creates, the other sources would not exist. Not far above the Earth's surface the effect of other sources becomes as strong or stronger than that of the geomagnetic dynamo. In the discussion that follows, each of these sources is considered and their respective causes explained. The Earth's magnetic field is subject to variation on all time scales. Each of the major sources of the so-called steady field undergo changes that produce transient variations, or disturbances. The main field has two major disturbances: quasi-periodic reversals and secular variation. The ionospheric dynamo is perturbed by seasonal and solar cycle changes as well as by solar and lunar tidal effects. The ring current responds to the solar wind (the ionized atmosphere of the Sun that expands outward into space and carries with it the solar magnetic field), growing in strength when appropriate solar wind conditions exist. Associated with the growth of the ring current is a second phenomenon, the magnetospheric substorm, which is most clearly seen in the aurora borealis. An entirely different type of magnetic variation is caused by magnetohydrodynamic (MHD) waves. These waves are sinusoidal variations in the electric and magnetic fields that are coupled to changes in particle density. They are the means by which information about changes in electric currents is transmitted, both within the Earth's core and in its surrounding environment of charged particles (see below). Observations of the Earth's magnetic field Representation of the field Electric and magnetic fields are produced by a fundamental property of matter, electric charge. Electric fields are created by charges at rest relative to an observer, whereas magnetic fields are produced by moving charges. The two fields are different aspects of the electromagnetic field, which is the force that causes electric charges to interact. The electric field, E, at any point around a distribution of charge is defined as the force per unit charge when a positive test charge is placed at that point. For point charges the electric field points radially away from a positive charge and toward a negative charge. A magnetic field is generated by moving chargesi.e., an electric current. The magnetic induction, B, can be defined in a manner similar to E as proportional to the force per unit pole strength when a test magnetic pole is brought close to a source of magnetization. It is more common, however, to define it by the Lorentz-force equation. This equation states that the force felt by a charge q, moving with velocity v, is given by In this equation bold characters indicate vectors (quantities that have both magnitude and direction) and nonbold characters denote scalar quantities such as B, the length of the vector B. The x indicates a cross product (i.e., a vector at right angles to both v and B, with length vB sin q). Theta is the angle between the vectors v and B. (B is usually called the magnetic field in spite of the fact that this name is reserved for the quantity H, which is also used in studies of magnetic fields.) For a simple line current, the field is cylindrical around the current. The sense of the field depends on the direction of the current, which is defined as the direction of motion of positive charges. The right-hand rule defines the direction of B by stating that it points in the direction of the fingers of the right hand when the thumb points in the direction of the current. In the International System of Units (SI), the electric field is measured in terms of the rate of change of potential, volts per metre (V/m). Magnetic fields are measured in units of tesla (T). The tesla is a large unit for geophysical observations and a smaller unit, the nanotesla (nT; one nanotesla equals 10-9 tesla), is normally used. A nanotesla is equivalent to one gamma, a unit originally defined as 10-5 gauss, which is the unit of magnetic field in the centimetre-gram-second system. Both the gauss and gamma are still frequently used in the literature on geomagnetism even though they are no longer standard units. Both electric and magnetic fields are described by vectors, which can be represented in different coordinate systems, such as Cartesian, polar, and spherical. In a Cartesian system the vector is decomposed into three components corresponding to the projections of the vector on three mutually orthogonal axes that are usually labeled x, y, z. In polar coordinates the vector is typically described by the length of the vector in the x-y plane, its azimuth angle in this plane relative to the x axis, and a third Cartesian z component. In spherical coordinates the field is described by the length of the total field vector, the polar angle of this vector from the z axis, and the azimuth angle of the projection of the vector in the x-y plane. In studies of the Earth's magnetic field all three systems are used extensively. Figure 5: The components of the magnetic induction vector, B, are shown in three coordinate The nomenclature employed in the study of geomagnetism for the various components of the vector field is summarized in Figure 5. B is the vector magnetic field, and F is the magnitude or length of B. X, Y, and Z are the three Cartesian components of the field, usually measured with respect to a geographic coordinate system. X is northward, Y is eastward, and completing a right-handed system, Z is vertically down toward the centre of the Earth. The magnitude of the field projected in the horizontal plane is called H. This projection makes an angle D (for declination) measured positive from the north to the east. The dip angle, I (for inclination), is the angle that the total field vector makes with respect to the horizontal plane and is positive for vectors below the plane. It is the complement of the usual polar angle of spherical coordinates. (Geographic and magnetic north coincide along the agonic line.) The major geologic features of the Earth's exterior Deformation of the crust The Earth's crust has been subjected to widespread deformation over geologic time. This deformation results from forces applied to the rocks that make up the crust. Such forces create stresses within the rocks, and these stresses in turn produce strain, which is manifested in the form of folds, faults, and joints. Force, stress, and strain Force