GEOPHYSICS

major branch of the Earth sciences that applies the principles and methods of physics to the study of the Earth. Geophysics deals with a wide array of geologic phenomena, including the temperature distribution of the Earth's interior; the source, configuration, and variations of the geomagnetic field; and the large-scale features of the terrestrial crust, such as rifts, continental sutures, and mid-oceanic ridges. Modern geophysical research extends to phenomena of the outer parts of the Earth's atmosphere (e.g., the ionospheric dynamo, auroral electrojets, and magnetopause current system) and even to the physical properties of other planets and their satellites. Many of the problems of geophysics are analogous to those of astronomy because the subject studied is rarely under direct observation, and conclusions must be drawn largely on the basis of mathematical interpretation of physical measurements. These include measurements of the Earth's gravitational field with gravimeters on land and sea and artificial satellites in space; magnetometric measurements of the planet's magnetic field; and seismological surveys of subsurface geologic structures using reflected and refracted elastic waves produced by earthquakes or by artificial means (see seismic survey). Research conducted with geophysical techniques has proved extremely useful in providing evidence in support of the theory of plate tectonics (q.v.). Seismographic data, for instance, have demonstrated that the world's earthquake belts mark the boundaries of the enormous rigid plates that constitute the Earth's outer shell, while the findings of paleomagnetic studies have made it possible to trace the drift of the continents over geologic time.