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.
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
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
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.
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
- 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).