This is a vector quantity that changes or tends to produce a change in the motion of a body. It is a push or pull and must be specified as to its direction and magnitude. If there is no acceleration of a body, the forces that act on it are balanced. There are four types: compression, tension (or extension), shear, and torsion. During compression, rocks are squeezed together by two equal forces acting toward each other along the same line. During tension, rocks are pulled apart by two forces acting away from each other along the same line. Shear is produced by a force couple; i.e., rocks are subjected to two equal forces acting in opposite directions but not along the same line. Torsion is a twisting action produced by two opposed force couples acting in parallel planes. The structure and composition of the solid Earth The basic structure and composition of the Earth's interior have been known since the mid-20th century. In a landmark paper published in 1952, the American geophysicist Francis Birch described the constitution of the planetary interior based on a broad array of seismological, experimental, and geochemical observations. Although there have been numerous advances in the intervening years, such developments have served largely to reinforce or extend the picture described by Birch. Thus, it is worth summarizing this basic picture before describing the detailed evidence on which current models of the Earth's interior are based. Figure 15: Schematic cross section illustrating the shell structure of the Earth. The Earth consists of two major regions: a central core, which is almost completely molten, surrounded by a predominantly solid shell comprising the mantle and crust together (Figure 15 ). The chemical compositions of these two regions are entirely different. The core is made of a dense, iron-rich metallic alloy, in contrast with the outer shell that consists of rocky (or ceramic-like) material. Oxidesspecifically silicate minerals (compounds of silicon and oxygen)make up this rocky material, with the crust containing somewhat more silicon, aluminum, and calcium relative to the mantle (Table 3). Examples of such minerals that are common in the outer few hundred kilometres of the planet (roughly down to 400 kilometres beneath the surface) include olivine, pyroxene, and garnet. Pressures and temperatures increase with depth inside the Earth, reaching maximum values of 364 gigapascals (GPa; 3,640,000 atmospheres) and about 6,000 kelvins (K; 10,300 Fahrenheit) at the centre. The interior temperatures are high enough to partially melt a small fraction of the crust and mantle and to completely melt the outer core. Most of the interior, however, including the inner core, is at a temperature below the melting point. Figure 15: Schematic cross section illustrating the shell structure of the Earth. At depths of about 300 to 700 kilometres, pressures and temperatures become sufficiently high that the minerals of the upper mantle transform to more tightly packed crystal structures such as that of perovskite (see below). Because of the occurrence of these pressure-induced transformations, such physical properties as density and elastic-wave velocities are observed to increase rapidly with depth. This depth interval, called the transition zone, occurs about one-fifth of the way down into the mantle, separating the upper mantle from the lower mantle (Figure 15 ). Few, if any, significant mineral transformations seem to occur in the lower mantle, which is the single largest uniform region of the interior. Figure 15: Schematic cross section illustrating the shell structure of the Earth. The fluid nature of the outer core has one consequence that can be felt directly at the surface. Turbulent flow of the liquid metal in the core effectively produces a dynamo. As noted above, the result is the main geomagnetic field (in Figure 15 the magnetic field and the fluid flow by which it is created are schematically indicated by H and , respectively). Although the mantle is solid in the conventional sense that its temperature is almost entirely below the melting point, this crystalline region is found to behave like a fluid over geologic time. Large-scale deformation of the mantle results in plate tectonics at the surface and the related phenomena of earthquakes and volcanoes. This dynamic picture of the solid interior has been recognized since the 1960s, deriving largely from the realization that the crystalline solids making up the planet are ultimately weak. That is to say, laboratory experiments reveal that rocks can undergo substantial deformations over geologic time scales, particularly at the high temperatures of the interior. Also, innumerable geologic and geophysical observations at the Earth's surface are found to fit neatly into this picture of global tectonics involving large-scale deformations of the solid interior (see below The surface of the Earth as a mosaic of plates). Thus, it is concluded that the primary way by which heat is lost from the deep mantle and core is by convective flow, as though the mantle consisted of a highly viscous fluid. Viewing the planet as a large heat engine, one can see that it is convection that governs the thermal and chemical evolution of the Earth's interior. Zonal structure as reflected by variations in physical properties Seismology: wave-velocity and density distributions Figure 15: Schematic cross section illustrating the shell structure of the Earth. The main difficulty with studying the Earth's interior is apparent from Figure 15 . Rock fragments (called xenoliths) are brought up volcanically from depth, thus providing samples of the upper mantle. These samples, however, seem to originate at depths no greater than 150 to 200 kilometres. Therefore, the material making up more than 90 percent of the interior is inaccessible to direct observation, and investigators must instead turn to indirect, geophysical approaches. Of these, seismology offers by far the most precise and detailed picture of the interior. Seismic waves are essentially elastic deformations that are generated by earthquakes, natural explosions (e.g., volcanic eruptions), or artificial explosions (either nuclear or large chemical-explosive blasts). The waves propagate around or through the Earth, thereby revealing its internal structure.