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Geophysics

Geophysics, one of several earth sciences, is the application of the principles and techniques of physics to the study of the Earth and its environment. It encompasses not only studies of the solid Earth but also of the oceans and atmosphere, the outer atmosphere, fields, particles, and planets within the solar system, and the relationship of the Sun and the planets.

SOLID-EARTH GEOPHYSICS

Studies of the solid Earth--its shape, composition, physical properties, and fields--are the subjects of several disciplines.

Geodesy is concerned with the shape of the Earth, its gravity field, and its orbital parameters, and with changes in the shape of the Earth brought about by tidal and tectonic forces. Studies of this shape and the gravity field lead to an understanding of the distribution of mass within the Earth. Variations of orbital parameters, such as changes in the pole of rotation and the length of the day, are measured at International Latitude Observatories established in a number of countries. The Earth's shape is measured both by conventional geodetic instruments and by satellites that measure the geopotential surface (geoid). The gravity field is studied by direct surface measurements and by analyses of the effect of the field on satellite trajectories.

Seismology is the study of earthquakes and related phenomena. Central to the field is the study of seismic waves generated by earthquakes and other disturbances, such as underground nuclear explosions. Seismic waves, sound waves that travel through the solid Earth, are of two principal types: body waves, which travel directly through the Earth; and surface waves, which travel along an interface of contrasting sound velocities, most prominently the surface of the Earth. Body waves propagate in two phases, as P-waves and as S-waves. In P, or compressional, waves, particle motion is parallel to the direction of wave propagation; in S, or shear, waves, particle motion is perpendicular to the direction of wave travel. Earthquakes generate these waves. By carefully timing the arrival of the waves, seismologists are able to study the physical properties of the deep interior of the Earth. Similarly, by locally monitoring seismic waves generated artificially by explosions, exploration geophysicists can determine underground structures that may indicate mineral deposits.

Seismology also involves the study of earthquakes themselves: the mechanisms, modes, and locations of occurrence, as well as their prediction. This part of seismology overlaps with the field of tectonophysics, which is concerned with deformations in the Earth. These range from the small deformations produced by tides to plate tectonics and mountain building (orogeny). Central to this field is the science of rock mechanics, the study of the physical processes responsible for the deformation of rock under the temperature and pressure conditions of the Earth's interior. Knowledge gained in this area is applied to study of the forces that generate earthquakes, volcanoes, and plate motions, and to the deformations produced by these processes.

Geomagnetism and paleomagnetism have to do with the nature of the Earth's magnetic field and the history of this field over geologic time. The Earth's main field, a dipole with poles offset approximately 11 deg from the rotational poles, is thought to result from the convective motion of conductive fluids, principally iron, in the Earth's outer core. This motion produces the field by induction. When a rock containing magnetic minerals (for example, magnetite) is formed, these minerals retain a magnetism that is oriented in the direction of the Earth's magnetic field at that time. Scientists have shown that the magnetic poles have wandered with respect to the location of the present continents (polar wandering) and that the polarity of the main magnetic field reverses periodically. These discoveries have made it possible to measure continental drift and to devise a geologic time scale for such events.

The study of volcanoes--their origin, behavior, and mode of occurrence--is the subject of volcanology. This science, especially with regard to the origin of the magmas that generate volcanoes, overlaps with the fields of geochemistry, petrology, and mineralogy.

OCEANS AND ATMOSPHERE

Investigation of the fluid envelopes of the Earth, both liquid and gaseous, have more in common with each other than with solid-Earth geophysics.

Meteorology is concerned with the behavior of the lower atmosphere, below the stratosphere. In that region the atmosphere is sufficient to behave according to the laws of fluid mechanics. Thus the general circulation of the atmosphere and the origin of weather and climate come under the heading of meteorology. Surface observations are now made in conjunction with global monitoring of the atmosphere by satellite.

The study of the oceans is usually broken into chemical, biological, and physical branches; only the latter, physical oceanography, is considered a branch of geophysics. Oceanography, concerned with the fluid part of the oceans, is distinct from marine geophysics, the study of the ocean floors. Major fields of study are the structure of the oceans--as expressed by variations of salinity, pressure, and temperature--and ocean circulation, currents, tides, and waves.

The study of the hydrosphere of the Earth--water that is not only in the oceans and atmosphere but on land, in lakes, rivers, and underground--is the domain of the hydrologic sciences. In studies of the hydrological cycle, hydrology extends into and merges with both oceanography and meteorology; other areas include groundwater hydrology and limnology (the study of lakes).

OUTER ATMOSPHERE AND SOLAR SYSTEM

Beyond 50 km (30 mi) above the Earth and extending into interplanetary space, matter is sufficiently diffuse to be strongly ionized by solar radiation; its behavior is determined largely by electromagnetic fields. The Sun gives off a continuous stream of charged particles (plasma) called the solar wind. The Earth's magnetic field forms a repulsive sheath about 100,000 km (62,000 mi) in diameter, called the magnetosphere, that changes continuously in response to changes in the solar wind arising from solar flares and other variations in the solar cycle. Magnetospheric physics is concerned with the nature of these fluctuations.

Within the magnetosphere, the upper atmosphere (50 to 2,000 km/30 to 1,250 mi) is diffuse and highly ionized. The study of the upper atmosphere, called aeronomy, includes many electrical phenomena, such as auroras and the Van Allen Radiation Belts, as its subjects.

The fields of solar and interplanetary physics involve the processes that occur within the Sun and their effects on solar radiation, the solar wind, and planets and other members of the solar system. Planetology is the study of the major and minor planets, with a view toward reconstructing the evolution of the Earth and of the solar system in general. Earth satellites and planetary probes have greatly expanded this field through the return of samples of lunar materials, the landing of scientific instruments on Venus and Mars, and flights past Mercury, Jupiter, Saturn, and Uranus and several satellites of the last three planets. Evidences of volcanism on Mars and Venus, for example, and even the very different volcanic processes at work on Jupiter's satellite Io, all aid in understanding the forces that shaped the Earth's own unique volcanic history.

INTERNATIONAL COOPERATION

Geophysics requires extensive data-gathering systems, such as global seismograph networks, weather satellites, oceanographic research vessels, and space probes. The first fully international cooperative venture began with the International Geophysical Year (IGY) of 1957-58. This highly successful project has led to numerous international programs, most of which are sponsored by the International Union of Geodesy and Geophysics. Like most member nations, the United States participates in these programs through its national organization, the American Geophysical Union.

Christopher H. Scholz

Bibliography: Allegre, Charles, The Behavior of the Earth (1988);

Bates, C. C., and Gaskell, T. F., Geophysics in the Affairs of Man (1982);

Foster, R. J., Physical Geology, 4th ed. (1983);

Mares, Stanislav, Introduction to Applied Geophysics (1984);

Stacy, Frank D., Physics of the Earth, 2d ed. (1977);

Wesson, P. S., Cosmology and Geophysics (1979).